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Old   March 16, 2007, 09:20
Default RSM mesh requirement
  #1
Felix
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Hello,

I would like somebody to confirm I understood well what I've read so far.

As I understand it, the SSG Reynolds stress model uses wall functions to solve the boundary layer. The Y+ values can there for be quite large, say 30-100. On the otherside, the BSL Reynolds stress solves down to the wall and so the Y+ should be kept below 2, as far as it is possible. The latter model should therefore be more accurate on the amount of separation predicted.

Is there anything else I should know concerning mesh requirement for the RSM models ? I usually use 2-eq models.

Thanks a lot,

Felix
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Old   March 18, 2007, 17:24
Default Re: RSM mesh requirement
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Glenn Horrocks
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Have a read of this thread:

http://www.cfd-online.com/Forum/cfx.cgi?read=19468

Glenn Horrocks
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Old   March 18, 2007, 22:29
Default Re: RSM mesh requirement
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Felix
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Hi Glenn,

I had already read this post. The BSL model uses the automatic wall-treatment just as the SST does, this is well explained in the documentation. However I didn't find the information for the SSG. I supposed that it works with the same wall functions as the standard K-e but I would like a confirmation on this.

Thanks,

Felix
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Old   March 19, 2007, 17:11
Default Re: RSM mesh requirement
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Glenn Horrocks
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Hi,

In that case then your original post describes the basics of the models correctly. Obviously to get an accurate turbulence modelling result you need to consider many other issues and you could write a textbook on that. For more details see "Turbulence Modelling for CFD" by Wilcox.

Glenn Horrocks
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Old   March 20, 2007, 08:28
Default Re: RSM mesh requirement
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Felix
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OK,

Thanks Glenn, it is obvious that turbulence modelling is both difficult and very important. This is why I am looking for the one model that is best suited to my case.

I tried the RSM models and got some stability problems. So far every calculation I have run "exploded" around 30-40 iterations, one turbulent quantity becoming incredibly large, even when I specified a good initial guess (the K-E solution) and reasonable boundary conditions. I will investigate this.

Well, thanks again for your answers. I will read a little in Wilcox to see what he says about those models.

Felix
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Old   March 20, 2007, 16:55
Default Re: RSM mesh requirement
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Glenn Horrocks
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Hi,

Instability is the main problem with RSM models. Additionally, even though they try to model more terms then 2-equation models they rarely increase accuracy. This is why 2-equation models are most commonly used in industry.

There are certain classes of flows where RSM is required to capture important physics, such as strongly swirling flows. Unless you have a specific reason to use RSM models I would in general recommend using the SST or k-e models.

Glenn Horrocks
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