CFD Online Logo CFD Online URL
Home > Forums > Software User Forums > OpenFOAM

Mesh Convergence Study - simpleFoam: Confusing Results

Register Blogs Members List Search Today's Posts Mark Forums Read

LinkBack Thread Tools Search this Thread Display Modes
Old   February 5, 2018, 07:34
Default Mesh Convergence Study - simpleFoam: Confusing Results
New Member
Karsten Hagemann
Join Date: Nov 2017
Posts: 4
Rep Power: 5
Karpfen is on a distinguished road
Hello all,

i'm very inexperienced with OpenFoam and tried to do a mesh convergence study for a 2D-simulation with the simpleFoam solver. The geometry is pretty simple and the mesh is created with blockMesh.
The perhaps not so easy features of the case are a rotating wall resulting in backflow at the outlet-patches (or more precise mostly outlet-patches due to this).
The Reynolds number is pretty high at ~100,000.
Of main interest are the forces at two stationary walls.
I used the kOmegaSST turbulence model.

My problem:

Basically, there is no clear picture or pattern. The forces vary greatly with the different solutions both undershoot and overshooting the results of a CFX-simulation done for cthe sake of comparisons at times by half an order of magnitude and even changing the sign.

Basically, i could not say at all which choice of resolution, boundary-layer grid spacing and other parameters would be fitting for this case.

Comparison to CFX-results:

I had the opportunity to to the same simulation in Ansys CFX. There it did not show the same behavior. The CFX results seem to be "converging" with increasing resolution.
CFX seems to be really insensitive for this case as changing discretization schemes and turbulence models did not change the results significantly.

While the resulting forces of CFX and OpenFOAM are of comparable magnitude for some parameter combinations, i did not manage to get OpenFOAM-results which are particularly close to the CFX-results.

What i tried: resolutions, residuals, RAS-models, some discretization schemes

I tried different spacings at the wall boundary layers: Uniform spacing, half and quarter the grid spacing at the wall compared to the biggest grid spacing.
I looked at 4 different resolutions. The coarsest just a little bit over the smallest resolution where the simulation still converges. Up to the finest mesh the resolution in each direction doubled two times.
Just to get an idea: The smallest mesh had ~5500 cells and converged after a couple of seconds, the biggest had ~90,000 and needed ~10 hours. (1 core always)

I systematically looked at different target residuals and the picture there is pretty clear and the forces not changing significantly from the residual 1e-5 to 1e-7.

I tried the k-omega and the k-epsilon turbulence model for some resolutions. The resulting forces were noticably different most of the time.

I did not get the solution to converge with some other discretization schemes.

Does anybody have any advice or has made similar observations for some cases?
Since i'm new to OpenFOAM, but i don't really know what to try to get a clear picture/understanding of the problem.

Further increasing the resolution is always a possibility, but for a 2D-case the simulation already took pretty long with the finer meshes and CFX does not seem to care even with the coarser grids.

Thank you and kind regards,
Karpfen is offline   Reply With Quote


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

Similar Threads
Thread Thread Starter Forum Replies Last Post
how to set periodic boundary conditions Ganesh FLUENT 15 November 18, 2020 06:09
[snappyHexMesh] No layers in a small gap bobburnquist OpenFOAM Meshing & Mesh Conversion 6 August 26, 2015 09:38
(New to CFD) How to do a mesh refinement study w/ Fluent? JAviles FLUENT 3 June 27, 2014 13:55
Force can not converge colopolo CFX 13 October 4, 2011 22:03
early stall, poor convergence, and mesh quality everest CFX 2 May 12, 2010 16:27

All times are GMT -4. The time now is 14:57.