Vortex Method Inlet BC for LES

nima_nz · March 29, 2021, 12:46

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

shalabhks · March 29, 2021, 13:40

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

nima_nz · March 29, 2021, 16:24

Quote:

Originally Posted by shalabhks

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?

aero_head · March 29, 2021, 16:41

Quote:

Originally Posted by nima_nz

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.

nima_nz · March 29, 2021, 16:44

Quote:

Originally Posted by aero_head

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

Thanks a million for your help

sbaffini · March 30, 2021, 04:59

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 $\sigma$ , 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:

$N = \frac{A_{INLET}}{\pi \sigma^2}$

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

You should instead adapt $\sigma$ 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).

nima_nz · March 31, 2021, 11:24

Quote:

Originally Posted by sbaffini

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 $\sigma$ , 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:

$N = \frac{A_{INLET}}{\pi \sigma^2}$

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

You should instead adapt $\sigma$ 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

March 29, 2021, 12:46	Vortex Method Inlet BC for LES	#1
nima_nz Member Nima Join Date: May 2014 Location: Ireland Posts: 51 Rep Power: 11	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

March 29, 2021, 13:40		#2
shalabhks New Member Join Date: Sep 2018 Location: Bangalore Posts: 6 Rep Power: 7	Number of faces/4 makes sense because smallest eddy that can be resolved in 2D mesh needs 4 cells. nima_nz likes this.

March 30, 2021, 04:59		#6
sbaffini Senior Member Paolo Lampitella Join Date: Mar 2009 Location: Italy Posts: 2,151 Blog Entries: 29 Rep Power: 39	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 $\sigma$ , 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: $N = \frac{A_{INLET}}{\pi \sigma^2}$ where you basically assume that, on average, the inlet face is covered by vortices. You should instead adapt $\sigma$ 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). nima_nz and aero_head like this.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
UDF for inlet temperature and species concentration	haidory	Fluent UDF and Scheme Programming	18	September 1, 2019 04:57
interFoam wave propagation and explosion of Courant number and residuals	ChiaraViola	OpenFOAM Running, Solving & CFD	1	June 26, 2019 05:36
Are there any reasons why Vortex Method doesn't perturb the flow using SAS	kingjewel1	FLUENT	0	June 1, 2018 11:42
LES Vortex Method by Mathey/Sergent	MrFrog	OpenFOAM Running, Solving & CFD	3	January 20, 2016 07:20
Vortex method vs High Reynolds number	Weili	Main CFD Forum	1	August 4, 2006 01:15