Roller Characteristics of Preaerated High-Froude-Number Hydraulic Jumps

A hydraulic jump in a stilling basin at the base of spillway is typically characterized by preaerated inflow conditions. This paper presents an experimental study of high-Froude-number hydraulic jumps with different preaeration levels. Two characteristic lengths of the jump, namely the jump length and roller length, which are defined in various studies but sometimes not clearly distinguished, are obtained from the same flows by means of free-surface and air-water flow velocity measurements, respectively. The effects of changes in preaeration level are found to be opposite on the jump and roller lengths. Enhanced preaeration allows for a shorter length of the hydraulic jump, and the jump roller is elongated to achieve the same energy-dissipation rate in a shorter distance. The results imply a predominant effect of energy-dissipation enhancement over reductions associated with the extra air–water mixing and bubble–turbulence interplay. Tailwater wave characteristics are presented, showing similar frequencies for the far-field wave fluctuations and upstream jump toe oscillations on the two ends of the jump.