Ribbed duct
 

Hello everyone, i have a doubt about mesh size. Is is required that the mesh size of both smooth duct and ribbed duct should be same? I have a mesh size with inflation layers 13 for smooth duct. Is it necessary that the inflation layer of ribbed duct should also be 13? I tried, but the meshing fails for ribbed one.

Assuming that the rib height is not small relative to the boundary layer, I suspect that you will need a lot more cells than the smooth case. Boundary layer meshes, by design, have wall-normal cell sizes that are often orders of magnitude smaller than in the other directions. This is justified because the flow is generally assumed to have very significant gradients in the wall-normal direction and comparatively small gradients in the other directions. But, if you have ribs on the scale of the boundary layer or larger, you are apt to have flow separation behind those ribs and thus violate the assumption of primarily wall-normal behavior dominating the flow.

I remember long ago reading about DNS (or maybe LES) of flow over shark skin and the drag reduction associated with the turbulence generated by the grooves in the surface of their skin. There are likely other ways to model your situation without resorting to DNS, but it should indicate that at minimum you have to assume 2D or 3D dimensional flow near the ribbed walls and mesh accordingly.

ribbed duct
 

thanks for the reply. one more thing I want to clarify. Do I need grid independency test for ribbed duct also? For smooth duct I have performed grid independency test. My project is "effect of rib structure on forced convection". so for each and every configuration, do i need to perform the grid independency test?