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October 20, 2015, 05:34 
PIMPLE pressure residual behaviour

#1 
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Hi,
I am doing some test cases in incompressible ke transient simulation with PIMPLE solver. The test case is a inletoutlet 200mm radius pipes and 1m cubic space with 1m/s inlet and zero pressure outlet. See attached image. The simulation converge with PG an PBiCG solvers and 50 inner interations, but the pressure residual is very high (0.2). At first I had a short pipe length at the outlet and thought that could be caused due to a forced "developed" BC too near the whirl, but extended to 10D length and results are similar. I donīt see why the pressure residual donīt decrease, I think I can be doing something basic wrong but canīt realise what. Any comment is welcomed, thanks for reading. 

October 20, 2015, 09:07 

#2 
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Philipp
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Ok!
1) The problem seems to be stationary, so why do you use a timedependent solver? 2) Can you post the last few iterations of your log file? Then, the fvSolution, fvSchemes and boundary settings would be usefull. Cheers, Philipp.
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October 20, 2015, 09:08 

#3 
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And please run checkMesh and post the output.
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October 20, 2015, 16:44 
Solver log, 3 last iterations

#4 
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PIMPLE: iteration 48
DILUPBiCG: Solving for Ux, Initial residual = 3.66943999509e15, Final residual = 3.66943999509e15, No Iterations 0 DILUPBiCG: Solving for Uy, Initial residual = 3.64742134734e15, Final residual = 3.64742134734e15, No Iterations 0 DILUPBiCG: Solving for Uz, Initial residual = 1.63696948347e15, Final residual = 1.63696948347e15, No Iterations 0 DICPCG: Solving for p, Initial residual = 8.49930096518e06, Final residual = 8.49930096518e06, No Iterations 0 time step continuity errors : sum local = 2.65405500279e09, global = 3.71852880882e10, cumulative = 1.46329427866e05 PIMPLE: iteration 49 DILUPBiCG: Solving for Ux, Initial residual = 1.76897567744e15, Final residual = 1.76897567744e15, No Iterations 0 DILUPBiCG: Solving for Uy, Initial residual = 1.74878398492e15, Final residual = 1.74878398492e15, No Iterations 0 DILUPBiCG: Solving for Uz, Initial residual = 8.10180631031e16, Final residual = 8.10180631031e16, No Iterations 0 DICPCG: Solving for p, Initial residual = 8.49930096373e06, Final residual = 8.49930096373e06, No Iterations 0 time step continuity errors : sum local = 2.65405500236e09, global = 3.71852880864e10, cumulative = 1.46325709337e05 PIMPLE: iteration 50 DILUPBiCG: Solving for Ux, Initial residual = 1.26944090701e15, Final residual = 1.26944090701e15, No Iterations 0 DILUPBiCG: Solving for Uy, Initial residual = 1.25329761635e15, Final residual = 1.25329761635e15, No Iterations 0 DILUPBiCG: Solving for Uz, Initial residual = 6.14630453987e16, Final residual = 6.14630453987e16, No Iterations 0 DICPCG: Solving for p, Initial residual = 0.222409325308, Final residual = 0.00214878147056, No Iterations 40 time step continuity errors : sum local = 8.16664635027e07, global = 2.69513888778e10, cumulative = 1.46323014198e05 DILUPBiCG: Solving for epsilon, Initial residual = 0.000308926995915, Final residual = 3.73817035895e06, No Iterations 1 DILUPBiCG: Solving for k, Initial residual = 0.000837974833799, Final residual = 2.28092083849e07, No Iterations 2 ExecutionTime = 1769.42 s ClockTime = 1806 s End 

