Low Reynolds LES Simulation questions
 

Hi,

I am starting a new thread, because things have changed (boundary conditions etc.)

I am trying to evaluate the drag coefficient of a cylinder with a Reynolds number of 180 in OpenFoam.
I created a 2D case out of the tutorial (motorbike les).
I created a 3D mesh and converted it with extrudeMesh into a 2D mesh.
The mesh is in the attachment.
At such a low reynolds number the flow should be laminar and the Karman vortex street
should be visuable (axis parallel eddies).
In my case the dead water area is getting bigger and bigger with each additional timestep and there is no laminar flow of the fluid. Instead there are huge eddies. I am not sure if I have to use a higher refinement of the mesh, to visualize the Karman vortex street, but imho the mistake of the case should be solved first.

A use the solver pisoFoam and the turbulencemodel SpalardAllmaras.
I used according to the tutorial the following wallfunktions:

nuSgs
nuSgsUSpaldingWallFunction;

k
kqRWallFunction


My max. Courant number is under 0.30 and my y+ values are far below 1.
I have tried to modify the case, but I am not able to get the right laminar behavior of the flow. The drag coefficient is around 1.08 and should be around 1.2.


Can somebody give me an advise, if I have done a mistake or if the used wallfunktions are not applicable at such low reynolds numbers?

Thank you for your time,

Matthias

Hi,

I have increased the simulation length and the increasing dead water area converted into a gradually changing vortex system.
In my oppinion the vortex system should be much smaller.
The drag coefficient is in the right range.

Regards Matthias

Hi,

I got my results of the cfd simulation.
They are in the attachment.
The only value I am not very satisfied with, is the cd coefficient of Re10.
Probably I am able to get a better value with a mesh refinement.
Are there any further suggestions ?

Thanks,
Matthias