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Old   August 14, 2001, 07:22
Default Mesh--Boundary layer.
  #1
Question Man.
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Hi all,

Can anyone tell me how critical the y+ value at the boundary layer is with regard to standard wall functions as implemented in fluent? For example, would a y+ value below 30....even below 11.25 (I think this is the absolute lower limit value), be a major problem. I have a rotating system with higly curved flow. IN other words, if my mesh is too well refined, is it a major problem. Thanks.
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Old   August 14, 2001, 15:05
Default Re: Mesh--Boundary layer.
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John C. Chien
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(1). There are several issues here: (a). For the wall function treatment to be valid, the matching point (the first Y+ value) must be inside the law of the wall region, (which varies from flow to flow, depending on the pressure gradient). If your results show that you are outside this region (too small), the wall function is not valid. (b). The mesh refinement is important for mesh independent solution. It should not be related to the first matchng point value. But because of the improper implementation of the wall function in most codes, when you refine the mesh, the code also reduce the first matching point location, thus create a coupled problem. (c). If you have questions about the first Y+ value in your solution and are worrying about the mesh size, you should try to use a low Reynolds number model or a two-layer model. There, you can refine the mesh and keep the first matching point Y+ to well below 1.0 (2). In other words, the wall function treatment is for simple attached boundary layer flow problems, where coarse mesh is adequate for obtaining a good solution. (3). I have named this wall function implementation (coupled through mesh refinement) as "black art in CFD".
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