Cavitation microstreaming is a well-known phenomenon; however, few flow visualizations or measurements of the velocity fields have been conducted. In this paper micro-PIV (particle image velocimetry) measurements and streak photography were used to study the flow field around a single and two oscillating bubbles resting on a solid boundary. The mode of oscillation of the bubble was also measured in terms of the variation in the radius of the bubble and the movement of the bubble's centroid so that the streaming flow field could be accurately related to the bubble's oscillatory motion. The mode of oscillation was found to vary primarily with the applied acoustic frequency. Several modes of oscillation were investigated, including translating modes where the bubble's centroid moves along either a single axis, an elliptical orbit or a circular orbit. The flow field resulting from these oscillation modes contains closed streamlines representing vortical regions in the vicinity of the bubble. The translating modes were observed to occur in sequential order with the acoustic excitation frequency, changing from a translation along a single axis, to an elliptical orbit and finally to a circular orbit, or vice versa. Following this sequence, there is a corresponding transformation of the streaming pattern from a symmetrical flow structure containing four vortices to a circular vortex centred on the bubble. Despite some inconsistencies, there is general agreement between these streaming patterns and those found in existing theoretical models. Volume and shape mode oscillations of single bubbles as well as several different cases of multiple bubbles simultaneously oscillating with the same frequency and phase were also investigated and show a rich variety of streaming patterns.

A series of experimental studies have been made of the fluid behaviour in a completely filled, precessing, right circular cylinder. The tank was spun about its axis of symmetry and subjected to a forced precession at various excitation frequencies ω, nutation angles θ and at various Ekman numbers. This forcing excites a subset of the modes, called inertia waves, that are made possible by the Coriolis force that arises in a spinning environment. In these experiments, the fluid flow breakdown phenomena are investigated. Here the fluid, when forced near a resonant frequency, exhibits a transition to disordered or turbulent flow. This paper presents a categorization of some of the breakdown regimes, of which the ‘resonant collapses’ (McEwan 1970) are the most catastrophic members. The studies reported in this paper used entirely visual observations and measurements. The experimental observations employed a visualization technique that gave no information on fluid velocities, but provided an excellent picture of the flow structure. Quantitative data were extracted in the form of the time for the breakdown to occur. The breakdown phenomena, while readily produced over a large region of parameter space, are complex and varied. The observations show that our system is extraordinarily rich, exhibiting, for example, recurrent breakdowns which may be explained in terms of chaotic intermittency. A detailed description of some of the different breakdown regimes indicates that no single model will explain the behaviour throughout parameter space. This research is motivated by the instability problems of spinning spacecraft containing liquid fuels.

A passive acoustic method of detecting breaking waves of different scales has been developed. The method also showed promise for measuring breaking severity. Sounds were measured by a subsurface hydrophone in various wind and wave states. A video record of the surface was made simultaneously. Individual sound pulses corresponding to the many individual bubble formations during wave-breaking events typically last only a few tens of milliseconds. Each time a sound-level threshold was exceeded, the acoustic signal was captured over a brief window typical of a bubble formation pulse, registering one count. Each pulse was also analyzed to determine the likely bubble size generating the pulse. Using the time series of counts and visual observations of the video record, the sound-level threshold that detected bubble formations at a rate optimally discriminating between breaking and nonbreaking waves was determined by a classification-accuracy analysis. This diagnosis of breaking waves was found to be approximately 70%–75% accurate once the optimum threshold had been determined. The method was then used for detailed analysis of wave-breaking properties across the spectrum. When applied to real field data, a breaking probability distribution could be obtained. This is the rate of occurrence of wave-breaking events at different wave scales. With support from a separate, laboratory experiment, the estimated bubble size is argued to be dependent on the severity of wave breaking and thus to provide information on the energy loss due to the breaking at the measured spectral frequencies. A combination of the breaking probability distribution and the bubble size could lead to direct estimates of spectral distribution of wave dissipation.