Shear instabilities in a tilting tube

Shear instabilities were investigated in an exchange flow in a tilting channel. The channel is connected to a freshwater reservoir at one end and a salt water reservoir at the other end. When the channel is initially horizontal, a steady two-layer flow occurs that supports symmetric Holmboe instabilities. When the channel is tilted, as in closed tilting channel experiments, shear increases and Kelvin-Helmholtz instabilities are generated. The KH instabilities break down the sharp interface between the two-layers resulting in a broad region of mixing. Subsequent levelling of the tube results in a three-layer exchange flow. The upper and lower layers are nearly pure fresh and salt water respectively. Separating these two layers is a relatively thin layer of homogeneous mixed fluid. Within this layer a third mode of instability forms that resembles a Taylor-Caulfield instability. The wave characteristics and appearance of these instabilities are compared with the results of linear stability analysis and direct numerical simulations.