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Experimental Research in Aerodynamic Control With Electric and Electromagnetic Fields | Magnetohydrodynamics | Plasma (Physics)
Fifty years ago, publications began to discuss the possibilities of electromagnetic flow control (EMFC) to improve aerodynamic performance.This led to an era of research that focused on coupling the fundamentals of magnetohydrodynamics (MHD) with propulsion, control, and power generation systems. Unfortunately, very few designs made it past an exploratory phase as, among other issues, power consumption was unreasonably high. .Recent proposed advancements in technology like the MARIAH hypersonic windtunnel and the AJAX scramjet engine concepts have led to a new phase of MHD research in the aerospace industry, with many interdisciplinary applications. Compared with propulsion systems and channel flow accelerators, EMF Cconcepts applied to control surface aerodynamics have not seen thesamelevelofadvancementthatmayeventuallyproduceadevice that canbeintegratedwithanaircraftormissile.Thepurposeofthispaperistoreviewtheoverall feasibilityofthedifferentelectricandEMFCconcepts.EmphasisisplacedonEMFCwithhighvoltage ionizationsourcesandexperimentalwork.
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Progress in Aerospace Sciences 45 (2009) 3049
Experimental research in aerodynamic control with electric and electromagnetic elds$
E.M. Braun , F.K. Lu, D.R. Wilson
Department of Mechanical and Aerospace Engineering, Aerodynamics Research Center, University of Texas at Arlington, Arlington, TX 76019, USA
Available online 7 January 2009 PACS: 47.65.d 47.85.lb 47.85.ld 52.80.Hc 52.80.Pi
Fifty years ago, publications began to discuss the possibilities of electromagnetic ow control (EMFC) to improve aerodynamic performance. This led to an era of research that focused on coupling the fundamentals of magnetohydrodynamics (MHD) with propulsion, control, and power generation systems. Unfortunately, very few designs made it past an exploratory phase as, among other issues, power consumption was unreasonably high. Recent proposed advancements in technology like the MARIAH hypersonic wind tunnel and the AJAX scramjet engine concepts have led to a new phase of MHD research in the aerospace industry, with many interdisciplinary applications. Compared with propulsion systems and channel ow accelerators, EMFC concepts applied to control surface aerodynamics have not seen the same level of advancement that may eventually produce a device that can be integrated with an aircraft or missile. The purpose of this paper is to review the overall feasibility of the different electric and EMFC concepts. Emphasis is placed on EMFC with high voltage ionization sources and experimental work. & 2008 Elsevier Ltd. All rights reserved.
Contents 1. 2. 3. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 Magnetohydrodynamic interaction and scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 EMFC surface actuator design issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.1. Channel ow and open ow experimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 3.2. Power consumption and packaging. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 3.3. Selection of EMFC magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3.4. Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 3.5. Overall feasibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 Flow control by glow discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 Flow control by DBD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Conclusions and future outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47
1. Introduction Fifty years ago, an article was written describing the prospects for Magneto-Aerodynamics [1]. In it, Resler and Sears stated that an electromagnetic eld could be coupled with an ionized gas ow to accelerate or decelerate it, delay boundary layer
$ An abbreviated version of this paper was presented as AIAA Paper 2008-3788 at the 39th AIAA Plasmadynamics and Lasers Conference, Seattle, Washington, June 2326, 2008. Corresponding author. Tel.: +1817 272 5120; fax: +1817 272 5124. E-mail address: eric.braun@mavs.uta.edu (E.M. Braun).
separation, or to control skin friction and heat transfer. With several additions since that time, these goals remain the same. The authors also discussed several advancements critical to the progress of electromagnetic ow control (EMFC). Among them was the ability to solve the complex magnetohydrodynamic equations, which has eased tremendously stemming from the development of powerful computing hardware and numerical methodologies. Next, Resler and Sears mentioned that powerful magnets would be needed for ionized uid ow control. This requirement of high strength magnets has been achieved to some extent. Electromagnets can produce elds of several tesla, and superconducting magnets can reach tens of tesla. However,
0376-0421/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.paerosci.2008.10.003
E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049 31
Nomenclature b B cf DBD E e EFC EMFC F FL I IBL IEM IM J L n p q induced magnetic eld, T magnetic eld, T skin friction coefcient dielectric barrier discharge electric eld, V/m electron charge, 1:602 1019 C electrohydrodynamic ow control electromagnetic ow control force, N Lorentz force, N (single particle), N=m3 (ionized particles per unit volume) current, A sB2 L=ru1 cf =21=2 , boundary layer magnetic interaction parameter BEsL=ru2 , electromagnetic interaction parameter sB2 L=ru, magnetic interaction parameter current eld, A=m2 characteristic length, m number density, 1=m3 static pressure, Pa electric charge, C
ReM r S u V WIG x y z Z
Subscripts BL p 1 boundary layer particle freestream
the size of these magnets makes their integration into an aerospace vehicle problematic. Also, the magnets are dependent upon large power supplies. Similarly, research into rare-earth materials has progressed considerably since 1958, with inexpensive neodymium-based magnets currently available with maximum surface elds in the 0.51.0 T range. However, their use for aerodynamic control is limited since their magnetic elds are reduced as temperature is increased, making their incorporation into applications like scramjet inlets difcult if not impractical. In addition to the strength of the magnetic eld, EMFC is also dependent upon the conductivity of the ionized airow. Resler and Sears believed that articial seeding of the airow to create higher plasma conductivities would need development. At the time, plasma jet sources were capable of creating high values of conductivity for ground testing. As an example, consider a linear Lorentz force accelerator developed in the 1960s [2]. The accelerator had a square cross-section of 2.54 cm sides at the inlet that diverged to about 2.54 5 cm at the exit with an overall length of 76 cm. The 60 electrode pairs in the accelerator were powered by a warehouse of 1700, 12 V automotive batteries. The current draw of the accelerator electromagnets reached up to 900 A at 80 V. Finally, the plasma generator operated with a 10 MW power supply, and could create a ow with a conductivity of up to 500 =m (with seeding). Note that units of conductivity are labeled using =m which is equal to O1 =m. Historically, conductivity has been described using mhos per meter although the unit name siemens (S) has been designated for O1 to make =m equal to S/m. In order to reduce the power requirement, seeding the plasma jet with low ionization energy potassium and cesium compounds was explored, which resulted in a tremendous increase in conductivity relative to the unseeded gas. For instance, a hypersonic vehicle ying at an altitude of 30 km at Mach 16 would ionize the air after a bow shock to s  0:05 =m. Adding 0.1% potassium by weight could boost the conductivity to roughly 1 =m [3,4], a 20-fold increase. However, the vehicle-scaled power requirement of an air-breathing engine incorporating thermal ionization and thrust generation by an electromagnetic accelerator currently may only be met by a system such as an onboard nuclear reactor. Research in this eld waned by 1970.
For control surface aerodynamics, thermal ionization, whether augmented by seeding or not, may not be feasible or even desirable. Its benets and drawbacks have been discussed for concepts like the MARIAH hypersonic wind tunnel facility [5] and the AJAX scramjet power generator [68]. In particular, seeding may contaminate the ow of the MARIAH wind tunnel. At speeds below that which result in signicant shock-induced ionization, EMFC may have serious limitations compared to its overall benets since a separate non-thermal ionization system must be utilized. A few situations do exist in which ionization is currently experienced by an aerospace vehicle. For instance, the Space Shuttle interacts with ionized particles while in low Earth orbit [9] and during re-entry, where the re-entry environment has resulted in numerous studies (e.g., Refs. [10,11]) of how to improve current vehicle designs with the addition of electromagnetic elds. However, with the Space Shuttles impending retirement and no new prospects for the incorporation of actuators into full-scale space vehicles (assuming the Orion design is nearly nalized), less grandiose platforms like hypersonic missiles may currently be the best near-term candidate for EMFC systems. With that in mind, the ight Mach number may be limited to below about 15, for which articial creation of an adequate amount of ow conductivity is necessary. Recently, generation of a conductive gas, also known as a weakly ionized gas (WIG), has been accomplished using high voltage elds, laser beams, or perhaps directed microwaves [12,13]. However, these ionization methods produce a far lower level of conductivity when compared with results from 50 years ago. Experimentally, the maximum realized values of s in air are currently in the 0:121 =m range with high voltage elds. Raising the gas conductivity and minimizing power consumption are obviously priorities if practical aerospace systems are to be realized. Also of signicant note has been the development of ow control systems utilizing only an electric eld to create plasma. The design of an EFC device with only a high voltage eld is much less complex since the eld will ionize the air itself. Electrohydrodynamic ow control techniques can be divided into two categories: glow discharges and dielectric barrier discharges (DBDs). The physics for the two categories is similar. An air gap
0 m0 r re s
m0 suL, magnetic Reynolds number distance, m cross-sectional area, m2 ow speed, m/s voltage, V weakly ionized gas streamwise Cartesian coordinate spanwise Cartesian coordinate transverse Cartesian coordinate number of net charges on a particle permittivity of vacuum, 8:854 1012 F=m permeability of vacuum, 1:257 106 N=A2 density, kg=m3 charge density, C=m3 conductivity, =m
32 E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049
exists between the anode and cathode region of a glow discharge, while a much thinner dielectric material barrier is used for DBD systems which limits arcing. Paschens law states that the electrical breakdown voltage is based on gap distance and pressure. Because the anode and cathode of a DBD are separated by a thin dielectric gap, the operating pressure is higher and additionally the high electric eld signicantly raises the output Coulomb body force. Because of a larger air gap distance, most glow discharge research has occurred with low pressure and the control mechanism is thought to be more of a thermal effect. Although both systems solve the conductivity generation problem by ionizing the air without a separate system, the value of s is very low, even 105 2107 =m for some DBD actuators. Both systems also operate often using low power requirements that may be met by current on-board generators. One may assume the magnitude of the force generated by electric or electromagnetic elds is naturally a reection of the amount of power consumed. Considering the potential use of each in the aerospace industry, there is a tendency to associate electromagnetic elds with systems consuming a large amount of power. Systems based solely on the Coulomb force have been proposed and used for applications with relatively less power (i.e., electrostatic ion thrusters and ion lifters). Therefore, a fundamental issue to address for the future of ow control using these elds is dening how much power is needed for an appreciable control force. Put another way, while an electromagnetic eld is generally associated with larger scale aerospace applications and may be more robust, an electric eld may be all that is necessary to generate a satisfactory control force. The answer to this question will likely cause one to eventually be far more appropriate for use over the other. Recent experimental results have not entirely addressed this issue, although some progress has been made. Aerospace research involving electromagnetic elds has focused on hypersonic ows [14], particularly for the augmentation of scramjet propulsion systems. Most work involving MHD and scramjets has been computational, which is clearly understandable due to the cost and complexity of testing. However, this situation has led to signicant differences between experimental and computational work. Experimental values of pressure, conductivity and magnetic eld strength are usually below what is assumed analytically or in computational simulations. For example, Bruno et al. assumed a magnetic eld between 7 and 17 T as part of a rst-order electromagnetic hypersonic propulsion system [15]. In another design, Park et al. computed values of B 11:28 T, s 35:87 =m (with seeding) and p 1:25 MPa at the entrance to the scramjets MHD accelerator [16]. Recent experimental environments have been generally limited to 0.54.0 T, at most a few =m, and rareeld pressures. Low pressure testing has been a method to increase IM understandably as such conditions facilitate ionization. Little discussion has been articulated on the subject matter of selecting appropriate values of these parameters for aerodynamic control surfaces. Interestingly, these experimental values of B, s and p might be better suited for control surfaces rather than for propulsion systems. Practical values of crucial scaling parameters must be established so as to dene what value ranges should be associated with larger, propulsion-associated systems and smaller control surface systems. Conversely, electrohydrodynamic ow control surfaces have mostly been experimentally demonstrated with low freestream speeds and Reynolds numbers [17]. These demonstrations represent a signicant departure from the high-speed ight regime where electric and electromagnetic elds were rst considered for use. This situation is especially true for DBDs. High-speed experimentation has appeared with glow discharges, but the studies have historically centered upon bluff bodies more
than aerodynamic surfaces. Again, static pressure is often in the range of a few torr to facilitate the creation of a diffuse electrical discharge. Although the high-speed control effect of glow discharges appears to depend more on an increase in of the local speed of sound caused by Joule heating rather than the presence of an electrohydrodynamic force, rapid control actuation is desirable. Increasing the Reynolds number while still providing effective control with these systems is crucial. For ow environments besides very low speeds where DBDs appear best suited, glow discharge and EMFC concepts are competitive for a wide range of control possibilities.
