Some three-dimensional effects during vortex generation at a straight edge

Abstract

For vortices generated by an impulsively started flow about a straight sharp edge bounded by side-walls, one might expect the vortex-flow in the mid-plane to remain unaffected by the walls for a time. Experiments in water using rectangular nozzles with generally moderate width-to-height ratios showed that a flow was initiated from the walls into the vortex core and towards the mid-plane. This flow set in at the same time as the main flow began. The fastest mass transport took place near the junction between the edge and the walls. This water moved within the vortex axis with an initially constant velocity approximately a third of that of the main flow, independent of the width, the height and the edge-angle within a surprisingly large range of these parameters. A further feature of the wall-near flow is the appearance and growth of a region of vortex breakdown in the core near the wall. In the mid-plane a flow was initiated directed radially outwards from the centre of the vortex. This flow was also short lived, beginning both before the axis became significantly distorted, as well as before any noticeable axial velocity gradient near the mid-plane existed. This radial motion seems thus to be the most sensitive measure of the flow in the mid-plane becoming three-dimensional. During this time the forces associated with the axial and radial flow may be significant. Despite the abovementioned relatively fast secondary flow, the trajectory of the vortex-centre in the mid-plane seems unaffected.