Wall y+, surface roughness and inflation layer CFX
Hi All,
I am doing a radial turbine simulation in Ansys CFX. I have meshed my domain and achieved a wall y+ of 4 for the rotor domain in case of a smooth wall simulation. I tried imposing sand grain roughness( about 70 microns) on my domain with the same mesh and my wall y+ exploded to 250. Now, surface roughness would result in an increase in the wall shear stress and cause a downward shift of the log layer. Now according to the equation: y+ = rho*y*ut/nu, I should refine my near wall mesh more (i.e. decrease the first layer thickness) to get a low y+ in case of the rough wall simulation. Am I correct in saying that? Also, I read in CFX guide that in case of large roughness magnitudes "The idea is to place the wall physically at 50% height of the roughness elements". Does this mean that my first layer height should be larger than the roughness element height? I am sorry if this question has been asked before but it's not really clear to me. I would be grateful if someone could explain to me how to model my near wall mesh in case of the rough wall simulation so as to get a low y+ value. Regards Prakhar |
You should do a sensitivity analysis to determine what near wall mesh size you require. So do a simulation where you have double the mesh size (y+=500) and half the mesh size (y+=125) and see what is required to obtain results independent of the mesh size. If you can't see it converging then refine your mesh further until you do obtain convergence.
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I intend to do that and have played around with the mesh size too. However, I want to understand how the roughness modelling works in general in k-w sst. Especially, the dependence on the inflation layer. I would ideally like to have an y plus for the rough case also around 5. Also, I tried increasing the first layer height to half the roughness height (about 0.035 mm) and that gave me a lower y plus of about 40. However, I couldn't really understand the reason as to why that should work. Despite the increase in my shear stress that should increase my friction velocity u*, I also increase my y that is my first layer thickness. This should increase my y plus. Instead, I see a drop in y plus value. Please let me know if I am wrong. I want to be sure that i apply the right conditions for application of the roughness equation for the kw-sst model. Apologies if there is some thing wrong in my consideration, but I am trying to understand what's going on in the model. Regards Prakhar |
I have not looked into that bit of the theory, so I will leave you to do that. Have a look at the definitions of y+ and the effect of wall roughness on the inputs to y+. It should all be in the CFX theory manual.
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