An experimental study of free-surface dynamics and internal motions in fully aerated hydraulic jumps

Abstract

Hydraulic jumps occur at the sudden transition from supercritical to subcritical flows. They are characterised by three-dimensional motions, instabilities and strong flow aeration. Due to the large energy dissipation capacity, hydraulic jumps are widely used in energy dissipators downstream of flow conveyance structures. The complexity of hydraulic jumps has fascinated researchers for centuries and numerous experimental studies provided important insights into the hydraulic jump phenomenon. However, the energy dissipation processes associated with complex three-dimensional motions and interactions of entrained air with the flow turbulence are still poorly understood due to limitations in experimental techniques. Herein, novel technologies were applied in the present study to enhance the current understanding of free-surface dynamics and three-dimensional motions inside fully aerated hydraulic jumps. Extensive experiments were conducted at the UNSW Water Research Laboratory in large-scale open channel flow facilities for fully and partially developed inflow conditions upstream of the hydraulic jumps. Distributions of air-water flow properties were measured with state-of-the-art double-tip conductivity probes identifying a strong effect of the inflow conditions on the flow aeration resulting in larger void fractions and bubble count rates for fully developed inflow conditions. World-first use of LIDAR technology in fully aerated hydraulics jumps provided the most detailed information of time-varying free-surface features and jump toe movements to date. The results highlighted the strong effect of the jump toe oscillations on the free-surface fluctuations, characteristic frequencies and turbulent scales. Unique measurements of internal three-dimensional motions in hydraulic jumps were conducted with a submerged sphere connected to a six-axis load cell. The results provided the first direct measurements of transverse and vertical motions inside hydraulic jumps highlighting the strong three-dimensionality of the internal motions and the need to consider flow processes in all directions to better understand the complex dynamics of hydraulic jumps. The combination of the new experimental results provided the most detailed picture of the dissipative flow processes inside hydraulic jumps to date. The study demonstrated that the use of advanced instrumentation can provide new insights into complex hydraulic phenomena such as hydraulic jumps.