2. Magnetohydrodynamic interaction and scaling The central difference between electrohydrodynamics (EHD) and magnetohydrodynamics (MHD) is the force produced during the interaction of ionized particles with the electric or electromagnetic elds, respectively. For EHD, it is the Coulomb force while for MHD it is the Lorentz force. These interactions are often summed up in one equation written as F qE u B. (1)
With research in electric and EMFC beginning to focus mainly within the boundary layer where E is high and u is low, there is a tendency to observe Eq. (1) and conclude that the presence of a magnetic eld has little effect on the magnitude of the body force from a simple order-of-magnitude comparison of the two terms on the right-hand side. This conclusion is incorrect since both RHS terms of Eq. (1) actually contain an electric eld component. Note that the u B product is actually an electric eld, usually referred to as the internal-induced electric eld in channel ow applications. The single E term is referred to as the applied or external electric eld for accelerators and generators, respectively. Interaction of only an electric eld with ionized particles will produce a body force, but its relative magnitude with respect to the force produced by an electromagnetic eld cannot easily be determined by Eq. (1). A strong current eld interacting with ionized particles can create an induced magnetic eld, but that eld and any body force generated from that interaction is likely negligible for EMFC as will be shown. The concepts above have been well established in the literature [18,19]. In order to provide insight to MHD interaction and scaling, a derivation of the EHD and MHD forces is useful. Coulombs law states that two charged particles exert a mutual force in a direction parallel to the line connecting each particle. If one of these particles is held stationary in the reference frame of the other one, it creates an electric eld and exerts a force on the other particle, written as   eZ 1 r2 r1 F2 r2 eZ 2 , (2) 3 4p0 jr2 r1 j where the bracketed term represent the electric eld. Similarly, the magnetic force law states that a force is developed between two current carrying wires which is dependent on distance and, additionally, the orientation of the wires. Over a length of wire dl this force is written as dFr
m0 I I 0 0 dl dl r r0 , 4pjr r0 j3
where the prime is used to denote the properties of one wire from another. Invoking the BiotSavart law leads to an expression for the magnetic eld at r, namely, Z 0 0 0 m I r dl r r0 . (4) Br 0 4p l0 jr r0 j3
E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049 33
Next, using Amperes law, Eq. (3) can be simplied to dFr Ir dlr B. (5)
Table 1 Resulting dimensionless numbers for several EMFC variable combinations. Variable combinations B; E; s; u; p B; E; s; u; r B; E; s; L; p Dimensionless numbers E Bu E Bu BEsL p B3 sL Er E BEsL ; Bu p E s B2 L BEsL ; ! Bu ru ru2
If S denotes the cross-sectional area of the wire, n denotes the number of particles and the subscript p denotes a particle along the wire, then the force is dF np eZ p S dl up B. Furthermore, the force on one particle is Fp eZ p up B. (7) (6)
B; E; s; L; r B; E; s; L; p; u B; E; s; L; r; u
Therefore, the combined electric and magnetic forces on a particle are Fp eZ p E up B. (8)
Neglecting polarization and magnetization effects, the body force components on an ionized gas or liquid are shown in Eq. (9), which also has omitted the Hall effect (which can be signicant) and ion slip: F re E J B. (9)
Next, the order of magnitude of these electric and electromagnetic Lorentz force components can be approximated, allowing for a comparison between the two [18]
minimized to raise the conversion efciency of energy used to accelerate the ow (rather than letting it contribute to Joule heating). From the last combination of variables, E=Bu and IM are formed. As is common with Buckingham Pi theorem results, two dimensionless numbers can be multiplied to form further dimensionless parameters. Usually only a few will have signicance, and one to highlight from that set is distinguished as the electromagnetic interaction parameter: IEM BEsL . ru2 (14)
re E
0 E2
F L j J Bj  sE uBB.
The relative magnitude of the two terms approximating F L in Eq. (11) is critical for EMFC characterization. From Eqs. (10) and (11), the ratio of the electrohydrodynamic force to the magnetohydrodynamic force is F EHD re E 0 E2 . F MHD j J Bj sLE uBB (12)
With the potential difference between electrodes on MHD accelerators and recent EMFC actuators usually on the order of 1000 V or less, it is apparent that the electrohydrodynamic force will be negligible, unless the characteristic length is very small. Aerodynamic control systems will operate in far larger environments. For EMFC characterization, it appears that much progress is needed to support experimental research. Historically, the magnetic interaction parameter IM has been used to dene the ratio of magnetic body force to the inertia of the uid: IM EM forces sB2 L . Inertia forces ru (13)
This parameter, like IM , is a ratio of the magnetic body force to the uid inertia. It is interesting to note their individual similarity to one of the respective Lorentz force components shown in Eq. (11). Although E  Bu has been assumed in the derivations of many publications (e.g., Refs. [18,21]), this assumption is not necessarily valid for small-scale environments like boundary layer EMFC where E is high and u is low. In fact, it appears that IEM and IM are useful over separate design spaces depending on where EbuB or E5uB, respectively. The term B3 sL=rE has not seen use, and is another product of the MHD loading factor and the interaction parameter. The term developed in the third row is interesting since it relates a magnetic force to static pressure, which may prove to be useful with further experimental testing. Another modied interaction parameter that has appeared in the literature is [12,2224] IBL
sB2 L
ru1 cf =2
The prospects of reaching IM  1 were discussed as a performance benchmark in literature decades ago (e.g., Ref. [20]) and has seen use again with many recent EMFC publications. However, these recent EMFC environments usually demonstrate IM 51 and it takes a combination of very high ow speed and low density to reach unity. Despite having a low value, these same experimental results still demonstrate appreciable changes to the ow. As Elsasser remarked, dimensional relations in MHD are often much larger or much smaller than unity [21]. Perhaps other dimensionless numbers are more suitable for characterizing and scaling the effects of EMFC. With pressure changes often measured to conrm the effect of the Lorentz force, variables to consider include B, E, r, p, u, L, and s. Table 1 shows several resulting dimensionless numbers derived using the Buckingham Pi theorem. The most common term developed from Table 1 is E=Bu, referred to as the MHD loading parameter. It can be seen as a ratio of the total power per unit volume added to the ow to the directed kinetic energy [2]. As such, the parameter should be
This term denes u as the friction velocity, whereby u p u1 cf =2. As research into boundary layer EMFC increases, this term and velocity proles using the inner-law variables should be useful in comparing effects over different test facilities. As an example of the importance of these parameters, consider a at plate, EMFC actuator with ve surface electrodes each separated by 1.59 cm of dielectric material as shown in Fig. 1. The electrodes alternate with embedded magnets. The gure is a computer rendition of an actuator constructed and tested [25], with the dielectric material shown as transparent. Such actuator geometry may be typical for surface EMFC as the eld progresses. Flat plate actuators with alternating magnets and electrodes have been previously demonstrated with salt water environments as can be seen in [22] along with a discussion of related research. The middle and outer electrodes are grounded, while the potential of the other two is 500 V. The length and width of the electrodes are 1.27 and 0.51 cm, respectively. The permanent, NdFeB magnets, 1.27 cm square by 2.54 cm long, are embedded about 1 mm below the surface. The measured magnetic eld across the surface of the actuator B is shown in Fig. 2. Next, assume this device is operating under a freestream airow of 1000 m/s. The conductivity of the air
34 E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049
Fig. 1. Image of the ve electrode, four magnet actuator plate with dielectric material shown as transparent. Fig. 3. The common logarithm of the Lorentz force (N=m2 ) across a spanwise slice over the actuator at x 1:8 cm.