October 20, 2015, 17:06 

#5 
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The case is really stationary, but it is the first case I run as transient and wanted start with something simple to avoid run into errors (not much sucess as you can see..)
I use openfoam with the SimScale web frontend, so donīt have access to the config files, but I will put the input parameters of the frontend that are used to generate them. Relax. P 0.3 Relax. U 0.7 Relax. k 0.7 Relax. e 0.7 Outer correctors 50 Correctors 1 Non orth correctors 0 Press. ref. cell 0 Press. ref. value 0 Correct the flux off pressure solver PGC pressure solver final PGC all rest solvers PBiCG Numerical SchemesTime differentiation scheme for default: euler Gradient scheme for default: gauss linear Gradient scheme for grad(p): gauss linear Gradient scheme for grad(U): gauss linear Divergence scheme for default: none Divergence scheme for div(phi,U): gauss upwind Laplacian scheme for default: none Laplacian scheme for laplacian(rAUf,p): gauss linear corrected Interpolation scheme for default: linear Interpolation scheme for interpolate(U): linear Surfacenormal gradient scheme for default: corrected Divergence scheme for div(phi,k): gauss upwind Divergence scheme for div(phi,epsilon): gauss upwind Divergence scheme for div((nuEff*dev(T(grad(U))))): gauss linear Laplacian scheme for laplacian(nuEff,U): gauss linear corrected Laplacian scheme for laplacian(DkEff,k): gauss linear corrected Laplacian scheme for laplacian(DepsilonEff,epsilon): gauss linear corrected Start time value [s] 0 End time value [s] 2 Time step length [s] 0.005 Adjustable timestep YES Max Courant number 0.9 Max timestep 0.1 Meshing log: Checking final mesh ... Checking faces in error : nonorthogonality > 65 degrees : 0 faces with face pyramid volume < 1e13 : 0 faces with concavity > 80 degrees : 0 faces with skewness > 4 (internal) or 20 (boundary) : 0 faces with interpolation weights (0..1) < 0.02 : 0 faces with volume ratio of neighbour cells < 0.01 : 0 faces with face twist < 0.01 : 0 faces on cells with determinant < 0.001 : 0 Finished meshing without any errors Finished meshing in = 17.94 s. End Thanks! 

October 21, 2015, 01:27 

#6 
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Philipp
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From the log file it looks like your linear solver settings are just not tight enough. You see, they all "converge" in zero iterations, which means, that the initial residual is lower than the tolerance setting of the solver and so the solver just doesn't start at all. Velocity residuals are already low enough, but pressure tolerance seems to be set to 1e5. You need to reduce that value to some very low value, or even better set it to "0" to deactivate it. Better work with relative tolerance of something as relTol=0.1  then the solver iterates each time to reduce the initial residual by a factor of 10.
If you have no clue what I am talking about please ask.
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October 22, 2015, 16:03 

