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Causes of checkerboarding

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Old   April 16, 2018, 09:17
Default Causes of checkerboarding
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Hi all

I'm attempting to develop a new solver and keep running into checkerboarding issues. I've been looking online and trying to find the cause. In the wiki describing buoyantBoussenesqPisoFoam, it mentions:

Quote:
Most importantly, lines 17--20 set up the Poisson equation for the pressure correction and can be seen to be the face-located analog to Equation 27. This shows that pressure is being predicted at the cell centers using face-centered information. This procedure effectively applies Rhie-Chow interpolation which avoids the checkerboard pressure oscillations that can occur on a co-located grid
http://openfoamwiki.net/index.php/Bu...sinesqPisoFoam

However, I closely followed the methods described in the above link when creating the solver and used the same way of building the equations to apply Rhie-Chow interpolation, so I don't think that is it.

In this thread (How to avoid checkerboard?), a user mentions checkerboarding caused by using slip and noSlip boundary conditions, which I am using. However, his issue is solved by increasing corrector loops (which doesn't help me) and another user points out that this means his case just hadn't converged yet. So I'm not sure if his assessment that his boundary conditions were causing checkerboarding or not is accurate.

In another thread (Checkerboarding with interFoam) a user mentions from literature that there's a minimum courrant number allowable to avoid checkerboarding. I couldn't find the literature in question but I tried messing around with cell size and timestep to remove checkerboarding, but unsuccessfully. Am I correct in understanding that there's in fact a minimum allowable courrant number? And is this the absolute minimum for any cell in the entire domain, or an average?

My case is a very simple one (2D, left inlet, right fixed pressure outlet, ceiling and floor are slip and noslip respectively) and my solver is a modification of buoyantBoussenesqPimpleFoam. I've disabled my modifications and it simply behaves like bBPF so I am fairly certain my coding isn't the issue. (Running the same case with bBPF produces identical results.)



Above is the vertical velocity profile at the inlet. There should be nothing really affecting the vertical velocity at all.

Now obviously I don't expect anyone to look through my code and case files to figure out why this is occurring - I was hoping someone could give me some insight as to why this kind of problem might be occurring, or expand upon the information I've found already

Cheers

EDIT: Also, I spent a lot of time messing around with the schemes but this also never seemed to make much difference. Courrant number was always kept well below 1

Second edit:

I found these threads: Oscillatory inlet velocity and Mesh problem/ coarse OK - fine not OK

The issues seem to be very similar. The fixedMean velocity as suggested in the first thread didn't make any difference but I'll attempt the other possible fixes related to the issue. If anyone could provide any insight into any of this,I would be grateful

Third edit:

I tried the method of having a patch of slip adjacent to the inlet, but this just caused the strange vertical velocities to be pushed further down the domain. I also used laminar instead of kEpsilon for turbulence and the vertical velocity anomaly in the corner was lessened by several orders of magnitude, but still present.

Last edited by sturgeon; April 16, 2018 at 14:50.
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