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-   -   AMG versus ICCG (https://www.cfd-online.com/Forums/openfoam-solving/59936-amg-versus-iccg.html)

msrinath80 November 7, 2006 13:46
For my approx 1.2 million cell
 
For my approx 1.2 million cells (single-phase) icoFoam vortex shedding case, here is the output after the first three time steps and the corresponding fvSolution pressure solver entries:

ICCG: (p ICCG 1e-06 0;)

/*---------------------------------------------------------------------------*\
| ========= | |
| \ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \ / O peration | Version: 1.3 |
| \ / A nd | Web: http://www.openfoam.org |
| \/ M anipulation | |
\*---------------------------------------------------------------------------*/

Exec : /home/madhavan/OpenFOAM/OpenFOAM-1.3/applications/bin/linuxAMD64Gcc4DPOpt/icoFoa m /home/madhavan/OpenFOAM/madhavan-1.3/run/tutorials/icoFoam/. one_sq_cyl_3d_unsteady
Date : Nov 07 2006
Time : 12:27:17
Host : masternode
PID : 21882
Root : /home/madhavan/OpenFOAM/madhavan-1.3/run/tutorials/icoFoam/.
Case : one_sq_cyl_3d_unsteady
Nprocs : 1
Create time

Create mesh for time = 0

Reading transportProperties

Reading field p

Reading field U

Reading/calculating face flux field phi

Reading probeLocations

Constructing probes


Starting time loop

Time = 0.1

Mean and max Courant Numbers = 0 0.0741732
BICCG: Solving for Ux, Initial residual = 1, Final residual = 7.60531e-08, No Iterations 3
BICCG: Solving for Uy, Initial residual = 0, Final residual = 0, No Iterations 0
BICCG: Solving for Uz, Initial residual = 0, Final residual = 0, No Iterations 0
ICCG: Solving for p, Initial residual = 1, Final residual = 9.73006e-07, No Iterations 825
time step continuity errors : sum local = 4.55914e-10, global = -5.75299e-13, cumulative = -5.75299e-13
ICCG: Solving for p, Initial residual = 0.00108292, Final residual = 9.32198e-07, No Iterations 685
time step continuity errors : sum local = 3.75695e-07, global = -7.83026e-10, cumulative = -7.83601e-10

Wall patch = 0
Wall patch name = ChannelWalls
Uav = (1 0 0)
Aref = 1
nu = nu [0 2 -1 0 0 0 0] 1.004e-06
DragCoefficient = 2.01178e-05
pressureDragCoefficient = 0
viscDragCoefficient = 2.01178e-05
LiftCoefficient = 1.41566e-11


Wall patch = 1
Wall patch name = ObstacleWalls
Uav = (1 0 0)
Aref = 1
nu = nu [0 2 -1 0 0 0 0] 1.004e-06
DragCoefficient = 4.79068e-06
pressureDragCoefficient = 4.62468e-06
viscDragCoefficient = 1.66004e-07
LiftCoefficient = 8.15603e-14

ExecutionTime = 347.02 s ClockTime = 347 s

Time = 0.2

Mean and max Courant Numbers = 0.108516 4.04533
BICCG: Solving for Ux, Initial residual = 0.823434, Final residual = 4.45773e-06, No Iterations 3
BICCG: Solving for Uy, Initial residual = 0.329552, Final residual = 3.46137e-06, No Iterations 4
BICCG: Solving for Uz, Initial residual = 0.362595, Final residual = 4.31759e-06, No Iterations 3
ICCG: Solving for p, Initial residual = 0.00371997, Final residual = 9.84174e-07, No Iterations 719
time step continuity errors : sum local = 1.53182e-07, global = 3.88971e-10, cumulative = -3.9463e-10
ICCG: Solving for p, Initial residual = 0.00419475, Final residual = 9.92882e-07, No Iterations 709
time step continuity errors : sum local = 7.36119e-08, global = -2.94289e-10, cumulative = -6.88919e-10

Wall patch = 0
Wall patch name = ChannelWalls
Uav = (1 0 0)
Aref = 1
nu = nu [0 2 -1 0 0 0 0] 1.004e-06
DragCoefficient = 2.11014e-05
pressureDragCoefficient = 0
viscDragCoefficient = 2.11014e-05
LiftCoefficient = 2.18023e-11


Wall patch = 1
Wall patch name = ObstacleWalls
Uav = (1 0 0)
Aref = 1
nu = nu [0 2 -1 0 0 0 0] 1.004e-06
DragCoefficient = -9.70849e-07
pressureDragCoefficient = -1.11824e-06
viscDragCoefficient = 1.47396e-07
LiftCoefficient = -2.70848e-11

ExecutionTime = 639.75 s ClockTime = 640 s

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

AMG (p AMG 1e-06 0 1000;):

