Experimental results on flow separation and transitions in the circular hydraulic jump

We describe experimental results on the circular hydraulic jump in a viscous fluid (ethylene-glycol mixed with water) in a setup, where the depth of the fluid far away from the jet is controlled. On increasing the depth, we find a transition from a state (type 1) with separation occurring only on the bottom to a state (type 2) with a "roller", i.e. separation occurring also on the free surface, as in a broken wave. "The system is laminar both before and after the transition and can thus be very accurately controlled. We present data for the height profile of the fluid layer, for the surface velocity and for the location of the separation points. Further, we show direct numerical simulations of the Navier-Stokes equations confirming our interpretation of the transition.