But in some cases if the wall effects are negligible then there is option of including semi-empirical formulae to bridge between the viscosity affected region and fully turbulent region and in this case the mesh need not to be dense or small near the wall i.e., coarse mesh would work.Ĭonsidering the first case i.e., near wall modelling it is well-known that the mesh size should be small enough, however then the question follows is how small ? Thus, here comes the concept of, and based on the value the first cell height can be calculated. Similarly coming back to our case if we intend to resolve the effects near the wall i.e., in the viscous sub layer then the size of the mesh size should be small and dense enough near the wall so that almost all the effects are captured. In this case even the large fishes are also captured. If he is trying to catch medium to big sized fishes the grids in the net he uses is somewhat big, but if he is trying to trap even small sized fishes then the grid size of the net should be small enough to capture them. A fisherman uses fishing net, a grid kind of structure to trap the fishes. Let us try to understand this with an example. One of the reasons for the need of is to distinguish different regions near the wall or in the viscous region, however how exactly it helps in turbulence modelling or in general CFD modelling need to be well understood. Thus is a dimensionless quantity, and is distance from the wall measured in terms of viscous lengths. The behaviour of the flow near the wall is a complicated phenomenon and to distinguish the different regions near the wall the concept of wall has been formulated. It is important to know about the concept of wall or in general how the flow behaves near the wall, to consider the effects near the wall as it is the basis on which choice of the turbulence model is governed. Basics of Y Plus, Boundary Layer and Wall Function in Turbulent Flowsīefore getting into the details of the turbulent models let us discuss an important concept known as and know how it is related to turbulence modeling, mesh generation process and how it is going to affect the CFD end result.
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