Mixing of thermally stratified fluids by injecting a series of vortex rings. A numerical simulation

The formation and transport of vortex rings in a thermally stratified fluid was studied numerically. A control-volume based finite difference method was used. The formation of vortex rings was simulated by periodically injecting a quantity of the fluid through a tube centrally located near the free surface in a tank initially occupied by stably stratified fluid. Buoyancy effects were examined. The mixing behaviour in the tank due to the formation and motion of the vortex rings were studied. The results showed that the ability of a buoyant vortex ring to penetrate into a tank with a step change in density is a function of the Richardson number Ri. For low values of Ri the buoyant vortex rings can penetrate the high density fluid causing significant mixing. However, For Ri > 0.1, the vortex rings cannot penetrate the high density fluid and mixing can only occur at the interface. The results obtained in this work can be useful in the design and analysis of vortex ring mixers.