Expert: Paul Soderman - 1/30/2008

Question
QUESTION: I am a senior M.E. major applying for a position in NASA's undergraduate student research program (USRP), and the application is due this Thursday. I have selected Dryden Flight Research Center and Langley Research Center as preferred locations since I'm interested in Aeronaughtics and fluid mechanics. An essay asks me to describe educational/work experiences that may enhance the application, and I have prominently featured my senior project, designing, building, and characterizing a low-speed, open-circuit wind tunnel, in the essay. As a NASA researcher, are there any general pointers you can provide me, particular aspects of the project or past work experiences you would want to hear about if asked to review some of the essays?

ANSWER: Sam
What a nice project.  If I were reviewing your essay, I would hope that you touch on two key aspects of a wind tunnel -  the energy ratio and the test section flow quality.  You should be able to estimate the energy ratio and show where you have tried to maximize the wind tunnel efficiency.  And you should be able to describe the methods you used to optimize the test section flow quality.  If you understand those two things, you can work in any wind tunnel complex. Oh, one other thing - an open-circuit wind tunnel throws energy out the exhaust.  It is nice to know how much.

Of course there is more you can talk about - flow calibration, inlet design, diffuser design, etc., but that may be a bit much for a senior project.

Both centers would be fun to work at, but Langley is where the wind tunnels are of course (not counting Ames and Glen).  Good luck.
Paul

---------- FOLLOW-UP ----------

QUESTION: Interestingly, for this tunnel, efficiency is a design driver not because it dictates operating costs (flow rates are fairly low at around 5 m3/s), but because we must obtain maximum flow rates from a pre-selected fan.
I have noticed a wide variety of differing design guidelines for contractions. Can you point me to a reliable source?
Thank you for your time.

Answer
I offer the same answer to this question I gave recently that I got from my former colleague 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