Advanced Turbulence Modeling in Fluent, Realizable k-epsilon Model

Jonas Larsson

This is really a question to the turbulence gurus at Fluent (Paul Malan, Sung-Eun Kim ...) concerning the implementation of the Realizable k-epsilon model. However, I think that the implications are of general interest to all Fluent users so I'll post the question here.

The realizable k-epsilon model that is available in Fluent has a variable Cmu. This variable Cmu works very well in the main flow and this model is my favourite model in Fluent. It avoids a lot of problems that you have with the standard k-eps model (overproduction of k etc.). However, it is not that self-evident how to handle the variable Cmu near walls. I think that Fluent just uses the variable Cmu in wall-laws, two-layer models and low-Re models. This might be okay, but I'm far from convinced. Recentely I have seen some anomalies in certain solutions that I think might be related to this. None of the wall-treatments available in Fluent were designed for a variable Cmu and using the variable Cmu can make them invalid. In the two-layer model for example, using the variable Cmu (dependent on k and epsilon) will create a strange mix of two models that were not originally intended to be used together. As far as I know the Realizable k-epsilon model was originally just a high-Re model and it did not include any low-Re treatment. The rapid-distortion theory used to derive the realizable model is also only valid in the high-Re case.

My question to Fluent is if you have validated the Realizable k-epsilon model for use together with all your near-wall treatments (wall-laws, two-layer, low-Re)? What kind of validations have you done? Are there any near-wall models that I definitely should avoid when using the Realizable k-epsilon model?