Comparison of Characteristics of Horseshoe Vortex at Circular and Square Piers

Abstract

This study presents an experimental investigation on the characteristics of horseshoe vortex system within the equilibrium scour hole at circular and square pier measured by an Acoustic Doppler Velocimeter (ADV). Two tests were 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 that had a median diameter of 0.825 m. The flow measurements by the ADV were taken within the equilibrium scour hole at a circular pier of width 0.11 m. In order to have a comparative study, the ADV measurements within an equilibrium scour hole at a square pier (side facing the approaching flow) of sides equaling the width of the circular pier were also taken. The contours and distributions of the time-averaged velocities, turbulence intensities, turbulent kinetic energy and Reynolds stresses at different azimuthal planes (0° i.e., at the upstream axis of symmetry, 45° and 90°) are presented. Velocity vector plots of the flow field at azimuthal planes are used to show further flow features. The circulation of the horseshoe vortex is determined by using Stokes theorem and Forward difference technique. Bed-shear stresses are also determined from the Reynolds stress distributions. The flow characteristics of the horseshoe vortex are discussed from the point of view of the similarity with the velocity and turbulence characteristic scales.