Expert: Paul Soderman - 11/1/2011

Question
In my university I'll design a contraction of a skydiving( closed tunnel with subsonic range), so I don't need concern about turbulence, my focus is pressure loss. I read many papers from NASA and some answers from you Paul, and I didn't find nothing like the maximum angle to not occur separation talking about convergent. And many papers donne have screens, honeycomb ...
I'm looking for experiments, datas from a clean contraction to compare with a CFD simulation. Could you indicate me some papers.
And exist grafics such as Kline and Sandborn did for diffuser, like giving a aspect ratio and area ratio you can predict if will have separation or not.

Answer
Vinicius
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. I am not sure exactly which angle you refer to in your question.

A common rule of thumb for a diffuser is the angle between the duct axis and wall shall be no greater than 4°. And the diffuser should be kept as short as is reasonable.

If you can find the shape of the DNW (German-Dutch Wind Tunnel) contraction to the open jet, I believe you will find a good design.  The shape might be found in:  Van Ditshuizen et al: Acoustic Capabilities of the German-Dutch Wind Tunnel DNW. AIAA Paper 83-0146, 21st Aerospace Sciences Conf., Jan. 1983.

The challenge for contraction design is that there are adverse pressure gradients at the entrance and exit.  Flow separation can occur. In addition, at the corner of rectangular contractions the boundary layer can get large. I offer the advice 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