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Unsteady problems - under-relaxation factors

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Old   March 28, 2013, 08:10
Default Unsteady problems - under-relaxation factors
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Stefanos Katifeoglou
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Dear community,

At unsteady problems where noticeable structures of the fluid interacting with other fluid or body zones might appear (alike vortex shedding) it is often necessary to decrease the under-relaxation factors of pressure, momentum, density, etc for avoiding "divergence problems" of the transferred properties. I would like to ask, if the under-relaxation factors of turbulent quantities are also decreased, is there a possibility that this may influence the appearance of these structures/or not?
In particular, i have been expecting to track a jet somewhere within my FSI problem, but it isn't very clear, beside the dense mesh, etc.
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Old   March 28, 2013, 08:41
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The URFs should not affect the final solution. Yet lower URFs mean that the solution advances slower and you need more Iterations to get convergence.

Phenomena like vortex shedding cannot be captured accurately with a RANS-type turbulence models. Maybe this is why your solution is not as you expected.
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Old   March 29, 2013, 05:31
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Thank you for your notification, sir.

Regards

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Old   July 21, 2013, 15:09
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Hi All,

Is it reasonable to use an under-relaxation factor for unsteady simulation? I think it is not, but I found that some of the Openfoam tutorials still have the options for under-relaxation factors for the unsteady solvers. Could anyone tell something about this issues ? Thank you so much!

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Dear community,

At unsteady problems where noticeable structures of the fluid interacting with other fluid or body zones might appear (alike vortex shedding) it is often necessary to decrease the under-relaxation factors of pressure, momentum, density, etc for avoiding "divergence problems" of the transferred properties. I would like to ask, if the under-relaxation factors of turbulent quantities are also decreased, is there a possibility that this may influence the appearance of these structures/or not?
In particular, i have been expecting to track a jet somewhere within my FSI problem, but it isn't very clear, beside the dense mesh, etc.
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Old   July 21, 2013, 15:23
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For openfoam please go openfoam forum.
For fluent yes you can change under relaxation factors in unsteady simulation in solve -------> control --------> solution
Discretization of equations are same for steady and unsteady.
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Old   July 21, 2013, 15:29
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Thank you very much. But I think this is a general CFD problem. Does anybody know something about this?

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For openfoam please go openfoam forum.
For fluent yes you can change under relaxation factors in unsteady simulation in solve -------> control --------> solution
Discretization of equations are same for steady and unsteady.
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Old   July 22, 2013, 04:25
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Why dont you think it is reasonable to use under-relaxation?
Without it, implicit solvers like SIMPLE become unstable.
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Old   July 22, 2013, 04:32
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Because, for example, when i use the time step is 1.0d-04, and then I use under-relaxation factor is 0.1, in terms of time marching, does it mean that the effective time step will be reduced? In other words, the simulation is marching with a time step less than 1.0d-04. Is that so?

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Why dont you think it is reasonable to use under-relaxation?
Without it, implicit solvers like SIMPLE become unstable.
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Old   July 22, 2013, 04:38
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This holds true only for the iterations within each timestep.
Given a sufficient amount of iterations per timestep (convergence!) the physical time step size remains unaffected of the under-relaxation.
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Old   July 22, 2013, 04:41
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OK, thank you very much. if I just use one iteration for each time step, but the convergence is reached for that iteration. So the under-relaxation is still can be used for unsteady time marching.

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This holds true only for the iterations within each timestep.
Given a sufficient amount of iterations per timestep (convergence!) the physical time step size remains unaffected of the under-relaxation.
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Old   July 22, 2013, 04:49
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No.
An iterative solver with only one under-relaxed iteration does not produce correct results.
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Old   August 14, 2020, 04:37
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I refer you all to the book "Computation of Unsteady Internal Flows: Fundamental Methods with Case Studies" by tucker, page 84: ""with unsteady problem it is desirable to AVOID under relaxation factor as much as it possible.""
I think it is due to the artificial time steps which make the time that you report for analysis invalidate.
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