LAUSR

Abstract We present new experiments of particle-laden turbulent fountains in a uniform horizontal crossflow, $u_a$, with momentum flux, $M_0$, and buoyancy flux, $B_0$. We use the ratio, $P$, of the crossflow speed to the characteristic fountain speed, $M_0^{-1/4}|B_0|^{1/2}$, and the ratio $U$, of the Stokes fall speed of the particles, $v_s$, to the characteristic fountain speed, to characterise the dynamics of a particle fountain in a crossflow. We find that the dynamics of these particle fountains can be categorised into three distinct regimes. In regime I when the fall speed of the particles is small in comparison with the characteristic fountain speed ($U\ll 1$), the particles remain well-coupled to the fountain fluid and the flow essentially behaves as a single-phase fountain in a crossflow. In the transitional regime II ($0.1< U<1$), when the fall speed of particles is comparable to the characteristic fountain speed, we observe some particles separating from the fountain fluid during the descent of the flow which leaves some fluid neutrally buoyant. As $U>1$ (regime III), we observe particles separating from the fountain as it rises from the source. We measure the average dispersal distance of the particles and the speed of the descending particles as a function of $U$ and $P$ and compare these results with models of a single-phase fountain in a crossflow. We build a regime diagram to describe the effect of $U$ and $P$ on the flow dynamics and consider our work in the context of deep-submarine volcanic eruptions.