Bistability and hysteresis in a highly turbulent swirling flow

We report on experimental observations of a gravity-wave instability forced by a highly turbulent free-surface Taylor-Couette flow. We observe a subcritical bifurcation from a highly turbulent base state, the mean flow of which is axisymmetric, to a turbulent gravity-wave state, the mean flow of which has a azimuthal number m=1. The flow is characterized by ultrasound velocimetry, by imaging of the surface profile, local free-surface disturbance measurements and by the dynamic pressure at the outer cylinder.