Dynamics of large droplets in turbulent channel flow

In this work the behaviour of large deformable droplets dispersed in a wall bounded turbulent flow has been investigated focusing on the role of the droplets deformability. A simplified physical problem has been considered: density differences and viscosity differences between the two fluids have been neglected while surface tension effects and coalescence/breakup events have been considered. A wide range of Weber numbers We (ratio between inertial forces and surface tension) have been investigated at moderate friction Reynolds numbers Re_\tau = 100 \div 150. Under these assumptions the problem represents an archetypal model of an industrial turbulent multiphase flow in which only hydrodynamics and capillary effects are considered. The problem has been studied applying a hierarchical decomposition: first the momentum transfer at the interface of a single large deformable non-breaking droplet in turbulence has been investigated, then the analysis has been extended to a swarm of large droplets: effects on the turbulent wall-drag and the coalescence rate have been measured. The results show flow field modifications in the vicinity of the droplet interface: the flow field deflections at the interface are smaller the larger is the interface deformability. As a consequence also an increment of the local shear stress is observed when decreasing the droplet deformability (namely, decreasing We). The role of deformability is central in the wall drag modifications produced by a large number of droplets; droplets with large Weber number can transport more efficiently small velocities from the near wall regions towards the high velocity regions of the channel center and vice-versa. The deformability is also a leading parameter in determining the coalescence rate and the possibility of breakup of the droplets swarm: when We<1 (inertia smaller than surface tension), the coalescence rate is almost universal, while different coalescence rates and breakup phenomena are observed when We>1 (inertial larger than surface tension). As a conclusion, large droplets in wall bounded turbulence can produce modifications of the flow field depending on their deformability. Significant Drag Enhancement (DE) is registered when the Weber number is smaller than a critical value (We<We_c), in particular the smaller is We, the larger is DE. Increasing We over a critical value We_c, no effects are observed on the flow. The coalescence events appear to be decorrelated from the DE: a universal coalescence rate is observed up to We>1, while We_c<1. This indicates that the wall drag modifications are weakly dependent on the droplet diameters d when their size is comparable to the channel height