[ohma0043]

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]

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]

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]

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]

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

[hamed1983]

Originally Posted by fastwave

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.

Would you plz to introduce me a reference for that, perhaps in Tutorial or everywhere else?

[hamed1983]

Would you plz to introduce a reference for that?

[ghorrocks]

Isn't it obvious? Why do you need a reference for that?

[hamed1983]

No unfortunately, I don't thing so, I couldn't find the relation between your explanation & the user named fastwave. I need a source in order to have more study on case. In my opinion your answer & fastwave's answer are quiet different.

[ghorrocks]

fastwave said this, and my comment also said the same thing:

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.

I cannot give you a reference for this as it is obvious.

fastwave also made comments about slip or no slip walls on the boundary - again these comments just come from a knowledge of the physics and seem obvious to me.