Experiments on Kelvin-Helmholtz billows influenced by boundaries

The effects of upper and lower boundaries on stratified shear flows with horizontally uniform velocity and density profiles are investigated through tilting tank experiments. Two geometries are considered: first horizontal boundaries at unequal distances from the shear layer; and second boundaries sloping at an angle, ⊘, in the spanwise direction. Asymmetric boundary separation is shown to retard the growth of Kelvin-Helmholtz billows and delay their transition to turbulence. However when the billows do evolve, they are shown to migrate vertically away from the closer boundary. This results in the billows causing mixing to a similar vertical extent as in the case of symmetrically placed boundaries, although this mixing is limited to patches in the streamwise direction. Boundaries tilted in the spanwise direction introduce a cross-tank slope to the orientation of the billows. This slope is found to vary linearly with ⊘, as predicted by Thorpe and Holt (1995). Of the 102 individual billows observed, only 21 evolve with a purely two-dimensional orientation, and 65 of the 81 sloping billows are oriented in the predicted sense. The across-lank structure of the billows is found to divide between two distinct regimes depending on the values of ⊘ and H, where H is the depth of the tank scaled by the characteristic width of the density interface, L. In cases where H is large and ⊘ small, the billows evolve with approximately spanwise sections displaced in the streamwise direction either side of the centre of the tank. These are connected by a sloping region of reduced amplitude. When H is small the whole of the billows slope across the tank with little Variation in their orientation. In both cases vortex tubes are commonly observed connecting neighbouring billows. These are always observed to slope in the opposite sense to the billow themselves.