Wave transformation and air entrainment by a forced plunging liquid jet*

Plunging liquid jets are ubiquitous in real-world flows, ranging from breaking waves to foundries. These jets are known to be important to mass exchange between the liquid and air, through their ability to entrain air within the liquid bulk. In the case of circular plunging jets, these entrainment events historically have been understood to occur with either very high velocities, or very large jet disturbances. Here, harmonic disturbances are imposed upon a circular plunging jet that is otherwise undisturbed, to determine the influence of such disturbances on air entrainment. This jet is found to entrain air at a low plunge velocity with small disturbances, the mechanism for entrainment being an interaction between subharmonic surface waves and the submerged jet flow. These surface waves appear due to harmonic forcing of the jet, a physical mechanism previously observed with surface-piercing bodies that lacked the submerged flow driven by the plunging jet. Experimental results concerning the onset of these waves, the inception characteristics and volume of their air entrainment, and their interactions with the jet flow will be presented, as well as comparisons to observations and measurements from the literature.