Fig. 2. Total magnetic eld located on the surface of the at plate over the 10:8 3:2 cm area. Fig. 4. The common logarithm of IM =L across a spanwise slice over the actuator at x 1:8 cm.
is arbitrarily set to be 1 =m, produced by a separate ionization system. The boundary layer thickness has been xed at 1.0 cm and the velocity prole follows a typical turbulent shape. The boundary layer velocity as a function of height off of the at plate is denoted as uBL uz. The magnetic eld components (Bx and By ), actuator geometry, and electrode potentials were used as boundary conditions in a computational MHD code to determine the Lorentz force and interaction parameters. The magnetic eld generated showed reasonable agreement with experimental magnetic eld measurements that were taken up to z 1:27 cm. Fig. 3 shows the common logarithm of the local Lorentz force for a two-dimensional spanwise slice over the actuator at x 1:8 cm. The locations of the magnets and electrodes are labeled at the bottom. The Lorentz force is most heavily concentrated over the electrodes and especially at their edges where electric charge builds. The magnitude of the Lorentz force produced in part with the NdFeB magnets drops signicantly with height, but remains viable for boundary layer applications. The Lorentz force is not as high over the outer electrodes because they are grounded. In this spanwise arrangement, it is important to keep the outer electrodes grounded to prevent a counteracting body force from developing past the edges of the actuator.
Figs. 4 and 5 show the common logarithm of the local interaction parameters IM and IEM , respectively. Logarithms are used for two reasons, rst, because the localized values range over several orders of magnitude. Second, the similarities and differences between the gures are more important than the actual values. For the calculation of the parameters, r was approximated as 1 kg=m3 throughout the boundary layer. (In actuality it will increase or decrease towards the surface depending on the wall conditions.) Additionally, L was removed from the parameters since it is not meaningful for the twodimensional slice of interest. Fig. 4, as would be expected, shows IM distinctly centered upon the embedded NdFeB magnets. If IM is truly a good choice as a non-dimensional representation of the Lorentz force for this actuator, then the localized contours of Figs. 3 and 4 would be similar. An evaluation of the data used for this example indeed shows that EbuB across the boundary layer. Fig. 5 shows that the Lorentz force more appropriately follows the same contours as IEM . In Fig. 5, the maximum values of IEM are centered on the electrodes. The only difference between the gures is IEM is more uniform near to the actuator surface because of the use of u2 BL .
E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049 35
With the aforementioned ranges of s, u, and L, the magnetic Reynolds number is far lower than unity within the boundary layer for this example actuator. This result should be true for most if not all EMFC actuators, as the creation of an induced magnetic eld for a ight vehicle needs a combination of very high speed and a current eld likely too large to be supplied by an on-board generator.
3. EMFC surface actuator design issues For electromagnetic elds to be successfully applied into a control surface actuator, several issues must be considered. First, while channel ow setups are ideal for understanding the physics of EMFC, open ow experiments must be considered in which the EMFC actuator is contained in a at plate or airfoil. Power consumption and packaging are important issues to address, with the selection of magnets and the method of ionization key to success. The selection of EMFC magnets is a signicant matter since rare-earth materials would be ideal for placement inside a thin control surface, except for the major problem in which their eld strength is relatively low and adversely affected by heat. Finally, unlike EFC, a sufciently high value of conductivity must be created by an additional system for applications in which thermal ionization is not encountered in the ight regime chosen. 3.1. Channel ow and open ow experimentation While one of the focal points of EFC has been for control surfaces, the same cannot necessarily be said for EMFC systems thus far. EMFC experiments applied to aerospace systems have typically been for scramjet-like systems and have taken place in a channel ow environment. Analytical approaches have also been well established for MHD channel ows while open ow modeling requires more sophistication. The design highlights and operating conditions of several recent facilities are discussed below. Ionization systems for each facility will be elaborated upon further into the text. One recent channel ow facility has been developed with the intent to place an accelerating or retarding Lorentz force on a high-speed ow (Mach 34) of air or another mixture of gases [12,23,2837]. The walls of the test section each have electrodes mounted into them. Two electrodes mounted opposite of each other create a WIG in the test section using high voltage, low duty cycle pulsing while the other two are connected to a DC current system. That system provides the energy for the Lorentz force so long as the level of conductivity provided by the ionization electrodes is enough for current to cross the electrode gap. The Lorentz force may be applied with or against the ow depending on the electrode and magnet polarity. Experimental data collected have included ow visualization, ow uctuation measurements, Lorentz force-induced pressure changes, and the output of the ionization and Lorentz force systems. The test section static pressure of this facility ranges from 5 to 20 torr. The channel itself is small enough to be surrounded with an electromagnet that can reach B 2 T, while a NdFeB magnet conguration (B  0:4 T) was also demonstrated in earlier works [12,30]. A decrease was measured in the pressure uctuation due to the Lorentz force in Ref. [12], but no direct measurements of pressure were taken at the time. The ow conductivity from the ionization system has risen with more recent publications, and is usually on the order of 0:1 =m. Further studies with electromagnets have shown large pressure differences created by the Lorentz force. For instance, Nishihara et al. [29] have shown a static pressure increase of 1720% for a retarding force and 57% for an accelerating force of the same magnitude. Similar data of
Fig. 5. The common logarithm of IEM =L across a spanwise slice over the actuator at x 1:8 cm.
The consequences of selecting the correct interaction parameter for an actuator will have a larger impact than the comparison of these gures. Most important, the performance will scale differently with E, B, and u depending on the value of the MHD loading parameter. On a case-by-case basis, the average of the localized MHD loading parameter in the boundary layer should be known before the actuator and its operating conditions are characterized by IM or IEM. Although IM has been more widely used than IEM , the example shows that the design space over which IEM is applicable is signicant. Also, where E  Bu, neither term may be appropriate for correctly scaling the Lorentz force effects. Going back to Fig. 3, the Lorentz force is noticeably nonuniform across the span of the actuator. This may be unavoidable for surface actuators since the electric and magnetic elds have inherently large gradients. The simplest strategy for creating some uniformity is to match the maximum B eld points with the minimum E eld points and vice versa along the actuator surface. Earlier MHD studies have shown that the geometry of segmented electrodes has a large impact on the distribution of the electric eld [26], and that observation certainly applies to this example actuator. As the MHD loading parameter decreases, the maximum Lorentz force locations will gradually shift from over the surface of the electrodes to over the magnetic poles. Computing these local parameters is only practical in a computational environment, but doing so may be necessary before accurately scaling the performance of the actuator with any particular interaction parameter. Establishing u as the friction velocity in the manner of IBL for comparison purposes across different facilities is recommended. Continuing on, the magnetic Reynolds number is a measure of the ease with which an ionized gas moves through a magnetic eld, and is dened as ReM m0 suL. (16)
ReM J r b.
The number can also be seen as a ratio between convection and magnetic diffusion effects, where diffusion (and therefore B) dominates the system for ReM 51 [27]. When ReM 41, the motion of charged particles in a current eld can create an induced magnetic eld b, responsible for many astrophysical phenomena. A relationship between the magnetic Reynolds number, the current eld, and the induced magnetic eld is
36 E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049
Fig. 6. Normalized static pressure traces downstream of an EMFC actuator for M 3 dry air for four electromagnetic arrangements (from [35]).
greater magnitude from [35] are shown in Fig. 6. The gure shows the normalized pressure difference for dry air between unaltered ow, ow with a retarding force, and ow with an accelerating force. The retarding force is more effective than the accelerating force because it works with Joule heating to create the rise in pressure, while the accelerating force works against Joule heating. The magnitude of the pressure rise also appears to be dependent upon Lorentz force polarity, where it is suggested that the test section Mach number and pressure are affected by the electromagnetic force interaction. Another channel ow test section has been constructed to explore the effects of a constricted plasma column operating under the presence of a magnetic eld [38]. In the test section, two tapered electrodes were placed on the side of one of the tunnel walls. After actuating a high voltage DC circuit, a constricted plasma column forms between the electrodes and propagates downstream due to the tapering. A helium-cooled superconducting ring magnet surrounds the channel and can generate a B eld up to 7 T, which increases the velocity at which the plasma column travels [39]. Since EFC systems are based on momentum transfer due to collisions between the ions and a neutral ow, this system can be seen as a novel method to enhance the momentum transfer using magnetic elds. Instead of two separate power supplies for ionization and Lorentz force generation, a single 20 kV, current regulated power supply is used. The electric eld generated by the 20 kV potential ionizes the gas to the point of breakdown, and the resulting arc draws up to a specied current limit. Once the current limit is reached, the power supply voltage drops signicantly, so the power input into the ow is considerably less than its maximum value of 20 kW. Therefore, the initial 20 kV potential before breakdown acts like an ignition system for the EMFC actuator. The test section has a Mach number of 2.8 and static pressure of 28 torr. Experimental data collected consisted of static pressure measurements in addition to ow visualization. A wedge was placed inside the test section to study the effects of the system on shock/boundary layer interaction [39]. Results show that the actuator is able to move the separation bubble induced by the
shock interaction, and the static pressure across the wedge is affected. A third facility has been developed with a at plate secured inside a free jet test section designed for basic research in MHD [40]. An electromagnet with a maximum eld of 3.5 T surrounds the entire channel, and a NdFeB conguration has also been tested. The Mach number in this facility is about 5.0, and the test section static pressure is designed to simulate an altitude between 30 and 50 km (approximately 0.67 torr). Besides covering the altitude range mentioned, the low test section pressure has also been designed with consideration to raising the value of IM , which reaches approximately 1.5 per meter. The at plate, shown in Fig. 13, has two embedded electrodes which use high voltage DC, RF, or a combination of both elds to ionize the air up to 2:5 =m. The gure also shows the rareed air pressure in the test section allows for a relatively low voltage glow discharge to transmit a substantial amount of power to support the Lorentz force. Before incorporating a at plate into the facility, tests were conducted with blunt body congurations at Mach 5.8 where experimental results included plasma diagnostics and aerodynamic force measurement [41]. For the at plate conguration, surface pressure measurements indicated that a Lorentz force directed out of the plate has more of an effect than directing it into the plate, again due to ow coupling and Joule heating issues [42]. Both push the luminous region of the glow discharge onto or off the actuator surface. Although a glow discharge raised lift by up to 18% in one set of experiments, applying a magnetic eld can negatively affect the discharge and void the change in lift [43]. A more recent study has moved to testing rectangular and cylindrical inlets supported by computational modeling [44]. EMFC publications have increased in the past few years and other facilities are likely to join those above. The ionization systems and overall electrode design for the generation of a WIG or plasma column and interaction with a magnetic eld to produce the Lorentz force appear to be feasible for high-speed, boundary layer EMFC based on results from these facilities. However, the value of conductivity generated (0:122:5 =m) by the high-voltage systems as well as the test section pressure are several orders of magnitude below those that may be necessary for the AJAX engine concept. It would be very interesting to modify the geometry and examine the performance of these facilities under an open ow, at plate environment with pressures closer to what may be encountered by a wing or n during high-speed ight. Magnets for such applications should also be embedded in the surface. Control of slender wings and ns, and perhaps the initial stage of an inlet compression system, are likely the best applications for these systems. If changing the geometry and increasing p are not formidable obstacles, perhaps these types of systems could be placed on a high-speed missile for control purposes. With electromagnetic elds, experimental measurements can be difcult because of signal interference. Typically, one or more transducer ports are placed downstream of the electromagnetic arrangement to capture the change in static pressure resulting from the Lorentz force. These data, along with ow visualization, power input, and plasma diagnostics results provide the means to understand the basic physics of EMFC. The measurement of aerodynamic forces, conducted in a few studies, will need to become more widespread as the actuator designs become more representative of control surfaces. Since most EMFC studies have been primarily focused on boundary layer control, it is desirable to use more rened techniques to analyze changes to the boundary layer prole. The inherent non-uniformity of the Lorentz force eld likely adds unusual effects that must be measured as a function of height above the plate as well as in the spanwise direction. Fig. 7 shows an example of the change in boundary layer
E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049 37
Fig. 7. Boundary layer velocity prole downstream of a at plate EMFC actuator for salt water ow (from [45]).
