Two-phase air-water flow properties in hydraulic jump at low froude number: Scale effects in physical modelling

Abstract

A hydraulic jump is a stationary transition from an upstream torrential flow to a downstream fluvial motion. Most jumps involve a vigorously tumbling flow region, called roller, where much kinetic energy is being lost. In the present study, both the hydraulic properties and air-water flow characteristics were investigated experimentally in hydraulic jumps with a small Froude number Fr1 = 2.1 and several Reynolds numbers 0.078×105 0.4, the air bubble motion in the roller was mostly driven by buoyancy and the two-phase flow processes were dominated by interfacial aeration/de-aeration through the pseudo-free-surface. Similarity and scale effects were tested in terms of a wide range of hydraulic and air-water flow properties in a hydraulic jump with constant Froude and Morton numbers, i.e. Fr1 = 2.1 and Mo = 2.5.10^-11, with different Reynolds numbers. Altogether, more than two dozen parameters were tested systematically under Froude similar conditions. All the data demonstrated that the selection of relevant (air-water) flow property(ies) used to assess similarity and scale effects is uppermost important. Furthermore the concept of similarity and scale effects must be linked to specific flow conditions. In a hydraulic jump at low Froude number Fr1 = 2.1, the present finding demonstrated that many hydraulic jump properties could not be extrapolated from laboratory study to full scale hydraulic structures without substantial scale effects. These findings have profound implications for civil, environmental and sanitary engineering designs, commonly operating with Reynolds numbers in excess of 10^6.