On Tue, Nov 3, 2015 at 8:24 AM, Matthew Knepley <[email protected]> wrote:
> On Tue, Nov 3, 2015 at 9:12 AM, Zou (Non-US), Ling <[email protected]> > wrote: > >> Matt, thanks for the reply. >> The simulation is a transient simulation, which eventually converges to a >> steady-state solution, given enough simulation time. >> My code runs fine and I could tell the simulation reaches steady state by >> looking at the residual monitored by SNES monitor function. >> >> See an example screen output >> >> Solving time step 90, using BDF1, dt = 0.1. >> >> Current time (the starting time of this time step) = 8.85. >> >> NL step = 0, SNES Function norm = 1.47538E-02 >> >> NL step = 1, SNES Function norm = 8.06971E-04 >> >> total_FunctionCall_number: 0 >> >> converged, time step increased = 0.1 >> >> Solving time step 91, using BDF1, dt = 0.1. >> >> Current time (the starting time of this time step) = 8.95. >> >> NL step = 0, SNES Function norm = 1.10861E-02 >> >> NL step = 1, SNES Function norm = 6.26584E-04 >> >> total_FunctionCall_number: 0 >> >> converged, time step increased = 0.1 >> >> Solving time step 92, using BDF1, dt = 0.1. >> >> Current time (the starting time of this time step) = 9.05. >> >> NL step = 0, SNES Function norm = 7.21253E-03 >> >> NL step = 1, SNES Function norm = 9.93402E-04 >> >> total_FunctionCall_number: 0 >> >> converged, time step increased = 0.1 >> >> Solving time step 93, using BDF1, dt = 0.1. >> >> Current time (the starting time of this time step) = 9.15. >> >> NL step = 0, SNES Function norm = 5.40260E-03 >> >> NL step = 1, SNES Function norm = 6.21162E-04 >> >> total_FunctionCall_number: 0 >> >> converged, time step increased = 0.1 >> >> Solving time step 94, using BDF1, dt = 0.1. >> >> Current time (the starting time of this time step) = 9.25. >> >> NL step = 0, SNES Function norm = 3.40214E-03 >> >> NL step = 1, SNES Function norm = 6.16805E-04 >> >> total_FunctionCall_number: 0 >> >> converged, time step increased = 0.1 >> >> Solving time step 95, using BDF1, dt = 0.1. >> >> Current time (the starting time of this time step) = 9.35. >> >> NL step = 0, SNES Function norm = 2.29656E-03 >> >> NL step = 1, SNES Function norm = 6.19337E-04 >> >> total_FunctionCall_number: 0 >> >> converged, time step increased = 0.1 >> >> Solving time step 96, using BDF1, dt = 0.1. >> >> Current time (the starting time of this time step) = 9.45. >> >> NL step = 0, SNES Function norm = 1.53218E-03 >> >> NL step = 1, SNES Function norm = 5.94845E-04 >> >> total_FunctionCall_number: 0 >> >> converged, time step increased = 0.1 >> >> Solving time step 97, using BDF1, dt = 0.1. >> >> Current time (the starting time of this time step) = 9.55. >> >> NL step = 0, SNES Function norm = 1.32136E-03 >> >> NL step = 1, SNES Function norm = 6.19933E-04 >> >> total_FunctionCall_number: 0 >> >> converged, time step increased = 0.1 >> >> Solving time step 98, using BDF1, dt = 0.1. >> >> Current time (the starting time of this time step) = 9.65. >> >> NL step = 0, SNES Function norm = 7.09342E-04 >> >> NL step = 1, SNES Function norm = 6.18694E-04 >> >> total_FunctionCall_number: 0 >> >> converged, time step increased = 0.1 >> >> Solving time step 99, using BDF1, dt = 0.1. >> >> Current time (the starting time of this time step) = 9.75. >> >> NL step = 0, SNES Function norm = 5.49192E-04 >> >> total_FunctionCall_number: 0 >> >> converged, time step increased = 0.1 >> >> Solving time step 100, using BDF1, dt = 0.1. >> >> Current time (the starting time of this time step) = 9.85. >> >> NL step = 0, SNES Function norm = 5.49192E-04 >> >> total_FunctionCall_number: 0 >> >> converged, time step increased = 0.1 >> >> Solving time step 101, using BDF1, dt = 0.1. >> >> Current time (the starting time of this time step) = 9.95. >> >> NL step = 0, SNES Function norm = 5.49192E-04 >> >> total_FunctionCall_number: 0 >> >> I observed that after time step 99, the residual never changed, so I >> believe the transient simulation converges at time step 99. >> I wonder can I use the criterion "SNES converges and it takes 0 >> iteration" to say the simulation reaches a steady state. Such that I don't >> have to look at the screen and the code knows it converges and should stop. >> >> Put it another way, what's the common way people would implement a scheme >> to detect a transient simulation reaches steady state. >> > > I don't think so. The above makes no sense to me. You are signaling SNES > convergence with a relative > residual norm of 5e-4? That does not sound precise enough to me. > > I would argue that number (5.e-4) depends on the problem you are solving (actually I am solving). The initial residual of the problem starts at ~1e8. But you might be right, and I have to think about this issue more carefully. > As I said, I think the believable way to find steady states is to look for > solutions to the algebraic equations, > perhaps by using timestepping as a preconditioner. > > You still need a numerical criterion to let the code understand it converges, right? For example, "a set of solutions have already been found to satisfy the algebraic equations because ___residuals drops below (a number here)__". Thanks, Ling > Thanks, > > Matt > > >> Thanks, >> >> Ling >> >> >> On Tue, Nov 3, 2015 at 5:25 AM, Matthew Knepley <[email protected]> >> wrote: >> >>> On Mon, Nov 2, 2015 at 7:29 PM, Barry Smith <[email protected]> wrote: >>> >>>> >>>> > On Oct 30, 2015, at 12:23 PM, Zou (Non-US), Ling <[email protected]> >>>> wrote: >>>> > >>>> > Hi All, >>>> > >>>> > From physics point of view, I know my simulation converges if nothing >>>> changes any more. >>>> > >>>> > I wonder how normally you do to detect if your simulation reaches >>>> steady state from numerical point of view. >>>> > Is it a good practice to use SNES convergence as a criterion, i.e., >>>> > SNES converges and it takes 0 iteration(s) >>>> >>>> Depends on the time integrator and SNES tolerance you are using. If >>>> you use a -snes_rtol 1.e-5 it will always try to squeeze 5 MORE digits out >>>> of the residual so won't take 0 iterations even if there is only a small >>>> change in the solution. >>>> >>> >>> There are two different situations here: >>> >>> 1) Solving for a mathematical steady state. You remove the time >>> derivative and solve the algebraic system with SNES. Then >>> the SNES tolerance is a good measure. >>> >>> 2) Use timestepping to advance until nothing looks like it is >>> changing. This is a "physical" steady state. >>> >>> You can use 1) with a timestepping preconditioner TSPSEUDO, which is >>> what I would recommend if you >>> want a true steady state. >>> >>> Thanks, >>> >>> Matt >>> >>> >>>> > >>>> > Thanks, >>>> > >>>> > Ling >>>> >>>> >>> >>> >>> -- >>> What most experimenters take for granted before they begin their >>> experiments is infinitely more interesting than any results to which their >>> experiments lead. >>> -- Norbert Wiener >>> >> >> > > > -- > What most experimenters take for granted before they begin their > experiments is infinitely more interesting than any results to which their > experiments lead. > -- Norbert Wiener >
