A 3D Study of an Air-Core Vortex Using HSPIV and Flow Visualization

A free-surface vortex is a mass of water rotating around an axis perpendicular to the free surface. It can occur when withdrawing water from reservoirs or rivers at hydropower intakes with low submergence. Existing vortex models provide general information about the symmetric vortex structure. The vortex structure occurring in an approach flow at the critical submergence condition is examined in detail. In the laboratory, a steady air-core vortex over a bottom intake was created in a wide recirculating flume, in which the water depth, mean velocity of the approach flow and intake discharge could be adjusted. Flow visualization shows that the approach flow results in a non-symmetrical velocity distribution in the vortex throughout the flow depth. The detailed set of high-speed particle image velocimetry data in a series of horizontal and vertical planes was used to observe the formation and evolution of the three-dimensional flow structure of the strong air-core vortex and determine the origin of the vortex. Analysis of these data revealed a complex three-dimensional vortex structure due to the approach flow interacting with the air-core vortex forming a secondary vortex originating at the mixing zone upstream of the vortex, identified by a zero downstream velocity component, and feeding into the upstream side of the intake.