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-   -   AMG parallelisation and convection schemes (http://www.cfd-online.com/Forums/openfoam-solving/59279-amg-parallelisation-convection-schemes.html)

christian November 20, 2007 06:14

I'm running LES on internal fl
 
I'm running LES on internal flow passing an orifice plate. There are 3 million hexahedral cells. AMG is used with the following settings:

p GAMG
{
p GAMG
{
tolerance 1e-04;
relTol 0;
smoother GaussSeidel;
nCellsInCoarsestLevel 30;
mergeLevels 1;
agglomerator faceAreaPair;
cacheAgglomeration on;
// nPreSweeps 0;
// nPostSweeps 2;
// nFinestSweeps 2;
// scaleCorrection yes;
// directSolveCoarsest no;
};
pFinal GAMG
{
tolerance 1e-04;
relTol 0;
smoother GaussSeidel;
nCellsInCoarsestLevel 30;
mergeLevels 1;
agglomerator faceAreaPair;
cacheAgglomeration on;
// nPreSweeps 0;
// nPostSweeps 2;
// nFinestSweeps 2;
// scaleCorrection yes;
// directSolveCoarsest no;

I'm running on a dual core machine. I've compared running on 1 CPU and running on 2 CPU:s, see below:

-------------- 1 CPU -----------------------

Time = 0.0127

Courant Number mean: 0.029355 max: 1.3072
DILUPBiCG: Solving for Ux, Initial residual = 0.00181139, Final residual = 3.63695e-06, No Iterations 2
DILUPBiCG: Solving for Uy, Initial residual = 0.0207766, Final residual = 3.83961e-06, No Iterations 3
DILUPBiCG: Solving for Uz, Initial residual = 0.0209138, Final residual = 3.63663e-06, No Iterations 3
GAMG: Solving for p, Initial residual = 0.247526, Final residual = 9.88771e-05, No Iterations 62
time step continuity errors : sum local = 2.27516e-08, global = -5.17908e-09, cumulative = 1.71557e-07
GAMG: Solving for p, Initial residual = 0.0506679, Final residual = 9.04641e-05, No Iterations 12
time step continuity errors : sum local = 2.07179e-08, global = -3.83544e-09, cumulative = 1.67722e-07
ExecutionTime = 37679.3 s ClockTime = 45218 s

Time = 0.01275

Courant Number mean: 0.0293701 max: 1.3049
DILUPBiCG: Solving for Ux, Initial residual = 0.00182424, Final residual = 3.75133e-06, No Iterations 2
DILUPBiCG: Solving for Uy, Initial residual = 0.0208073, Final residual = 2.84017e-07, No Iterations 3
DILUPBiCG: Solving for Uz, Initial residual = 0.0209474, Final residual = 3.01873e-07, No Iterations 3
GAMG: Solving for p, Initial residual = 0.249727, Final residual = 9.40309e-05, No Iterations 61
time step continuity errors : sum local = 2.15569e-08, global = -4.70248e-09, cumulative = 1.63019e-07
GAMG: Solving for p, Initial residual = 0.0506867, Final residual = 9.95558e-05, No Iterations 12
time step continuity errors : sum local = 2.30218e-08, global = -4.36871e-09, cumulative = 1.58651e-07
ExecutionTime = 37848.4 s ClockTime = 45388 s

-------------------------------------------

------------- 2 CPU -----------------------

Time = 0.0142

Courant Number mean: 0.0298694 max: 1.25606
DILUPBiCG: Solving for Ux, Initial residual = 0.00415434, Final residual = 3.0531e-06, No Iterations 2
DILUPBiCG: Solving for Uy, Initial residual = 0.026599, Final residual = 1.81261e-07, No Iterations 3
DILUPBiCG: Solving for Uz, Initial residual = 0.0327937, Final residual = 9.15543e-06, No Iterations 2
GAMG: Solving for p, Initial residual = 0.27006, Final residual = 9.92377e-05, No Iterations 178
time step continuity errors : sum local = 3.20367e-08, global = -3.3541e-09, cumulative = -1.08914e-08
GAMG: Solving for p, Initial residual = 0.0598454, Final residual = 9.88563e-05, No Iterations 37
time step continuity errors : sum local = 3.10378e-08, global = -3.38161e-09, cumulative = -1.4273e-08
ExecutionTime = 2829.56 s ClockTime = 2969 s

