Hi, Dr. Zambri

There is a brief explanation on the function of the honeycomb in the wind tunnel in the follow link: http://navier.stanford.edu/bradshaw/tunnel/honeycomb.html.  In the link above, you can find  the ratio of the cell length to the cell diameter of the honeycomb and also some old references for designing the wind tunnel.

I am just curious why you want to know the ratio of the diameter of the honeycomb to the size of the wind tunnel.  Usually the design of the wind tunnel will be expressed in term of contraction ratio and turbulence level.  I managed to improve the turbulence level of the wind tunnel with the contraction ratio of 4:1 from 0.7% to 0.35% by replacing the existing wire mesh with the finer one.  However I have to sacrifice the maximum speed that can be achieved as the pressure drop across the wire mesh increases.  Thus, we need to know the maximum pressure drop that can be handled by the blower used in the wind tunnel and the estimation of the pressure drop due to the wire mesh and honeycomb.

Hatsari

Dear Dr. Aziz,

Thanks for your valuable reply. No, I have not seen the two papers yet. To answer your questions. This wind tunnel is meant for multi purpose research including aerodynamic study and boundary layer. It is supposed to have small turbulence intensities level. I am not happy with the current intensities. However, the mean flow profile is acceptable. It is an open type, blow down, constant pressure, max velocity is 25m/s. Test section size is 1.2m wide, 0.5m high and 2m long. Other than the high turbulence intensities, the boundary layer profile does not collapse well with published data especially in the near wall region y+ approx. 10 - 100 (measured with a CTA), that's kind of the main reason of me looking into honeycomb. I have uploaded, may be our prelim. paper for reference, just about the construction and dimensions of wind tunnel. 

https://www.researchgate.net/publication/285299096_The_Development_of_a_Multi-purpose_Wind_Tunnel

But yeah, thanks!

Conference Paper The Development of a Multi-purpose Wind Tunnel

Dear Dr. Hatsari,

Thanks, I saw the link before and some related papers. The authors did not elaborate clear tips on the honeycomb diameter : to wind tunnel ratio. The reason of this is to get an optimal honeycomb size, not too small diameters as I might be loosing speeds and not too big as it might not improve the flow at all. I saw the honeycomb used for the NASA facility, that is way too large. The other reason, as I mentioned in prev. message is that the near wall portion of the boundary layer does not collapse well with published data. Thanks for sharing you turbulence intensities results and other things. Regards.

You mention a desired reduction in turbulence intensity, do you mean only in the freestream direction?  If so, you need to change the number or nature of your screens.  This is also true if you are trying to better tailor the uniformity of the freestream entering the test section.

If you are tackling issues of flow angularity, cross-stream turbulence intensity (v',w') or cross stream physical turbulence scales then you want to investigate honeycomb further.  If you have done a multi-component hot-wire investigation then you can correlate cross-stream turbulence values and do a temporal decomposition to get an idea of the physical scales that need to be broken up.

 Hi. Prof. Altman,

Yes I meant turbulence intensities in the streamwise and in the freestream region. At this moment, I will just concentrate in one-sensor hotwire i.e. in the streamwise. It looks like I have to 'play around' with the size of our only screen i.e. approx. 2cm square mesh. Thanks for the valuable input.

2 cm or 2 mm??? (I hope the latter)

It is in fact approx. 2cm. We were worried about intensities.

What is your tunnel inlet contraction ratio?  And you have how many screens?  And what is (approximately) the screen wire diameter?  Also, I am assuming it is 2 cm square mesh?

I want to run a quick calculation based on loss factor and contraction ratio to let you know what to expect for freestream turbulence intensity.......

Dear Prof. Altman,

Here are the dimensions:

contraction ratio: 2.4 : 1 (space limitation)

mesh: 15 mm square mesh made of 1 mm diameter together with 4 mm square mesh made of 0.5 mm diameter. This is kind of small and big-grid mesh put together. It is located exactly in between the 2 section-diffuser of the same length. Please disregards the dimensions in my previous post.

Appreciate your efforts and thanks in advance!

Zambri,

OK, So, I've taken your wire mesh data and Weighardt (1953) and Mehta's (1985) result, that provides individual loss coefficients of 0.2876 and 0.3461 for the 15 mm and 4 mm screens respectively. 

Then applying Dryden and Shubauer's 1947 paper to get to turbulence intensity reductions of 0.8813 and 0.8619 for each screen respectively.  This results in a composite turbulence intensity reduction factor of 0.7596 due to the screens. (it is multiplicative, so if you want to add another 4 mm screen, for example, you would multiply this composite of 0.7596 by 0.8619 to get an overall reduction factor of 0.6547)

Now, using Batchelor's relation for turbulence intensity reduction due to the contraction, the result is a reduction factor of 0.0693 due to the contraction alone.

Hence the composite turbulence reduction factor for the combined screens and contraction is 0.0526.

If you assume your inlet turbulence intensity is of the order 10%, you should expect a test section turbulence intensity in the freestream direction to be on the order of 0.5% with your screens and contraction ratio.

The honeycomb section first breaks the large whirl into smaller ones at its entrance and then makes the flow align along its length. L/d of 7 – 8 is in the optimum range. The screens attenuate the turbulent fluctuations. Large-scale turbulent eddies are broken into smallers eddies and finally made to decay. The contraction accelerates and aligns the flow and feeds the test section with flow with reduced turbulence.

Harun, here are some very useful references:

1.       A very digested publication on wind tunnel testing can be found on Researchgate profile of Louis Cattafesta :

2.       Barlow, Rae and Pope.  Low-Speed Wind Tunnel Testing, John Wiley and Sons, NY 1999.

3.       Bradshaw &  Mehta.  Wind Tunnel Design, http://www-htgl.stanford.edu/bradshaw/tunnel