Ascending motion of drops in stratified fluids

In this paper, we examine spatio-temporal evolution of free moving drops in a vertically stratified fluid and study the role of stratification on the rise velocity of a single drop, two drops and a swarm of rising drops. Utilizing fully resolved three-dimensional Direct Numerical Simulations (DNS), we discuss the effects of density stratification on the interaction of two drops and microstructure of swarm of drops. We considered nearly spherical drops in our simulations. To study the drafting-kissing-tumbling (DKT) process in a stably stratified fluid, we carried out simulations with different initial angles of approach and found that for the side-by-side configuration, the drops separate from each other in both weakly stratified and homogenous fluids but stratification suppresses the rise velocity of the drops. For the case of multiple drops, we found that the time-averaged Reynolds number of multiple drops in a linearly stratified fluid is smaller than that of a homogenous fluid. After the initial transition period, the drops in the stratified fluid rise less than the ones in the homogeneous case.