Roller fluctuations of pre-aerated high-Froude-number hydraulic jumps

Abstract

This work presents an experimental study of the fluctuating properties of hydraulic jump rollers for high-Froude-number and pre-aerated inflow conditions, to investigate the validity of empirical relationships in the literature that are derived typically from experiments for smaller Froude numbers and clear-water approach flows. The jump roller fluctuations are evaluated in terms of the dimensions of the roller surface profile, the fluctuation magnitudes and frequencies of the instantaneous jump toe positions and water elevations, including at the tailwater surface, and the formation frequency and advection velocity of internal vortices in the shear layer. The effects of pre-aeration are found primarily in the length-scale properties such as jump length and surface fluctuation magnitudes, while the time-scale properties such as characteristic frequencies of the unsteady motions are not affected. Extrapolation of existing quantitative prediction of jump roller dynamic properties to high-Froude-number flow conditions should be performed carefully. An assessment of the fluctuating bubble counts within fixed time intervals yields close frequencies between the bubble grouping motions in the shear layer and the jump toe and tailwater wave motions, thus implying possible correlations between the roller surface deformations and air entrainment.