Reduction in time step leads to divergence of velocity

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kchinembiri
Posts: 18
Joined: Sun Feb 19, 2017 6:44 pm

Reduction in time step leads to divergence of velocity

Post by kchinembiri » Mon Feb 11, 2019 10:00 pm

Hi all,

I have puzzling problem. If i reduce the time step for my LES simulations the velocity diverges. If increase the time step back to its original value (2e-4), no divergence occurs and I can get past the point where the solution diverges with the smaller time step. I want to reduce the time step as the instantaneous cfl number goes above 1 (maxes out at around 2) with the original value.

I am using Code-Saturne 5.2.0 and I have attached the xml, listing files and subroutines for the simulation which diverges.

Any advice you can give would be much appreciated.

Kind regards,

Kenneth
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Yvan Fournier
Posts: 2578
Joined: Mon Feb 20, 2012 3:25 pm

Re: Reduction in time step leads to divergence of velocity

Post by Yvan Fournier » Fri Feb 15, 2019 7:52 pm

Hello,

This is strange, but could be linked to several factors, possible the Rhie & Chow filter, or some other instability (in some LES cases adding a small amount of upwind, say 2% can help avoid some issues while not adding significant error). Also, your max CFL constraint leads to a very small mean CFL, which is less risky but can lead to such issues with the Rhie & Chow filter (which is used to avoid checkerboarding with colocated finite volumes but introduces a time-step dependent error term).

I mentioned your case to a turbulence specialist so he might take a look.

Best regards,

Yvan

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