Two-Phase Flow Properties and Free-Surface Deformations in Hydraulic Jumps

In an open channel, a hydraulic jump is a sudden transition from a high-velocity to slower, fluvial flow motion. The impinging flow forms a turbulent roller with a highly fluctuating free-surface, while a large amount of air bubbles is entrapped at the jump toe. The present work presents some physical investigations of hydraulic jumps based upon a wide range of inflow Froude numbers (3.8 < Fr1 < 10.0) and Reynolds numbers (2.1×10 4 < Re < 1.6×10 5 ). Both the Froude and Reynolds similarities were tested independently. Free-surface deformations were measured non-intrusively with fast-response sensors. The shape of free-surface profile was clearly defined, and maximum fluctuations were observed in the first half of the roller. Characteristic fluctuation frequencies were seen between 0.5 and 3.5 Hz, including both dominant and secondary frequencies. The free-surface fluctuation data were analysed and linked to the longitudinal roller movement, showing some simultaneous motions of the roller surface in both horizontal and vertical directions. Wave propagation at the roller free-surface presented some celerity close to the advection speed of large vortices in the roller shear region. A phase-detection probe was used to measure the two-phase flow properties, including void fraction and bubble count rate. The interaction between the instantaneous two-phase flow properties and free-surface deformations was documented by some simultaneous surface location and sub-millimetric air-water flow measurements. Different features were exhibited depending upon the phase-detection location in different regions of the roller.