Characterisation of flow and wall-pressure statistics of permeable-rough surfaces

Permeable and rough walls are known to affect the turbulent boundary layers (TBLs) developing over them differently. The effect of roughness on a developing TBL can be characterised by the log-law shift (the Hama roughness function). The effect of permeability, on the other hand, depends on the pore size and the surface thickness. In terms of turbulent structures, near-wall cycles are replaced by coherent roughness-sized structures, while permeability weakens them. Independently, the effects of these wall conditions on their flow characteristics and wall-pressure statistics are relatively well-established. Their combined effects, however, are still largely unknown.

In this study, we aim to characterise the combined effects of both roughness and permeability on TBLs. Simultaneous hot-wire anemometry and microphone, as well as independent wall-shear stress measurements are conducted inside a boundary layer wind tunnel over a wide range of Reynolds numbers (9000 < Reτ < 38000), where the boundary layer develops over the following surface arrangements: permeable wall (polyurethane foam), rough walls (metal meshes with 81% and 73% open area, respectively), and permeable-rough walls (the foam topped with metal meshes). Present results suggest that both permeable-rough walls increase drag compared to that of permeable and rough walls, separately. Characterisation of the combined permeable-rough effects, decoupling of these effects on surface drag, and examination of coherence between velocity and wall-pressure fluctuations will follow.