I have a multiphysics problem in COMSOL involving the laplace equation, navier stokes equations and heat transfer. When I use the direct solver, it solves fine. When I switch to iterative, I cannot get it to solve. Basically, I will encounter an error saying that inf or NaN encountered in the solver for SOR. I have to use iterative due to the large number of elements (in real case), so direct solver is out of the question.
This problem generally occurs when there is some mistake in the physics or study section or wrong selection of the mesh size. Few days back i was also facing this problem in direct solver and found that there was a little mistake in the boundary conditions.
This problem generally occurs when there is some mistake in the physics or study section or wrong selection of the mesh size. Few days back i was also facing this problem in direct solver and found that there was a little mistake in the boundary conditions.
My experience, that is common in COMSOL. I believe it has something to do with their scheme. If I am correct, cfd module in COMSOL solve "continuity + momentum" eqs in one matrix in which the pressure and velocity are the unkowns. My experience of coding is that it will encounter stability problem. In my Ph.D dissertation, we took that approach in using Legendre collocation method to our problem. Very hard to converge. So, later we split the velocity and pressure into two stages. This stablize our scheme. I have similar experience in using COMSOL. If you took staggered approach, it becomes easier to converge. Another issue is that SOR is less stable than direct method. If you have to adopt SOR because of matrix size problem, the use it wisely. For example, make itseveral blocks. In each block, direct solve. In connection or while need to update the flow filed, use SOR. That why ADI is better than point-to-point SOR.
This's all my experience. But, looking back to the basic linear algebra, make sense.
Without using too much mathematics, nowaday in my advising graduate students, I'll tell them that: imaging there are errors in your flow field and the problem is non-linear, or the coefficient of your PDE is associate with your flow-field solution. Just imaging how the error can be propagate, then it will be very easy to choose the right matrix solver or the numerical scheme.
One last thing: COMSOL is a good package. But the problem is, hard to use for solving CFD problem using iterative solver. If you need to use COMSOL, buy a lot of memory to put the whole problem into one matrix. This is how I am doing now.
@Shiu-Hwa Shyu you've spoken about staggered mesh approach. How do you do that in comsol? As in how do you build a staggered grid?
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