Expert: Paul Soderman - 3/12/2009

Question
QUESTION: Dear Mr.Paul Soderman
I'm Kaveh Amiri, M.S. student of Aerospace in Sharif University of Technology, Tehran.
I have to design a honeycomb for a transonic wind tunnel as a part of my final thesis. that wind tunnel is a suction type. It's test section is 60cm*60cm and velocity in settling chamber is about 17m/sec.
I think that I have gathered all informations about honeycombs for wind tunnel such as length to diameter ratio, material and etc., but I have no idea about the cell size or hydraulic diameter of cell.
I'm grateful if you could help me. Thanks in advance.


ANSWER: Kaveh
I have the following answer from my friend Ken Mort, who is a wind tunnel designer and former NASA researcher:

I think the guy ought to use Barlow, Jewel B.: Rae Jr, William H.; and Pope, Alan: " Low-Speed Wind Tunnel Testing" and Pankhurst, R. C. and Holder, D. V. : "Wind-Tunnel Technique" for this even though he's designing it for a transonic wind tunnel. After all the airspeed is low in the settling chamber. If I understand Pope correctly the diameter of the honeycomb  should be less than or equal to the settling chamber diameter divided by 150. I think this results in a honeycomb diameter of on the order of a half inch or a little more than a cm. The biggest challenge is to find a cheap source of honeycomb. People have used straws and milk cartons stacked up for low speed tunnels to keep the cost down. He should locate the honeycomb upstream of the contraction at least a half diameter of the contraction if possible to minimize the wakes entering the test section.

Good luck.
Paul

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

QUESTION: Dear Mr.Paul Soderman
sorry, in sequel of my question,could you help me if there is any rule about the distance between that honeycomb and it's following?
I'm grateful if you could help me. Thanks in advance.


ANSWER: I don't quite understand your question.  What is following ?  According to my previous answer, the honeycomb should be upstream of the inlet contraction by a distance, if possible, of half diameter of the contraction.  In other words, the honeycomb will be in a constant area duct or settling chamber upstream of the contraction leading to the test section.
Paul

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

QUESTION: Dear Mr.Paul Soderman
sorry because of my fault. I sent my question again. I had forgotten to type the last word in the mail 'screen'. my question was: "in sequel of my question,could you help me if there is any rule about the distance between that honeycomb and it's following screen?
I'm grateful if you could help me. Thanks in advance."
thank's a lot.


Answer
Not that I know of, but it would make sense to keep the screens far enough downstream so much of the honeycomb wake turbulence has dampened.  I imagine one or two honeycomb cord lengths would be a good separation.  If you can get the following references, you can see what others have done with honeycomb and screens:

R. Loehrke and H. Nagib: Experiments on Management of Free-Stream Turbulence, AGARD Report 598, Sept 1972.

R. Loehrke and H. Nagib:  Control of Free-Stream Turbulence by Means of Honeycombs:  A Balance Between Suppression and Generation. J. Fluids Engineering, 342-353, Sept 1976.

It would be good if you could remove the screens for cleaning because they tend to trap dust and change their performance.  If multiple screens are used they should be spaced 30 times the mesh size or 500 times the wire diameter, whichever is larger, according to Rae and Pope.
Paul