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how to consider perforated wind tunnel wall in CFD simulation

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Old   June 10, 2009, 06:03
Default how to consider perforated wind tunnel wall in CFD simulation
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we are doing CFD transitional simulation to validate wind tunnel data. my computational domain is the same dimension as the wind tunnel. at wind tunnel sidewalls, I set freeslip or symmetry Boundary conditions (BCs). the calculated results are close to the experiemntal data. however, I then realize in wind tunnel, the sidewall are perforated (i.e. the air can flow in or flow out through the tunnel walls) to eliminate wall interference on flow field. (obviously, in my CFD model, i am considering solid free slip walls, they are different). So I am wondering how can I consider the perforated wall. May I set opening BCs at sidewalls to represent the flowin /flowout behaviour? I got a little confusion here becasue in my understanding the opening BCs in CFX is for far field Boundaries, while the wind tunnel walls are far field boundaries? am I right? Is there any good choice?

thank you for any of your comments

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Old   June 10, 2009, 08:09
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Glenn Horrocks
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Hi,

There are a few options to consider. I am not sure which will work best. I don't think using openings at the side walls will work too well as openings don't like cross flow much (but I could be wrong). My suggestions are:
1) Use a mass flow or prescribed velocity outlet boundary.
2) Use a mass source to gobble up a small amount of air next to the wall.

I don't know how well these would work. Give them a go and see what works best.
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Old   June 10, 2009, 10:26
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thanks, Glenn,

I will think about your ways, the only problem is I don't know how much the mass flow through the walls. actually, I have already tried opening BCs at sidewalls, and the result looks good and reasonable (i.e. it eliminate the wall interference effect, there is no pressure suction as in the solid wall case).

I also checked the velocity vectors on the sidewalls in above opening BCs cases, they are all parallel the walls, it seems there there is no flow through the walls. am I right? how can I visualize/check if there is the flow through the walls.

anyway, thank a lot for your reply

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Old   June 10, 2009, 19:22
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Glenn Horrocks
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Hi,

If you don't know how much suction is being used at the side walls then how can you model it?

If you just want to eliminate the side wall effect why don't you just run with a larger domain with the side walls further away?

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Old   June 11, 2009, 05:51
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Thanks, Glenn,

No, we don't have the exact flow data through the walls. i.e. we can't simulate the wall interference effects precisely. In the real wind tunnel test, these data is not available too, it is stochastic for the flowin or flow out. however, we need to know how much the effect is, which I don't think would be very much. that's why we are trying to find a way of modelling the porosity on the walls, e.g. allow the air flow in and/or flow out.

actually, I have already increase the domain size (but only for inlet and sidewalls location, the outlet keep the same), then I use opening (static entrainment) BCs, unfortunately, the result is unrealistic. there are pressure suction at some points, which is not supposed to be there becasue the wall interference effects are assumed to be eliminated in this large domain model. also for this model, the convergence is terribly slow and bad. I tried to improve the mesh, but no change at all. I also tried freeslip BCs for this large domain model, it works well.

another option i have tried is running on the small domain (the same size as wind tuneel working section), while using open BCs, is it reasonable? becasue in my understanding, opening BCs are for far field boundary, which are not the case for wind tunnel walls (they are not far enough), am I right?

so now, which way is better (no best), can you give me some suggestion on it?

many thanks,

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