[houssame222]

Hi All, I have a question concerning the RANS model,

1- is that a time average or space average or a ensemble average in the equations ?

2- when writing the RANS equation and using some models as k omega or k epsilon are we doing the assumption of isotropic turbulence ?

I thank you guys in advance

[Hamidzoka]

hi
1- in RANS time averagde is done, in which the time scale is comparable with time span of fluctuating part of velocity components. in LES space averaging is done the length space of which depends on filtration scale selected.

2- RANS equations can either use isotropic turbulence assumption or non-isotropic turbulence. It depends on the turbulence model selected. in case of K-Epsilon or K-Omega turbulence is assumed to be isotropic while in RSM or ASM models is non-isotropic.

regards

[FMDenaro]

Originally Posted by houssame222

Hi All, I have a question concerning the RANS model,

1- is that a time average or space average or a ensemble average in the equations ?

2- when writing the RANS equation and using some models as k omega or k epsilon are we doing the assumption of isotropic turbulence ?

I thank you guys in advance

1. RANS is based on the time averagin over an infinite time, leading to a steady state solution. Versions like URANS are based on reduced time-interval or ensemble averaging.
2. It depends on the assumptions of the model. But as the averaging affects all the scale of the motion, there is no separation of large scales from isotropic assumption of small scales (as conversely is assumed in LES)

[houssame222]

Originally Posted by FMDenaro

1. RANS is based on the time averagin over an infinite time, leading to a steady state solution. Versions like URANS are based on reduced time-interval or ensemble averaging.
2. It depends on the assumptions of the model. But as the averaging affects all the scale of the motion, there is no separation of large scales from isotropic assumption of small scales (as conversely is assumed in LES)

I thank you very much for the answer, but I have one more question ..the turbulent eddy viscosity is defined as an isotropic property in the eddy viscosity model, is that what makes the turbulence isotrope localy ?

[FMDenaro]

the key is in the construction of the eddy viscosity by means of a unique characteristic lenght (Prandtl mixing lenght), that is an isotropic assumption.