/*---------------------------------------------------------------------------*\
| ========= | |
| \ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \ / O peration | Version: 1.3 |
| \ / A nd | Web: http://www.openfoam.org |
| \/ M anipulation | |
\*---------------------------------------------------------------------------*/

Exec : /home/madhavan/OpenFOAM/OpenFOAM-1.3/applications/bin/linuxAMD64Gcc4DPOpt/icoFoa m /home/madhavan/OpenFOAM/madhavan-1.3/run/tutorials/icoFoam/. one_sq_cyl_3d_unsteady
Date : Nov 07 2006
Time : 11:29:35
Host : masternode
PID : 21242
Root : /home/madhavan/OpenFOAM/madhavan-1.3/run/tutorials/icoFoam/.
Case : one_sq_cyl_3d_unsteady
Nprocs : 1
Create time

Create mesh for time = 0

Reading transportProperties

Reading field p

Reading field U

Reading/calculating face flux field phi

Reading probeLocations

Constructing probes


Starting time loop

Time = 0.1

Mean and max Courant Numbers = 0 0.0741732
BICCG: Solving for Ux, Initial residual = 1, Final residual = 7.60531e-08, No Iterations 3
BICCG: Solving for Uy, Initial residual = 0, Final residual = 0, No Iterations 0
BICCG: Solving for Uz, Initial residual = 0, Final residual = 0, No Iterations 0
AMG: Solving for p, Initial residual = 1, Final residual = 9.80164e-07, No Iterations 105
time step continuity errors : sum local = 4.59248e-10, global = 2.47297e-14, cumulative = 2.47297e-14
AMG: Solving for p, Initial residual = 0.00108292, Final residual = 9.69421e-07, No Iterations 48
time step continuity errors : sum local = 3.90696e-07, global = -1.15826e-11, cumulative = -1.15579e-11

Wall patch = 0
Wall patch name = ChannelWalls
Uav = (1 0 0)
Aref = 1
nu = nu [0 2 -1 0 0 0 0] 1.004e-06
DragCoefficient = 2.01179e-05
pressureDragCoefficient = 0
viscDragCoefficient = 2.01179e-05
LiftCoefficient = -5.64623e-09


Wall patch = 1
Wall patch name = ObstacleWalls
Uav = (1 0 0)
Aref = 1
nu = nu [0 2 -1 0 0 0 0] 1.004e-06
DragCoefficient = 4.79158e-06
pressureDragCoefficient = 4.62556e-06
viscDragCoefficient = 1.6602e-07
LiftCoefficient = 1.45852e-10

ExecutionTime = 146.7 s ClockTime = 147 s

Time = 0.2

Mean and max Courant Numbers = 0.108519 4.04592
BICCG: Solving for Ux, Initial residual = 0.823441, Final residual = 4.46106e-06, No Iterations 3
BICCG: Solving for Uy, Initial residual = 0.329408, Final residual = 3.4621e-06, No Iterations 4
BICCG: Solving for Uz, Initial residual = 0.362669, Final residual = 4.339e-06, No Iterations 3
AMG: Solving for p, Initial residual = 0.0037213, Final residual = 9.14087e-07, No Iterations 82
time step continuity errors : sum local = 1.42274e-07, global = -4.78435e-13, cumulative = -1.20363e-11
AMG: Solving for p, Initial residual = 0.00419589, Final residual = 9.94429e-07, No Iterations 75
time step continuity errors : sum local = 7.37282e-08, global = 1.93804e-13, cumulative = -1.18425e-11

Wall patch = 0
Wall patch name = ChannelWalls
Uav = (1 0 0)
Aref = 1
nu = nu [0 2 -1 0 0 0 0] 1.004e-06
DragCoefficient = 2.11013e-05
pressureDragCoefficient = 0
viscDragCoefficient = 2.11013e-05
LiftCoefficient = 8.69834e-09


Wall patch = 1
Wall patch name = ObstacleWalls
Uav = (1 0 0)
Aref = 1
nu = nu [0 2 -1 0 0 0 0] 1.004e-06
DragCoefficient = -9.72347e-07
pressureDragCoefficient = -1.11973e-06
viscDragCoefficient = 1.47384e-07
LiftCoefficient = -2.83295e-10

ExecutionTime = 294.14 s ClockTime = 295 s

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


AMG (p AMG 1e-06 0 100;):

/*---------------------------------------------------------------------------*\
| ========= | |
| \ / F ield | OpenFOAM: The Open Source CFD Toolbox |
| \ / O peration | Version: 1.3 |
| \ / A nd | Web: http://www.openfoam.org |
| \/ M anipulation | |
\*---------------------------------------------------------------------------*/

