[Prak_32]

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