Physical mechanisms governing drag reduction in turbulent Taylor–Couette flow with finite-size deformable bubbles

Abstract

The phenomenon of drag reduction induced by injection of bubbles into a turbulent carrier fluid has been known for a long time; the governing control parameters and underlying physics is however not well understood. In this paper, we use three dimensional numerical simulations to uncover the effect of deformability of bubbles injected in a turbulent Taylor-Couette flow on the overall drag experienced by the system. We consider two different Reynolds numbers for the carrier flow, i.e. Re i = 5 × 10 3 and Re i = 2 × 10 4 ; the deformability of the bubbles is controlled through the Weber number which is varied in the range W e = 0.01 − 2.0. Our numerical simulations show that increasing the deformability of bubbles i.e., W e leads to an increase in drag reduction. We look at the different physical effects contributing to drag reduction and analyse their individual contributions with increasing bubble deformability. Profiles of local angular velocity flux show that in the presence of bubbles, turbulence is enhanced near the inner cylinder while attenuated in the bulk and near the outer cylinder. We connect the increase in drag reduction to the decrease in dissipation in the wake of highly deformed bubbles near the inner cylinder.

Bubble puzzles: From fundamentals to applications

“…(a) Drag reduction in turbulent TC flow as function of Weber number for two different Reynolds numbers: Increasing Weber number implies increasing bubble deformability which is crucial for the drag reduction. Taken from the numerical simulations of ref [220]…”

Drag reduction in boiling Taylor–Couette turbulence

“…To achieve large drag reduction in high-Re flows with relative small gas bubble volume fraction, the gas bubble deformability has been identified as one of the crucial ingredients. This view is supported by experimental and numerical studies in high Reynolds number TC flows and other turbulent canonical flows: (Merkle & Deutsch 1992;Moriguchi & Kato 2002;van den Berg et al 2005;Lu et al 2005;Shen et al 2006;van der Berg et al 2007;Murai et al 2008;van Gils et al 2013;Murai 2014;Verschoof et al 2016;Rosenberg et al 2016;Spandan et al 2018) Whether a bubble is deformable or not is determined by the corresponding Weber number which compares inertial and capillary forces, and it also influences the mobility of the bubbles. It is defined as…”

Bubbly and Buoyant Particle–Laden Turbulent Flows

“…Recent work by Spandan et al (2018) have used DNS to investigate the physical mechanisms of drag reduction in the turbulent regime (up to Reic = 2 × 10 4 ). They connected the increase in drag reduction to a decrease in the dissipation in the wake of highly deformed bubbles near the inner cylinder.…”