An Experimental Investigation on the Air Entrainment by Plunging Jets

Abstract

The air entrained by the impact of jets ejected by plunging breakers was studied both theoretically and experimentally using laboratory plunging jets which are in horizontal relative motion with respect to the free surface. In order to investigate the influence of the horizontal translation on the air entrainment, a vertical, circular, laminar jet that plunges continuously onto a receiving water pool was utilized. Three different air entrainment regimes were identified for different values of the non-dimensional parameters that control the process: the Fr number based on the translation velocity and jet diameter, and the ratio between the jet translation velocity and the jet impact velocity. The underwater flow produced by this jet was further investigated by marking the jet water with small particles and recording it with a high-speed camera. These experiments reveal the existence of vortical structures resulted from the shear between the incoming jet water and the pool water that play an important role in the air entrainment process.

As a second degree of approximation to the wave problem, a planar, translating water jet that suddenly impacts on the pool free surface was investigated. The inertia of the impacting jet was observed to create two open air craters at either side of the impact site which are driven into the pool water and remain attached to the underwater jet. Simple scalling arguments based on quasi-steady potential flow theory were utilized to predict the underwater jet tip velocity and the evolving shapes of the open craters. It was found that the dynamics of the different regions of the crater walls was dominated by either inertial or gravitational effects depending on the local radius of curvature, the local inclination of the crater wall and the velocity of the particles parallel to the wall given in a reference frame fixed with the jet tip. Far from the jet tip, the gravitational effects are dominant. In these regions, the hydrostatic pressure forces the crater walls to move towards the jet with a deformation velocity that increases with the square root of depth.