Fig. 8. Smoke visualization of a DBD control surface composed of rows of actuators creating an electrostatic force that acts from left to right (from [52]).
prole for a low-speed salt water freestream ow of about 18 mm/s as measured with a particle image velocimetry (PIV) system. Since salt water is naturally conductive (a few =m), at plate Lorentz force actuators have been much easier to build, test, and characterize [22,45]. PIV imaging directly over an EMFC actuator for gas ow may be difcult because of the luminosity of the ionized gas and Lorentz force energy addition. Although the ow speed in Fig. 7 is very low, other research with salt water has been conducted at higher speeds. The concept of EMFC and propulsion for naval applications has existed just as long as it has for aircraft [46]. The concept has also been proven with subscale submarines and ships [47,48]. A collection of papers in this eld concerning electromagnetic drag reduction can be found in Ref. [49]. Studies of MHD propulsion have concluded that it is feasible and desirable because of stealth [50], but effects from bubble formation at the electrodes in salt water and the generation of hydrogen and chlorine gas will need mitigation. MHD propulsion for a full-scale submarine will require signicant power and new developments in efciency for on-board nuclear reactors. 3.2. Power consumption and packaging For EFC systems, power consumption and packaging are relatively simple issues. Glow discharges require high voltage, but they are generally low power phenomena. DBDs also require high voltage, but the low operating current again leads to low power consumption. Corke and Post report a power level of approximately 6.5130 W per spanwise linear meter for DBD actuators [51]. Fig. 8 shows the ease at which these DBD actuators can be placed onto a surface as long as the material they are embedded in is dielectric [52]. Since the actuators are thin in the streamwise direction, the power requirement is anywhere between several hundred watts and a few kilowatts per square meter of a hypothetical control surface built from placing them into rows. In the gure, the separation between DBD strips is 1.0 cm, and the RF signal input is 4.5 kV RMS at a frequency of 3.3 kHz. At these conditions, a ow speed of 6 m/s is induced at the edge of the panel. The thrust from these DBD arrangements rises with dissipated power [53]. Assuming the induced ow speed for ight-ready control surface actuators will need to be higher than a few meters per second, they will eventually have signicantly higher power consumption than what was reported in Ref. [51]. Since glow discharge and DBD systems often use these thin sets of electrodes, packaging into control surfaces is also straightforward. The largest components of these systems may indeed be the high voltage circuit elements. As Jayaraman et al.
Fig. 9. A water-cooled electromagnet surrounds an EMFC free jet test section (from [43]).
discuss, increased interest in low Reynolds number aerodynamic control for micro air vehicles (MAVs) has brought the use of DBDs into consideration [54]. Although current computational and experimental research appears promising, scaling down to smaller, low-speed vehicles that t into the useful design space for these actuators may pose a problem due to the mass and volume of high voltage circuit components. Studies on the integration of these components to small-scale aircraft appear limited and it is recommended that future research efforts cover this topic. Concerning EMFC systems, the packaging issue is more complex and dependent not only on the power requirement but the choice of magnets. Fig. 9 shows a large electromagnet surrounding a hypersonic test section. As used in Ref. [43], it generated a magnetic eld of 0.9 T. Although electromagnets used for experimental channel ow EMFC are useful in that they can provide a steady magnetic eld inside the test section (whereas a eld from NdFeB magnets will vary as shown in Fig. 2), their ability to be integrated into ight vehicles is questionable due to their mass and weight. The use of superconducting magnets only exacerbates the problem. The use of rare-earth magnets will lead to the most compact EMFC actuator that may be placed on the surface of a wing or at the beginning of an inlet compression system. Clearly, the drawback of efcient packaging with embedded permanent magnets is the relative reduction in magnetic eld strength across the control surface. However, going back to the discussion of the interaction parameters, if the
38 E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049
actuator performance is not wholly dependent on B2 , a reduction in B is easier to offset with one of the other variables. Assuming EMFC surfaces themselves can be made compact enough for ight, there is still a problem with the possibility that the Lorentz force power requirements will exceed the output of a reasonable on-board generator. As discussed, ionization systems may also require a separate power supply. Lorentz force power requirement estimates similar to the one presented above for DBD actuators is still relatively unknown. The pressure increase of 1720% in Ref. [29] was created with about 2 kW of power where the electrodes enclosed a volume of approximately 40 cm3. This gure does not appear unreasonably high as many aerospace systems currently require power of that scale. The power supply requirements are also heavily based upon the time of use of the actuator. If the purpose is long-term drag reduction or lift enhancement, an innovative generator is probably required. Perhaps a short duration missile (with EMFC actuators for nal, rapid course corrections) can operate based off a thermal battery typical of current technology. The fact that many other potential sources (MEMS microturbine generators, fuel cells, ywheels, capacitors, etc.) of compact on-board power supplies are under development is encouraging [55].
Fig. 10. Temperature versus Mach number for lines of constant wedge angle (11, 51, 101, 201) after an oblique shock wave (based on an initial temperature of 220 K) along with neodymium and samariumcobalt maximum operating temperatures.
3.3. Selection of EMFC magnets Concerning the viability of EMFC, an inquiry must be made to understand exactly what range of magnetic surface eld values is needed. This inquiry leads back to the reason why magnets are needed for EMFC in the rst place, namely, that the cross product of magnetic eld and the electric eld produces the Lorentz force. The presence of a magnetic eld acts as a facilitator for energy addition into the uid ow from the electrodes. The energy addition is then split into Joule heating and the kinetic energy (rate of work done by the Lorentz force) of the uid. The selection of appropriate magnets for EMFC systems is a current topic of debate. As was mentioned previously, many recent experimental EMFC facilities have used powerful electromagnets capable of surrounding a test section since it is a straightforward way to increase the magnetic interaction parameter. However, some of these electromagnets and superconducting magnets have masses up to hundreds of kilograms (e.g., Ref. [39]) which will make ight applications problematic. The advancements in several new rare-earth magnetic alloys between 1970 and 2000 [56] have made permanent magnets a competitive choice for aerospace applications of MHD. Preliminary demonstrations of their capabilities have been seen in several publications [12,22,25,30,40,45]. Permanent magnets, where possible, should be considered instead of electromagnets since they consume no power and demonstrate much higher values of energy density making their strength-to-weight ratio relatively superior. Unfortunately, one major drawback of using permanent magnets for aerospace applications is the fact that high temperatures drastically weaken their overall surface eld strength, which is already low when compared to electromagnets. Permanent magnets lose their magnetic properties at a specied point called the Curie temperature. Prior to that point there is another temperature called the maximum operating point, after which a magnet will experience permanent losses to its original strength [57]. For AJAX-style scramjet engines, it is unlikely that permanent magnets could be used in the high-temperature environments even with robust active cooling systems. Fig. 10 shows the maximum operational temperatures of samariumcobalt and neodymium magnets charted along with typical post-shock temperature curves as a function of Mach number for different wedge angles with an incoming stream at
220 K. Neodymium magnets are operationally limited to temperatures just over 400 K, while some samariumcobalt alloys can be used at temperatures exceeding 800 K. While these temperatures are still far below the requirements of implementation into, for instance, a multi-shock scramjet inlet, these magnets could be used for slender control surfaces to some extent on supersonic and hypersonic vehicles. Moreover, while it would appear that samariumcobalt alloys are superior to neodymium for highspeed aerodynamic control because of the higher operating point, Fig. 11 shows that the high-temperature alloys typically have less overall magnetic eld strength [58]. Note that the magnetic ux density is measured using teslas, but the values from Fig. 11 are not representative of the maximum magnetic eld (also measured in teslas) that will be present on the surface of the magnets. Neodymium and samariumcobalt magnets are widely available, but they rarely demonstrate maximum surface elds over 0.5 T. Fig. 12 shows that it is common for permanent magnets to lose the bulk of their surface eld before reaching their maximum operational temperatures [59]. Typically, these magnets will see a slight linear decline in surface eld for a limited temperature range before reaching a point of rapid decline extending to the maximum service temperature. One of the focal points of current research in magnet development has been to broaden the temperature range in which only a slight linear decline is present, with signicant improvements made to samariumcobalt alloys [6063] and apparently much less work carried out with neodymium alloys. Literally tens of thousands of other alloys have been tested to determine if they produce a stronger magnetic eld, with no signicant ndings. Another method that has been used in an effort to improve rare-earth magnets is to use exchange-coupling to effectively combine the benets of so-called hard and soft phases of different magnetic materials [64]. A hard magnetic phase resists permanent demagnetization (i.e., B eld reduction by heating), and a soft phase exhibits a high level of magnetization. This concept has been experimentally proven to raise the energy product of rare-earth magnets where the phases are combined in nanoparticle assemblies [65]. One of the key issues to resolve in this eld is uniform nanoparticle dispersions of the hard and soft phases. As far as the surface eld is concerned, one can conclude that neodymium magnets are the better choice for applications with temperatures ranging up to 350 K. However, the typical decline of the neodymium surface eld strength as shown in Fig. 12 indicates that the performance of samariumcobalt magnets becomes comparable for higher temperatures. It appears that many, if not all, high-speed aerodynamic control concepts involving permanent magnets will require active cooling. This
E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049 39
Fig. 11. Magnetic ux density charted as a function of the maximum operating temperature for several neodymium and samariumcobalt alloys [58].
