Author(s): Claire Biggs; Roger Nokes; Ross Vennell
Keywords: Shallow jet; Turbulence; Harbour outflow; PTV; Macro-vortices
Abstract: Preliminary results from the study of a steady shallow turbulent jet are presented. The jet, intended to approximate the tidal jet exiting a harbour, has a small aspect ratio (depth/width) as it leaves a smooth channel into a large rectangular pond. The aim is to simulate shallow water jets where bottom friction is important. A novel experimental setup enables an unbounded lateral flow domain to be simulated using variable flow lateral boundaries within the pond. The flow characteristics of the jet are measured using a particle tracking velocimetry (PTV) system based on the tracking of surface drogues. Mean and turbulent flow statistics are measured up to 9 channel widths downstream of the inflow and approximately half of this measurement region lies in the zone of flow establishment for the jet. Mean velocity profiles, originally close to top hat in shape gradually transform to Gaussian profiles with a decreasing maximum velocity along the centre line. Spread rates for the mean velocity profiles show two distinct behaviours–an average spread rate of 0.037 up to approximately 4 widths downstream of the source, and an average spread rate of 0.083 beyond this point. This dramatic change in spread rate correlates with a similar increase in growth rate for turbulent kinetic energy and turbulent stress. The lateral instabilities that form in the shear layers on either side of the jet are shown to develop into coherent macro-vortices by x/W~4 and beyond this point the vortices resemble a vortex street with staggered vortices on either side of the jet centreline. These vortices are shown to grow with distance and dramatically distort the trajectory of the jet, resulting in wandering and flapping of the jet centreline. This modulation of the jet motion by the vortices is believed to be responsible for the change in flow characteristics beyond x/W~4.