Understanding multiphase flows through multi-scale numerical simulations: an insight into bubbly Taylor-Couette flow

Numerical simulations of turbulent flows have been gaining immense popularity over the last few decades owing to the availability of mature solution algorithms, powerful multi-core super computers and efficient visualisation tools. This, however, does not apply yet to multiphase turbulent flows in which the carrier fluid is laden with particles, drops or bubbles (for example ash dispersed into the atmosphere by a volcanic eruption, turbulent mixture of water and entrapped air bubbles, clouds etc.). The primary cause behind the gap between advances in simulations of single phase and multiphase flows is that in addition to the scale separation inherent to turbulence itself, the dispersed bubbles, drops and particles bring into play new time and length scales into the flow, and this makes turbulent multiphase flows an exciting yet challenging field of study. In this talk, I will discuss how this problem can be tackled using multi-scale numerical simulations and how we can take a step forward in studying new multiphase flow regimes. In particular, we will look at the simple yet physics-rich system of two-phase Taylor-Couette flow which comprises of bubbles or drops dispersed into a carrier fluid. The carrier fluid is confined and driven using two co-axial independently rotating cylinders. We will also look at how the bubbles and drops play a crucial role in modifying the underlying turbulence in the carrier fluid and in turn affect the overall drag or skin-friction on the rotating cylinders.