1,720,995 research outputs found
Acoustic and photographic studies of injected bubbles
No full textWhen gas bubbles are injected into a liquid, their behaviour can be markedly different from the single, isolated, spherical oscillating bubble, since shape oscillations can distort the bubble. In addition, the process of bubble formation can lead to multiple excitations of the bubble oscillations, for example when the newly formed bubble contacts and absorbs the bubble being formed at the nozzle. This gives rise to corresponding features in the acoustic output. In this study the photographic and acoustic data from such events are correlated. This undergraduate-level experiment provides useful data in a field of current interes
High-speed photography of transient excitation
No full textTransient excitation can cause the unstable growth and collapse of bubble nuclei, as well as enhancing the stable cavitation of larger bubbles. These unstable collapses have been photographed at 8000 f.p.s., the resulting pictures agreeing well with predicted numerical solutions. These cavities, unlike those normally studied with high-speed photography, are generated by purely acoustic methods, and so the events are as would occur during in situ acoustic cavitation
Acoustic bubble sizing by the combination of subharmonic emissions with an imaging frequency
No full textThe non-linear response of a bubble to an acoustic field results in the scattering of sum- and difference-frequencies when two frequencies are incident. This has in the past been used to size bubbles. since it is only when the pump frequency, Wp, is near the bubble resonance that the non-linear effects are observed. In addition, the subharmonic frequency emitted by the bubble can combine with the imaging frequency, Wi, to produce scattered signals at Wi ± (Wp /2). In this study, the ability of these signals to accurately size and resolve bubbles, both singly and in arrays, has been studied
Studies of non-linear bubble oscillations in a simulated acoustic field
No full textGas bubbles in liquids are capable of forced oscillations in a sound field Photographic study of these non-linear oscillations is difficult. It is technically most feasible at low acoustic frequencies, where lower framing rates are required, and where the resonance bubble size is larger. However, the acoustic pressures required to elicit a non-linear response from the bubble might, if generated at audible frequencies, be damaging to the hearing. This paper outlines an experimental solution to the problem, whereby a volume of liquid containing bubbles is vibrated vertically The oscillating pressure in the water mimics an acoustic field at the vibration frequency and the bubble responds to it as such The apparatus has minimal audible emissions, yet the oscillating pressure is of such an amplitude as to generate highly non-linear oscillations in the bubble. The experiment provides a good example of forced oscillation, as well as illustrating a subtle solution to a difficult experimental problem Radius-time data from a given bubble were obtained using high-speed and stroboscopic photography, and were compared with the numerical predictions obtained from the Rayleigh-Plesset equatio
Cavitation luminescence from flow over a hydrofoil in a cavitation tunnel
No full textThis paper describes a photon-counting study of the cavitation luminescence produced by flow over a hydrofoil. This has previously been identified in water saturated with xenon. The four objectives of this study are: to determine whether luminescence can be obtained using air-saturated water; to quantify this emission, if it is present, as a function of flow parameters; to determine whether the photon arrivals occur with random timing, or in ‘bursts’; to put limits on the rates associated with any bursts. The flow experiments were performed in a cavitation tunnel capable of achieving flow velocities of up to about 50 m s-1 in the test section. The experimental hydrofoil was a NACA 009 blade. Parameters varied were the flow velocity, the incident angle of the hydrofoil and the cavitation index. The results show that significant photon counts are recorded when leading-edge cavitation takes place and U-shaped vortices (cavities) are shed from the main cavity. The photon count increases dramatically as the flow velocity increases or the cavitation index is reduced. Departures from a Poisson distribution in the arrival times of photons at the detector suggest the presence of ‘bursts’. These may be related to the way vortices are shed from the main cavity. Limits are inferred on the detection rates associated with bursts
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Beta-delta phase change during dropweight impact on cyclotetramethylene-tetranitroamine
No full textThe secondary explosive cyclotetramethylene-tetranitroamine (HMX) exists in a variety of crystal structures; the most widely used being the ?-phase which is stable at room temperature and pressure. On heating, a more impact sensitive form (? phase) is produced. The nonlinear optical technique of second harmonic generation (SHG) can be used as a probe of phase since ?-phase HMX generates a second harmonic at 532?nm when 1064?nm laser light is incident upon it. We present high-speed photography of SHG in HMX samples during dropweight impact and show that this technique can provide good spatial information and time resolution. We find evidence for small areas of ?-phase HMX appearing in the period from 13??s before ignition to 10??s afterwards, demonstrating that the heating on impact is sufficient to overcome the loading conditions and cause the phase change.<br/
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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