# Viscosity ratio in gamma-theta transition model based on k-w sst turb model

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 June 6, 2012, 06:24 Viscosity ratio in gamma-theta transition model based on k-w sst turb model #1 New Member   Join Date: Jul 2011 Posts: 6 Rep Power: 15 In Langtry's parer on gamma-theta transition model,viscosity ratio is defined as rho*k/(w*mu),where mu is laminar viscosity and rho is density.however, in k-w sst turb model,turbulence viscosity is not defined as rho*k/w but max( rho*k/w,something else). why viscosity ratio in gamma-theta transition model is not calculated from turbMu/laminarMu ? supplement:usually,turb model equations and transition model equations are solved alternately,so,i don't understand why not use turb viscosity directly that already given by turb model. Last edited by Qiaol618; June 6, 2012 at 23:36. Reason: additional explanation

 June 6, 2012, 21:51 #2 Senior Member   Mehdi Baba Mehdi Join Date: Jan 2011 Posts: 158 Rep Power: 15 because Mut is the property that we want to obtain it, and we use turbulence model to do it. I think what you mention about viscosity ratio is used for initial and boundary conditions.

June 6, 2012, 23:30
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 Originally Posted by mb.pejvak because Mut is the property that we want to obtain it, and we use turbulence model to do it. I think what you mention about viscosity ratio is used for initial and boundary conditions.
Maybe i didn't say my question clear.Transition model calculates viscosity ratio in the mentioned manner at every grid cell and time step.usually,turb model equations and transition model equations are solved alternately,so,i don't understand why not use turb viscosity directly that already given by turb model.

 June 7, 2012, 00:36 #4 Super Moderator   Sijal Join Date: Mar 2009 Location: Islamabad Posts: 4,553 Blog Entries: 6 Rep Power: 54 I encourge you to ask this question directly Dr. Florain Menter florian.menter@ansys.com; martin.kuntz@ansys.com, robin.langtry@ansys.com and let us know as well. http://www.ansys.com/staticassets/AN...Turbulence.pdf

June 7, 2012, 02:48
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 Originally Posted by Qiaol618 Maybe i didn't say my question clear.Transition model calculates viscosity ratio in the mentioned manner at every grid cell and time step.usually,turb model equations and transition model equations are solved alternately,so,i don't understand why not use turb viscosity directly that already given by turb model.
Dear Qiaol618
according what I understood from your question and what I have studied about gamma- Retheta, transition model finally calculates effective gamma and this value is used in K equation in turbulence model for finding trigger of transition (this value shows if specific point is in turbulence region or not.) and Mut is abtained. so transition model doesn't calculate Mut.
if you read it, please introduce me in which papre and which section.

June 7, 2012, 12:10
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 Originally Posted by mb.pejvak Dear Qiaol618 according what I understood from your question and what I have studied about gamma- Retheta, transition model finally calculates effective gamma and this value is used in K equation in turbulence model for finding trigger of transition (this value shows if specific point is in turbulence region or not.) and Mut is abtained. so transition model doesn't calculate Mut. if you read it, please introduce me in which papre and which section.
In Langtry's doctor thesis,chapter 3,calculate of F_onset used viscosity ratio,which defined as rho*k/(w*mu).

 June 7, 2012, 13:53 #7 Senior Member   Mehdi Baba Mehdi Join Date: Jan 2011 Posts: 158 Rep Power: 15 I think Mut=min(a1*k/w,a1*k/F2/S) is used only for preventing overprediction of shear stress in turbulence model in strong adverse pressure gradient (As menter mentioned in his paper) and used second term of this formula for this purpose. so I think it is not necessary to use it in Fonset in transition modeling.

June 8, 2012, 02:55
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Sijal
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 so I think it is not necessary to use it in Fonset in transition modeling.
My two cents for this. Same goes for the Reynolds stress model.

June 9, 2012, 06:43
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 Originally Posted by mb.pejvak I think Mut=min(a1*k/w,a1*k/F2/S) is used only for preventing overprediction of shear stress in turbulence model in strong adverse pressure gradient (As menter mentioned in his paper) and used second term of this formula for this purpose. so I think it is not necessary to use it in Fonset in transition modeling.
I think this is reasonable.
approval