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Old   November 7, 2003, 03:43
Default anisotripic results
Matthias Finke
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Hi all,

im trying to model flow instabilities in an internal flow. I was suggested to choose a turbulence that is known to produce ansiotropic results (e.g. Launder eddy viscosity model).

What does it means "anisotropic results" ?

I dont have the Launder eddy viscosity model. The turbulence models that i have are the follows:

Standard k-eps Model

Zero Equation Model

RNG - (Re-normalized Group Model)

NKE - (New k-eps Model due to Shih)

GIR - (Model due to Girimaji)

SZL - (Shi, Zhu, Lumley Model)

which one should i choose ?
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Old   November 7, 2003, 12:01
Default Re: anisotripic results
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Anisotropic turbulence is turbulence that is dependent on direction, i.e. (u'u')x != (u'u') y != (u'u') z. The standard k-e model for instance assumes isotropic turbulence, ie. turbulence that is independent of direction.

What u need is a model that is not based on the conventional eddy-viscosity assumption by Boussinesq (i.e. Reynolds stress tensor = (eddy viscosity x mean strain rate)-2/3 x ro x k x g^(beta x alpha)) but one which extends the stress-strain relationship by introducing non-linear elements. These models are called anisotropic or non-linear eddy-viscosity models (NLEVM's).

Since u seem to be using ANSYS/FLOTRAN I am pretty sure (except of SZL) that none of the mentioned models incorporate the anisotropic eddy-visocity approach.

To make sure, though, I suggest u check out the code's manual and look out for the relationship between stress and strain, e.g. Reynolds stress tensor = (eddy viscosity x mean strain rate)- (2/3 x ro x k x g^(beta x alpha))+ (2 x eddy viscosity) - ((4 x eddy viscosity/(ro x cp x k)*(A x ...)) + (B x ...) + (C x ...) where A, B and C are the closure coefficients. These coefficients were determined to get a good match for shear flows.

Hope this helps.
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Old   November 20, 2003, 09:17
Default Re: anisotripic results
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Just stumpled across a few pieces of info and thought of your inquiry. It's late but better late than never:

You can definitely use the following models to model anisotropic problems:

a) GIR - (Model due to Girimaji) - non-linear (probably differential) Reynolds stress model

b) SZL - (Shi, Zhu, Lumley Model) - algebraic Reynolds stress model

Still not sure about the new K-e model due to Shih so better don't ...

Hope it's not too late, let me know how u r getting on ...

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