Time = 0.01425

Courant Number mean: 0.0298782 max: 1.21613
DILUPBiCG: Solving for Ux, Initial residual = 0.00416074, Final residual = 1.55868e-06, No Iterations 2
DILUPBiCG: Solving for Uy, Initial residual = 0.0264671, Final residual = 3.28098e-07, No Iterations 3
DILUPBiCG: Solving for Uz, Initial residual = 0.0325682, Final residual = 3.72951e-07, No Iterations 3
GAMG: Solving for p, Initial residual = 0.277771, Final residual = 9.83602e-05, No Iterations 178
time step continuity errors : sum local = 3.08924e-08, global = 3.23832e-09, cumulative = -1.10347e-08
GAMG: Solving for p, Initial residual = 0.0579332, Final residual = 9.59878e-05, No Iterations 37
time step continuity errors : sum local = 3.10846e-08, global = 3.44964e-09, cumulative = -7.58509e-09
ExecutionTime = 3115 s ClockTime = 3269 s

-------------------------------------------

1) How come the AMG solver needs to do more iterations when running parallel? This is not what I've seen before on this machine and a similar case. Then there was a little speed-up when running on 2 CPUs. In this case, I'm losing time running parallel.

2) What are recommended settings for the AMG solver? I'm especially interested in the number of cells in the coarsest level when running serial and parallel, respectively.

3) In general, isn't it possible to get a scaling of 2 when running on two CPUs with AMG? What kind of case will give me a scaling of two?

4) I'm using Crank-Nicholson for time discretisation. From your experience, what is the gain in accuracy (compared to all sources of errors in the computation) and increase in computational time compared to Backward?

5) What would you say to be a reasonable convergence criteria (for p)? I don't want a situation where p is converged to much to drown in all other errors.

6) For convection, both midpoint and linear discretisation give me the velocity field in the plot below (after a few time steps). I assume what is seen are wiggles??? I used vanLeer for the RANS solution to start from. Do you believe vanLeer to be to diffusive for LES?

http://www.cfd-online.com/OpenFOAM_D...ges/1/5958.jpg

Best regards,
Christian

msrinath80 November 20, 2007 14:53

Hi Christian, I can only an
 
Hi Christian,

I can only answer some of your questions:

Q2) What are recommended settings for the AMG solver? I'm especially interested in the number of cells in the coarsest level when running serial and parallel, respectively.

A2) See response by Hrv in this[1] post.


Q3) In general, isn't it possible to get a scaling of 2 when running on two CPUs with AMG? What kind of case will give me a scaling of two?

A3) Yes. I get it all the time. Some of the important requirements for getting a scaling of two are:

i) Decent problem size (at least 0.25 million cells)
ii) Make sure you have a decent interconnect (gigabit is good, infiniband is better, SMP is best)
iii) Don't run your simulation on the following configurations as they at best give 1.2 speedup:

a) hyperthreaded CPUs (a load of crap is what this is for parallel CFD)
b) Dual/Quad/Octo-core CPU offerings from Intel/AMD (These CPUs steadily approach the performance of hyperthreaded CPUs as you move from left to right)

References:

[1] http://www.cfd-online.com/OpenFOAM_D...es/1/3360.html

christian December 17, 2007 07:45

When running this LES of the f
 
When running this LES of the flow past the orifice plate I'm using a turbulentInlet. Running with fluctuations of 0 % gives me a solution that runs smoothly with a few pressure iterations per time step. However, when running with 5 % fluctuations I get temporal oscillations and a lot of pressure iterations are needed. Why is this? How can I come up with a remedy?

I using a fluctuation scale of (0.05 0.05 0.05). The inlet uses a mapped RANS profile and is normal to the x axis. As a reference scale (to the fluctuations) I'm using the mapped velocity profile.

If you can help me somehow, please respond ASAP.

Best regards,
Christian Svensson

eugene December 17, 2007 09:21

The turbulentInlet boundary pr
 
The turbulentInlet boundary produces inlet velocities that are not divergence free, i.e. they do not obey continuity. The pressure solver therefore has to work a lot harder to converge. turbulentInlet is a very poor choice of inlet condition for velocity, since the random oscillations will be damped out before they can transition to proper turbulence anyway.


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