Kelvin-Helmholtz instability in a continuously forced shear flow

Kelvin-Helmholtz instability is a fluid instability frequently observed in atmospheric and astrophysical situations. Previous experiments were mainly conducted in a tilting closed tube or in open-flow experiments. We present a study of Kelvin-Helmholtz instability in a closed annular two-layer experiment where the shear across the layers is maintained at a steady rate by rotating the lid in contact with the upper fluid at a constant speed. The flow can be estimated from distortions of the interface which are visible due to differences in the refractive indices of the two fluid layers. This method is applied to immiscible layers of silicone fluid and water. The initial instability of the fluid interface and the wave number of the emerging flow will be compared with linear stability calculations. The experimental results indicate that two types of instability may have been observed; one dominated by viscous effects, the other (the Kelvin-Helmholtz instability) by interfacial tension.