Turbulent-wall pressure fluctuations in permeable-rough surfaces

Permeable and rough walls are known to affect the turbulent boundary layers (TBLs) developing over them differently. Permeability relaxes wall blockage, while roughness increases surface drag. A realistic representation of a permeable wall might consist of both permeability and roughness, to a varying degree. In the presence of rough walls, the near-wall turbulent structures in smooth wall TBLs are replaced by coherent roughness element-sized structures, while the flow penetration into a permeable wall has been observed to alter the dynamics of coherent structure formation above the wall. 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 explore and decouple the effects of permeability and roughness in the flow and wall-pressure statistics. Simultaneous hot-wire anemometry and microphone measurements are conducted inside a boundary layer wind tunnel over a wide range of Reynolds numbers, where TBLs develop over various porous and rough walls (with a systematic variation of the roughness effects). Present results suggest that the footprints of permeability and roughness are apparent in the energy spectra of turbulent fluctuation for isolated permeable and rough walls. Explorations of this phenomenon in porous-rough combinations, the coherence between turbulence and wall-pressure statistics, as well as decoupling of roughness and permeability effects, will follow.