Expert: Paul Soderman - 10/13/2011

Question
Hello Sir,
I am doing BSc. in Aeronautical Engineering in Emirates Aviation College, Dubai. I've got my graduation project in order to design and construct a wind tunnel. I cannot determine the contraction ratio, could you please help me how to find the contraction ratio?
Thank You

Answer
Masimuddin
There is no single answer to your question because wind tunnel contraction ratio depends on many things.  In fact, a system analysis is called for.  Contraction ratio depends on available real estate, construction cost, circuit losses, fan power available, test section size and speed, type of wind tunnel (open or closed circuit), primary diffuser angle and length, energy ratio, and so on.  Subsonic wind tunnels around the world have contraction ratios that vary from 4.0 to 14.0 more or less.  W. Rae and A. Pope:  Low-Speed Wind Tunnel Testing list the geometries and performance parameters of numerous wind tunnels around the world (table 1.1 in 2nd edition).  Perhaps you can find one that is close to what you want and start with that design.  Or you can scale one down to your size requirements with consideration of Reynolds number changes and effects on boundary layers. The design procedure is also described in that book.
Paul