RANS Limitations (2)
 

THIS IS A CONTINUATION OF A THREAD MADE ABOUT A MONTH AGO ENTITLED RANS LIMITATIONS.

IT READ: "they don't (two-equation models) correctly take into account the anisotropy of the Reynolds Stress Tensor since the Reynolds Stress Tensor is suppose to be directly proportionnal to Deformation tensor. "

i understand that they are linearly related, but why can it not take acount of anisotropy???

Please correct my mistake from what i say below:

* the deformation tensor is symmetric and often the trace is added to the pressure term. this leaves the deviatoric or anisotropic part - is this correct??? then surely the linear relation is between the stress tensor and a possible anisotropic rate of strain tensor.

then the stress tensor will be anisotropic!?!?!?!

Re: RANS Limitations (2)
 

You are right, the deformation tensor is anysotropic, but the principal axis of the deformation tensor and the principal axis of the Reynolds stress tensor are supposed to be ALWAYS aligned when the Boussinesq assumption is used, which is not true in the "real world". Just imagine a rapid distorsion, or look at the C55 ercoftac's test case.

Sylvain

Re: RANS Limitations (2)
 

Thanks for the swift reply sylvain.

So is it true to say that strongly swirling flows or flows with strong streamline curvature (where boussinesq ideas fails) are generally highly anisotropic?

therefore we could say that boussinesq ideas are poor when dealing with strongly anisotropic flows???