‘I-YPICAL SUPERSONIC FLOW PATTERNS 
When supersonic flow is clearly established, 
all changes in velocity, pressure, density, flow 
direction, etc., take place quite suddenly and 
in relatively confined areas. The areas of flows 
change are generally distinct and the phenom- 
ena are referred to as “wave” formations. All 
compression waves occur suddenly and are 
wasteful of energy. Hence, the compression 
waves are distinguished by the sudden “shock” 
type of behavior. All expansion waves are not 
so sudden in their occurrence and are not waste- 
ful of energy like the compression shock waves. 
Various types of waves can occur in supersonic 
flow and the nature of the wave formed depends 
upon the airstream and the shape of the object 
causing the flow change. Essentially, there 
are three fundamental types of waves formed 
in supersonic flow: (1) the oblip shock wave 
(compression), (2) the normal shock wave 
(compression), (3) the expansion wave (no 
shock). 
OBLIQUE SHOCK WAVE. Consider the 
case where a supersonic airstream is turned 
into the preceding airflow. Such would be 
the case of a supersonic flow “into a comer” 
as shown in figure 3.3. A supersonic airstream 
passing through the oblique shock wave will 
experience these changes: 
(1) The airstream is slowed down; the 
velocity and Mach number behind the wave 
are reduced but the flow is still supersonic 
(2) The flow direction is changed to flow 
along the surface 
(3) The static pressure of the airstrea:m 
behind the wave is increased 
(4) The density of the airstream behind 
the wave is increased 
(5) Some of the available energy of the 
airstream (indicated by the sum of dynamic 
and static pressure) is dissipated and turned 
into unavailable heat energy. Hence, the 
shock wave is wasteful of energy. 
A typical case of oblique shock wave forma- 
tion is that of a wedge pointed into a super- 
sonic airstream. The oblique shock wave 
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will form on each surface of the wedge and the 
inclination of the shock wave will be a func- 
tion of the free stream Mach number and the 
wedge angle. As the free stream Mach number 
increases, the shock wave angle decreases; as 
the wedge angle increases the shock wave 
angle increases, and, if the wedge angle is in- 
creased to some critical amount, the shock 
wave will detach from the leading edge of the 
wedge. It is important to note that detach- 
ment of the shock wave will produce sub$onic 
flow immediately after the central portion of 
the shock wave. Figure 3.4 illustrates these 
typical flow patterns and the effect of Mach 
number and wedge angle. 
The previous flow across a wedge in a 
supersonic airstream would allow flow in ;UU 
dimensions. If a cone were placed in a super- 
sonic airstream the airflow would occur in 
three dimensions and there would be some 
noticeable differences in flow characteristics. 
Three-dimensional flow for the same Mach 
number and flow direction change would pro- 
duce a weaker shock wave with less change in 
pressure and density. Also, this conical wave 
formation allows changes in airflow that con- 
tinue to occur past the wave front and the 
wave strength varies with distance away from 
the surface. Figure 3.5 depicts the typical 
three-dimensional flow past a cone. 
Oblique shock waves can be reflected like 
any pressure wave and this effect is shown in 
figure 3.5. This reflection appears logical and 
necessary since the original wave changes the 
flow direction toward the wall and the reflected 
wave creates the subsequent flow change to 
cause the flow to remain parallel to the wall 
surface. This reflection phenomenon places 
definite restrictions on the size of a model in a 
wind tunnel since a wave reflected back to the 
model would cause a pressure distribution not 
typical of free flight. 
NORMAL SHOCK WAVE. If a blunt- 
nosed object is placed in a supersonic airstream 
the shock wave which is formed will be de-