DNS of turbulent bubble-laden channel flows

Abstract

This thesis deals with numerical simulation methods for multiphase flows where different fluid phases are simultaneously present. In particular, the subject of interest is a system in which the carrier fluid is a liquid that transports dispersed gas bubbles. The simultaneous existence of physical phenomena spanning a wide range of scales of motion is certainly one, if not the most, complex aspect of bubbly multiphase flows. In this context, numerical simulation is a useful and powerful tool for a better understanding of the physics of such systems. The method applied in this thesis is direct numerical simulation (DNS), where all the details of the flow, up to smallest scales, are resolved by the computational grid and time steps. The aim of this thesis is to develop an accurate and computationally efficient tool for DNS of turbulent bubbly channel flow, starting from the volume-of-fluid formulation. While single phase flow has been studied for a considerable time, the presence of a second phase in the flow drastically changes the structure of the turbulence making the problem significantly more complex and accessible to detailed simulations only much more recently. An important current limitation is that a mathematical fluid is often used. In particular, the mass density ratio between the fluid and gas phase is often set to approximately 10 since high mass density ratios are notoriously challenging from a numerical standpoint. The method developed in the present work attempts to take a step forward in the direction of DNS of turbulent channel flows loaded with thousands of bubbles and mass density ratios closer to a real air-water system at atmospheric pressure. In order to achieve this, efficiency, parallel scalability and accuracy are essential. A careful validation of the numerical method employed has been carried out establishing the reliability of the method which was subsequently used to investigate the flow statistics at higher mass density ratios of up to 100 and different bubble sizes.