Unsteady problems  underrelaxation factors
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 underrelaxation factors of pressure, momentum, density, etc for avoiding "divergence problems" of the transferred properties. I would like to ask, if the underrelaxation 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. 
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 RANStype turbulence models. Maybe this is why your solution is not as you expected. 
Thank you for your notification, sir.
Regards S 
Hi All,
Is it reasonable to use an underrelaxation factor for unsteady simulation? I think it is not, but I found that some of the Openfoam tutorials still have the options for underrelaxation factors for the unsteady solvers. Could anyone tell something about this issues ? Thank you so much! Quote:

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. 
Thank you very much. But I think this is a general CFD problem. Does anybody know something about this?
Quote:

Why dont you think it is reasonable to use underrelaxation?
Without it, implicit solvers like SIMPLE become unstable. 
Because, for example, when i use the time step is 1.0d04, and then I use underrelaxation 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 1.0d04. Is that so?
Quote:

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 underrelaxation. 
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 underrelaxation is still can be used for unsteady time marching.
Quote:

No.
An iterative solver with only one underrelaxed iteration does not produce correct results. 
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