Hi, I used 2 packages to solve my poisson eqn, which is part of my NS unsteady solver. One is the NSPCG solver package which uses the compressed row format to store the A matrix. The other is PETSc. I found that using both solvers gave me similar answers for a number of time step. However, after that, the answer will suddenly change drastically for the PETSc case. This does not happen for the NSPCG solver.
For e.g. time step 1-315, oscillating airfoil case, pressure changes smoothly, similar answers in both cases at time=316, pressure changes from -3.22 to -3.2 for NSPCG, but pressure changes from -3.21 to -60.2 for PETSc This happens when I use HYPRE's AMG or PETSc's direct solver LU. I have been trying to find out what's the cause and I can't find the answer in debugging. I would like to compare the values of the matrix of the 2 different solvers and see if there's any difference. However, NSPCG's matrix is in compressed row format while PETSc's one is just an address and it can't be viewed easily. Moreover, it's a big matrix so it's not possible to check by inspection. I'm thinking of subtracting one matrix by the other and find if it's zero. What's the best way to solve this problem? Btw, I'm using fortran and there's no mpi Thank you.
