LES mesh requirement

sharif88 · May 22, 2014, 14:00

Hello everybody
I am simulating turbulent flow in an annulus using LES in CFX
the pipe is 8 meter long and in the meshing I used hexahedral mesh along with inflation layers at both walls of the annulus,
in the streamwise direction I have meshes of constant length (5 mm) but much finer mesh in cross sectional area to capture near wall effect.
No as I now LES filter applies based on mesh size I want to know which characteristic length is taken as cut off size between large eddies and smaller one which are not simulated directly.
I thought this length is (V)^(1/3) where V is the volume of each control volume but I am not sure of it.
I appreciate your time in advance

ghorrocks · May 22, 2014, 18:47

Look in a turbulence textbook like "Turbulence Modelling for CFD" by Wilcox for the answer to this.

But be aware that it is not a cut-off (which implies to me a sharp transition). The filter is done by mesh resolution and the cut-off is occurs over a range, more like a low-pass filter.

sharif88 · May 23, 2014, 10:10

Thanks Glenn
I think the filter is a box volume filter which dictates the smallest scale of the resolved eddies and that tells me volume is more important than one characteristic length.

Cheers,

May 22, 2014, 14:00	LES mesh requirement	#1
sharif88 New Member Majid Join Date: Jan 2014 Location: Canada Posts: 23 Rep Power: 12	Hello everybody I am simulating turbulent flow in an annulus using LES in CFX the pipe is 8 meter long and in the meshing I used hexahedral mesh along with inflation layers at both walls of the annulus, in the streamwise direction I have meshes of constant length (5 mm) but much finer mesh in cross sectional area to capture near wall effect. No as I now LES filter applies based on mesh size I want to know which characteristic length is taken as cut off size between large eddies and smaller one which are not simulated directly. I thought this length is (V)^(1/3) where V is the volume of each control volume but I am not sure of it. I appreciate your time in advance

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May 22, 2014, 18:47		#2
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Look in a turbulence textbook like "Turbulence Modelling for CFD" by Wilcox for the answer to this. But be aware that it is not a cut-off (which implies to me a sharp transition). The filter is done by mesh resolution and the cut-off is occurs over a range, more like a low-pass filter.

May 23, 2014, 10:10		#3
sharif88 New Member Majid Join Date: Jan 2014 Location: Canada Posts: 23 Rep Power: 12	Thanks Glenn I think the filter is a box volume filter which dictates the smallest scale of the resolved eddies and that tells me volume is more important than one characteristic length. Cheers,