[hulli]

Dear Foamers,


maybe one of you can help me with this, PLEASE. I am trying hard to simulate a flow around an obstacle by a LES, incompressible, pimple approach. At this stage I am trying to get the model right without the obstacle, but just the flow in the domain. The only difference to most of the tutorial cases is that I am using inflow boundary condition (fixed profile) and two refinement boxes that I generate with SHM. Please see the attached pictures for an overview.



The model is performing well, when I use a uniform fixed value. So I am quite sure that everything related to the solution, schemes, turbulence models (of which I tested several LES / RANS etc.), is performing fine, unless I test cases with the fixed profile boundary condition at the inlet.
This brings me to my problem. Every time, the “front of the flow” passes through a box with lower to higher refinement I observe a velocity pattern in a stripe like manner (please see the pictures attached). These stripes do not disappear throughout the entire simulation, regardless what I tired (e.g. double checking the schemes, changing the solvers, increasing the number of corrector loops, decreasing the timestep … ).


I am so confused, because when I tested other cases with a continuous mesh i.e. the same level of regiment ranging from the inlet to the outlet, regardless in which resolution, I never observed these stripes.


So why do I get these stripes when the flow passes a coarse and enters a fine mesh? The other way around this does not happen. Could this be an interpolation issue?



Any suggestion is highly appreciated!!!
Best H

[hulli]

any ideas ??? Please these stripes drive me crazy !!!!! aaaaaahhhhh

[hulli]

No ideas??? Nobody?????

[RobertHB]

I've got a similar probelm in my case. I'm also using a fixed boundary profile and my mesh generates strips according to the coarsest cell level. Any idea how to fix that?

[hulli]

Originally Posted by alberto

The choice of the scheme for LES should be based on the numerical diffusion it introduces. Ideally you should use linear, or cubic. You need good and prett uniform mesh anyways for LES, since you are assuming the filter operator and the differential operator are commutative, which is not true on non-uniform grids!
The filtered schemes are a last resort scheme in my view, if you can't really do better with your mesh. Upwind and linearUpwind, but also limitedLinear and QUICK should be avoided because they are too dissipative.

For the time scheme, backward is the prmary choice among the schemes available in OF. The constraint on he time step is generally not an issue in LES because you are tied by the requirement of resolving the flow large-scale time scales.

Best,

Mesh in LES

I guess these two treads kind of explain my instabilities....