# LES dynSmagorinsly model average

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 June 5, 2006, 04:14 Hello, I would like t know #1 Member   anne dejoan Join Date: Mar 2009 Location: madrid, spain Posts: 66 Rep Power: 10 Hello, I would like t know how does exactely the "average" function operate: I have read on a discussion forum that "average" performs a global avarage: does it mean that it also applies in non-homogeneous directions ? As an example, when simulating the channel flow, the average is performed in the three directions ? OR only in the homogeneous ones ? Thanks you to let me know more about this, Anne

 June 5, 2006, 10:38 Well in the programmer's guide #2 Member   Pierre Le Fur Join Date: Mar 2009 Location: UK Posts: 60 Rep Power: 10 Well in the programmer's guide, it says: "Average, fvc::average produces an area weighted average of surfaceField face values, i.e. Sum(Af*Xf)/Sum(Af) , and returns a volField." With Af the face area, and Xf the field face value. Pierre

 June 5, 2006, 11:00 Hello Pierre, Indeed there #3 Member   anne dejoan Join Date: Mar 2009 Location: madrid, spain Posts: 66 Rep Power: 10 Hello Pierre, Indeed there is a fvc::average function, but this is not the one I talk about. The avarage function I asked about is used in LES modelling subroutine for computing the dynamic constant. usually for these models, averages are done over homogeneous directions. BUT, In openFoam, I read that it operates "global" averages and I am still without knowing to what exactely corresponds "global". If someone could inform me ... Thanks Anne

 February 23, 2014, 08:35 #4 New Member   jav.amn Join Date: Nov 2013 Posts: 6 Rep Power: 6 Hi Please help me about the concept of homogeneous direction. In dynamic smagorinsky model we should have homogeneous averaging, whats that mean? please help me thanks

 February 23, 2014, 09:25 #5 Senior Member   ArathoN Join Date: Jul 2011 Posts: 137 Rep Power: 9 If i remember correctly the smagorinsky model use a similar hypothesis to the boussinesq, where there is proportionality between the SGS stres and the rate-of-strain tensor, so for the point of view of the matrices it means that the "principal vectors" (sorry don't know the exact English term but it should be the base of the matrix) are aligned, and so you impose the homogeneous condition. This was the problem for the standard smagorinski model because you'll have only one variable to play for each flow (Cs) then Germano came up with the dynamic concept and there you could (auto)adapt better the model to the problem you are studying. This is what i know unfortunately my turbulent flow course didn't give us a detailed information about LES, even thought we studied for les mainly the Germano method (he was my teacher in another course before retiring).,

February 24, 2014, 05:45
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Bernhard
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 Originally Posted by javad.amnian Hi Please help me about the concept of homogeneous direction. In dynamic smagorinsky model we should have homogeneous averaging, whats that mean? please help me thanks
In addition to Arathon's answer. In, for example channel flow, you don't expect any statistical differences in the streamwise direction (homogeneous direction), therefor you can average the coefficient for C_S in planes parallel to the wall. Of course, this only works for simple geometries where you have these kind of planes. In OpenFOAM you have the homogeneousDynamicSmagorinsky model, which averages C_S over the complete domain.

February 24, 2014, 07:43
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jav.amn
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Quote:
 Originally Posted by Bernhard In addition to Arathon's answer. In, for example channel flow, you don't expect any statistical differences in the streamwise direction (homogeneous direction), therefor you can average the coefficient for C_S in planes parallel to the wall. Of course, this only works for simple geometries where you have these kind of planes. In OpenFOAM you have the homogeneousDynamicSmagorinsky model, which averages C_S over the complete domain.
Thanks Bernhard
you introduce an example about Homogeneous direction. but i think homogeneous directions are directions that have no any body force act on it, for example in the simulation of buoyant heat transfer we can averaged in the directions perpendicular to gravity directions, but can't average in gravity direction!!!
do you think, this is true?

 February 24, 2014, 10:54 #8 Senior Member   Bernhard Join Date: Sep 2009 Location: Delft Posts: 790 Rep Power: 15 Uhm, it is complicated enough without body forces. At the boundaries you introduce shear force. Wouldn't they be more relevant? So averaging towards the wall does not make sense then. javad.amnian likes this.

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