Flow around surface-mounted permeable cubes on solid and deformable surfaces

Abstract

A wind tunnel experiment was carried out to characterize the flow surrounding rectangular prisms of varying permeability, each set mounted on a stationary plane-bed surface and subsequently on an erodible bed. Laser-Doppler anemometer measurements of the horizontal and vertical velocity components were obtained in a grid that included an area adjacent to the windward face, enveloped the free end of the form, and extended ≈6.5 element heights downwind of the rear wall. From these component measurements, the total velocity (Tuw), turbulence intensity (TI), Reynold Stress (RS) and the turbulence kinetic energy (TKE) were calculated throughout the sampling array. As compared to an impermeable same-sized cube, the near-surface TKE and RS were substantially reduced within the wake flow behind the permeable elements. In the plane-bed experiments, TI generally increased downwind of the permeable cubes, opposite to the trend for the impermeable form. The distinction in TI was less pronounced, however, when the bed morphology developed scour marks. The impermeable cube had the largest amount of erosion relative to its volume, in response to strong downwash along its windward face and the development of an energetic horseshoe vortex. This coherent flow structure was not detected for all permeable forms and the amount of scour was orders of magnitude less. This study would suggest that for restricting erosion, the efficiency of a surface-mounted element can be improved by making the walls of the form permeable rather than solid, thereby increasing energy dissipation in the wake flow while reducing vortex impingement and bed shear stress.