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-   -   Non Equilibrium (http://www.cfd-online.com/Forums/fluent/49086-non-equilibrium.html)

Pipin August 24, 2008 13:22

Non Equilibrium
 
I want to know why the wall treatment Non Equilibrium need an Y+ > 30? and what does it do when Y+<10?

Thanks in advance

Jason B August 28, 2008 15:37

Re: Non Equilibrium
 
I recommend doing some research on boundary layers and the component layers of a boundary layer.

The non-equilibrium treatment assumes you're in the log-layer of fluid. Below y+ of 30 the boundary layer profile does not follow a logarithmic shape and therefore the log-layer assumption is not valid. The code will calculate the flowfield for any y+ values (around y+ of 11 it switches from a log calculation to a linear calculation... don't quote me on the exact transition point, I'm a bit rusty, but know that at some point it switches how it's calculating). But in real boundary layers the non-dimensionalized velocity distribution is approximately linear below y+ of 5, and then from 30 to 300 it's logarithmic. Between 5 and 30 is the "buffer layer" which is poorly defined (although there have been emperical curve fits and such, few tools take advantage of these, including Fluent). It's recommended that you size your mesh so that your first cell does not fall in this region. You can either tighten mesh and switch to enhanced wall treatments (y+ ~ 1) or losen your mesh (shooting for y+ ~ 30 or above).

Hope this helps, and good luck, Jason


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