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Under-Relaxation of Transport Equations

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Old   July 12, 2007, 04:06
Default Under-Relaxation of Transport Equations
  #1
Florian Fruth
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Hello all, although I've read older responses to such a question I'd like to add a comment but also append a question. So as far as I understood it, there is an underrelaxation possible for the velocity and the pressure (see Solver options - ) But does anyone know if it is possible to set these rel. values independently of each other, cause what I found was one parameter for v and p, but I'd need different ones for a comparison with openFOAM, which just converges with these certain underrelaxation factors!?

Cheers Florian
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Old   July 12, 2007, 04:25
Default Re: Under-Relaxation of Transport Equations
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Florian Fruth
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Sorry I forgot to mention where these underrelaxations are possible to my opinion:

Solver - Expert Parameters - Linear Solver - Solver Relaxation Fluids

Here the relaxation for v and p can be done, but I' also searching something like that for k, epsilon...

Cheers Florian
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Old   July 12, 2007, 10:36
Default Re: Under-Relaxation of Transport Equations
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opaque
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Dear Florian,

There are only two relaxation options available, one for the Cont+Mom equation (called fluids) and a second one for all other scalar equations (kε are scalars).

There is no access on a per equation basis. That means, that if you change "solver relaxation scalar" you are also changing energy relaxation as well as additional variables, mass fraction, etc..

I would not touch the default values unless the solver is having problems and you have been adviced to do so. That is why they are "kind of hidden" in the expert parameters section.

Good luck,

Opaque

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Old   July 12, 2007, 10:42
Default Re: Under-Relaxation of Transport Equations
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Florian Fruth
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Thanks for your help, I'll try it nevertheless once! I somehow guessed that k and \epsilon are "other scalars" but wasn't sure. Usually I wouldn't touch these parameters, but as I already told in the first thread, I need to do this, such that the computations of openFOAM and cfx are somehow "similar"....

Cheers Florian

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Old   July 12, 2007, 11:06
Default Re: Under-Relaxation of Transport Equations
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Gert-Jan
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As far as I know, using small (false) timesteps is the same as using an underrelaxation factor.

What do you want to compare? Solver speed or the solution?

Gert-Jan
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Old   July 12, 2007, 11:45
Default Re: Under-Relaxation of Transport Equations
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Florian Fruth
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Well I wanted to compare the solver speed on the one hand and the stability/convergence on the other. Additionaly of course the solution of the backward-facing step which is almost always the same/very similar.But as you can image there are plenty of problems due to the differences between openfoam and cfx. As it is a steadyState simulation I can't really change the timestep size.

Florian
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Old   July 16, 2007, 09:59
Default Re: Under-Relaxation of Transport Equations
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Phil
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Be careful that you don't compare apples and oranges. These solver relaxation factors are internal to the linear solver, not the nonlinear solver; they are completely different from the velocity and pressure underrelaxation used in SIMPLE-based solvers. The only mechanism to apply nonlinear relaxation in CFX is the Physical Timescale (which for a steady state simulation is a false timestep, and is therefore still applicable).
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