Wave Transformation And Air Entrainment By A Harmonically Forced Plunging Jet

It is known that air may be entrained by a plunging jet even when that jet is relatively small and slow, so long as it has significant disturbances. In the current work, harmonic disturbances of controlled frequency and amplitude are used to produce air entrainment. At sufficient frequency and amplitude, the harmonic jet disturbances cause azimuthal subharmonic waves on the plunge pool surface to appear, oscillating at half of the forcing frequency. These subharmonic waves interact with the jet flow to entrain air at lower disturbance amplitudes and jet velocities than previously documented. The dependence of the wave shape and inception criteria on jet parameters, namely frequency and diameter, is investigated, giving results that agree well with past investigations of azimuthal waves driven by surface-piercing bodies. The number of azimuthal wave lobes may be understood as the ratio of the perimeter length of the forced jet and the wavelength of the resulting azimuthal water waves, given by nwaves = Ckf/2rj, where C is the ratio of the wave and jet diameters. In past experiments, C was approximately 1.2, but with much smaller liquid jets, we found that C can be as high as 1.4-2. The volumetric flow rate of entrained air was measured directly, by way of a capture hood, enabling investigation of air entrainment behavior, and it is concluded that the resultant air flow rate is dependent on several factors, including jet frequency, azimuthal wave mode number, jet velocity, and the jet disturbance amplitude. The influence of several of these factors was determined by the use of an effective bubble size parameter, hypothesizing that every wave trough during every wave cycle was equally likely to entrain air, an assumption that appears borne out in fact. This effective bubble size, free of the influences of forcing frequency and wave mode number, was found to depend on the forcing intensity in an approximately linear fashion, at least for forcing amplitudes close to those required for entrainment inception. Jet and wave measurements were conducted using high speed videography, enabling quantitative measurements of the nearjet wave amplitude, as well as the size and spectral nature of the jet disturbances.