[RodriguezFatz]

My original question was about a discussion in this thread where fredo490 said that OpenFOAM doesn't have a coupled solver and thus can't handle arbitrarily large time steps. But Fluent can handle these time steps without using a coupled solver, so my question was why. Also why isn't there a solver like that in OpenFoam, or is there any way I can tune pimpleFoam for larger time steps?

Also, what I am wondering: PISO, Simple, Simplec, Simpler... they are all more or less the same as I understand them. Just different approximations or some extra loops. But how can that lead to such different behaviour?

[olivierG]

Hello,

Just a bad trick, if you solution don't change to much over time

- try a "compressible" solver instead of incompressible,, even if you don't have much compressible effect => may run faster
- and try with Local Time Stepping (LTS), ex: rhoLTSPimpleFoam instead of pimpleFoam.

in fvSolution
- use "nOuterCorrectors 10" <- 10 or more
with residualControl to the wanted tolerance.

... And always check if the solution is correct ...

But you still can't get as high dT as in Fluent.

regards,
olivier

[akidess]

Phillip, read Jasak's comments on transient SIMPLE here:
http://www.cfd-online.com/OpenFOAM_D...es/1/1807.html

Then have a look at:
http://openfoamwiki.net/index.php/Ma...ransientSimple

Also read more theory on PISO and SIMPLE. It is not as simple as just adding or removing extra loops.

[RodriguezFatz]

Originally Posted by akidess

Also read more theory on PISO and SIMPLE. It is not as simple as just adding or removing extra loops.

Of course, I did. I read in Ferziger, Peric:
"Still another method of this general type is derived by neglecting ui' in the first correction step as in the SIMPLE method but following... it is known as the PISO algorithm"
And in Versteeg, Malalasekera:
"PISO involves one predictor step and two corrector steps and may be seen as an extension of SIMPLE, with a further corrector step to enhance it."

That's why I was stating that all these methods base on more or less the same principle. I am curious why you think they are essentially different (or not "just you", but why they actually are).

[akidess]

Yes, I agree the methods are related, but I don't think that's a strong argument to assume very similar numerical behavior (side-note: similarly, you could argue first and second order time discretization should have the same properties because they only differ by a factor theta).

PISO involves explicit correction steps, which is why your time step ends up being limited. These explicit correction steps are not part of the SIMPLE method.

Another side note - the differences between PISO and SIMPLE grow when looking at compressible flows, as now also the density is updated in a different manner. Issa brings this up in his 1986 paper on the development of PISO.

[RodriguezFatz]

I still got more questions, I really appreciate your help here!
I am wondering why I get this behaviour in pimpleFoam:
residuals.png
You can see that after 24 iterations pimple residuals jump at the last iteration to pretty much the initial value. Why? I thought this must be from under relaxation, which isn't done in the last iteration. But I then set relaxation factors to

relaxationFactors
{
    fields
    {
        p                 0.8;
    }
    equations
    {
        "(U)"             0.7;
        "(k)"             0.8;
        "(epsilon)"       0.8;
        "(omega)"         0.8;
    }
}

So there is nearly no under relaxation. Why are the iterations 24 and 25 so different? Do you think it will still come from this? And why is it so high?
This is the log output:

