Negatively buoyant fluid projectiles

We describe the rise-height behaviour of a finite volume saline release dispensed vertically upwards into a still fresh-water environment. The dynamics of the non-continuous release, or projectile, differ significantly from the continuous version that produces a turbulent fountain. The projectile can be characterised in terms of the release aspect ratio $L/D$ (the length $L$ of the dispensed column to the nozzle diameter $D$) and the source Froude number $Fr_0$, expressing the ratio of inertia and buoyancy. In a continuous high Reynolds number fountain $L/D \rightarrow \infty$ and the behaviour is characterised solely by $Fr_0$. We dispensed, over a time $t_d$, each release and recorded the extent of its maximum vertical propagation as a function of $L/D$ and $Fr_0 = (L/t_d)/ (g' D/2)^{1/2}$, where $g'$ denotes the reduced gravity of the fluid released. For $Fr_0 \rightarrow \infty$, the release propagates as a vortex ring with a trailing jet for $L/D > 4$. As $Fr_0$ decreases, gravitational effects limit the vertical propagation and a maximum rise height $z_m/D$ is reached. We find that the releases are sensitively dependent upon $Fr_0$ and $L/D$ and three rise height regimes, `the weak fountain regime', `the vorticity development regime' and `the forced release regime', are identified by considering rise heights and morphologies. Finally, we discuss some aspects of the transition from a non-continuous release to the continuous fountain as achieved on increasing $L/D$.