Vortex flow around the bases of obstacles

Abstract

This thesis presents the results of experimental and theoretical investigations of the horseshoe vortex systems caused by boundary layer separation upstream of obstacles mounted on a wall. Flow visualization enabled these vortex systems to be described in detail for both laminar and turbulent flow upstream of the obstacles, for flow velocities between 0.3 ms-1 and 25 ms-1. Pressure distributions on the wall beneath such systems are presented and the variation of the vortex and separation positions upstream of various obstacles is described in detail. A complex unsteady behaviour of laminar horseshoe vortex systems is described, and the frequency spectra of turbulent vortex systems are presented. For laminar vortex systems a theoretical study of the equations of motion has been made and the distributions of vortex circumferential velocity and pressure within the vortex cores have been derived, plausible assumptions having been made for the vertex radial velocity distribution. Some measurements have been made of the flow velocity within laminar horseshoe vortex systems using smoke flow visualization techniques. The scour around obstacles mounted in a bed of sand beneath a water flow, caused by such horseshoe vortex systems, is also studied. A dimensional analysis has been carried out to determine what dimensionless combinations of the flow variables might influence the scour depth. Small scale experiments were carried out to determine how the scour depth varied with these dimensionless groups. The results of these experiments are compared with the results of previous investigations and the way the scour depth varies with these dimensionless groups has been clearly shown. A theoretical analyisis of one aspect of the phenomenon of scour around obstacles is also presented.