Multiscale Euler/Lagrange approach to simulate finite-sized solid particles and bubbles as well as numerical and experimental studies to improve the modeling of complex bubble motion

This work comprises both numerical simulations and experiments and consists of three main parts. The first part describes the development of a numerical method that expands the point-particle approximation of the classical Euler/Lagrange approach by introducing a spatially extended coupling to take the finite particle size into account. This method can be applied to dispersed multiphase flow and has been tested and validated in this work for solid particles and bubbles with a deterministic Euler/Lagrange approach. To further improve the available modeling of bubble motion and gain insights into the complex bubble motion under different flow conditions, direct numerical simulation (DNS) and experiments of isolated bubbles submitted to different flow conditions were carried out subsequently. The second part therefore comprises the DNS of less and more deformed bubbles, which have been simulated by means of a front-tracking method (Bunner & Tryggvason, 1999, 2003). In order to investigate the influence of turbulence on bubble motion, deformation, orientation and oscillation properties, a pseudo-spectral method was implemented to generate velocity fluctuations of the liquid phase. The third part describes experiments of single bubbles which either move in still water or in turbulent upward or downward flow. The developed experimental design is based on stereoscopic imaging with a high-speed CCD camera and a mirror to reconstruct the motion behavior of individual air bubbles three-dimensionally and to investigate oscillation properties. The bubble motion was evaluated by using particle tracking velocimetry (PTV) and the flow and turbulence properties of the liquid phase were measured by laser Doppler velocimetry (LDV).