I studied a manual of Ansys CFX about turbulence models and I came across the term "non equilibrium boundary layer". What does this term mean?

  • $\begingroup$ I am not sure because your quote is so out of context I might get it all wrong. However, I would guess the equilibrium refers to the state of production and dissipation of turbulent kinetic energy. This means the characteristics of the boundary layer are changing in stream wise direction and therefore ordinary turbulence models might have problems predicting the flow correctly. $\endgroup$
    – rul30
    Commented Oct 13, 2015 at 19:37
  • $\begingroup$ Thanks, What is your exact meaning of "stream wise"? It means "In the direction of a stream"? Am I right? $\endgroup$
    – user19061
    Commented Oct 13, 2015 at 20:38
  • $\begingroup$ Exactly, however it broadly means where the flow is coming from and is usually not referring to a specific streamline. Which in this particular case would actually make some sense as well. $\endgroup$
    – rul30
    Commented Oct 13, 2015 at 20:43

1 Answer 1


For the equilibrium boundary layer, the local skin friction coefficient is independent of two parameters, both the streamwise distance and the Reynolds number, based on the momentum thickness, and the boundary layer thickness is proportional to the streamwise distance. On the other hand, for the non-equilibrium boundary layer, the local skin friction coefficient depends on the above two parameters.

This is from here

PS: This seems self explanatory. Please comment if you think an explanation is required. I have highlighted the parameters in bold myself.


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