#7 
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I changed the pressure solvers to 0 abs. tolerance as suggested and 0.01 relative tolerance, from the log it seems it is working, but pressure residual behaves in the same way remaining stable but very high.
any clue that what is happening? Last log iterations: PIMPLE: iteration 46 DILUPBiCG: Solving for Ux, Initial residual = 3.48133791804e12, Final residual = 3.48133791804e12, No Iterations 0 DILUPBiCG: Solving for Uy, Initial residual = 3.49851495228e12, Final residual = 3.49851495228e12, No Iterations 0 DILUPBiCG: Solving for Uz, Initial residual = 9.25990915442e13, Final residual = 9.25990915442e13, No Iterations 0 GAMG: Solving for p, Initial residual = 1.3857838272e08, Final residual = 1.27475120605e10, No Iterations 10 time step continuity errors : sum local = 3.98048688524e14, global = 3.64980793544e17, cumulative = 1.31043940799e06 PIMPLE: iteration 47 DILUPBiCG: Solving for Ux, Initial residual = 2.4256915443e12, Final residual = 2.4256915443e12, No Iterations 0 DILUPBiCG: Solving for Uy, Initial residual = 2.43791479004e12, Final residual = 2.43791479004e12, No Iterations 0 DILUPBiCG: Solving for Uz, Initial residual = 6.46925707426e13, Final residual = 6.46925707426e13, No Iterations 0 GAMG: Solving for p, Initial residual = 9.66587424372e09, Final residual = 9.2695754586e11, No Iterations 10 time step continuity errors : sum local = 2.89449026233e14, global = 2.6438740825e17, cumulative = 1.31043940802e06 PIMPLE: iteration 48 DILUPBiCG: Solving for Ux, Initial residual = 1.68332763499e12, Final residual = 1.68332763499e12, No Iterations 0 DILUPBiCG: Solving for Uy, Initial residual = 1.69200223919e12, Final residual = 1.69200223919e12, No Iterations 0 DILUPBiCG: Solving for Uz, Initial residual = 4.48119295379e13, Final residual = 4.48119295379e13, No Iterations 0 GAMG: Solving for p, Initial residual = 6.74367452056e09, Final residual = 5.98224072053e11, No Iterations 11 time step continuity errors : sum local = 1.86801068143e14, global = 1.77539045341e17, cumulative = 1.31043940803e06 PIMPLE: iteration 49 DILUPBiCG: Solving for Ux, Initial residual = 1.17285513915e12, Final residual = 1.17285513915e12, No Iterations 0 DILUPBiCG: Solving for Uy, Initial residual = 1.17901812847e12, Final residual = 1.17901812847e12, No Iterations 0 DILUPBiCG: Solving for Uz, Initial residual = 3.12713589318e13, Final residual = 3.12713589318e13, No Iterations 0 GAMG: Solving for p, Initial residual = 4.70462071566e09, Final residual = 4.29905977241e11, No Iterations 11 time step continuity errors : sum local = 1.34243811453e14, global = 1.24364706896e17, cumulative = 1.31043940805e06 PIMPLE: iteration 50 DILUPBiCG: Solving for Ux, Initial residual = 1.19431805646e12, Final residual = 1.19431805646e12, No Iterations 0 DILUPBiCG: Solving for Uy, Initial residual = 1.2004567811e12, Final residual = 1.2004567811e12, No Iterations 0 DILUPBiCG: Solving for Uz, Initial residual = 3.1916640893e13, Final residual = 3.1916640893e13, No Iterations 0 DICPCG: Solving for p, Initial residual = 0.222418912957, Final residual = 0.00215247161149, No Iterations 40 time step continuity errors : sum local = 8.18033215972e07, global = 1.0803896483e10, cumulative = 1.31033136908e06 DILUPBiCG: Solving for epsilon, Initial residual = 0.000308933503258, Final residual = 3.71821064209e06, No Iterations 1 DILUPBiCG: Solving for k, Initial residual = 0.000838036948419, Final residual = 2.28589230167e07, No Iterations 2 ExecutionTime = 2467.63 s ClockTime = 2503 s End 

October 23, 2015, 02:54 

#8 
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Philipp
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I actually don't see any major mistake. Maybe you should lower the time step. PIMPLE isn't really made for steady state problems, but should normally converge anyway into the steady state solution. And you can use less "outer corrections", maybe 5 or so.
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October 23, 2015, 03:42 
My $0.02

#9 
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Hi,
Unfortunately I was not able to find your fvSolution file in the thread (except rather vague "pressure solver PGC pressure solver final PGC", which contradicts with your log output). But... 1. Your script for plotting residuals is something SPECIAL (well, I guess it is for SIMPLE output, yet you are trying to use it with PIMPLE). If you compare your plot with log file, well, they are quite different. 2. Linear solvers in general doing nothing: Code:
PIMPLE: iteration 46 DILUPBiCG: Solving for Ux, Initial residual = 3.48133791804e12, Final residual = 3.48133791804e12, No Iterations 0 DILUPBiCG: Solving for Uy, Initial residual = 3.49851495228e12, Final residual = 3.49851495228e12, No Iterations 0 DILUPBiCG: Solving for Uz, Initial residual = 9.25990915442e13, Final residual = 9.25990915442e13, No Iterations 0 GAMG: Solving for p, Initial residual = 1.3857838272e08, Final residual = 1.27475120605e10, No Iterations 10 time step continuity errors : sum local = 3.98048688524e14, global = 3.64980793544e17, cumulative = 1.31043940799e06 3. And finally your 50th (final) iteration is rather funny: Code:
PIMPLE: iteration 50 ... DICPCG: Solving for p, Initial residual = 0.222418912957, Final residual = 0.00215247161149, No Iterations 40 ... 

