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Old   September 30, 2005, 10:34
Default SST Convergence
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new@cfx
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Hi,

I am facing problems in getting good convergence with an SST model. whereas for the same geometry the K-E model converges easily. The expert parameter section in the documentation mentions a parameter tef numerics option which limits the flux in regions of high omega gradient. Could someone kindly clarify what this option does exactly?

Regards,

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Old   September 30, 2005, 10:57
Default Re: SST Convergence
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Robin
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Are you sure your convergence problem is numerical? Quite often, using the SST model will capture boundary layer separation that the k-e model does not. A separated boundary layer results in a detached shear layer, which can be very unstable.

Before looking for an expert parameter, write a sequence of backup files and look for fluctuating flows downstream of separation points. YOu can also try increasing/decreasing the timestep by factors of 10. In some cases a larger timestep will be enough to get rid of the disturbance.

Regards, Robin
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Old   October 1, 2005, 02:29
Default Re: SST Convergence
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Vikram
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Hi Robin, Thanks a lot for that incisive reply. I will be trying that out and I will keep you posted. Regards, Vikram
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Old   October 1, 2005, 06:28
Default Re: SST Convergence
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Vikram
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Hi, The original question was about the tef numerics option. The documentation says that "the k-omega and SST turbulence models give convergence difficulties in areas of high gradients in "omega" , for example at a blunt trailing edge or sudden rearward facing step, in conjunction with a highly refined boundary layer grid ( generally resolved into the viscous sub-layer). In such situations a flux limiting numerics implementation can be used for the omega equation, activated by setting this parameter to 1."

So what does this flux limiting implementation do exactly? Can someone kindly enlighten me on this ? Robin?

Thanks in advance, Vikram
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