Effect of a bottom gap on the mean flow and turbulence structure past vertical solid and porous plates situated in the vicinity of a horizontal channel bed

Abstract

The paper analyzes how the structure of the flow past solid (porosity P=0%) and porous (P=36%) plates containing parallel solid elements changes with increasing gap distance from the bottom surface. Fully three-dimensional large-eddy simulations are conducted in a straight channel with a horizontal smooth bottom surface and a vertical thin plate of height H positioned at a distance G from the channel bottom for 0≤G/H≤1. For small G/H values, the main features of the wake flow are the formation of a bottom-attached recirculation region behind the plate and of corotating vortices originating from eddies shed inside the separated shear layer at the top edge of the plate. For sufficiently high G/H values, the wake flow is dominated by the shedding of counter-rotating wake vortices behind the plate (antisymmetric vortex shedding). For G/H≤1, the wake vortices originating in the bottom separated shear layer lose their coherence after they start interacting with the bottom surface. The effects of increasing G/H on the turbulent kinetic energy, nondimensional streamwise drag force acting on the plate and friction velocity distributions on the bottom surface are analyzed. The paper also discusses how the bleeding flow present in the porous plate cases affects the dynamics of the large-scale coherent structures and its effect on the spectral content of the flow, streamwise drag force, and bed friction velocity distributions.