to temperature effects since the temperature inside the wind tunnel could vary considerably after continuously running. Common difficulties encountered when using PSP include the inability to accurately measure the leading and trailing edge effects in areas where there is high curvature due to limitations in the cameras ability to gain an advantageous viewing angle. Additionally application of PSP on the leading edge is sometimes avoided because it introduces a finite thickness that could cause early flow separation thus corrupting results. Since the pressure variations at the leading edge is typically of primary interest, the lack of accurate results in that region is very problematic. Once a model is painted with pressure sensitive paint, certain paints have been known to adhere and continue to perform for a matter of months after initially applied. Finally PSP paints have been known to have certain frequency characteristics where some require a few moments to stabilize before achieving accurate results while others converge rapidly. In the latter instance paints that have ability to reflect rapid changes in pressure can be used for Dynamic PSP applications where the intent is to measure unsteady flow characteristics. Particle Image Velocimetry (PIV): PIV is a technique in which a laser sheet is emitted through a slit in the wall of the tunnel where an imaging device is able to track the local velocity direction of particles in the plane of the laser sheet. Sometimes this technique involves seeding the airflow with observable material. This technique allows for the quantitative measurement of the velocity and direction of the flow across the areas captured in the plane of the laser. Model Deformation Measurement (MDM): MDM works by placing markers at known geometric locations on the wind tunnel model and taking photographs of the change in the marker's location as the wind in the tunnel is applied. By analyzing the change in marker positions from different camera viewing angles, the translational change in location of the marker can be calculated. By collecting results from a few markers, the degree to which the model is flexibly yielding due to the air load can be calculated. Classification There are many different kinds of wind tunnels. They are typically classified by the range of speeds that are achieved in the test section, as follows: Low-speed wind tunnel High speed wind tunnel Subsonic and transonic wind tunnel Supersonic wind tunnel Hypersonic wind tunnel High enthalpy wind