October 23, 2015, 03:49 

#10 
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Philipp
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It's the last iteration of PIMPLE that runs without under relaxation. I always thought the residuals jump to a higher value because of that. I always see that in my calculations with PIMPLE.
Alexey, can you help to understand this? Here is an older post of me with the same problem: http://www.cfdonline.com/Forums/ope...tml#post512566
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October 23, 2015, 04:09 

#11 
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Yes, it runs without relaxation. I could agree with residual jump, here is an example:
Code:
Time = 403.570434783 Courant Number mean: 0.0254108903838 max: 0.399601783095 deltaT = 0.0017391304348 PIMPLE: iteration 1 ... smoothSolver: Solving for p_rgh, Initial residual = 5.27025050486e07, Final residual = 5.27025050486e07, No Iterations 0 ... PIMPLE: iteration 2 ... smoothSolver: Solving for p_rgh, Initial residual = 1.2707579238e06, Final residual = 3.96394665144e07, No Iterations 2 ... smoothSolver: Solving for p_rgh, Initial residual = 4.75110200449e07, Final residual = 4.75110200449e07, No Iterations 0 ... PIMPLE: converged in 2 iterations In fact the case is mysterious and description lacks lots of information. We can try to guess, we can suggest to change linear solvers, number of outer, inner, and nonorthogonal iterations. YET the problem could be just in rather meaningless boundary conditions. 

October 23, 2015, 04:15 

#12 
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Philipp
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The problem is he uses some fancy tool to feed openFoam instead of the linux files. That's why we can't see all the files.
I am interested in this, too, since I had the same problem in the link I posted above.
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October 23, 2015, 08:46 

