An experimental investigation of a horseshoe vortex induced by a bridge pier

Abstract

The horseshoe vortex induced at a bridge pier is considered to be one of the principal causes of local scour at the pier. Knowledge of the location, size, shape and rotational velocity of the vortex is of paramount importance in modelling scouring processes and designing a suitable scour protection system. An experimental investigation was carried out to study the dominant features of the horseshoe vortex during the scouring processes with the help of a specially devised vortex probe and a simple method of vortex visualisation. It has been found that the shape of the primary horseshoe vortex is dependent on the pier Reynolds number (ReD). The vortex is of circular shape only at lower Reynolds numbers (ReD ≤ 5000), but it becomes more and more elliptical as ReD increases. The horizontal dimension of the vortex along the mean flow direction has been found to be a function of the pier geometry whereas its vertical dimension is mainly governed by the parameters related to the upstream velocity profile such as the boundary layer thickness. The size of the vortex increases with the development of the scour hole whereas its elliptical shape remains fairly invariant during the scouring process. The mean velocity of the vortex is approximately 50% of the mean approach flow velocity on the rigid bed for 104 ≤ ReD ≤ 1·4 × 105. Its variation with the development of the scour hole indicates an increasing trend in the initial stages of scouring whereas a decreasing trend has been found in the later scouring stages.