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 ohma0043 February 9, 2011 12:47

Wind Tunnel Blockage question

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Hi all, I am trying to run a CFD test for an Inflatable Aerodynamic Decelerator (essentially a blunt body) in a wind tunnel. For this wind tunnel model, I have approximately 20% tunnel blockage. I will be looking for drag and turbulent flow characteristics around the decelerator. The question I have is, does CFX already incorporate tunnel blockage into its calculations or do I need to set it up to achieve accurate results. Any help towards this problem will be of great use.

Also, I will be running at low subsonic speeds, apprx. 38 m/s, and I will be using the K-Omega turbulence model. At the inlet I have a velocity boundary condition of 38 m/s and at the outlet I have a bound condition of 0 Pascal relative pressure. Along the walls I have used no-slip bound conditions.

To simplify the calculations required I have used a symmetry plane to cut the model in half. The wind tunnel is approximately 25 inches wide and 40 inches high. The model has a diameter of 16 inches.

Thanks,
Eric

 ghorrocks February 9, 2011 19:31

Are you trying to model the thing in free air or reproduce a wind tunnel model? If free air then you should widen the domain. If a wind tunnel, then no, CFX does not have any blockage factor compensation built in. You should ask the wind tunnel people to give you the raw test results with no blockage factor compensation. Then you will be comparing CFD to the wind tunnel results and be doing a direct comparison.

 fastwave February 11, 2011 06:42

Not quite true. If you model the WT size and use a wall condition on your boundaries the blockage is included in the simulation and can be compared to the WT results.

The choice of using slip or no-slip condition on the walls is a tradeoff betweeen simplicity and accuracy. If the boundary layer thickness is a small percentage of the overall WT dimensions then the effect on blockage is small. In small tunnel with thick WT wall boundaries it could be significant.

 ghorrocks February 11, 2011 07:12

Quote:
 Not quite true. If you model the WT size and use a wall condition on your boundaries the blockage is included in the simulation and can be compared to the WT results.
Isn't that what I said? It is at least what I tried to say.

 asan09 February 13, 2011 10:12

like you

Hi
My name is Ghassan Sharraf.
I am a student in the 5th year of the mechanical engineering
I make a project about modeling a flow around bodies in the wind tunnel
I ask if you can help me to compare my results with experimental data
So if you can submit me from your knowledge
Note : my tool is Ansys
And thanks
my email : ghassan.s.a@hotmail.com

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