Kinematics of horseshoe vortex development in an evolving scour hole at a square cylinder

Abstract

This paper presents an experimental investigation on the characteristics of the development of turbulent horseshoe vortex flow in an evolving (intermediate stages and equilibrium) scour hole at square cylinder measured by an acoustic Doppler velocimeter (ADV). As the primary objective was to study the turbulent flow characteristics of horseshoe vortex in an evolving scour hole, the flow zone downstream the cylinder was not considered. Experiments were conducted with a square cylinder of width 12 cm embedded in the bed of uniform sand of median diameter 0.81mm under the approaching flow having undisturbed flow depth (= 25 cm) greater than twice the width of the cylinder and the depth-averaged approaching flow velocity (= 35.7 cm/s) equaling approximately 0.95 times the critical velocity for the uniform bed sand. The ADV flow measurements were taken inside the intermediate scour holes (having depths of 0.25, 0.5 and 0.75 times the equilibrium scour depth) and the equilibrium scour hole (frozen by spraying glue). The contours of the time-averaged velocities, turbulence intensities and Reynolds stresses at different azimuthal planes (0?, 45? and 90?) are presented. The change of the characteristics of horseshoe vortex flow associated with a downflow from intermediate stages to equilibrium condition of scour hole is revealed through the vector plots of the flow field at different azimuthal planes. Also, the flow characteristics of the horseshoe vortex are analyzed from the point of view of similarity with the velocity and turbulence characteristic scales. The important observation is that the flow and the turbulence intensities in horseshoe vortex flow in a developing scour hole are reasonably similar.