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Coriolius November 17, 2004 04:58

Oscillating residual
 
If a flow is inherently unsteady (e.g. vortex shedding) but a steady state solution is being solved for, will the residue still converge or oscilate at some point?


Glenn Horrocks November 17, 2004 17:27

Re: Oscillating residual
 
Hi,

It depends. If you are using a small physical timestep size in the steady state solution the residuals probably will oscillate. A test for this is to change the timestep size and see if the oscillations scale with it. If it does then you probably have an oscillatory solution.

If you use a larger timestep you can damp out the oscillations and get a steady state answer, but if the timestep becomes too large the solution will diverge. It is not always possible to get a steady state solution. Depends on the problem.

Glenn Horrocks

Mark November 18, 2004 01:26

Re: Oscillating residual
 
Try to change the relax factor in the expert parameter. May help with the convergence.

Glenn Horrocks November 18, 2004 17:28

Re: Oscillating residual
 
Hi Mark,

I think it unlikely changing the relax factor will help for most simulations. Usually oscillating residuals are indicating something more fundamental than that, either an oscillatory physical solution or incorrect pseudo timestep.

Glenn Horrocks

Ian November 19, 2004 11:54

Re: Oscillating residual
 
Yes, and.... for "weak" oscillations the turbulence model can dampen them out giving you a well converged steady state solution.

Pete November 20, 2004 07:05

Re: Oscillating residual
 
I have similar experience for this. While using KE model, no oscillation is observed. If you switch to SST or SSG model, the solution is still converged but with oscillation after the residuals drop to a certain level. The weak oscillation seems to have little effect on the solution. Anyone has different result?


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