Turbulent Boundary Layer on flat plate
Hello,
I am trying to simulate a boundary layer on flat plate in a 3D space. My model consists of a "brick" volume with geometric caracteristics 124x44x22, with one velocity inlet of 8.3 m/s, a pressure outlet, 3 symetries and one wall. I have meshed my model using gambit 2.4.6, with hexaedric elements. When I do not use the boundary layer on the wall the calculations in Fluent, with the kepsilon model, converge. When I do use a boundary layer with the distance to first node of 0.01m and standart wall functions the calculations do not converge at all even after 400 iterations. I do not have any clue why. My Reynolds number is in the order of magnitude of 10^5 , i have chosen y+=30 and the fluid is air. Can somebody help me? 
what's the length of your plate?

Are you sure your y+ is appropriate for this kind of simulation? I simulate turbulent boundary layers and I use a RANSbased model, and I am aiming for a y+ which is less than 2.
The thing with turbulent boundary layers is that there is a very thin viscous (or laminar) sublayer which is in contact with the boundary wall. In order for a simulation to give accurate results, the first row of elements from the wall need to be embedded entirely inside this viscous sublayer, hence the small value of y+. In this link you can find how to calculate the thickness of your first row of elements given your maximum (not average!) Re and your length scale (I work with tubes so I choose the diameter as my length scale): http://geolab.larc.nasa.gov/APPS/YPlus/ Hope it helps. 
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