Effect of spur dike length on the horseshoe vortex system and the bed shear stress distribution

Abstract

ABSTRACTTurbulent flow structures forming around isolated spur dikes in a horizontal channel are investigated in this study. In the analysis detached eddy simulation is used under fully turbulent incoming flow conditions at a channel Reynolds number of 45,000. Changes in the structure of the horseshoe vortex system, bed shear stress and pressure standard deviation on the bed are investigated for three different spur dike lengths. In all of the cases the main horseshoe vortex undergoes bimodal oscillations, which leads to an amplification in the turbulence quantities such as turbulent kinetic energy and pressure fluctuations along its axis. The main horseshoe vortex disappears over a much shorter distance in the flow direction for the short spur dike than those of medium and long spurs. Large bed shear stress values and pressure standard deviation values observed around the tip of the spur dike, beneath the upstream part of the main horseshoe vortex and beneath the separated shear layers increase with the increasing length of the spur dike. Different from the long and medium spur dike, in the short spur dike case, it is shown that the secondary horseshoe vortex is as coherent as the main horseshoe vortex and it contains bimodal oscillations together with the main horseshoe vortex.