Towards decoupling the effects of permeability and roughness on turbulent boundary layers

Boundary-layer flow over a realistic porous wall might contain both the effects of wall-permeability and wall-roughness. These two are typically examined in context rough-wall flow, i.e. by defining ‘roughness’ length or equivalent to capture effect surface on momentum deficit/drag. In this work, we examine hypothesis Esteban et al. ( Phys. Rev. Fluids , vol. 7, no. 9, 2022, 094603), that turbulent boundary layer could be modelled as superposition roughness permeability using independently obtained information roughness. We carry out wind tunnel experiments at high Reynolds number $14\ 400 \leq Re_{\tau } 33\ 100$ ) various combinations walls where different roughnesses overlaid given permeable wall. Measurements also conducted well rough obtain corresponding scales. Analysis mean data across all these measurements suggests an empirical formulation can deficit $\Delta U^+$ is combination This assumes presence outer-layer similarity surfaces, which shown valid numbers. Finally, decoupling approach area-weighted power-mean respective scales, consistent with recently suggested Hutchins Ocean Engng 271, 2023, 113454) heterogeneous surfaces.