Effects of inflow conditions on plunging jet dynamics

Abstract

We present an experimental study on the influence of inflow characteristics on the dynamics of plunging jets. Several nozzle configurations were tested to establish a link between upstream conditions, the development of a water jet in air, and the subsequent bubble cloud generated upon impact with the water pool. Our study concerns jets with a nozzle diameter of $7.6\mathrm{mm}$ and fall heights up to 1.6 m. Both the impact of the nozzle geometry and the upstream pipe length were evaluated, covering inflow velocities within the range $(1.2,10.4)\mathrm{m/s}$, Reynolds numbers in the range $(24\times 10^3,79\times 10^3)$ and Weber numbers in the range $(1.0,11.2)$. Complementary experimental techniques were applied to investigate several properties of the jet before and after the impact, that include optical probes, high-speed imaging and pressure sensors.

Differences between jets from a conical nozzle and a cylindrical nozzle with a sharp section change were examined. Our results show that the length of the pipe connected to the nozzle, that covers from 27 to 131 nozzle diameters, influences the jet’s breakup length, a phenomenon that increases with the pipe’s length. Moreover, this effect presents differences in terms of the nozzle properties, being stronger for the conical configuration. Furthermore, the breakup length is greater for the conical nozzle, whereas the cylindrical nozzle exhibits higher variability and more disturbances, leading to more extreme events. High-speed camera footage confirms that the jet from the cylindrical nozzle is rougher than that from the conical nozzle. Nevertheless, axial turbulence measurements do not differentiate between the two configurations, as all cases present small and similar values of turbulence intensity. Finally, the jet’s entrained airflow was analyzed in terms of penetration depth, bubble size, and mean velocity. This was achieved by means of an optical probe that can quantify the local void fraction and also the velocity of the air/water interface. We find that the conical nozzle produces a jet plume with a larger penetration depth, containing bubbles with smaller chords and velocity.

Globally, our study shows that the upstream conditions of a plunging jet are relevant for all studied quantities. Even a fully developed pipe inflow will be strongly affected by secondary currents generated at the nozzle.