simpleFoam instability
 

hi community,

I'm running my first models using OpenFoam, the problem is a confined water flow, initial velocity is 0.1m/s, the solver is simpleFoam stationary

when postprocessing and animating the results through time steps I observe:
- first 6 to 10 time steps show big oscillations in the velocity field, with changes in velocity even reversing directions
- then these oscillations reduce and the flow field evolves to an each time more stable solution and reaches a visually quasi-converged solution
- after that, additional time steps produce an instabilization that yields in and error stop after 4 - 5 time steps
- cumulative time step continuity errors dramaticaly rise to e+60 order values

I attach the solver log file for the last iterations

thank's in advance for your comments :)

You have an issue with your turbulence model. Look at epsilon. You are already diverging when you are considering the "solution quasi converged...".

thank you very much, Aurelien, for your help

can you give me any orientation about what I'm doing wrong?

a) I prepare the case importing the mesh from a UNV file into Discretizer GUI
b) in Discretizer I specify boundary conditions
c) and set several parameters:
- viscosity NU
- RNGkEpsilon model
- turbulence ON
- WALLKAPPA 0.4187 (default value)
- WALLE 9 (default value)

Discretizer offers further wall models: kEpsilon, realizableKE, LaunderSharmaKE, LamBremhorst; for all of them default values are the same as above

I didn't know Discretizer.

But what you can do :
- Launch a checkMesh : The command is "checkMesh > log.checkMesh" (or whatever you want for the output log). It will check the quality of your mesh (upload the log file if you don't understand the output) : skewness, cell's aspect ratio, nonorthogoality...

If the quality of your mesh is low, you'll have to change your fvScheme file (in the system sub-directory).

I use salome for geometry construction & meshing, then I export to UNV file that I import into Discretizer...
I try to use as much as possible open-source tools with Graphic User Interface in order to work in a more friendly environment
I'm familiar with mesh distortion parameters, but don't know where to launch the command checkMesh, I've tried it in a linux shell, at the open-foam case directory, but command is not found
How do you set your openfoam cases up, what tools do you use?
thank's

1/ You have to go in a linux shell to your case directory.
2/ Then you have to source OpenFoam :
"source ~/OpenFOAM/OpenFOAM-2.0.0/etc/bashrc (adapt the command)
3/ And finally type : "checkMesh > log.checkMesh"

I'm doing all the stuff within a simple linux command shell. It's a bit difficult at first, but I think you learn faster how to set up cases properly by this way than with a GUI.

OK, it worked, thank you :)
I attach the log.checkmesh of my model
it seems everything is correct
what do you think?

Yes, everything is good with your mesh. Upload your boundary condition files (from the folder 0/, you should have something like U, p nut, k and epsilon).

hello,

Is your domain 250 m wide, 200 m high and 200 m deep? Are you using the right units?
I needed to change in constant/polyMesh/boundary for the wall the patch type from patch to Wall. I do not know discretizer does this.

hope this helps

Wouter

thanks again, Aurelian
here I upload a the openfoam case files, I've omitted those big files listing points, faces,...

thanks as well, Wouter
model units are mm-g-s
I use consistent units for NU=MU/RHO, being for water at 20ºC:
MU = 1e-03 g/(mm·s)
RHO = 1e-03 g/mm3
then NU = 1 mm2/s = 1e-06 m2/s (SI)

all the case files uploaded are generated by Discretizer, where you enter the values through its GUI

hope this may help :)

Lluís