Fig. 12. A typical plot of the surface eld decline versus temperature for a neodymium magnet with a maximum operating temperature of 423 K [59].
need for active cooling is not solely associated with the use of rare-earth magnets. As exemplied by Fig. 9, some electromagnets also require cooling just to operate.
3.4. Conductivity In 1968, Garrison stated that the performance of MHD accelerators depends directly upon the magnitude of the electrical conductivity of the seeded working gas [66]. This statement remains true for aerodynamic control, only with more emphasis placed on improving low-temperature, WIGs. Before then, the concept of propulsion using electric and magnetic elds had appeared in the literature for several decades. Jahn presented a short review of early literature in electric propulsion [67]. Efforts at experimentation began in the late 1950s beginning with the implementation of plasma jets for propulsion systems [68]. Plasma jets were certainly capable of generating highly conductive gases through thermal ionization, but the temperature and power requirements were too high for viable aerospace applications at the time. Alkali salt seeding was introduced into the plasma jet in order to achieve the same level of conductivity at a considerably lower temperature [3]. Extensive experimentation with different low ionization energy seed materials (potassium and cesium) with air or noble gases (argon and helium) appeared in the literature through the end of the 1960s [6977]. Generating a bulk ow conductivity on the order of 1000 =m was achievable. The experimental gas pressures reported were usually on the order of
one atmosphere. At higher pressures around 10 atmospheres, electron attachment by positive oxygen ions signicantly reduces s [78]. Like many other elds, research in MHD was affected by the direction of the Apollo program. It appears that engineers may have assumed that megawatts of power produced by an on-board nuclear reactor would be available for future MHD acceleratorbased propulsion systems, but the nuclear prospect never materialized with the exception of the Project Pluto engine testing program [79]. Although nuclear-powered ramjet ground demonstrators were built and successfully tested during Project Pluto, the environmental concerns outweighed their strategic advantage. Additionally, the success of controlled ablation reduced the need for further research into EMFC for use on reentry capsules [80]. Arc jets were then applied to ground testing systems with many integrated into wind tunnels as a source of high enthalpy, high velocity ow [6]. Seeding is still a viable method for increasing the performance of those wind tunnels, but it can lead to undesirable contamination of the ow. Simmons et al. concluded that discrepancies in the air chemistry caused by seeded MHD accelerator concepts for the MARIAH hypersonic wind tunnel made non-MHD options more appealing [81]. The lack of post-Apollo funding and interest in on-board nuclear power effectively halted the prospects of MHD systems for aerospace vehicles. Non-nuclear power generation became the new focus of MHD research [82,83]. Despite the fact that electric propulsion engines make use of a comparatively weaker force, concepts [84] developed simultaneously with those of MHD propulsion and eventually ourished with help of a low power requirement capable of matching with radioisotope thermoelectric generator technology then emerging [85]. The past decade has certainly seen a reemergence of MHD research applied to aerodynamics. The history above shows that generating and controlling a ow with s4100 =m is difcult because of the power requirements. Creating ionization from a thermal source such as a plasma jet is not desirable, and is not possible for aerodynamic control surfaces. Fortunately, ionization can also be achieved through high voltage elds, laser beams, microwaves, and radiation (any method of transferring energy to cause molecular excitation of the gas). Of the EMFC facilities mentioned thus far, all have at minimum employed high voltage elds. However, a difference exists between each on how the high voltage elds are applied. The easiest method of creating plasma is to apply an electric eld with a large potential difference between two electrodes. Based on factors like separation distance, potential difference, geometry, and the gap medium, plasma will develop between the electrodes. The current is based on the effective resistance of the gap. When plasma lls the gap, the resistance is immediately and signicantly lowered. Normally, the end result is an arc discharge where the gap becomes a short circuit and draws maximum power from the potential source. Because of their difculty to control and destructive nature, arc discharges are most often seen as detrimental for engineering applications. Thus for aerodynamic control, ionization with electric elds has focused on producing discharges of a more diffuse nature, known as glow or corona discharges. However, Zaidi et al. have in fact used a plasma column control concept operating in a constant current, variable voltage mode [38]. The power supply potential was 20 kVDC, and the maximum current was 1.0 A. Fixing the current and activating the power supply causes a high voltage eld to be applied until electrical breakdown occurs and a plasma column forms. Once the column forms, the voltage required to maintain the set point current can be very low. When a plasma column forms between the path of least resistance where the tapered electrodes are closest to each other, it travels downstream where the gap
40 E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049
Fig. 13. A DC voltage discharge between two electrodes at freestream conditions of M 5:15 and p 0:59 torr. The applied voltage is 880920 V at a current of 50 mA. The addition of a magnetic eld signicantly affects the plasma and creates a virtual hypersonic leading edge strake (from [44]).
Fig. 14. Voltage oscillogram for 40 kHz pulsed ionization of a Mach 3 nitrogen ow. The test section pressure is 8.4 torr and B 1:5 T (from [36]).
between the electrodes gradually increases, visually similar to a Jacobs ladder. The boundary layer control is provided by the momentum transfer from the plasma column propagating downstream. During the wind tunnel experiments, the plasma column that forms between the electrodes was found to be periodic with a frequency of 110 kHz depending on the current and magnetic eld strength [86]. While applying a 1.7 kV eld at 35 mA with no magnetic eld, the plasma column travels downstream at 360 m/s. When a magnetic eld of B 2:0 T is applied, the column speed increased to 2000 m/s [87] to generate the control results previously discussed. Thus the electric and magnetic elds combine to allow for greater momentum addition to the ow. It appears that the performance of this facility can be increased simply by raising the set point current. Unfortunately, Joule heating has worked against the plasma column in the case of moving a shock wave-induced separation bubble downstream and will begin to suppress results while current is increased beyond a certain point [39]. To conduct further studies with higher plasma column power, an assembly consisting of a sapphire base plate and high-temperature arc corrosion-resistant electrodes was constructed [87]. The DC ionization system rst presented by Shang et al. [41] in 2002 was part of an EMFC actuator designed to affect the shock wave structure around a blunt body. A diffuse WIG is created instead of a plasma column due to a lower applied potential difference and a lower static pressure. The blunt body study was followed by a at plate model constructed of a ceramic base and two embedded copper electrodes [42]. Fig. 13 shows these electrodes, with the upstream cathode experiencing a more intense glow [44]. This diffuse glow discharge begins to constrict and transition to an arc when the current surpasses 100 mA or when B is greater than 0.2 T [42]. In addition to using only a DC discharge, pulsing the discharge with a frequency between 5 Hz and 10 kHz was also used to explore the response of the Mach 5 ow to plasma actuation with a magnetic eld present [88]. Volumetric heating of the air by the plasma was found to occur faster than the 3 ms response time of the pitot probe. In another case, RF radiation was added to augment the ionization created by the DC glow discharge, resulting in a reduction in the impedance across the electrode gap [40]. Accounting for all of the ionization methods, the maximum power requirement remained a few kilowatts or less and can result in a conductivity of a few =m in the static pressure environment of 0.67 torr. The research presented in the various publications for this facility certainly shows that a low-temperature WIG created by a
DC discharge ionization system allows for energy addition in the boundary layer of high-speed air ow. As a result, the energy addition allows for an EMFC actuator to signicantly affect the surface pressure distribution. Direct current discharges do not easily remain diffuse as pressure rises [89], but this system should be operational for high-altitude ight conditions. Research has shown that the degree of ionization produced by an electric eld may be higher for pulsed discharges rather than for a steady DC discharge of similar potential difference. Discharges with periodic high voltage pulses simply can withstand a higher applied electric eld before a transition to arcing occurs, so long as they are sufciently short enough to sustain streamers before transitioning to sparks [90]. This property allows more power to be transmitted during the pulse, which will increase s. Very short duration, low duty cycle pulses can maintain a conductive path between electrodes as long as the frequency is additionally high enough to counteract the WIG decay. However, the applied Lorentz force should be continuous and therefore generated with a DC power supply. Palm et al. [12] addressed these issues by creating an EMFC channel ow facility with RF WIG generation and simultaneous DC Lorentz force application using the electrode conguration previously discussed. The facility generated a diffuse WIG for Mach 24 ow originally by using a 13.56 MHz, 600 W RF power supply to create the conductive path for the Lorentz force energy addition [30]. Conductivity (0:121 =m) scales with the power draw of the system. Since that time, a more complex ionization system has been constructed to raise the attainable level of conductivity without raising the power draw. Nishihara et al. reported using this system to attain s  0:1 =m in a Mach 3 ow by compressing a 500 V, 1 ms pulse into a high frequency (up to 50 kHz), high peak voltage (20 kV), short duration pulse (1020 ns) [29]. During the peak voltage application, the current may reach 100 A, but the short duty cycle results in a reasonable overall power consumption of, for instance, 4080 W in Ref. [29]. The ionization power requirement is therefore much less than what is used to apply the Lorentz force. According to Ref. [34], raising the frequency of the system from 40 to 50 kHz increases the ow conductivity along with lowering the ballast resistance. The frequencies in this range match reasonably well with the WIG decay and provide fairly steady ow conductivity. The life of the WIG can be observed by measuring the current draw from the DC Lorentz force power supply. Fig. 14 shows four
E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049 41
pulses measured from the ionization system operating at 40 kHz. Fig. 15 shows two current oscillograms measured from the DC Lorentz force circuit for the same conditions as Fig. 14, created with constant electrode potentials of 2 kV (one at each polarity). The current rises at the initiation of each ionization pulse, and then falls with the WIG decay. With slightly different test section conditions, a current oscillogram appears in Ref. [37] for an ionization frequency of 100 kHz, which results in a more constant Lorentz force over time because the WIG decays less. As one can see from Fig. 15, the Lorentz force system has an average power consumption of about 2 kW and it is capable of creating the pressure changes shown in Fig. 6.
