Direct numerical simulations of bubbly flows

The work presented considers direct numerical simulation (DNS) based studies of bubbly
flows in turbulent channel and pipe flows. A variety of bubble sizes are simulated by
means of Lagrangian tracking (LPT) and the volume of fluid (VOF) approaches to gain
an in-depth knowledge of the physical phenomena involved in, and provide a predictive
capability for, turbulent bubbly flows.
DNS coupled with LPT is used to investigate microbubble dynamics in four-way coupled
channel and pipe flows. Microbubbles, assumed to be non-deformable and spherical, in
channel flows show that one- and two-way coupled predictions demonstrate different
trends, with microbubbles pushed by the lift force towards the channel wall in upflow and
towards its centre in downflow. Analysis of bubble collision behaviour highlights that
binary collisions most frequently occur at very small approach angles and with low
relative approach velocities. This trend is confirmed in pipe flows, with bubble
coalescence in both geometries predicted using film drainage and energy-based models.
DNS of turbulent channel flow with large deformable bubbles is studied using the VOF
method. The motion of single bubbles is considered, with 8 mm bubbles travelling in
roughly rectilinear paths in upflow owing to their higher deformability than smaller
bubbles which move towards the walls. A method for estimating the drag coefficient is
proposed, with good agreement found with experimental data. Time-averaged liquid
turbulence statistics are evaluated to quantify bubble-induced turbulence, and bubble
clustering is analysed by simulating bubble swarms with a horizontal alignment of
colliding bubbles found.
The work reported contributes new understanding of the phenomena involved in bubbly
flows down to the smallest scales, with the VOF method, in particular, allowing highly
accurate results to be generated that improve understanding. The DNS-LPT technique,
for the foreseeable future, will remain the main predictive tool for modelling practically
relevant bubbly flows.