On Jan 8, 2012, at 9:33 PM, Mohamad M. Nasr-Azadani wrote: > How do you know it is diverging? Because it looks weird? You are comparing > it to something? > > While marching in time, it only takes 20-30 iterations to solve for pressure. > After a very long integrattion in time, all of a sudden the pressure lsys > does not converge even in 10,000 iterations. So, that's why I am saying it is > converging.
Are you using the exact same matrix at each time-step or does the matrix change? > > How accurately are you solving the linear system? > I tried rtol = 1e-10 and 1e-12. > > What strange behavior? Is the right hand side for the pressure solution > reasonable but the solution "strange"? How do you know you are not giving > "bad stuff" to the pressure solve? > For my case, I get huge pressure gradients close to the solid boundaries. > That, indeed, causes very small delta_t's when I keep integrating. That > should not happen cause at this stage of my simulations, nothing is really > happening in the flow field. Very small velocities and velocity gradients. > I am trying to find the problem, since I have only seen this for such big > problem size, i.e. 100 million grid points. That makes it really hard to see > if the I am feeding back rhs into the pressure linear system. > > Best, > Mohamad > > > > > > > On Sun, Jan 8, 2012 at 6:47 PM, Barry Smith <bsmith at mcs.anl.gov> wrote: > > On Jan 8, 2012, at 5:13 PM, Mohamad M. Nasr-Azadani wrote: > > > Thanks Barry and Matt, > > > > Barry, > > Also if you are really solving the Poisson problem you should use > > multigrid; if simple geometry then geometric multigrid if complicated > > geometry probably easier to use hypre BoomerAMG. No sane person solves > > Poisson problem with anything but a multigrid or FFT based solver. > > > > In my main code, I am actually doing what you suggested, i.e. GMRES + > > boomerAMG to solve for my Poisson equation. I have not used the > > KSPSetNullSpace() though. > > The problem is that my code (CFD, incompressible flow 3D) diverges after a > > long time integration > > How do you know it is diverging? Because it looks weird? You are comparing > it to something? > > > How accurately are you solving the linear system? > > > > and I am trying to find out why. > > The system that I have is a fairly big one, i.e. 100 million grid points > > and more. > > I see that pressure solution (which is obviously coupled to the velocity > > field) starts showing strange behavior. > > What strange behavior? Is the right hand side for the pressure solution > reasonable but the solution "strange"? How do you know you are not giving > "bad stuff" to the pressure solve? > > Barry > > > > That's why I tried to first double check my pressure solver. > > > > Based on your experience, do you think that not using a nullspace() for the > > pressure solver for that linear system size could have caused it to diverge? > > > > > > Matt, > > 1) Matlab could be doing a lot of things. I am betting that they scale the > > problem, so -pc_type jacobi. > > > > That could be right. The reason that I relied on the MATLAB's gmres solver > > to behave exactly similar to PETSc was just their "help" saying that > > ************ > > X = GMRES(A,B,RESTART,TOL,MAXIT,M1,M2) use preconditioner M or M=M1*M2 > > and effectively solve the system inv(M)*A*X = inv(M)*B for X. If M is > > [] then a preconditioner is not applied. > > ************ > > > > Best, > > Mohamad > > > > On Sat, Jan 7, 2012 at 5:39 PM, Barry Smith <bsmith at mcs.anl.gov> wrote: > > > > On Jan 7, 2012, at 4:00 PM, Mohamad M. Nasr-Azadani wrote: > > > > > Hi guys, > > > > > > I am trying to narrow down an issue with my Poisson solver. > > > I have the following problem setup > > > > > > Laplace(f) = rhs(x,z,y) > > > 0 <= x,y,z <= (Lx,Ly,Lz) > > > > > > I solve the Poisson equation in three dimensions with the analytical > > > function f(x,y,z) defined by > > > > > > f(x,z,y) = cos(2*pi*x/Lx)*cos(2*pi*y/Ly)*cos(2*pi*z/Lz) + K > > > where Lx = Ly =Lz = 1.0 and K is a constant I use to set f(Lx,Ly,Lz) = > > > 0.0. > > > > > > Second order descritization is used for the Poisson equation. > > > Also, Neumann boundary condition is used everywhere, but I set the > > > top-right-front node's value to zero to get rid of the Nullspaced matrix > > > manually. > > > > Please don't do this. That results in a unnecessaryly huge condition > > number. Use KSPSetNullSpace.() > > > > Also if you are really solving the Poisson problem you should use > > multigrid; if simple geometry then geometric multigrid if complicated > > geometry probably easier to use hypre BoomerAMG. No sane person solves > > Poisson problem with anything but a multigrid or FFT based solver. > > > > Barry > > > > > I use 20 grid points in each direction. > > > > > > The problem is: > > > I use GMRES(20) without any preconditioners (rtol = 1e-12) to solve the > > > linear system. > > > It takes 77,000 iterations to converge!!!! > > > > > > For the size of only 8,000 unknowns, even though the lsys is not > > > preconditioned, I guess that is a LOT of iterations. > > > Next, I setup the exact same problem in MATLAB and use their GMRES solver > > > function. > > > I set the same parameters and MATLAB tells me that it converges using > > > only 3870 iterations. > > > > > > I know that there might be some internal differences between MATLAB and > > > PETSc's implementations of this method, but given the fact that these two > > > solvers are not preconditioned, I am wondering about this big difference? > > > > > > Any ideas? > > > > > > Best, > > > Mohamad > > > > > > > > >