3.5. Overall feasibility A few examples of recent EMFC facilities have been presented in this section. These facilities have emphasized the use of electromagnetic forces within the boundary layer and they have
Fig. 15. Current oscillogram for Lorentz force power supply with the test conditions of Fig. 14 and different 2 kV electrode polarities (from [36]).
shown that EMFC can have a considerable effect at supersonic and hypersonic ow speeds. However, the pressures at which these experiments have been conducted, as well as the magnitudes of s and B, are far below what may be necessary for a hypothetical AJAX scramjet engine. Consequently, these systems are more applicable for boundary layer control of an aerodynamic surface or inlet system. Several steps must be taken to transition to a feasible electromagnetic virtual control surface. Experimental facilities have demonstrated success using low pressure core ows often surrounded by large magnets, and it is time to consider more compact congurations that can simulate environments like external ow over a wing or the beginning of an inlet compression system. In these environments, the magnets can be embedded below the surface and between the electrodes. Novel cooling methods must be developed for permanent magnets to survive the high-temperature environment. EMFC actuators may be placed in regions where the surface is actively cooled or is relatively cool. Considering the maximum operating temperature of rare-earth magnets, cooling to a temperature as low as 350 K may be necessary. Test section pressures must be increased, not necessarily to atmospheric, but perhaps to simulate the pressure after a shock over a thin wedge. The Mach 5 EMFC facility test section pressure reported by Shang et al. is meant to simulate an altitude from 30 to 50 km (0.67 torr) [40], but accounting for a real ight vehicle with a leading shock leads to much higher pressures in that altitude range (i.e., a 101 wedge at that speed would lead to a static pressure of 220 torr). Although it appears challenging, experimental generation of a Lorentz body force is not particularly difcult for EMFC actuators. The key problem is creating non-thermal ionization to supply a conductive working uid for the actuator [24]. Under rareed conditions, DC ionization systems are capable of creating a diffuse WIG for which s can reach a few =m at high speeds. As pressure rises, the voltage required to sustain a glow discharge rises. This relationship makes arcing with DC discharges more probable for EMFC as charge builds up on the electrodes. High voltage, highfrequency pulsed ionization sources are another available method for creating the same value of conductivity. These discharges with high energy transfer during low duty cycle pulses can be applied in systems to produce non-thermal ionization, and s is sustained by using a frequency fast enough to counteract the WIG decay.
Table 2 EMFC experimental environment summary. Ref. [22] [41] [30] [12] [32] [45] [33] [43] [34] [88] [40] [23] [28] [35] [38] [36] [42] [29] [86,87] [37] [39] [44] Year 1995 2002 2002 2002 2003 2003 2004 2005 2005 2005 2005 2005 2005 2006 2006 2006 2006 2006 2007 2007 2008 2008 Mach num. Low-speed 5.8 4 4 3 Low-speed 3.4 5.1 3.4 5.1 5 3 3 3 2.8 3 5.3 3 2.8 4 2.6 5.15
3.2 12.5  0.01 0.050.1 0.020.12 3 0.080.18 1 0.140.23 1 2 0.1  0.07  0.1 N/A  0.1 0.06  0.1 N/A  0.1 N/A 1
p (torr) 1300 1.22 68 210 1215 760 720 0.6 720 0.8 0.67 720 720 720 28 720  0.6 720 28 4.8 28 0.6
B (T)  0.4 02  0.4 0.45 01.5  0.4 01.75 0.10.2 01.5 0.2 01 01.5 01.5 01.5 02 01.5 01 01.5 04.5 01.63 04.5 00.2
Magnet type NdFeB Electromagnet NdFeB NdFeB Electromagnet NdFeB Electromagnet Electromagnet Electromagnet Electromagnet Electromagnet, NdFeB Electromagnet Electromagnet Electromagnet Superconducting Electromagnet Electromagnet Electromagnet Superconducting Electromagnet Superconducting Electromagnet
Ionization source N/A High voltage DC High voltage RF High voltage RF High voltage RF N/A High voltage RF High voltage DC High voltage RF HV, 10 Hz-RF HV DC, RF rad. High voltage RF High voltage RF High voltage RF Plasma column HV RF, seeding High voltage DC High voltage RF Plasma column High voltage RF Plasma column High voltage DC
Medium Salt water Air He, N2 He N2 Salt water N2 , air Air N2 , air Air Air N2 , He N2 , He, air N2 , air Air N2 , air Air N2 , air Air N2 Air Air
Conguration Flat plate Blunt body Channel Channel Channel Flat plate Channel Flat plate Channel Flat plate Flat plate Channel Channel Channel Channel Channel Flat plate Channel Wedge Channel Wedge Flat plate, inlet
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Ionization sources for EMFC actuators should trend towards using RF or square wave signals, as long as the packaging of the pulsing circuit elements does not lead to size and weight requirements much greater than a DC system. Fridman et al. postulated that voltage pulse durations less than 100 ns/cm of anode and cathode separation can sustain streamers without transformation into arcs [90]. Rening that estimate and determining the plasma decay rate between pulses will allow for the optimization of the ionization source and will minimize uctuations to the ow conductivity. The constant development of power semiconductors should make high-frequency pulsing systems smaller and more cost effective. Separate ionization and Lorentz force power supplies can be combined over the same at plate electrodes with the use of rectiers or diodes. The largest difference in pressure is obtained when the Lorentz force and Joule heating effectively work together. Since Joule heating thickens the boundary layer and decreases the local speed of sound, a Lorentz force applied to retard the freestream ow or direct it off of the surface has the greatest effect. With these issues properly addressed, EMFC could potentially be used in place of traditional control surfaces at high altitudes. It should not be questioned if the Lorentz force is powerful enough to provide high-speed aerodynamic control. The main issue is the determination of whether or not the ionization and Lorentz force power requirements of an actuator result in a system compact enough to be implemented into a ight vehicle. Table 2 provides a summary of literature in this section with experimental environments highlighted.
4. Flow control by glow discharge Although much of the previous discussion has been dedicated to ow control by electromagnetic elds, it must be noted that considerable interest for ow control with only plasma or electric elds has developed. Techniques for aerodynamic ow control by electric elds can be categorized into glow discharges and DBDs, covered in the next two sections, respectively. A glow discharge is formed across a gap of air or another gas between two electrodes with a difference in electric potential. The presence of a glow discharge is based on factors such as electrode geometry, ambient pressure, the gap medium, and the voltage. The glow discharge essentially means that the gap is lled with free radicals and electrons traveling between the electrodes. As such, the current increases rapidly after initial formation. Increasing the voltage after the glow discharge is formed eventually leads to electrical breakdown and arcing. A diffuse discharge is desirable since it indicates that the WIG effects will be uniform throughout the glow region. Often, experimentation with this phenomenon has occurred in low pressure environments where it is easier to create a diffuse discharge with a relatively low voltage. High pressure glows are also possible, and applying the potential difference with an increasing frequency has shown that the same current is maintained with a lower voltage [91]. Bletzinger et al. have provided a review of plasmas related to high-speed aerodynamics, containing a short history of the development of experimentation with glow discharges in recent decades [92]. In summary, initial shock tube experiments were conducted by measuring the drag and shock wave structure of objects (often blunt) while recording the differences with and without the actuation of a plasma source. As shown in Fig. 16, plasma ow as opposed to typical ow can drastically change the standoff distance of a bow shock around a blunt body [93]. Similar results were demonstrated as early as 1959 by Ziemer [94]. The change in shock wave geometry is important because a change in standoff distance can reduce heating and drag. Furthermore, the
Fig. 16. Split image of a bow shock around a sphere with and without plasma for a ow velocity on the order of 1600 m/s (from [93]).
properties of a glow discharge may be used to improve the offdesign performance of a high-speed inlet compression system by manipulating the shock wave structure. These features of glow discharges are signicant for the future development of re-entry vehicles and hypersonic airbreathing propulsion. Although some of the early shock tube literature makes a case for electrohydrodynamic effects as the reason behind some of the shock wave alteration [9597], the general consensus is that most of the effects seen are a product of the heating from the plasma [92]. Computational studies also indicate this result [98]. If the bulk of the glow discharge control effect is from heating, then the next logical step in the process of estimating its feasibility is to determine if there are benets of plasma as opposed to other heating sources. One benet of heating by glow discharges when compared to a typical heating element is rapid actuation. This may be a large enough benet to continue experimentation with surface glow discharges for aerodynamic control. For instance, Shin et al. measured a glow discharge actuation time of less than 220 ms using pin electrodes on a at plate in a Mach 2.85 ow environment [99]. This at plate plasma actuator is capable of creating a weak shock wave over the actuator when the plasma is diffuse. A more constricted plasma formation in that environment, although produced with higher power, does not have the same shock wave control effect. The difference between plasma heating and surface resistance heating is noticeable, whereby the plasma has more of a volumetric effect and is not exclusively characterized as typical surface heating [100]. Flat plate experimentation with glow discharge plasma actuators for aerodynamic control has yielded promising results.
