How to I include surface roughness of the CAD model

I can include roughness height in the Custom boundaries, but is it necessary to include surface roughness in the buildings that were included in the Geometry through the CAD mode? If needed, how do I include this?

Thanks for your help.

Hello @jjansman, and thank you for reaching out to us!

I believe the best response here would be: it depends on the level of accuracy you require. When surface roughness is not accounted for and no boundary conditions are assigned to building faces, a no-slip boundary condition is applied to these surfaces. It is assumed that these surfaces are smooth, and a velocity profile is computed on them using the corresponding turbulence model.

If you are indeed considering a rough surface, surface roughness should be included as it significantly impacts the computation of turbulent flows.

I recommend examining the surface roughness values provided here and comparing them with the material type you are dealing with. If the value seems negligible for your material type (e.g., glass, smooth concrete), you can proceed with the no-slip approach. However, you will find that the roughness value becomes significant when dealing with materials such as grass or rough concrete. Ultimately, the choice will be yours.

I hope this explanation proves helpful.

Thanks for your hints. Very worthwhile.
In my case. I have a built environment (Rigde height by jjansman | SimScale ) and the area between (and in front/behind) the buildings is rough as in every town/estate. The ABL has at the Inlet a Z_0 of 0.5m.
So should I define the ground with Custom with an equivalent roughness height, or is it better to use No-slip Wall? I would have thought the best to define Custom with a roughness height (and the roughness height between 10 to 30 times Z_0). Is this ok-ish?

I did some exploration here: Workflow to determine Impington Mill's biotope

All the best,

Hello again @jjansman, and thank you for getting back to me.

Whatever you define here for the buildings or areas between the buildings will be independent of the ABL you defined at the inlet. The Z_0 value defined at the inlet will account for the general roughness of the terrain, such as sand, water, concrete, etc. If you define an additional custom boundary with a roughness height, you will indeed be able to specify additional roughness, as you anticipated.

As a side recommendation in that case, the areas should be represented by a separate face. :slight_smile: