# need help (SGS viscosity)

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 July 30, 2009, 02:53 need help (SGS viscosity) #1 New Member   LiuYuxuan Join Date: Jun 2009 Posts: 7 Rep Power: 10 is there are anyone know the different between SGS viscosity (LES model) and turbulent viscosity (K-omega model)? Thank you very much!

 July 31, 2009, 10:48 #2 Senior Member   Aroon Join Date: Apr 2009 Location: Racine WI Posts: 148 Rep Power: 10 The difference is the way in which the turbulent stresses are filtered or averaged. In any RANS model the turbulent viscosity is derived from the difference between the actual value and the time averaged value. In SGS, the turbulent viscosity is derived from the difference between teh actual values of velocities and the resolved velocities (obtained from sub-grid filters).

 August 2, 2009, 23:32 #3 New Member   LiuYuxuan Join Date: Jun 2009 Posts: 7 Rep Power: 10 Thank you very very much!

August 21, 2009, 12:07
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federico
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 Originally Posted by liuyuxuan is there are anyone know the different between SGS viscosity (LES model) and turbulent viscosity (K-omega model)? Thank you very much!
SGS is the Sub Grid Scale stress, the part of the stress which is not resolved by your grid because for example it is too coarse. This contribution provide the effect of just a small fraction of the Reynolds stress because most of it is already resolved. Instead in RANS model the turbulent viscosity must provide all the Reynolds stress

August 23, 2009, 04:59
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LiuYuxuan
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 Originally Posted by aatman SGS is the Sub Grid Scale stress, the part of the stress which is not resolved by your grid because for example it is too coarse. This contribution provide the effect of just a small fraction of the Reynolds stress because most of it is already resolved. Instead in RANS model the turbulent viscosity must provide all the Reynolds stress
you said that sub grid scale stress provide the effect of just a small fraction of reynolds stress, so is this mean, change another word, the sub grid scale stress just divide my grid into more smaller and then caculate this smaller grid? By the way, I am a beginner in CFD, I am not so sure that is a silly question!

August 24, 2009, 12:01
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federico
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 Originally Posted by liuyuxuan Thank you for answer! you said that sub grid scale stress provide the effect of just a small fraction of reynolds stress, so is this mean, change another word, the sub grid scale stress just divide my grid into more smaller and then caculate this smaller grid? By the way, I am a beginner in CFD, I am not so sure that is a silly question! Thank you in advance!
No, this mean that with your grid you are solving vortex with dimension comparable to your grid cell dimension, so there are several vortex with dimension lower than your grid cell that you are not directly resolving. These small vortex give effect to the flow and because you are not directly solving them you have to provide their effect through a model. The SGS eddy viscosity for example is a type of model. A model needs some assumption that are not always valid. For example if you calculate the SGS eddy viscosity through a Smagorinsky model you made the assumption that the production and destruction of turbulence are in equilibrium. Then you have other assumption but it start to be hard to explain without formula.

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