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Experimental results from Ref. [100] show the plasma and surface heater both cause a 50% change in pitot pressure in Mach 5 ow over the cathode, with the glow discharge heater acting an order of magnitude faster. As was previously mentioned, the EMFC actuator in Ref. [43] produced an 18% change in lift while using only a DC glow discharge. Many other studies show similar results. Although efciency improvements have been made, the performance of these plasma control systems is still uncertain across wide ranges of ight speeds and vehicle congurations [92]. If indeed the plasma effect is thermal and increases the local speed of sound, than it can intuitively be expected that the effect will be lessened as speeds increase and the post-shock air temperature and enthalpy increase. Hence, a at plate actuator immersed in a low-temperature wind tunnel ow is a best case scenario for demonstrating glow discharge control. The power of the plasma source can be raised in order to compensate for a ow of higher enthalpy, but the efciency of control may be drastically reduced. Experimental studies using plasma discharges with wedges and blunt bodies are discussed next to elaborate on this issue. Although the presence of plasma can change the structure of a shock wave, it appears that most literature involving inlet compression systems also contains magnetic elds for the full Lorentz force effect. A detailed literature review appears in Ref. [92]. Some research with only plasma has been reported. For a Mach number of two and a ow mixture of nitrogen and helium, a glow discharge yielded a signicant change in the oblique shock angle over a wedge [101]. The change in the shock angle indicates a change in the Mach number from 2 to 1.8 due to the plasma heating. The WIG source was located on the walls of the wind tunnel. Since the initial ramps of an inlet compression system are often not surrounded by an outer wall (e.g., X-43A design), it would be benecial to test if this effect could be duplicated with a WIG source located entirely on the surface of the wedge. Placing a diffuse plasma source on the tip of a wedge and creating an effect on the oblique shock angle is a logical direction to move in to determine if these systems can be placed on a vehicle. Such a design was recently attempted by Gnemmi et al., where plasma discharges on a conical tip of a projectile were used to disturb a shock wave at freestream conditions of Mach 4.57 and 54 kPa static pressure [102]. More research is needed to determine if such systems can produce an appreciable aerodynamic control force, but it appears that ramp and inlet concepts of low to moderate turning angles are just as viable as at plate concepts. Closely related to that design is the concept of a virtual cowl that can be created by plasma heating [103,104]. The plasma source is more likely to be high-energy electron beams or microwaves rather than glow discharges. The heat addition specically can alter the upstream oweld in order to reduce the inlet spillage. This concept will require a considerable amount of power to operate, but one must consider that any system with plasma heating may be used only during a (presumably short) transition process by acceleration to the design Mach number. Although most of these system designs are analytical models, some preliminary experimental studies have demonstrated the concept [105]. Concerning blunt bodies, many studies focusing on drag reduction have appeared in the literature. One initial study showed that the drag coefcient for a sphere in the presence of a WIG was signicantly reduced for subsonic ow [106]. The same experiment for supersonic speeds showed that the drag coefcient was higher using a WIG than with typical airow, attributed to an increase in the pressure integral on the front of the model at certain conditions. Other plasma sources constructed for drag reduction have proven to be more effective since then. Ganiev et al. reported a reduction in the drag coefcient of about 50% from Mach 0.59 to 4 using a plasma jet placed at the tip of a
somewhat blunt body [107]. The reduction in the drag coefcient was found to depend on the stagnation temperature of the counterow jet. Plasma jets appear to be inefcient for streamlined shapes [108]. At the time of Ref. [107], other publications also described drag reduction with plasma jets and other forms of focused energy addition. A thorough list of these can be found in Ref. [109]. However, the large drag reduction by the plasma jet injection appears to be more directly related to the counterow jet instead of the thermal effects of the plasma. Fomin et al. experimentally determined that uid dynamics instead of plasma is the dominant effect using the jet for moderate supersonic Mach numbers [110]. Those experiments were conducted using a truncated cone cylinder at Mach 2, 2.5, and 4. As was discussed, the use of plasma jets was eventually deemed unrealistic for MHD ight applications in the 1960s because of the power requirement. Many of these current systems have been met with enthusiasm, but again scaling the power requirements to ight vehicles or missiles may pose insurmountable problems with current technology. Although new publications continue to emerge with different plasma sources and test geometries, very little of it is predominantly different from what was carried out at the beginning of this decade. Concerning the use of plasma ow control systems as part of realizable ight vehicles, some appear more feasible than others. In order to overcome problems including but not limited to power consumption, scaling, and hypersonic interaction at true ight conditions, the next step for plasma control for aerodynamics is a transition into realistic systems. It is understandable that some of the models of full-scale hypersonic systems have not been constructed due to the cost, but plasma control needs to be better proven experimentally as part of more ight-ready systems instead of basic shapes. Manipulating the bow shock wave around blunt bodies with plasma has been experimented with for 50 years, but no concrete applications are yet practical. It appears that ground testing of aerodynamic surfaces and inlet systems is moving forward, with the rapid plasma heating effect showing promise for control applications. The main challenge is producing systems that make use of current technology while maintaining power and packaging considerations.
5. Flow control by DBD Considering the physics involved, a DBD is similar to a glow discharge. Where a glow discharge has an air gap, a DBD contains a gap of dielectric material between the anode and cathode. Typical materials like glass, polymers, and ceramics have a much higher resistivity than air, allowing for the electrodes to be placed closer to one another. Closer placement increases the electric eld around the electrodes and ultimately raises the Coulomb force in Eq. (10) without the occurrence of electrical breakdown. The dielectric barrier is self-limiting as it prevents charge accumulation over the barrier material to prevent arcing. DBDs have been recognized since the mid-19th century, with the their rst application being the production of ozone [111]. Since that time, research has continued to grow and now applications include surface treatment, reduction of pollutants, lasers, and plasma display panels. Systems using glow discharges often use low pressure, but these discharges were stabilized across the barrier at atmospheric pressure beginning in the 1980s [112]. DBDs constructed specically for aerodynamic ow control applications appeared in the literature near the end of the 1990s [113,114]. In the decade since those reports, research into aerodynamic ow control with DBDs has rapidly increased both experimentally and computationally. A number of reviews have been written [51,115117], which probably indicates a variety of
44 E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049
Fig. 18. Typical spanwise cross-section geometry of a dielectric barrier discharge actuator for aerodynamics applications (from [120]).
Fig. 17. Smoke visualization shows ow reattachment on a NACA 0015 airfoil at a 121 angle of attack by an array of EFC actuators. The freestream ow speed is 2.6 m/ s (from [118]).
opinions on their applicability. Most conclude the DBD control effect is applicable for low Reynolds number ows below the general aviation range, and improvements to their strength will have to be made for them to be applied to ight aerodynamics. At low speeds, DBD actuators have a signicant effect on boundary layer ow. Fig. 17 shows a notable image by Roth et al. of ow reattachment made possible by an array of DBD actuators [118]. This actuator system works at atmospheric pressure, and has been named the One Atmosphere Uniform Glow Discharge Plasma (OAUGDPTM ). The ionization is created with a high voltage RF signal and the barrier material is Kapton. An RF signal is used rather than a DC signal because it creates a cycle of charge exchange between the electrodes that increases the control effect. The waveform shape and frequency along with the dielectric material choice may be optimized to some extent, with many different congurations reported. Several studies with the system in Fig. 17 have resulted in successfully increasing or decreasing drag on a at surface, adding momentum to the boundary layer ow, reducing the boundary layer thickness, and inducing a ow (also known as the ionic wind) of up to 6 m/s [119]. Fig. 17 raises the immediate question about the ability to apply a DBD system to high-speed ow where ow reattachment, drag reduction, and turbulence suppression are all major concerns. Although DBD actuators are studied by several institutions, the spanwise electrode geometry is always fairly similar and is depicted in
Fig. 18 [120]. This linear arrangement also can be modied into an annular jet source (known as a plasma synthetic jet actuator), where pulsed operation can generate vortex rings [121]. The maximum jet velocity for that study was on the order of 1 m/s. The electrodes also may be wrapped around the internal diameter of an axisymmetric jet. Benard et al. demonstrated that this conguration may be used for jet mixing enhancement, where experiments increased mixing in a ow up to 30 m/s on a model with an exhaust diameter of 72.5 mm [122]. No modication was seen at a jet speed of 40 m/s. The self-limiting DBD allowed about 10 W of power to be transferred into the ow, and it was noted that this value must be increased for the DBD to have more effect on the jet ow. The relative strength of current systems can be compared by their ability to induce a certain ow speed of air passing over the actuator. The ion wind speed measured in most recent surface DBD actuators is only a few meters per second, and efcient control results are obtained when u1 is less than 30 m/s [115]. However, some experiments have been conducted using higher freestream speeds. Opaits et al. [123] investigated DBD control of a NACA 0015 airfoil with freestream speeds of 2075 m/s at atmospheric pressure. The stall angle was raised with the DBD actuators at u1 75 m=s, and a change in pressure distribution was also recorded. Similarly, Roupassov et al. [124] measured changes in the pressure distribution for a NACA 0015 airfoil at speeds up to 110 m/s. In this case, the electrodes were placed parallel to the ow, and it appears that the pressure distribution incurs a greater change with the DBD actuator when the airfoil is close it its stall angle. One attempt was made recently to mount a DBD actuator on the leading edge of the wing of a Jantar Standard SZD-48-3 sailplane [125]. It appears that the DBD system was able to affect the separation and lift characteristics of the wing surface, but the data collected were not particularly reliable and rened tests are needed. A study by Corke et al. stated a DBD actuator was able to excite three-dimensional boundary layer instabilities on a sharp cone at Mach 3.5, but no similar results have appeared since [126]. For current DBD actuators, it is clear that a speed of 30 m/s represents the freestream ow limit in which a noticeable control effect is demonstrated. For freestream speeds over 30 m/s, DBD actuators may slightly delay stall, assuming they are placed on the location where the ow will normally begin to separate. In order to maximize DBD actuator performance for highspeed ow control, one may initially assume that the anode and cathode should have minimal size and be placed as close as
E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049 45
possible to each other and separated by a very thin layer of dielectric material. This geometry would maximize the electric eld, where the Coulomb force grows with E2 in Eq. (10). However, it has been argued that the force induced from DBD actuators should not be associated simply with E2 and requires a more detailed analysis [127]. The ion wind is a momentum transfer between neutral particles and heavy ions whose motion is induced by the Coulomb force. Consequently, the electric eld is signicantly affected from the charge accumulation and particle interaction over the dielectric gap. This interaction dampens the electric eld and is known as Debye or electrostatic shielding. Currently, numerical simulations are unable to simulate the observed random microdischarges in time and space that may help to resolve this issue. It does appear that the effectiveness of the exposed electrode is increased when it is thinner [120]. However, the ion wind increases with the width of the insulated electrode until it reaches a limit based on the applied voltage. Perhaps new efforts into geometric optimization or a method of resolving the E eld dampening will yield a viable design for freestream ows of higher speeds. An increase in the ion wind speed would no doubt expand the usefulness of DBD actuators for aerodynamics. They may become capable of improving the efciency of turbine blades as one example [128,129]. For those studies, DBD actuators were placed on the tip of low-speed turbine blades in a linear cascade. The chord Reynolds number was in the range of 104 105 . An actuator placed close to the separation region of the blades was determined to have an effect similar to using tabular vortex generators, where the advantage of the DBD actuators is they are used only when necessary. The lack of effectiveness of DBD actuators for higher freestream speeds is based more on the low energy input to the air ow rather than a reduction in the output ion wind due to electric eld dampening. Going back to the work of Benard et al., axisymmetric jet mixing was achieved with DBD actuators for a Reynolds number limited to about 128,000 [122]. Similarly, axisymmetric jet mixing was obtained with RF frequency plasma actuators acting on a Mach 1.3 ow with a Reynolds number of approximately 1.1 million [130]. The RF frequency plasma actuators added considerably more energy to the ow at a rate of 160 W where the exit diameter was 25.4 mm. Several options are potentially available to raise the strength of DBDs. First, the potential difference could be raised between the DBD electrodes. Enloe et al. have estimated that the induced velocity increases with V 3:5 [120], but very high voltage DBD actuators with an ion wind speed over 68 m/s still have not been demonstrated. Increasing the voltage ultimately leads to greater instability and possible signal interference if actuators are eventually placed on aircraft. Conceivably, altering the barrier material may lead to a design with higher power input, but doing so may increasingly indicate that high frequency plasma actuators without any barrier material at all are more practical for high-speed ow control. Certainly attempts are being made to limit electrostatic shielding and increase DBD efciency, but little discussion appears on a concrete methodology behind pursuing this strategy. Without signicant improvements to their overall strength, current DBD actuators are suited only for low Reynolds number aerodynamic control applications. A noticeable increase in work with applying DBD actuators to unmanned air vehicles (UAVs) and even MAVs has occurred in the past few years. One major issue with the development of small aircraft is the design of low Reynolds number airfoils that produce useful lift. Leading edge actuators have been demonstrated experimentally and computationally to increase the ight envelope of some familiar airfoil designs by modifying lift and drag [54,131,132]. DBD control for MAVs is advantageous for airfoils with high sweep and angle of attack where the use of conventional aps and ailerons is either
troublesome or completely ineffective. However, the effectiveness of low Reynolds number DBD active ow control actuators is very much dependent on the airfoil leading edge geometry [132]. These actuators are also limited to the low Reynolds number range because they lose their effectiveness in rareed atmospheric pressure environments normally encountered by high-speed vehicles. Fortunately, a momentum transfer study by Abe at al. has shown that DBD actuator thrust actually increases for a certain pressure range below atmospheric [133]. Until pressure was reduced to about 60 kPa, the performance was greater than or equal to what was recorded for atmospheric pressure conditions. Afterwards, performance dropped as pressure was reduced further and sparks began to develop which damaged the barrier material. This study indicates the actuators should perform well to a ight ceiling of perhaps 5 km. The power requirement of these UAVs and MAVs appears again to be only on the order of tens of watts or less. The next logical step in the development of these systems is to determine the mass requirements of the actuator circuit. The scalability of the mass and volume requirements of DBD systems must be determined for potential integration into UAVs and MAVs. Although it is well known that DBDs are lightweight and compact enough to be placed on a thin airfoil, the same has not yet been veried for corresponding on-board high voltage transformer and control circuit elements.
