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Old   July 13, 2016, 08:35
Default Boundary layer issues
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Hello dear freinds,
I am modelling a De-laval Nozzle flow with K-e model and i am very suspicious about the velocity at the Wall.
I've read the FAQ and the manual about hybrid and conservative values but i would like to hear some opinion.
According to the Y+ values(<1) my near Wall mesh is fine enough, the Residuals also look good in all the Volume.
The velocity in the centre of the flow is as expected 1350 m\s but the conservative value at the Wall is 650 m\s. Can it be that the gradien of the velocity in the first nearwallelement is so high that the averege nearwallelement velocity is 650 m\s. Tried to make the first nearwallelement even thinner the velocity just slightly decreases.
Would be very thankfull and have a nice day!
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Old   July 13, 2016, 16:27
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As initial advice, it makes no sense to use meshes with Y+ < 11 for K-Epsilon model. Meshes with very small Y+ are meant to be used with Omega based models, either K-Omega, SST or similar.

For meshes with Y+ < 11, ANSYS CFX uses a scalable wall function and you can read the details in the documentation

From the documentation:

4.2.2. Scalable Wall Functions

Scalable wall functions overcome one of the major drawbacks of the standard wall function approach in that they can be applied on arbitrarily fine meshes. If the boundary layer is not fully resolved, you will be relying on the logarithmic wall function approximation to model the boundary layer without affecting the validity of the scalable wall function approach. If you are not interested in the details of the boundary layer, then it may not be worth fully resolving it. However, if you have produced a very fine near-wall mesh to examine details of the boundary layer, then you should use the SST model with Automatic near-wall treatment to take advantage of the additional effect in the viscous sublayer.
Hope the above helps,
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Old   July 14, 2016, 04:53
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Thank you very much! Than helped so much! Have a nice day!
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