1,720,966 research outputs found
The subharmonic oscillations and combination-frequency emissions from a resonant bubble: their properties and generation mechanisms
No full textSizing bubbles using a two frequency technique which examines the coupling of a resonant subharmonic oscillation to an imaging beam is not prone to some of the inaccuracies and ambiguities of other methods. This combination frequency subharmonic signal is parametric in nature, and the amplitude onset threshold has been quantified for thirty tethered air bubbles in water. This paper details work which aims to investigate the generation mechanism of a bubble's subharmonic oscillations, and presents results from theoretical consideration of three potential mechanisms: (i) The possibility that at high amplitudes a bifurcation appears in a bubble's volumetric pulsations; (ii) the potential for bubbles driven at twice their resonant frequency to pulsate with a component at their resonant frequency (which is a subharmonic of the driving field); (iii) that surface waves are responsible. The first two mechanisms are examined using a numerical solution to the Gilmore-Akulichev model, and the third using plane surface theory. Whilst all three mechanisms show the potential for generating subharmonic oscillations, the first two demonstrate onset thresholds four and three orders of magnitude higher than the experimental threshold respectively. However, the threshold predicted using surface wave theory shows very good agreement with the experimental results
The Rayleigh-like collapse of a conical bubble: simultaneous pressure and photographic records
No full textOn the relation between surface waves on a bubble and the subharmonic combination-frequency emission
No full textThe characterisation of bubbles using a two-frequency excitation technique is known to accurately detect and size certain bubble samples. This is done through the generation of a signal at Wi ± Wp/2 when the bubble is insonified by a fixed MHz imaging signal Wi and a variable pumping frequency, Wp , tuned to the bubble's resonance. Recent work has suggested that the principal mechanism for the generation of the Wi ± Wp /2 signal is linked to the onset of surface waves on the bubble's surface This paper strengthens this argument through the comparison of published experimentally measured thresholds for the Wi ±Wp /2 signal with recent theoretical models which predict the driving sound field pressure amplitudes required for the onset of surface waves on a spherical surface
Acoustic detection of gas bubbles in a pipe
No full textThis report describes an acoustic system designed to detect gas bubbles in a pipe. The system makes simultaneous use of multiple techniques for detection and sizing; these include six resonance-based indicators, as well as off-resonance scattering. The abilities of these acoustic indicators to detect, size and count bubbles are compared. It was envisaged that using several simultaneous bubble detection techniques would compensate for restrictions inherent in the isolated use of individual techniques. For example, whilst geometric scattering tends to be more successful at locating bubbles than sizing them, the converse tends to hold for techniques which exploit the bubble resonance. In the latter case, the greater the tolerance allowed in the estimation of the bubble size from the measured resonance frequency, the more approximate may be the conversion algorithm. Single bubbles tethered to a wire in the pipe, and a freely rising stream of similar size bubbles, are examined. Results measured in these two cases are presented to indicate the potential of the location and sizing techniques. The effect on bubble resonance frequencies of confining the bubble in a pipe is considered in detail. Standard interpretations of bubble resonances in terms of bubble size assume free-field conditions, and require modification for the pipe environment. The necessary corrections are presented for the frequency range below the first transverse acoustic resonance of the pip
Theory and preliminary measurements of the Rayleigh-like collapse of a conical bubble
No full textKey to the dynamics of the type of bubble collapse which is associated with such phenomena as sonoluminesence, and the emission of strong rebound pressures into the liquid, is the role of the liquid inertia. Following the initial formulation of the collapse of an empty spherical cavity, such collapses have been termed 'Rayleigh-like', and today this type of cavitation is termed 'inertial', reflecting the dominant role of the liquid inertia in the early stages of the collapse. Whilst the inertia in models where, at these early stages, spherical symmetry can be assumed can depend primarily on the liquid density, experimental control of the liquid inertia has not readily been achievable without changing the liquid density, and consequently changing other liquid properties. In this text, novel experimental apparatus is described whereby the inertia at the early stages of the collapse of a conical bubble can easily be controlled. The collapse is capable of producing sonoluminescence. The similarity between the collapse of spherical and conical bubbles is investigated analytically, and compared with experimental measurements of the pressures generated by the collaps
The Rayleigh-like collapse of a conical bubble
No full textKey to the dynamics of the type of bubble collapse which is associated with such phenomena as sonoluminescence and the emission of strong rebound pressures into the liquid is the role of the liquid inertia. Following the initial formulation of the collapse of an empty spherical cavity, such collapses have been termed "Rayleigh-like." Today this type of cavitation is termed "inertial," reflecting the dominant role of the liquid inertia in the early stages of the collapse. While the inertia in models of spherical bubble collapses depends primarily on the liquid, experimental control of the liquid inertia has not readily been achievable without changing the liquid density and, consequently, changing other liquid properties. In this paper, novel experimental apparatus is described whereby the inertia at the early stages of the collapse of a conical bubble can easily be controlled. The collapse is capable of producing luminescence. The similarity between the collapses of spherical and conical bubbles is investigated analytically, and compared with experimental measurements of the gas pressures generated by the collapse, the bubble wall speeds, and the collapse times
Comparison of the abilities of eight acoustic techniques to detect and size a single bubble
No full textThis paper details the preliminary results from the Characterisation Of Bubbles Using Simultaneous Techniques (COBUST). There are a range of acoustic techniques for characterising bubble populations within liquids. Each technique has limitations, and complete characterisation of a population requires the simultaneous use of several, so that the limitations of each find compensation in the others. Eight acoustic signals were scattered from a single bubble to determine how easily and accurately they can, individually or together, measure the bubble size
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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