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 Qiaol618 June 6, 2012 06:24

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

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.

 mb.pejvak June 6, 2012 21:51

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.

 Qiaol618 June 6, 2012 23:30

Quote:
 Originally Posted by mb.pejvak (Post 365151) 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.

 Far June 7, 2012 00:36

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

 mb.pejvak June 7, 2012 02:48

Quote:
 Originally Posted by Qiaol618 (Post 365160) 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.

 Qiaol618 June 7, 2012 12:10

Quote:
 Originally Posted by mb.pejvak (Post 365179) 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).

 mb.pejvak June 7, 2012 13:53

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.

 Far June 8, 2012 02:55

Quote:
 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.

 Qiaol618 June 9, 2012 06:43

Quote:
 Originally Posted by mb.pejvak (Post 365319) 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

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