Author(s): Hang Wang; Hubert Chanson
Keywords: Hydraulic jump; Characteristic frequency; Free-surface; Air-water flow; Physical modelling
Abstract: In a hydraulic jump, the turbulent flow region between the upstream supercritical flow and downstream subcritical flow is called the jump roller. The turbulence development in the jump roller leads to substantial energy dissipation as well as free-surface fluctuations, oscillations of jump position, air entrainment and unstationary velocity field. The time scales of these motions cover a broad range of frequencies, which may be critical to the design and maintenance of hydraulic structures. This paper presents a physical study of hydraulic jumps based upon a series of measurements of the free-surface and two-phase flow properties. Some long-term change in jump position was documented. The fluctuations of free-surface water level and longitudinal jump toe position were measured non-intrusively, corresponding to the relatively slow unstationary processes with frequency magnitude of about 1 Hz. Characteristic frequencies of these motions were reported, and the surface deformation pattern was characterised. The intrusive measurement of local void fraction and bubble frequency in the roller provided information on the turbulence properties corresponding to high frequency (greater than 10 Hz) processes. The air entrapment rate in the roller was calculated based on the void fraction and velocity distributions. The coupling between the free-surface position data and instantaneous void fraction revealed some interaction between the surface deformation and air entrainment process, though they were of different frequency magnitudes. The present study provided detailed physical description of the low- and high-frequency processes in hydraulic jumps. The understanding of these properties and their potential effects are fundamental to practical hydraulic engineering.