Vortex Method Inlet BC for LES
 

Hi everyone

Does anyone know how to calculate the number of Vortices in Vortex Method Velocity Inlet bc in LES simulation?

I read somewhere that it should be (Number of Facets at inlet)/4 which is too much (more than 1000) in my case.

I’d appreciate any help

Number of faces/4 makes sense because smallest eddy that can be resolved in 2D mesh needs 4 cells.

Oh! That’s right; so it means it is better to produce the smallest possible vortexes right?

That is correct, you must account for the smallest possible vortices.

Thanks a million for your help

There isn't, to the best of my knowledge, any paper discussing this with the proper theoretical rigor, or probably discussing this at all.

In general, reasoning on the grid resolution alone is not rock solid, for the simple reason that such vortex methods have also a prescribed size , which can be assigned independently from the grid resolution. Also, the vortex positions are random, so you can't actually control anything at the local level as the Nfaces/4 rule might seem to suggest.

One approach I have seen is to use something like:



where you basically assume that, on average, the inlet face is covered by vortices.

You should instead adapt to the grid resolution. But, again, this requires more than simply reasoning on a pure number. Different schemes will require different resolutions, but sometimes a magic number exists that makes things work even if they shouldn't.

So, in conclusion, N is not directly linked to the resolution. Indeed, some have found reasonable solutions with a fixed number O(100).

Thank you so much for your comprehensive reply