6. Conclusions and future outlook Flow control with electric or electromagnetic elds is an exciting topic due to its multidisciplinary nature, the possibility to solve difcult high-speed aerodynamics problems, and the overall design challenges. Also, another long-term factor can be added. It has long been theorized that research into new sources of atomic energy will eventually produce an extremely high power, yet compact generator system. The engine tested during Project Pluto shows that a nuclear reactor with 1960s technology was close to being capable of supporting a Lorentz force accelerator with thermal ionization. However, the radiation makes their implementation into a ight vehicle unacceptable. When a major breakthrough eventually happens, these new on-board generators will make all forms of MHD ow control realizable. Lorentz force engines may even someday replace conventional turbojet and ramjet engines. Until that time, an inquiry must be made as to what EFC and EMFC technologies can be supported with on-board power generators with todays technology. Thermal ionization for bulk ows does not appear achievable, leaving the non-thermal WIG sources as the best prospects for creating an appreciable amount of conductivity. Also, the ow speed range in which electromagnetic, glow discharge, and DBD systems are applicable does not appear to be clearly dened. Electromagnetic actuators are optimal for very high speeds where the ow downstream of the leading shock becomes ionized by itself and the thermal effect of using only plasma is likely to be negligible. This leaves a sizeable subsonic and supersonic gap where none of these concepts have been found to be relatively superior yet. EMFC actuators can be characterized in more detail with a better use of dimensionless parameters. For MHD accelerators, reaching IM  1 is achievable. For control surfaces, reaching IM  1 requires extremely low pressures and unrealistic magnetic elds. However, reaching that value is not necessary for boundary layer EMFC. The parameter dened as IEM may be more appropriate, not just because it results in a higher value but because it includes the electric eld and more accurately depicts the contours of the Lorentz force. The use of one dimensionless number over the other is based on the MHD loading parameter which indicates if one Lorentz force term out of EB and uB2 is dominant.
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DBDs, because of their geometric simplicity and compact size, seem ideal for high-speed ow control. For years, their applications have been growing and DBDs can be found in most households and ofces in plasma display panels. The concept of utilizing DBDs for aerodynamic control has existed for a little more than 10 years. Although this concept is under active research, it appears as though DBDs are limited to affecting freestream ow speeds of less than 30 m/s despite many optimization efforts to improve their strength. Limited control has been seen for speeds over 100 m/s when, for instance, the actuators are located directly where separation begins to occur over an airfoil. Therefore, current DBD actuators do not appear to be robust enough for all but very low-speed ight applications. The systems may be integrated into UAVs and other small vehicles to improve the airfoil ight envelope, but it is unclear if there will be a distinct advantage using this particular control concept due to the weight of on-board electrical circuits. The power requirement itself is low enough for small vehicles, but the supporting high voltage pulse equipment may lead to scaling problems. A tradeoff study should be considered for this issue. While DBDs have generally been researched with atmospheric pressure and low speeds, glow discharge phenomena have operated in low pressure, high-speed environments. As was discussed, the effect of glow discharges is generally thermal, which changes the local Mach number and can affect drag and the shock wave geometry. Although glow discharges have demonstrated several capabilities during subscale ground tests, some of their trends may be troublesome for high-speed ight. For instance, the glow discharge thermal effect will likely be reduced for higher speeds which produce higher aerodynamic heating. However, the rapid actuation ability is desirable and has been experimentally demonstrated to produce a signicant change in surface pressure. More emphasis should be placed on surface actuators and inlet systems in an effort to advance from blunt body testing which does not appear to have led to any engineering applications. Counterow plasma jets do not appear to be practical with thermal ionization. Another area where plasma will make a major impact in aerodynamics is the use of actuators to assist ignition and combustion. Ref. [134] provides a thorough experimental review of that eld. It appears that EMFC actuators have considerable potential for further research into high-speed ow control. EMFC systems have one major disadvantage when compared to DBD and glow discharge control in that a separate ionization system is needed to generate conductivity for the Lorentz force to take effect for typical high-speed aerodynamic conditions. However, new methods of creating non-thermal conductivity by high-frequency pulsed discharges, electron beams, microwaves, radiation, and various combinations are promising. Increased research into improving the conductivity seen with these systems and operating with higher pressures is recommended. Also, proof-of-concept testing of these systems will lead to further understanding of their effectiveness for control surface implementation. Magnet selection is another critical issue. Inexpensive NdFeB magnets can be placed into thin control surfaces, but they may need considerable active cooling in hot aerodynamic environments. It is unknown if the eld strength of these magnets will be high enough at this time. Electromagnets and superconducting magnets provide much higher surface elds than NdFeB magnets, but they carry a large weight penalty and must additionally be powered. Although NdFeB magnets have been discussed and tested to some extent in recent publications, more research should be conducted with them contained in compact control surface actuators. If NdFeB magnets prove to be powerful enough and can be cooled, the EMFC control should be more effective than control only with
Fig. 19. Example of a future high-speed missile with EMFC actuators that could potentially replace conventional control surfaces. Magnets may be embedded beneath and between the electrodes (colored black) and diffuse plasma (colored purple) is observed when the controls are actuated.
plasma. If not, plasma control with the rapid actuation effect becomes more advantageous. Perhaps the best prospects for on-board EMFC with current technology are for improved control of high-speed missiles, which would benet from surfaces that can actuate rapidly with reduced heating and drag when compared to mechanical actuators. A futuristic depiction is shown in Fig. 19. Power requirements will be raised, but improvements in on-board generators like state-ofthe-art MEMS microturbines may be able to provide the same power input for 5% of the weight of current batteries [55]. With actuators creating a body force to control the direction of the missile, perhaps ns may even become unnecessary. Control may be initiated from many locations around the missile to maneuver it when segmented electrodes are placed around the diameter as shown. Magnets may be placed under the surface between the electrodes if EMFC is proven more effective than control only with plasma discharges. As the elds stand now, active ow control with electromagnetic, glow discharge, and DBD actuators are at different phases of development. EMFC is the least developed, with much to be studied about its performance in simplied ow environments like channels, wedges, and at plates. Flow control with glow discharges has been studied for decades with simple ow environments, but more detailed and improved concepts appear to be trending away from being applied to blunt bodies to being used for aerodynamic control surfaces. Control with DBDs is the most advanced of the three with detailed experimental analysis of both the physical aspects and integration into potential ight systems. All have major design hurdles to overcome before they can be labeled ight-ready technology, where perhaps the largest hurdle is the integration of the power supply and additional electric circuit components into a vehicle. However, technical needs coupled with demonstrations of the potential that exists with EMFC and EFC systems make the prospects for further research in this eld promising.
Acknowledgments The authors would like to thank Dr. J. Craig Dutton of the University of Illinois at Urbana-Champaign for many useful discussions regarding EFC and EMFC as well as Dr. Ramakanth Munipalli of HyPerComp, Inc., for providing a computational MHD code that has been used for the EMFC electric eld calculations presented in this paper. The authors are also grateful to Dr. Joseph
E.M. Braun et al. / Progress in Aerospace Sciences 45 (2009) 3049 47
Shang of Wright State University for providing helpful comments on the manuscript prior to publication in conference paper form. This work was partially supported by the Texas Advanced Research Program, Project no. 003656-0013-2006. References
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