Aeronautical Engineering/Wind tunnel honeycomb design
I'm assigned to design the honeycomb for a close return wind tunnel and now I have trouble choosing the diameter of the honeycomb. I've read some of your responses and recommended reference, such as Low-speed wind tunnel testing.It is said that "The operating state parameter is the Reynolds number with the characteristic length scale typically chosen to be the cell hydraulic diameter". However, I donít know how to determine the Reynolds number here cause the speed of air varies in a large range in this wind tunnel.
Some suggested that the diameter should be less than the diameter of the tunnel divided by 150. But it leads to a ridiculously large cell for us.
So, what is actually the rule for choosing the honeycomb cell size?
Frank - I know of no rule for choosing honeycomb for large wind tunnels. For small wind tunnels the rule you cite is common. For more information, see:
P. Bradshaw and R. D. Mehta, "Design Rules for Small Low Speed Wind Tunnels,"
The Aeronautical Journal of the Royal Aeronautical Society,
pp. 443-49, 1979
James Scheiman and J.D. Brooks. "Comparison of Experimental and Theoretical Turbulence Reduction from Screens, Honeycomb, and Honeycomb-Screen Combinations", Journal of Aircraft, Vol. 18, No. 8 (1981), pp. 638-643.
Low-speed Wind Tunnel Testing by Alan Pope and William H. Rae,Wiley, 1984
However, for large wind tunnels the cells should probably be smaller than D/150. I have never seen honeycomb cells smaller than 16.5 mm diameter (not that I have seen that many). After all, the cell size is related to the lateral turbulence scale that you want to filter, and that goal may not change much with wind tunnel size.
I did not understand your question about Reynolds number. I assume the key Reynolds number is based on the diameter of the honeycomb cell and the velocity at the cell face. That should not vary greatly over the honeycomb cross section or the test section velocity distribution will be too uneven. See the above references for more detail.
Most wind tunnel developers do component testing before building the wind tunnel.
p.s. I should have said "I have never seen honeycomb cells larger than 16.5 mm diameter..."