#13 
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Well, it seems relaxation has the strongest influence on this pressure residual jump. There's output of simple von Karman vortex street simulation (Re=100). Here's PIMPLE dictionary:
Code:
PIMPLE { ... nCorrectors 2; nOuterCorrectors 200; nNonOrthogonalCorrectors 1; residualControl { "(Up)" { tolerance 1e4; relTol 0; } } } Code:
Courant Number mean: 2.50232221546 max: 9.96200968481 deltaT = 0.120358962239 Time = 2.09713302882 ... PIMPLE: iteration 27 DILUPBiCG: Solving for Ux, Initial residual = 4.15327775911e05, Final residual = 1.23514504456e09, No Iterations 4 DILUPBiCG: Solving for Uy, Initial residual = 6.09455422471e05, Final residual = 8.18928362675e09, No Iterations 3 GAMG: Solving for p, Initial residual = 0.000371321428441, Final residual = 3.42843494044e08, No Iterations 21 GAMG: Solving for p, Initial residual = 0.00017741155415, Final residual = 1.23005016557e08, No Iterations 14 time step continuity errors : sum local = 4.62132670435e12, global = 6.16715518034e13, cumulative = 1.92439390203e11 GAMG: Solving for p, Initial residual = 0.000225325680736, Final residual = 1.90339707e08, No Iterations 21 GAMG: Solving for p, Initial residual = 9.76761067809e05, Final residual = 6.79675647185e09, No Iterations 14 time step continuity errors : sum local = 2.55627318983e12, global = 3.32714190268e13, cumulative = 1.95766532106e11 PIMPLE: iteration 28 DILUPBiCG: Solving for Ux, Initial residual = 0.000181363144351, Final residual = 7.60441245621e09, No Iterations 22 DILUPBiCG: Solving for Uy, Initial residual = 0.000258495496672, Final residual = 8.54422172066e09, No Iterations 16 GAMG: Solving for p, Initial residual = 0.0545226923199, Final residual = 3.37862610543e06, No Iterations 18 GAMG: Solving for p, Initial residual = 0.0066508532655, Final residual = 5.55033630196e07, No Iterations 13 time step continuity errors : sum local = 2.38614106372e10, global = 4.54598804251e13, cumulative = 2.00312520148e11 GAMG: Solving for p, Initial residual = 0.016294518945, Final residual = 9.24578676737e07, No Iterations 22 DICPCG: Solving for p, Initial residual = 0.00450585596504, Final residual = 9.57540140063e09, No Iterations 385 time step continuity errors : sum local = 4.6638163735e12, global = 1.0568861233e14, cumulative = 2.00206831536e11 PIMPLE: converged in 28 iterations Code:
Courant Number mean: 2.4947035938 max: 9.96027202621 deltaT = 0.120062962341 Time = 2.09487802595 ... PIMPLE: iteration 44 GAMG: Solving for p, Initial residual = 0.000238872288081, Final residual = 2.32647216666e08, No Iterations 21 GAMG: Solving for p, Initial residual = 0.000118346041252, Final residual = 8.49113866312e09, No Iterations 14 time step continuity errors : sum local = 3.14221423572e12, global = 4.04831295648e13, cumulative = 1.42930177289e09 GAMG: Solving for p, Initial residual = 0.000194168070676, Final residual = 1.90414144434e08, No Iterations 21 GAMG: Solving for p, Initial residual = 9.76214328148e05, Final residual = 6.79906293134e09, No Iterations 14 time step continuity errors : sum local = 2.51459825656e12, global = 3.18921759336e13, cumulative = 1.42898285113e09 PIMPLE: iteration 45 GAMG: Solving for p, Initial residual = 0.0480809612314, Final residual = 3.73088388333e06, No Iterations 10 GAMG: Solving for p, Initial residual = 0.000716015292718, Final residual = 4.39791075468e08, No Iterations 16 time step continuity errors : sum local = 2.16396947525e11, global = 1.24748574368e12, cumulative = 1.43023033687e09 GAMG: Solving for p, Initial residual = 0.00688525707745, Final residual = 5.66825871087e07, No Iterations 13 DICPCG: Solving for p, Initial residual = 0.000271098422015, Final residual = 9.20746002634e09, No Iterations 323 time step continuity errors : sum local = 4.56109711273e12, global = 2.39087820596e14, cumulative = 1.43025424565e09 PIMPLE: converged in 45 iterations Code:
Courant Number mean: 2.48139995258 max: 9.94371386088 deltaT = 0.119771881886 Time = 2.09124724549 ... PIMPLE: iteration 17 GAMG: Solving for p, Initial residual = 0.000214021152968, Final residual = 1.75470894623e08, No Iterations 20 GAMG: Solving for p, Initial residual = 0.000106143523768, Final residual = 8.48570411037e09, No Iterations 14 time step continuity errors : sum local = 4.29085749707e12, global = 5.52875805525e14, cumulative = 2.0568156925e10 GAMG: Solving for p, Initial residual = 0.000179002131243, Final residual = 1.61128110673e08, No Iterations 20 GAMG: Solving for p, Initial residual = 9.48553480382e05, Final residual = 6.35308637381e09, No Iterations 14 time step continuity errors : sum local = 3.20037924877e12, global = 5.77498637594e14, cumulative = 2.05739319114e10 PIMPLE: iteration 18 GAMG: Solving for p, Initial residual = 0.000223613422704, Final residual = 1.65083115521e08, No Iterations 20 GAMG: Solving for p, Initial residual = 9.53518776637e05, Final residual = 7.61412355603e09, No Iterations 14 time step continuity errors : sum local = 3.82654997504e12, global = 4.14200408574e14, cumulative = 2.05697899073e10 GAMG: Solving for p, Initial residual = 0.000169727491343, Final residual = 1.49042004301e08, No Iterations 20 DICPCG: Solving for p, Initial residual = 8.52713742618e05, Final residual = 9.20630885865e09, No Iterations 269 time step continuity errors : sum local = 4.61128666844e12, global = 2.58903829271e13, cumulative = 2.05438995244e10 PIMPLE: converged in 18 iterations 

October 24, 2015, 14:35 

#14  
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I find how to get the openfoam config files. I tried some changes in number of correctors as suggested but still same result, pressure residual stabilizes to 0.28??
BC are (see image): inlet velocity 1m/s (short tube inlet face) oulet pressure = 0 (long tube outlet face) all other walls (no slip with wall functions) fvSolution Quote:
Quote:


October 24, 2015, 16:14 

#15 
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Hi,
Well, this Code:
pFinal { relTol 0.05; preconditioner DIC; tolerance 0.0; solver PCG; } Last edited by alexeym; October 25, 2015 at 02:35. 

