Learning Forum

[admin]

How to select appropriate wall function for turbulence modeling and what is the requirement for y plus?

[Vishal Ganore]

A mesh which provides accurate results at laminar flow conditions may not be acceptable for turbulent flow situations. The turbulent boundary layer can be divided into several regions. Based on the region that needs to be resolved, the location of first cell adjacent to the wall is determined. When flow characteristics in the viscous sub-layer need to be captured (e.g., separation, turbulence-laminar transition, better drag and pressure-drop prediction), enhanced wall treatment should be used. Standard wall functions can be employed when the flow resolution starts from the log-layer region. Depending on the choice of near wall treatment, some constrains on the placement of the first cell from the wall are imposed. To use enhanced wall treatment, the first cell should be in the viscous sublayer, and the y plus should be around 1 and can go up to 5. To use standard wall function, the first cell should be in the log layer, and the y plus should be between 30 and 300. The non-equilibrium wall function has the same limits as standard wall function. When generating the mesh, care should be taken such that the first cell adjacent to the wall doesn’t fall in the buffer layer, i.e. y plus not between 5 and 30. Cell height calculations are based upon the cell centroid location. Enhanced wall treatment is recommended for the accurate prediction of frictional drag, pressure drop, separation, transition etc.