Ventilated oscillatory boundary layers

Abstract

Boundary layers arising from flows which oscillate parallel to a permeable bed, and are subject to oscillating percolation of the same frequency as the bed parallel flow, referred to here as ‘ventilated oscillatory boundary layers’, are the subject of this laboratory study. These boundary layers are intended to approximate naturally occurring wave boundary layers over permeable beds. Measurements of boundary-layer velocities, bed stress and turbulent flow properties are presented. It is observed that suction (flow into the bed) enhances the near-bed velocities and bed stress while injection (flow out of the bed) leads to a reduction in these quantities. As the ventilated oscillatory boundary layer experiences both these phenomenon in one full cycle, the result is a net stress and a net boundary-layer velocity in an otherwise symmetric flow. While production of turbulence attributable to injection is enhanced, the finite time required for this to occur leads to a greater vertically averaged turbulence in the suction half-cycle. Turbulence generated in the suction half-cycle is maintained in a compact layer much closer to the bed. These effects appear to hold for$\widetilde{Re}$ranging from 105to 106and for oscillations other than sinusoidal.