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source in SA model(or one equation model)

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Old   August 27, 2018, 06:29
Default source in SA model(or one equation model)
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phd
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Hello,

May I ask something about the source term implementation of the one-equation model?

In k-omega SST model, when we add the source term into the simulation, there will be source term for k and omega equations due to the two additional equations brought by the SST model.

However while using source term in SA model(or all the other one-equation model), there are only places to add source for continuity, momentum and energy equations, but no place to add source for eddy viscosity. Could anyone tell me how can I insert the source for the eddy viscosity? Otherwise the implementation of source term with one-equation turbulence model seems to be incomplete because the source should also contain the source from the eddy viscosity equation.

Or is there any method we can assign the eddy viscosity directly to the solver so that we can bypass the eddy viscosity computed by CFX? Such as using User Fortran or CCL command?

Thank you very much! I really appreciate your help!
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Old   August 27, 2018, 07:14
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The SA turbulence model does not implement the turbulence dissipation by a source term. It does it by introducing a turbulent viscosity which is added to the molecular viscosity.

A small piece of advice about the software - it is written by CFD experts, extensively tested and thoroughly validated. There is zero chance of them forgetting something fundamental like the effect of the turbulence model on the momentum equations. If something fundamental appears to be missing the mistake is almost certainly with the user, not the software.

If you want to do your own turbulence model then develop your own equation for viscosity as a function of whatever you like, and add source terms to handle the other factors. This way you can completely bypass any built in turbulence model and do whatever you like.
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Old   August 27, 2018, 07:32
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Hi, Ghorrocks, thanks for your fast reply!

I know SA doesn't implement turbulence dissipation by source term. What I mean is in CFX when we add 'source' in the subdomain, we can only add source to the Mass, Momentum and Energy, but not to the Eddy viscosity.

The contrast of that is SST model. While adding 'source' in the subdomain with SST, we can add source to Mass, Momentum, Energy, 'k' and 'omega'.

Shouldn't SA model have a function to add the source for Eddy viscosity?

Thanks!
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Old   August 27, 2018, 07:39
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If I recall correctly, SA is a beta model in CFX. This means it does not have all functionality in place, so it would not surprise me if the source terms are not included on the CFX-Pre GUI. You probably need to use CCL to put them in. If you want to put source terms in have a look at the CCL for adding source terms to other variables and other supported turbulence models and you should be able to figure out the syntax for SA.
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