The concept of an equivalent surface roughness provides an established procedure of how to include homogeneous surface roughness into large scale flow predictions of forced convection flows.

This seminar presentation addresses the question whether the drag of heterogeneous rough surfaces can be included in this type of predictive correlation. Heterogeneous surfaces are often present as boundary conditions for the atmospheric  boundary layer and may impact the Obukhov length scale.

In terms of forced convection we consider results from laboratory experiments with different surface topologies. Moreover, the effect of spanwise heterogeneous surfaces on mixed convection is discussed based on Direct Numerical Simulations (DNS) data. It is shown that the flow organization classically found for smooth-surface mixed convection conditions can be influenced by the presence of heterogeneous surfaces. For example, streamwise aligned ridges favor the transition from streamwise rolls to convective cells at significantly lower Richardson number than in the  smooth surface case.