The fan motor sizing is determined by carrying out a series of pressure loss calculations in each part of the wind tunnel. Considerations that go into the design of the wind tunnel are:

1. Low versus high speed: Low speed tunnels are those in which air may be treated as incompressible. For air at atmospheric pressure the limiting speed is one-third the speed of sound i.e., about 100 m/s. In high speed tunnels the Mach number may exceed unity. These are useful in studying shock formation in gas flows. Supersonic conditions are generally obtained as choked flow in nozzles from a high pressure reservoir. The design of the nozzle profile is critical, if supersonic speeds are required at its exit plane.

2. Open and closed circuit: In large wind tunnels the kinetic energy in the air stream is so large that it is uneconomical to discharge it into the atmosphere. In such cases the wind tunnel operates in a closed circuit and air is recirculated through the test cell. The reduced operating cost of such tunnels must be balanced against the capital cost of making a bend as well as the cost of heat exchangers that may be needed to control air temperature. For large wind tunnels an important design parameter is the energy ratio defined as the ratio of the energy in the air stream to the energy input to the fan. The former is calculated as

Dynamic pressure Area of cross-section air velocity

This ratio is normally in the range 3-5 in well-designed tunnels.