PIMPLE: iteration 23
DILUPBiCG:  Solving for Ux, Initial residual = 1.679199e-06, Final residual = 5.2347394e-09, No Iterations 2
DILUPBiCG:  Solving for Uy, Initial residual = 2.1944388e-05, Final residual = 1.8097067e-07, No Iterations 2
DILUPBiCG:  Solving for Uz, Initial residual = 4.1885199e-05, Final residual = 1.8273755e-07, No Iterations 2
GAMG:  Solving for p, Initial residual = 0.0034921225, Final residual = 0.00032813228, No Iterations 1
time step continuity errors : sum local = 1.3572061e-08, global = 1.4769898e-11, cumulative = 2.9324676e-09
GAMG:  Solving for p, Initial residual = 0.0012103037, Final residual = 0.00011895051, No Iterations 3
time step continuity errors : sum local = 4.9195925e-09, global = 7.4839779e-11, cumulative = 3.0073074e-09
DILUPBiCG:  Solving for epsilon, Initial residual = 1.0191608e-05, Final residual = 8.6961352e-08, No Iterations 2
DILUPBiCG:  Solving for k, Initial residual = 1.9168529e-06, Final residual = 9.0455142e-09, No Iterations 5
PIMPLE: iteration 24
DILUPBiCG:  Solving for Ux, Initial residual = 1.6236176e-06, Final residual = 5.5279907e-09, No Iterations 2
DILUPBiCG:  Solving for Uy, Initial residual = 2.1084076e-05, Final residual = 1.7394592e-07, No Iterations 2
DILUPBiCG:  Solving for Uz, Initial residual = 4.0269511e-05, Final residual = 1.4442665e-07, No Iterations 2
GAMG:  Solving for p, Initial residual = 0.0033965334, Final residual = 0.00033571154, No Iterations 1
time step continuity errors : sum local = 1.3884555e-08, global = 4.8871526e-12, cumulative = 3.0121945e-09
GAMG:  Solving for p, Initial residual = 0.0012140916, Final residual = 0.00010521305, No Iterations 4
time step continuity errors : sum local = 4.3514973e-09, global = 8.6495586e-11, cumulative = 3.0986901e-09
DILUPBiCG:  Solving for epsilon, Initial residual = 9.9529664e-06, Final residual = 8.4810541e-08, No Iterations 2
DILUPBiCG:  Solving for k, Initial residual = 1.8475951e-06, Final residual = 5.5701423e-09, No Iterations 4
PIMPLE: iteration 25
DILUPBiCG:  Solving for Ux, Initial residual = 2.7349049e-06, Final residual = 2.3595166e-08, No Iterations 7
DILUPBiCG:  Solving for Uy, Initial residual = 4.7743527e-05, Final residual = 3.7953704e-07, No Iterations 7
DILUPBiCG:  Solving for Uz, Initial residual = 9.2807878e-05, Final residual = 6.4070125e-07, No Iterations 9
GAMG:  Solving for p, Initial residual = 0.2261028, Final residual = 0.020367173, No Iterations 3
time step continuity errors : sum local = 1.0216869e-06, global = -2.317465e-08, cumulative = -2.007596e-08
GAMG:  Solving for p, Initial residual = 0.065461729, Final residual = 0.0063586886, No Iterations 4
time step continuity errors : sum local = 2.6827385e-07, global = -4.9828708e-08, cumulative = -6.9904668e-08
DILUPBiCG:  Solving for epsilon, Initial residual = 3.4064378e-05, Final residual = 3.3123049e-07, No Iterations 5
DILUPBiCG:  Solving for k, Initial residual = 5.9163051e-06, Final residual = 3.2299708e-08, No Iterations 8
PIMPLE: not converged within 25 iterations

[akidess]

I don't really use the pimple solver, so I can only guess. To me it looks like while your solution seems to be ok with the relaxation, the final result in iteration 24 is still not a great initial guess for a solution without underrelaxation. Thus in iteration 25, where no relaxation is applied, things break down. Hope someone with more experience with pimple chimes in too.

[be_inspired]

Would it be possible to use OF3.1-ext for pimpleDyMFoam to achieve higher CFL? That is, an overal implicit approach.
I am trying to simulate a wind turbine with 3 blades. The mesh just at trailing edge is really small so the time step should be really low to match a CFL=1.
For perfom 1 iteration per 1 degree of rotation, the max CFL is around 700 while the average is around 0.007.
For CFL=1, the simulation time maybe could last 2 months...

[be_inspired]

What about transientSimpleDyMFoam solver?
What are the diferences wrt pimpleDyMFoam?

[wyldckat]

Greetings be_inspired,

For some details about "transientSimpleFoam": http://openfoamwiki.net/index.php/Ma...ransientSimple

As for using a CFL higher than 0.5 or 1:

• Why don't you simply use a steady-state solver?
• Or use the CoBlended feature that was introduced in OpenFOAM 2.2: http://www.openfoam.org/version2.2.0/numerics.php

Best regards,
Bruno