Expert: Paul Soderman - 6/24/2009

Question
QUESTION: Hello Paul

My background has been the design and build of simple automated machinery .
I been instructed to build a wind tunnel .
Having looked around on the web , there is alot of stuff I don't understand , especially formulas . My education is high school level .
Would it be possible to get your opinion of this cross section ? I'm open to any changes you might suggest .
Regards
Greg

ANSWER: Greg
You have a good contraction ratio for a subsonic wind tunnel.  The length of contraction should be as short as practical.  For the test section, an octagonal or circular test section is better than a square or rectangular test section because of the flow in the corners.  Here is advice on the contraction from my friend Ken Mort:

"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."

It would also help to get the following book:  Low-Speed Wind Tunnel Testing by W. Rae and A. Pope.  You need to read about diffusers, corner vanes, drive fans, etc.
Paul

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

QUESTION: Thanks Paul for the quick response .
I have the length of the contraction drawn at 20.75".Should I shorten it more ?
Should I increase the radius size at the inlet (3") ?
I'm restricted to a square test area .
On the diffuse end , I'm going from 5.5" (sq) to 10" round to house the fan.....should this end be shorter or longer ?
I'm shooting for 6' as an oal .
Any input is appreciated .

Answer
Greg
Designing a wind tunnel is a complicated process beyond the scope of my effort here.  For one thing you need objectives and requirements that I don't know.  I suggest getting the references I cited and copy the geometry of a likely wind tunnel or develop the process used to answer the questions you raise.  You will find that, in general, the diffuser wall angle should be no greater than a certain angle.  And the inlet shape should be as described in the references.  Many test sections have rectangular shape.  You just have to avoid testing in the corners.  Test section blockage (ratio of model to test section area) is an important constraint. As I don't know what you want to study in the wind tunnel, I can't advise further.
Paul

p.s.  If this is for an employer, I might be able to provide consultant service for a fee.  Contact me at psoderman@gmail.com.