Expert: Paul Soderman - 10/7/2011

Question
hello sir

sir want to know about the low speed subsonic open circuit wind tunnel.i want to know how to design the contraction section and diffuser section. i am using a test section of square cross section of length 600mm and 300*300 mm with a test section mach no 0.2&0.3.kindly inform me about that or refer some easily available books where i can get the formulas.
waiting for your kind reply.

Answer
Abdul
You can find a discussion and guidelines for wind tunnel circuit design including contractions and diffusers in W. Rae and A. Pope:  Low-Speed Wind Tunnel Testing. Although the guidelines are pretty straightforward, the subject is too complex to get into here. Following is the advice for contraction design from my friend and expert wind tunnel designer Ken Mort:

There have been hundreds (thousands ?) of papers and reports written on contractions and 90% are mostly BS. They use CFD and don't have much experimental data. The shape should be an S shape with cubics at each end. The inflection point should be much closer (1/4-1/3 of the length) to the inlet than the exit.  I like the guides given in the following.

Rouse, Hunter and Hassan, M. M.: "Cavitation-Free Inlets and Contractions". Mechanical Engineering, Vol. 71, March 1949, pp 213-216.

The same guides are also given in NASA TN D-8243.

I've used these guides for all of the tunnels I've worked on with good success.
Paul