Dynamics of bubbles across scales

Doctoral thesis, 2023

This thesis presents numerical investigations of bubbly flow phenomena across a wide range of relevant spatial and temporal scales. The aim is to increase our understanding of a great variety of underlying phenomena and to facilitate improved predictions of bubbly flows at all relevant scales. The investigations start at small spatial scales (size of individual bubbles and below). We focus on the evolution of vapour bubbles by formulating a multiphase Direct Numerical Simulation (DNS) framework and a computationally inexpensive 1D framework, which both consider phase change- and thermal effects. These frameworks are used to study laser-induced thermocavitation bubbles that are a part of a promising technology to achieve good control of the properties of the formed crystals in the crystallisation process. Our findings identify plausible mechanisms that induce crystallisation and give guidelines for selecting suitable system parameters to maintain and control the crystallisation process. We continue to larger scales by focusing on the dynamics of individual rising bubbles. An efficient multiscale methodology is developed in an Eulerian-Lagrangian framework that predicts the liquid-phase fluctuations experienced by a bubble rising in a turbulent flow field. The dynamics and deformation of the bubble due to the liquid-phase fluctuations are resolved using a multiphase DNS framework together with a formulated Moving Reference Frame (MRF) technique. This multiscale approach is useful for studying numerous small-scale processes where bubbles are smaller than the Kolmogorov scales and can be used for bubbles, droplets or particles in both laminar and turbulent flows. We use the developed DNS framework with the MRF to study the lift force acting on deformable bubbles in steady shear flows. We formulate a theoretical framework and support it with DNS to provide a comprehensive explanation for the several identified mechanisms behind the lift force. The findings also elucidate the influence of the shear rate and governing parameters on the lift force. Finally, we study, using DNS, the dynamics and mixing properties of bubbly flows at large spatial scales (size of the entire system). We extract and analyse the dynamics and statistics of passive scalars involving O(10-100) bubbles in periodic domains. The results show a significant influence of the bubble-induced turbulence on the scalar spectra and elucidate the influence of the governing parameters on the scalar dynamics and mixing properties.