Exec : /home/madhavan/OpenFOAM/OpenFOAM-1.3/applications/bin/linuxAMD64Gcc4DPOpt/icoFoa m /home/madhavan/OpenFOAM/madhavan-1.3/run/tutorials/icoFoam/. one_sq_cyl_3d_unsteady
Date : Nov 07 2006
Time : 12:20:05
Host : masternode
PID : 21848
Root : /home/madhavan/OpenFOAM/madhavan-1.3/run/tutorials/icoFoam/.
Case : one_sq_cyl_3d_unsteady
Nprocs : 1
Create time

Create mesh for time = 0

Reading transportProperties

Reading field p

Reading field U

Reading/calculating face flux field phi

Reading probeLocations

Constructing probes


Starting time loop

Time = 0.1

Mean and max Courant Numbers = 0 0.0741732
BICCG: Solving for Ux, Initial residual = 1, Final residual = 7.60531e-08, No Iterations 3
BICCG: Solving for Uy, Initial residual = 0, Final residual = 0, No Iterations 0
BICCG: Solving for Uz, Initial residual = 0, Final residual = 0, No Iterations 0
AMG: Solving for p, Initial residual = 1, Final residual = 8.07098e-07, No Iterations 102
time step continuity errors : sum local = 3.78169e-10, global = 1.46199e-14, cumulative = 1.46199e-14
AMG: Solving for p, Initial residual = 0.00108292, Final residual = 8.70596e-07, No Iterations 48
time step continuity errors : sum local = 3.50868e-07, global = 3.39341e-10, cumulative = 3.39356e-10

Wall patch = 0
Wall patch name = ChannelWalls
Uav = (1 0 0)
Aref = 1
nu = nu [0 2 -1 0 0 0 0] 1.004e-06
DragCoefficient = 2.01179e-05
pressureDragCoefficient = 0
viscDragCoefficient = 2.01179e-05
LiftCoefficient = -4.35326e-09


Wall patch = 1
Wall patch name = ObstacleWalls
Uav = (1 0 0)
Aref = 1
nu = nu [0 2 -1 0 0 0 0] 1.004e-06
DragCoefficient = 4.79156e-06
pressureDragCoefficient = 4.62554e-06
viscDragCoefficient = 1.6602e-07
LiftCoefficient = 8.678e-11

ExecutionTime = 108.75 s ClockTime = 109 s

Time = 0.2

Mean and max Courant Numbers = 0.10852 4.04587
BICCG: Solving for Ux, Initial residual = 0.823445, Final residual = 4.46093e-06, No Iterations 3
BICCG: Solving for Uy, Initial residual = 0.329439, Final residual = 3.4626e-06, No Iterations 4
BICCG: Solving for Uz, Initial residual = 0.362502, Final residual = 4.33196e-06, No Iterations 3
AMG: Solving for p, Initial residual = 0.00372114, Final residual = 7.40435e-07, No Iterations 68
time step continuity errors : sum local = 1.15246e-07, global = -2.12212e-12, cumulative = 3.37234e-10
AMG: Solving for p, Initial residual = 0.00419577, Final residual = 9.12739e-07, No Iterations 60
time step continuity errors : sum local = 6.76724e-08, global = 1.6342e-11, cumulative = 3.53576e-10

Wall patch = 0
Wall patch name = ChannelWalls
Uav = (1 0 0)
Aref = 1
nu = nu [0 2 -1 0 0 0 0] 1.004e-06
DragCoefficient = 2.11013e-05
pressureDragCoefficient = 0
viscDragCoefficient = 2.11013e-05
LiftCoefficient = 7.399e-09


Wall patch = 1
Wall patch name = ObstacleWalls
Uav = (1 0 0)
Aref = 1
nu = nu [0 2 -1 0 0 0 0] 1.004e-06
DragCoefficient = -9.72242e-07
pressureDragCoefficient = -1.11963e-06
viscDragCoefficient = 1.47384e-07
LiftCoefficient = -2.05608e-10

ExecutionTime = 191.77 s ClockTime = 192 s

Clearly, the AMG solver is better than ICCG for this case. Are there any guidelines on what number to use at the coarsest level for the AMG solver?

hjasak November 7, 2006 14:49
On the top level of AMG you ha
 
On the top level of AMG you have a system of equations that needs to be solved to a tight tolerance and the question is how much work to leave for the top level. In serial runs, anything between a dozen and a couple of hundred cells will give you the same result (1000 is too much). In parallel, it is the communications that matter, which means you need to be more careful: make sure there's a sufficient number of cells for each CPU. You also need to worry about the case where you have multiple separate domains. In general, 20-30 cells per CPU in parallel will do you fine (until I do some more tricks to speed this up). :-)

Hrv

msrinath80 November 7, 2006 15:15
Thanks for that insightful rep
 
Thanks for that insightful reply Hrv. Much appreciated.


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