### 15.3.1. Physical Modeling Errors: YPLUS and Mesh Resolution Near the Wall

Unless you are specifically interested in resolving the boundary layer profile in your simulation, ANSYS CFX will use wall functions to model the near wall region in a turbulent flow simulation.

Wall functions are extremely useful in reducing computational load, but you should be aware of their limitations. These limitations are best understood in terms of the parameter (Yplus), which is a non-dimensional variable representing the distance from the wall to the first node away from the wall. It is therefore dependent on the size of the mesh in the wall region. If the value of is too large, then the wall function will impose wall type conditions further from the wall than would normally be physically appropriate. The use of scalable wall functions in ANSYS CFX has removed problems associated with the lower valid limit for .

You can plot the values of at the wall in CFD-Post to see where the mesh needs refining (or coarsening) to reduce (or increase) the value of in that region.

For more information see Scalable Wall Functions in the CFX-Solver Theory Guide and Solver Yplus and Yplus in the CFX-Solver Theory Guide.

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