Analytical and numerical investigation of an air entraining hydraulic jump

Abstract

A significant environmental concern associated with hydropower electricity generation is the effect of operations on water quality. Of particular interest is the need to maintain elevated concentrations of dissolved oxygen (DO) in water passed through or over the dam. The hydraulic jump is commonly employed as a cost-effective mechanism to transfer oxygen into the flow and increase downstream DO through air entrainment and air-bubble mass transfer. However, dam operators currently lack the ability to accurately predict gas transfer at a given structure due to largely unknown initial and boundary conditions. The hydropower industry has a strong desire for improved DO predictive models that can quantify air entrainment under a variety of flow conditions across multiple structures.The purpose of this thesis is to better characterize the air entrainment that occurs at a hydraulic jump. The investigation is carried out in two parts: (1) development of a predictive equation for gas transfer at low-head gated dams, and (2) numerical simulations of an air entraining hydraulic jump. In Part 1, field measurements are analyzed to develop a dimensionless relationship between inflow conditions, dam geometry, and oxygen transfer efficiency. An improved predictive equation for gas transfer efficiency is derived that incorporates bubble physics and hydraulic jump scaling factors. Part 2 will describe the development and investigation of numerical simulations of the hydraulic jump. The numerical method was designed to capture air entrainment in a laboratory scale hydraulic jump and resolve the breakup of large air pockets in the turbulent shear region. Examination of the 3D flow field provides an improved understanding of the flow physics, particularly the behavior of vortical structures and their effect on bubble transport.