Air Entrainment and Scale Effects in Hydraulic Jumps with Small Froude Numbers

摘要

The transition from supercritical to subcritical flow is characterised by a strong dissipative mechanism, a hydraulic jump. In the present study, the air-water properties were investigated experimentally in hydraulic jumps with relatively small Froude numbers (2.4 < Fr_1 < 5.1) and relatively large Reynolds numbers. A comparative analysis demonstrated that, for hydraulic jumps with Fr_1 = 5.1, the void fraction data obtained with Re < 4×10~4 could not be scaled up to Re = 1×10~5. Most air- water flow properties measured with Reynolds numbers up to 1.25×10~5 could not be extrapolated to large-size prototype structures without significant scale effects in terms of bubble count rate, turbulence and bubble chord time distributions. The findings have some major implications of civil, environmental and sanitary engineering designs, because most hydraulic structures, storm water systems and water treatment facilities operate with Reynolds numbers larger than 10~6 to over 10~8.