A large stratified shear flow water channel facility

Abstract.

A large multilayer, recirculating, salt-stratified water channel is described. The channel was similar to, but much larger than that used by Stillinger et al. (J Fluid Mech 131:73–87, 1983), and with a more compact and simpler recirculating system. Each layer was driven independently by a pump and had an adjustable velocity range from 0 to 120 mm/s. The density stratification can be readily obtained with a buoyancy frequency of 0 to 1.7 s^–1. Hot-film anemometry, a newly developed salinity probe, and a thermistor together with a PC were used to measure, collect, and process the data. The facility can also be used as a towing tank. The facility has also been used for grid-turbulence experiments in uniform and shear flows with stratification. These experiments were carried out to test the facility for changes in stratification. The experiments did not change the density gradient substantially, especially in the middle portion of the vertical profile. These experiments had gradient Richardson numbers typically larger than one and are not meant to cover an extensive range of experimental conditions. The main findings showed that the stratification hardly changes the rate of decay of longitudinal turbulence velocity fluctuation, but enhances the decay of vertical turbulence fluctuation; the decay of vertical velocity fluctuation in shear stratified experiments is even larger than for corresponding cases without shear; and the normalized buoyancy flux initially increases behind the grid, then falls off rapidly at the onset of buoyancy effects and goes to zero at about Nt=2, and then tends to oscillate about zero.