Observations of the Kelvin-Helmholtz Instability in Laboratory Models and Field Examples of Thermal Plumes

Abstract

Buoyant, rectangular thermal plumes were modelled in a 2 by 7 meter laboratory water table facility. The plume enters at right angles to a constant temperature, uniform crossflow of depth equal to that of the plume. Highly buoyant plumes (0.16 < Fr o < 2) were investigated. Velocity, temperature and dye measurements were made using a scanning infrared camera, thermistors, dye injection, and a temperature compensated hot film anemometer. A Kelvin-Helmholtz instability is observed in the plume thermocline for Fr o < 0.50. For the same values of Fr o a cold water wedge penetrates beneath the plume into the plume discharge channel. A series of transverse line vortices quickly grow in the thermocline to cover the vertical extent of the plume, moving nearly with a mean flow. These vortices are visible both by dye injection into the thermocline and surface infrared imagery. Local gradient Richardson numbers fall below 1/4 in the thermocline for all plumes observed (Fr o < 1), but above Fr o = 0.50 the Kelvin-Helmholtz instability is dominated by randomized turbulent mixing. This vortex instability was also observed in aerial infrared photographs taken by NASA of the Cuyahoga River mouth entering Lake Erie. Field measurements indicate that Ri < 1/4 in the thermocline at the river mouth as well. These results are compared with related experimental observations and previous theoretical inviscid stability analyses of stably stratified shear flows by other authors.