October 26, 2015, 05:09 

#16 
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Fabian Roesler
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You are just plotting the first iteration residual of your PIMPLE simulation.
When using high Courant Numbers and many PIMPLE iterations, the first residual will be high. However, the first residual is not important. The residual of your last PIMPLE iteration is the one that has to be small. As far as I can see this is fine for all your logs. If you want to avoid PIMPLE iterations with zero inner iterations, just specify a residual control for all fields and the solver will stop the PIMPLE loop if converged. With this procedure it is possible to run simulations with Co = 200 with only 10 to 15 PIMPLE iterations. 

October 26, 2015, 11:31 

#17 
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Thanks Fabian,
If I undersoot correctly you think that it is being plot the first residual per inner iteration, that happens to be stabilized about 0.2 due to any reason. It seems more sensible to see the final residual after the iterations , doesnīt it? why should I be interested in the first iteration residual? might be a residual plot bug of the tool I use... 

October 27, 2015, 02:08 

#18 
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Fabian Roesler
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I'd not say that it is a bug. The residuals show the variation of the conserved quantity. By using more PIMPLE iterations, you can increase the Courant number what leads to a larger time step. The larger time step results in a more unsteady fields and thus gives you larger (in your eyes inaccurate) residual/solution process.
I'd say your convergence is good. As I said before, try to use residual control. Code:
PIMPLE { momentumPredictor yes; transonic no; nOuterCorrectors 50; nCorrectors 1; nNonOrthogonalCorrectors 0; rhoMin rhoMin [ 1 3 0 0 0 ] 0.1; rhoMax rhoMax [ 1 3 0 0 0 ] 2.0; residualControl { "(Ukepsilonomega)" { relTol 0; tolerance 0.0001; } p_rgh { relTol 0; tolerance 0.001; } } turbOnFinalIterOnly on; } 

September 8, 2020, 06:05 

#19 
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Hello All,
I am simulating particleladen backward facing step using DPMFoam. which is based on pimple solver for fluid phase. the case is unsteady,incompresiible and turbulent (i am using komegaSST RANS model). I plotted initial residuals to check for the convergence. From the attached residual plot you can see that initial residuals for U.air(x), U.air(y), k.air, omega.air falls below 10^3. but the p has relatively higher value of rintitial residuals (`0.05). Is my solution converged? If not, how can I achieve the convergence? my fvSolutions directory reads: Code:
solvers { "(pkinematicCloud:theta)" { solver PCG; preconditioner DIC; tolerance 1e06; relTol 0.01; } "(pkinematicCloud:theta)Final" { solver PCG; preconditioner DIC; tolerance 1e06; relTol 0; } "(U.airk.airomega.air)" { solver PBiCG; preconditioner DILU; tolerance 1e05; relTol 0.1; } "(U.airk.airomega.air)Final" { solver PBiCG; preconditioner DILU; tolerance 1e05; relTol 0; } } PIMPLE { nOuterCorrectors 1; nCorrectors 2; nNonOrthogonalCorrectors 0; pRefCell 1001; pRefValue 0; } The output of last time step reads: Code:
PIMPLE: iteration 1 DILUPBiCG: Solving for U.airx, Initial residual = 1.33954e05, Final residual = 1.51627e07, No Iterations 1 DILUPBiCG: Solving for U.airy, Initial residual = 0.000181577, Final residual = 1.77745e06, No Iterations 1 DICPCG: Solving for p, Initial residual = 0.073047, Final residual = 0.000679366, No Iterations 26 time step continuity errors : sum local = 2.71727e08, global = 2.26846e10, cumulative = 3.70693e07 DICPCG: Solving for p, Initial residual = 0.00706675, Final residual = 9.44924e07, No Iterations 241 time step continuity errors : sum local = 3.78099e11, global = 1.41854e13, cumulative = 3.70693e07 DILUPBiCG: Solving for omega.air, Initial residual = 9.47518e06, Final residual = 9.47518e06, No Iterations 0 DILUPBiCG: Solving for k.air, Initial residual = 1.17881e05, Final residual = 2.07731e07, No Iterations 1 ExecutionTime = 1661.97 s ClockTime = 1662 s I will be thankful, if someone can guide me to get better my convergence of solution for p. Kind Regards Atul 

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