Circular hydraulic jumps triggered by boundary layer separation

When a high-flow-rate circular jet impinges vertically on a horizontal plane, it flows out radially and then undergoes a distinctive hydraulic jump on the plane because of boundary layer separation induced by hydrostatic back pressure. The jump radius is shown to be 0.37 a Re^1/3Λ^-1/8, where Λ = (ga³/v²) Re^-7/3 is a modified Froude number, Re = (Q/av) is the jet Reynolds number, a is the jet radius, and Q the liquid flow rate, which is favorably compared to experimental data in the limit of small Λ. When Λ exceeds 3.0 × 10^-4 at low flow rates, the jump radius decreases below a minimum in the film depth and our experiments detect a different jump mechanism that may be triggered by capillary pressure rather than hydrostatic pressure.