Turbulence and air entrainment in hydraulic jumps

Abstract

A hydraulic jump is a sudden transition from a supercritical flow to a subcritical flow. It is an extremely turbulent flow associated with the development of large-scale turbulence, surface waves and spray, air entrainment and energy dissipation. Hydraulic jumps are commonly encountered in natural waterways and hydraulic structures, and are often used for the purpose of enhancement of energy dissipation, fluid mixing or flow re-aeration. Despite relevant studies in the last two centuries, the hydraulic jump flow regimes are not fully understood because of the large number of parameters to describe the flow and the simultaneous presence of and interactions between various turbulent two-phase flow properties. This thesis presents a systematic study of classical hydraulic jumps with partially-developed inflow conditions based upon physical modelling. The nature of the flow was characterised with a range of flow properties related to the free-surface dynamics, air entrainment and transport, characteristic turbulent scales and bubble-turbulence interactions. The free-surface and air-water flow measurements were respectively facilitated with non-intrusive acoustic displacement meters and intrusive phase-detection probes. The turbulence properties were mainly derived from the air-water interface detection. The spatial distributions of two-phase flow properties and turbulent scales, as well as the effects of Froude and Reynolds numbers, revealed some interaction between turbulence development and air entrainment. The study covered a wide range of Froude numbers from 2.8 to 10 and Reynolds numbers from 2.1×104 to 1.6×105. A series of basic findings were in agreement with previous researches. The dimensions of hydraulic jump and the length scales of free-surface dynamics were only determined by the inflow Froude number, while the time scales, namely the characteristic frequencies of free-surface dynamics, were further affected by the Reynolds number and may experience scale effects. The jump roller was characterised by a turbulent shear flow region on the bottom and a free-surface recirculation region above. Typical void fraction and bubble count rate distributions reflected the unique aeration pattern which was a combination of singular air entrainment at the jump toe and interfacial air-water exchange through the free-surface. A number of characteristic air-water flow properties were quantified, of which the self-similar distributions enabled the theoretical prediction of two-phase flow structures. The results highlighted the linkage of the advection and diffusion of air bubbles to both buoyancy effects and dissipation of turbulence and kinetic energy. The bubble-turbulence interplay induced the occurrence of bubble clustering, which was analysed in both longitudinal and transverse dimensions. The turbulence characterisation based upon local phase detection in such an instationary two-phase flow was substantially improved by the application of triple decomposition technique to the phase-detection signal. The impact of pseudo-periodic fluctuations was identified, and the true turbulence was characterised with high-frequency turbulence intensity and integral length/time scales. An extensive discussion of characteristic turbulent scales was proposed, including the scale effects. The simultaneous total pressure measurements in the roller further provided a connection between water level fluctuations, jump toe oscillations, air entrainment and instantaneous velocity field variations. The total pressure fluctuations indicated different dynamic flow regimes in the main and lower shear regions. In a down-scaled model, most turbulence-related flow properties were not scaled accordingly based on a Froude similitude, and viscous scale effects could be significant for small Reynolds numbers. The present thesis provides a comprehensive description of the turbulent two-phase flow in hydraulic jump. The original work was contributed with advanced data collection and analysis methods. It is expected that this work would bring our knowledge of such a complex open channel flow to the cutting edge, and provide solid justification for future theoretical and numerical studies that have a long way ahead.