Vorticity and Circulation of Horseshoe Vortex in Equilibrium Scour Holes at Different Piers

Abstract

The present work deals with an experimental investigation on clear water local scour and horseshoe vortex flow field of cohesionless bed sediment around an equilateral triangular pier (side facing the approaching flow). The turbulence field within the equilibrium scour hole is experimentally measured in a laboratory tilting flume using an Acoustic Doppler Velocimeter. The experiment is conducted for the approaching flow having undisturbed flow depth (=0.125 m) greater than the pier width and the depth-averaged approaching flow velocity (=0.247 m/s) about 68 % of the critical velocity of the uniform bed sand of median diameter of 0.825 m. The triangular pier width is 0.11 m. The outcomes of triangular pier have been compared with the square pier (side facing the approaching flow) and circular pier obtained from previous study. The distribution of time-averaged velocity vectors, contours of the vorticities and turbulent kinetic energies at different azimuthal planes (0° i.e. at the upstream axis of symmetry, 45° and 90°) are presented. Vector plots of the flow field at azimuthal planes reveal the characteristics of the horseshoe vortex flow. The circulation of the horseshoe vortex is determined by using Stokes theorem and forward difference technique. An attempt has also been made to correlate the horseshoe vortex strength with the effective obstruction width of pier for equilibrium scour hole at different azimuthal angles. The presented data would be useful for the development and validation of turbulence field models that can be used to find out the strength of the horseshoe vortex.