[diggee]

Hello. I am doing a transient simulation and want to use PISO as the pressure velocity coupling scheme. After reading the theory guide, I understand that the skewness correction is for highly skewed meshes and neighbor correction is to get a more accurate solution. But I couldn't find as to what the values (default 1) of both these fields mean? My mesh has almost little to none skewness. So do I make the skewness correction value as 0? What about the neighbor correction value?

[LuckyTran]

Leave them as the defaults.

I actually can't discern the details either and it's not well documented. The problem is that there are too many things going on and these options are implemented in the style of under-relaxation whereas they are discrete steps in the PISO algorithm. I.e. you do an extra neighbor correction step and an extra skewness correction step so the values should be integers like 0,1,2,3, etc... But that's not the way the menu is set up. It probably has something to do with the AMG solver.

My guess is the corrections are always calculated and you apply an under-relaxation factor. If that is so, then setting neighborCorrection to 0 would turn PISO into SIMPLE. You could probably set the skewness correction to 0, but if it is always calculated then you would not get any speedup, and it would slow down your convergence. So unless someone knows better, I would leave them as they are because they don't hurt.

[Malthehh]

Good question diggee - I am in the exact same position as you are.
From that I have read in the user guide. I think LuckyTran is right in what he have described.

Did you experience any difference between SIMPLE or PISO in the convergence time or did you experience any problems when using the default values?

Best Malthe

[diggee]

Originally Posted by Malthehh

Good question diggee - I am in the exact same position as you are.
From that I have read in the user guide. I think LuckyTran is right in what he have described.

Did you experience any difference between SIMPLE or PISO in the convergence time or did you experience any problems when using the default values?

Best Malthe

Hey. I asked this question a long long time ago and I now know what they mean. For PISO, those numbers indicate how many iterations are done for the mesh skewness and neighbor correction (hence only integers are accepted as arguments). If you have very good mesh that has little to no skewness (eg pipe meshed with only hex elements) , you can make the skewness correction 0. If your mesh is highly skewed, you can increment the number as you want. Leave the neighbor correction to 1. Obviously, if you increment these numbers, computational time increases.

SIMPLE is always lighter computationally than PISO but at the same time SIMPLE can run into problems with a skewed mesh. Hence the convergence time with each method really depends on the problem setup. If you have a simple geometry and simple flow physics, go ahead with SIMPLE.

[LuckyTran]

Cool! That clears up a lot! Yes, they should be integers. So the interface is just oddly setup and documentation not so good.

Originally Posted by diggee

SIMPLE is always lighter computationally than PISO but at the same time SIMPLE can run into problems with a skewed mesh. Hence the convergence time with each method really depends on the problem setup. If you have a simple geometry and simple flow physics, go ahead with SIMPLE.

For complex (transient) problems, SIMPLE can also be better than PISO for the same reason. SIMPLE is faster per iteration than PISO because it doesn't solve the final pressure correction which takes a long time. If you have strong coupling between momentum and other equations, say energy equation due to strongly varying properties or other transport equations, running SIMPLE can save you a lot of time over PISO. The thing is PISO is good for solving the P-V coupling problem in 1 super-expensive step. But if you are coupling with other equations then you need to do many sweeps anyway.

[Malthehh]

Thanks a lot for your quick response LuckyTran and Diggee. It helped on the understanding.

Best Malthe