Expert: Paul Soderman - 12/4/2007

Question
Paul,

I am a postgraduate student in the Mechanical Engineering Department of Trinity College Dublin. I need to design a contraction section to be placed in an existing wind tunnel in order to increase the maximum flow velocity throughout the test section so I can achieve acoustic-strouhal coincidence of a small cylinder in cross flow. I need to contract the duct from a cross section of 300 mm x 300 mm to one of 300 mm x 100 mm over a spanwise length of roughly 300-400 mm.  What is the best way to approach this, using a parabolic contraction or one of a ellipse form? Also can you suggest any particular equations I should use to describe the profile of the contraction and maybe some good (and recent) publications on the design rules of a wind tunnel.

Thanks

Shane

Answer
Shane
I punted your question (American football term, sorry) to my former NASA colleague Ken Mort, an expert on wind tunnel design who offers the following:

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.

Good luck.  Please get back to me if you need clarification.
Paul