Abstract
Hydraulic jumps occur naturally when supercritical flow transitions into subcritical flows in a sudden, extremely turbulent three-dimensional process. Research of hydraulic jumps is a challenge and to date turbulence and air-water flow measurements are limited to hydraulic jumps in smooth rectangular channels. Past studies of above uniformly distributed roughness focused on the identification of the conjugate depth relationship and mean flow velocities. No study has investigated the bed roughness effects on the air-water flows in hydraulic jumps. The present study systematically researched the effects of macro-channel bed roughness upon the basic flow patterns and air-water flow properties. Detailed experiments of hydraulic jumps were conducted for three different channel bed configurations comprising smooth bed, and two different configurations of macro-roughness. Experiments were conducted for a range of discharges 0.012 ≤ Qw ≤ 0.106 m3/s, corresponding to upstream Froude numbers of 1.7 ≤ Fr1 ≤ 6.5 and to Reynolds numbers of 6.3 × 104 ≤ Re ≤ 2.1 × 105. The hydraulic jumps on the rough bed exhibited some characteristic flow patterns including a preaeration of the flow upstream of the hydraulic jump, an upwards shift of the jump roller resulting in a reduction of jump length, a clear water flow region underneath the jump and a stabilisation of the jump toe fluctuations. Air-water flow measurements were conducted with a newly designed conductivity probe from WRL. The overall distributions of air-water flow properties were similar for the rough and smooth bed hydraulic jumps. Comparative analysis highlighted some distinctive effects of the bed roughness including an upwards shift of the hydraulic jump and an increase in bubble count rate and void fractions in the region close to the jump toe. In the second half of the hydraulic jumps the rough bed lead to elevated levels of void fraction in the recirculation region suggesting a lesser aeration of the free-surface region. The
present study highlighted the potential that improved and non-standard designs may have for flow manipulations and design enhancements.
