Interaction between free-surface, two-phase flow and total pressure in hydraulic jump

• Free-surface, air–water flow and total pressure fluctuations measured simultaneously.
• Mean total pressure and its fluctuation both exhibited maxima in shear flow region.
• Surface dynamics and aeration interacted, modulating total pressure field in roller.
• Instantaneous flow pattern was time-variable and not temporally unique.
• Flow regimes differed spatially between main and lower shear regions and upper surface.

A hydraulic jump is characterised by intense turbulent flow patterns and substantial flow aeration. The flow turbulence, at both macroscopic and microscopic scales, interacts with the air entrainment process and the free-surface. A series of simultaneous measurements of the free-surface fluctuations, jump toe oscillations, void fraction and total pressure variations allowed for an investigation of the interactions between these characteristics. Experiments were conducted for a range of Froude numbers from 3.8 to 8.5. The total pressure measurements were justified for the air–water flow characterisation of the flow region with a positive time-averaged velocity. The interactions between roller surface deformation, air entrainment and diffusion, velocity variation, flow bulking, and the associated total pressure field modulation highlighted different flow regions, hence flow patterns, in the roller. The jump toe oscillation was found closely linked to the air entrapment at the toe and velocity variation in the shear flow. The instable total pressure distribution was primarily associated with the free-surface fluctuation for the bubbly roller region and with the velocity re-distribution for the lower shear region underneath. The present work provides new information on the physical characteristics of hydraulic jumps and a comprehensive insight into the nature of such complex turbulent two-phase flow.