Wind Tunnels

In many experiments in fluid mechanics one requires a base flow whose velocity profile, temperature distribution and turbulence level are fully known. Specifically, most experiments require that the incoming flow be at a uniform temperature and velocity, while the velocity fluctuations are minimal. A definite disturbance is introduced in this flow and the following questions are addressed:

1. Does the disturbance level grow with time and distance?

2. What is the force developed at the site of this disturbance?

3. What are the local pressure and temperature profiles and the global heat transfer rates?

Question 2 is one of fundamental importance in aeronautics where lift and drag forces developed on aerofoil sections must be determined. Wind tunnels are used to generate the prescribed base flow in a rectangular channel that is large enough with respect to the size of the affected zone around the disturbance. In special purpose wind tunnels, the base flow may have a uniform shear (i.e., a velocity gradient) or uniform stratification (i.e., density and temperature gradient). However the common requirements for a wind tunnel are:

1. Uniform flow over a large portion of its cross-section

2. Low background turbulence level.