understanding the PIMPLE algorithm

[shIxx]

hi guys,
I have a question regarding the transient algorithm.
I noticed that the size of the time steps for the solver does not matter. The solver calculates with another delta t
.......
.......
smoothSolver: Solving for omega, Initial residual = 0.004410836, Final residual = 7.8808988e-009, No Iterations 17
smoothSolver: Solving for k, Initial residual = 0.16740384, Final residual = 8.6197292e-009, No Iterations 37
ExecutionTime = 121.62 s ClockTime = 122 s

Courant Number mean: 0.20074245 max: 5.0545416
deltaT = 0.00027325827
Time = 0.00108677

PIMPLE: iteration 1
smoothSolver: Solving for Ux, Initial residual = 0.00016858864, Final residual = 1.5728663e-005, No Iterations 1
.......

should't delta t match the specified time step (in this case 0.05s) and should the graph of the residuals not look like this?

case 3b [https://openfoamwiki.net/index.php/Open ... n_OpenFOAM]

[thschrader]

I had a look at the pitzdaily-tutorial.
In the controlDict, parameter "adjustTimeStep" is set to "yes" by default.
Did you use this too?

[shIxx]

I had a look at the pitzdaily-tutorial.
In the controlDict, parameter "adjustTimeStep" is set to "yes" by default.
Did you use this too?

No I do not, because I have no idea of openFOAM in the text-based environment. I only used it with freeCAD as GUI so I do not know how to look/runs through at these tutorials.

At least I've found where they are
C:\Program Files\blueCFD-Core-2017\OpenFOAM-5.x\tutorials\basic\potentialFoam\pitzDaily\system\controlDict

However, I do not know what to do with these files

[thschrader]

... I have no idea of openFOAM in the text-based environment. I only used it with freeCAD as GUI so I do not know how to look/runs through at these tutorials.
At least I've found where they are
C:\Program Files\blueCFD-Core-2017\OpenFOAM-5.x\tutorials\basic\potentialFoam\pitzDaily\system\controlDict
However, I do not know what to do with these files

Create a new run-folder in ofuser-of5
Copy the pitzdaily tutorial into this folder
doubleclick on bluecfd-core terminal icon, terminal starts.
In terminal:
goto run folder with command "$FOAM_RUN" (dont type the ")
EDIT: type "cd $FOAM_RUN"
goto pitzdaily folder with command "cd pitzDaily"
Command "ls" lists all files
generate mesh: type "blockMesh"
check mesh: type "checkMesh"
view mesh: type "paraFoam"
close paraview and run sim: type "pimpleFoam"
view result: type "paraFoam" ==> feel good
"clear" removes all text from console, "exit" closes console

You should get familiar with the text files of OF by DOING the tutorials from this guy:
https://www.youtube.com/channel/UCjdgpu ... qheyE82Vvw
Need cfd-help in german? Go here:
https://forum.freecadweb.org/viewtopic. ... &start=170
Thomas

run_folder.JPG (40.59 KiB) Viewed 3451 times

[oliveroxtoby]

hi guys,
I have a question regarding the transient algorithm.
I noticed that the size of the time steps for the solver does not matter. The solver calculates with another delta t
.......
.......
smoothSolver: Solving for omega, Initial residual = 0.004410836, Final residual = 7.8808988e-009, No Iterations 17
smoothSolver: Solving for k, Initial residual = 0.16740384, Final residual = 8.6197292e-009, No Iterations 37
ExecutionTime = 121.62 s ClockTime = 122 s

Courant Number mean: 0.20074245 max: 5.0545416
deltaT = 0.00027325827
Time = 0.00108677

PIMPLE: iteration 1
smoothSolver: Solving for Ux, Initial residual = 0.00016858864, Final residual = 1.5728663e-005, No Iterations 1
.......

should't delta t match the specified time step (in this case 0.05s) and should the graph of the residuals not look like this?

case 3b [https://openfoamwiki.net/index.php/Open ... n_OpenFOAM]

There are a few different ways of using PIMPLE. With a fixed time step size, it is wise to solve to a particular residual tolerance. One can use a fixed number of iterations if one keeps a limit on the max Courant number. The latter is the mode we use at the moment in CfdOF, as it is not really dependent on appropriate user input of time step size. The first option can give you faster run times, however, and should perhaps be considered.

[shIxx]

You should get familiar with the text files of OF by DOING the tutorials from this guy:
https://www.youtube.com/channel/UCjdgpu ... qheyE82Vvw
Need cfd-help in german? Go here:
https://forum.freecadweb.org/viewtopic. ... &start=170
Thomas
run_folder.JPG

Thanks, he explains it very detailed. The problems are very academical and shows well the impact of different values.
I will watch his videos!

There are a few different ways of using PIMPLE. With a fixed time step size, it is wise to solve to a particular residual tolerance. One can use a fixed number of iterations if one keeps a limit on the max Courant number. The latter is the mode we use at the moment in CfdOF, as it is not really dependent on appropriate user input of time step size. The first option can give you faster run times, however, and should perhaps be considered.

I have already suspected such a behavior of the solvers.
So the solver works with a time step so that the condition C<1 is true (for the defined convergance criteria) and then solves all other equations with this time step. But then entering a time step is redundant in all cases where it is not small enough to satisfy C <1.
Thank you for your explanation, I will delve more deeply into this subject.