# roughness and y+ constraints

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 July 11, 2005, 11:56 roughness and y+ constraints #1 Jean Guest   Posts: n/a Hi, I am modeling an atmospheric boundary layer flow and I have a wall roughness of 1.6m I have a 2D domain of 1000m height. I meshed with a cell height of 4m, but I still have a viscosity ratio problem and my y+ is around 10000. If I refine my mesh to get smaller y+ going down to a cell height of 2m near the wall, my roughness height will exceed the center of my cell adjacent to the wall. I am using wall function k-epsilon. Please help Thank you

 July 12, 2005, 09:54 Re: roughness and y+ constraints #2 Razvan Guest   Posts: n/a Change your turbulence model to SST-kw. If this does not solve the problem, change some settings for multigrid: - set presweeps=postsweeps=2 - set pressure and all momentum to F-cycle - set turbulence quantities to V-cycle If this will not solve the problem, then I do not know what else you can do. Hope this will help you. Best wishes, Razvan

 July 12, 2005, 13:35 Re: roughness and y+ constraints #3 Jean Guest   Posts: n/a Thx Razwan, I am modeling wind turbine effect on the atmospheric layer. Is the k-w convenient for such a problem? The domain is rectangular. I chose a velocity inlet, an outflow and a symmetry boundary for the top of the domain at 1km height. Do you think those boundaries are convenient? Thank you

 July 14, 2005, 03:44 Re: roughness and y+ constraints #4 Razvan Guest   Posts: n/a Not k-w but SST-kw. You should better change the "outflow" boundary to "pressure-outlet" with outlet pressure value=local(desired) atmospheric pressure. If you are modeling wind turbine then I think 1km might be too much. I am not experienced in this tipe of situation, but I don't see no problem in lowering it to say, 150-200m top. That should be enough. Of course, if you are interested in high winds also, this could be an explanaition for a such big domain you have chosen. Best wishes, Razvan

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