Structure and evolution of tidal starting jet vortices at idealized barotropic inlets

Abstract

We present laboratory experiments for tidal starting jet vortices forming at idealized barotropic inlets using dye visualization and particle image velocimetry (PIV) of the surface velocity field. Vortices are identified in the PIV data using the local swirl strength, and metrics are calculated for each identified vortex, including position, equivalent circular diameter, maximum vorticity, circulation, and upwelling potential. These quantitative metrics are presented for four different inlet layouts, including narrow and wide barrier islands and short and long jetties. In each case, starting jet eddies initially form attached to the inlet mouth, with a rapid increase in vorticity, circulation, and size as water exiting the inlet flows directly into the starting jet dipole. Once the vortices reach a critical size, they are entrained into the tidal jet and detach from the inlet. As they advect away from the inlet, their size remains steady while the maximum vorticity and circulation gradually decrease because of the effect of bottom friction and the reduction of input to the vortices from the tidal flow as a result of their advection within the tidal jet. Secondary vortices shed from the inlet during the quasi‐steady tidal jet are also entrained into the starting jet vortices, decreasing the overall rate at which their vorticity and circulation decay downstream of the inlet. The quantitative results indicate that the starting jet eddies grow, detach from the inlet mouth, and decay at predictable non‐dimensional rates and times.