Mass transfer enhancement produced by laser induced cavitation

A microelectrode is used to measure the mass transfer perturbation and characteristics during the growth and subsequent collapse of a single bubble (which, following its initial expansion, achieved a maximum radius, Rm, of not, vert, similar500–1000 ?m). This mass transfer enhancement was associated with the forced convection, driven by bubble motion, as the result of a single cavitation event generated by a laser pulse beneath a 25 ?m diameter Au microelectrode. Evidence for bubble growth and rebound is gained from the electrochemical and acoustic measurements. This is supported with high-speed video footage of the events generated. A threshold for the formation of large cavitation bubbles in electrolyte solutions is suggested

High resolution bubble sizing through detection of the subharmonic response with a two frequency excitation technique

Sizing bubbles in fluid using a two-frequency excitation technique is not prone to the same drawbacks of some other sizing methods—it has a global maximum at the bubble resonance frequency and allows good spatial resolution. The bubble is insonated with a high fixed imaging signal and a variable pumping signal tuned to the resonant frequency of the bubble, which are coupled at resonance by the high-amplitude oscillation of the bubble wall, with the formation of sum-and-difference terms. This paper examines both the resonance and off-resonance behavior of such combination frequency signals. A coupling of the subharmonic bubble response with the imaging frequency is shown to be a much more accurate and unambiguous detector of the bubble resonance than couplings involving the fundamental resonance. The characteristics of this subharmonic signal are examined using an automated sizing method, and the dependence of the response on the pumping signal amplitude and the frequency step size between two successive pumping frequencies is examined. The location of a definite subharmonic threshold is reported and quantified both for single bubbles held on a wire and for free rising bubbles moving through the focus of the transducers. This amplitude is found to be orders of magnitude lower than that predicted by traditional volumetric pulsation models, but agrees very closely with the theoretical onset of surface wave

An activated fluid stream – new techniques for cold water cleaning

Electrochemical, acoustic and imaging techniques are used to characterise surface cleaning with particular emphasis on the understanding of the key phenomena relevant to surface cleaning. A range of novel techniques designed to enhance and monitor the effective cleaning of a solid/liquid interface is presented. Among the techniques presented, mass transfer of material to a sensor embedded in a surface is demonstrated to be useful in the further exploration of ultrasonic cleaning of high aspect ratio micropores. In addition the effect of micropore size on the cleaning efficacy is demonstrated. The design and performance of a new cleaning system reliant on the activation of bubbles within a free flowing stream is presented. This device utilised acoustic activation of bubbles within the stream and at a variety of substrates. Finally, a controlled bubble swarm is generated in the stream using electrolysis, and its effect on both acoustic output and cleaning performance are compared to the case when no bubbles are added. This will demonstrate the active role that the electrochemically generated bubble swarm can have in extending the spatial zone over which cleaning is achieved

Group behavioural responses of cyprinids to artificial acoustic stimuli: implications for fisheries management

Physical screens are considered a necessary mitigation strategy, restricting access to hazardous areas (e.g. turbine intakes or extraction points) and guiding fish to safer preferred routes. Whilst this precautionary approach is considered fit for purpose, fish interactions at such interfaces have been observed to cause stress, injury and mortalities where high sweeping approach velocities exist. Alternative or collaborative systems may allow for improvements where effectiveness of physical screens alone are limited (e.g. repelling small sized larval/ juvenile stages). A number of acoustic guidance systems have been deployed to date, however efficiencies of such devices are measured using relatively simplistic metrics (e.g.percentage deflection or attraction). Results are highly variable, with some systems observing little to no deflection. Current approaches testing efficiencies are based on methods which provide limited information surrounding the wild behaviour of migratory fish. Furthermore, many studies investigating fish bioacoustics pay limited attention to the entirety of the acoustic field to which fish are exposed. The auditory system is exceptionally important to most aquatic species due to its information provisioning, however various challenges need addressed to extrapolate useful informative data which could develop working behavioural deterrents. Life-history strategies, life-stages, species specific hearing capabilities (i.e. directivity index, sensitivity, and critical ratio), and the propensity for a sound to elicit a repeatedly effective behavioural response (i.e. signal detection theory and effects of habituation) are all areas that require thorough investigation. Utilising model cyprinid species (in possession of morphological hearing specialisations), groups of fish were video recorded under controlled conditions and exposed to differing acoustic stimuli. Behaviours were quantified and analysed, alongside appropriate mapping of tested acoustic fields. This talk summarises past and ongoing experiments of a PhD project investigating group behavioural responses to sound. Results may better inform fisheries engineering design of acoustic behavioural deterrents for conservation and control purposes

The detection of tethered and rising bubbles using multiple acoustic techniques

There exists a range of acoustic techniques for characterizing bubble populations within liquids. Each technique has limitations, and complete characterization of a population requires the sequential or simultaneous use of several, so that the limitations of each find compensation in the others. Here, nine techniques are deployed using one experimental rig, and compared to determine how accurately and rapidly they can characterize given bubble populations. These are, specifically (i) two stationary bubbles attached to a wire; and (ii) injected, rising bubble

Asymmetric transfer of CO2 across a broken sea surface

Most estimates of the climatically-important transfer of atmospheric gases into, and out of, the ocean assume that the ocean surface is unbroken by breaking waves. However the trapping of bubbles of atmospheric gases in the ocean by breaking waves introduces an asymmetry in this flux. This asymmetry occurs as a bias towards injecting gas into the ocean where it dissolves, and against the evasion/exsolution of previously-dissolved gas coming out of solution from the oceans and eventually reaching the atmosphere. Here we use at-sea measurements and modelling of the bubble clouds beneath the ocean surface to show that the numbers of large bubbles found metres below the sea surface in high winds are sufficient to drive a large and asymmetric flux of carbon dioxide. Our results imply a much larger asymmetry for carbon dioxide than previously proposed. This asymmetry contradicts an assumption inherent in most existing estimates of ocean-atmosphere gas transfer. The geochemical and climate implications include an enhanced invasion of carbon dioxide into the stormy temperate and polar seas

Anthropogenic sources of underwater sound can modify how sediment-dwelling invertebrates mediate ecosystem properties

Coastal and shelf environments support high levels of biodiversity that are vital in mediating ecosystem processes, but they are also subject to noise associated with mounting levels of offshore human activity. This has the potential to alter the way in which species interact with their environment, compromising the mediation of important ecosystem properties. Here, we show that exposure to underwater broadband sound fields that resemble offshore shipping and construction activity can alter sediment-dwelling invertebrate contributions to fluid and particle transport - key processes in mediating benthic nutrient cycling. Despite high levels of intra-specific variability in physiological response, we find that changes in the behaviour of some functionally important species can be dependent on the class of broadband sound (continuous or impulsive). Our study provides evidence that exposing coastal environments to anthropogenic sound fields is likely to have much wider ecosystem consequences than are presently acknowledge