Air-water Flow Characteristics in Hydraulic Jump on Pebbled Rough Bed

The hydraulic jump is the sudden transition from a supercritical flow to a subcritical flow motion which is characterised by strong turbulence, air entrainment, and energy dissipation. A hydraulic jump results in strong interactions and modulation between turbulence and air-water mixing, associated with free-surface coupling. Past research on uniformly distributed roughness focused on the identification of the conjugate depth relationship and mean velocities flow, and few studies measured the turbulent air-water parameters. No study was conducted on the effect of non-uniform bed roughness on the air-water flows in hydraulic jumps.

This study aims at investigating the basic parameters of air-water flow in hydraulic jump on pebbled rough bed. The experiments were performed in a channel with partially-developed inflow conditions. The flume was 3.2 m long, 0.5 m wide and 0.41 m high and consisted of a horizontal high-density polyethylene (HDPE) bed and glass sidewalls. The gravel materials were mixed natural river pebbles sieved between 9.5 mm and 13.2 mm, with d₅₀ = 0.011 m and ρ_s = 2530 kg/. The pebbles were installed on the bed for the whole length of the channel. A phase-detection double-tip conductivity probe was used to measure the basic air-water flow properties including the interfacial velocity. The experiments were conducted for a range of discharge from 0.06 to 0.1 m³/s, corresponding to an inflow Froude number Fr₁ ranging from 1.7 to 2.84 and a Reynolds number Re₁ from 170,000 to 220,000.

Comparison between the present rough and smooth bed configurations, as well as between the present data and those from literature on both smooth and rough beds highlighted some distinctive effects of the non-uniform bed roughness. These included a shorter roller length, an increase in the momentum exchange in the hydraulic jump and a modification of air-flow parameters in the region close to the jump toe. Compared to past studies on smooth bed and rubber mat roughness, a significant difference on the pebbled rough bed was the larger maximum bubble count rate.