Browsing by Author "Prosperetti, Andrea"
Now showing 1 - 20 of 117
- Results Per Page
- Sort Options
Item A fully resolved numerical simulation of turbulent flow past one or several spherical particles(The Physics of Fluids, 1/31/2012) Botto, L.; Prosperetti, AndreaThe flow past one or nine spheres arranged in a plane lattice held fixed in a stream of decaying homogeneous isotropic turbulence is studied by means of fully resolved Navier-Stokes simulations. The particle radius is 3–5 times the Kolmogorov length and about 1/3 of the integral length scale. The mean particle Reynolds number is 80 and the turbulence intensity 17% and 33%. Several features of the flow are described: the mean and fluctuating dissipation and its spatial distribution, the mean and fluctuating hydrodynamic forces on the spheres, stimulated vortex shedding, and others. A special attention is paid to the relation between the work done on the fluid by the particles (in the reference frame of the former) and the total dissipation. It is shown that these quantities, which are assumed to balance in many point-particle models, can actually be very different when inertial effects are important.Item A general derivation of the subharmonic threshold for non-linear bubble oscillations(The Journal of the Acoustical Society of America, 6/5/2013) Prosperetti, AndreaThe paper describes an approximate but rather general derivation of the acoustic threshold for a subharmonic component to be possible in the sound scattered by an insonified gas bubble. The general result is illustrated with several specific models for the mechanical behavior of the surface coating of bubbles used as acoustic contrast agents. The approximate results are found to be in satisfactory agreement with fully non-linear numerical results in the literature. The amplitude of the first harmonic is also found by the same method. A fundamental feature identified by the analysis is that the subharmonic threshold can be considerably lowered with respect to that of an uncoated free bubble if the mechanical response of the coating varies rapidly in the neighborhood of certain specific values of the bubble radius, e.g., because of buckling.Item A Generalization of the Rayleigh-Plesset Equation of Bubble Dynamics(The Physics of Fluids, 1982-03) Prosperetti, AndreaThe classical Rayleigh?Plesset equation of spherical bubble dynamics in an incompressible liquid is generalized to include the non?Newtonian behavior of the liquid and mass exchange processes at the bubble interface.Item A new mechanism for sonoluminescence(The Journal of the Acoustical Society of America, 1997-04) Prosperetti, AndreaIt is argued that a pulsating acoustically levitated bubble cannot possibly maintain a spherical shape. A jet forms during compression, and the sound amplitude such that the jet first strikes the other side of the bubble with sufficient energy is hypothesized to be the threshold for sonoluminescence. It is proposed that the connection between jet impact and light emission is a fracturing of the liquid that cannot flow during the extremely short time scale over which pressure is applied. With this hypothesis, sonoluminescence would therefore be a manifestation of the non-Newtonian nature of water and other simple liquids when stressed with sufficient intensity and rapidity.Item A Nonlinear Model of Thermoacoustic Devices(The Journal of the Acoustical Society of America, 2002-10) Karpov, Sergey; Prosperetti, AndreaThis paper presents a nonlinear, time-domain model of thermoacoustic devices based on cross-sectional averaged equations. Heat transfer perpendicular to the device axis—which lies at the core of thermoacoustic effects—is modeled in a novel and more realistic way. Heat conduction in the solid surfaces surrounding the fluid medium is included. Contrary to the previous versions of this model [Watanabe et al., J. Acoust. Soc. Am. 102, 3484–3496 (1997)], the present version does not require artificial damping and is numerically robust. The model performance is illustrated on several examples: a prime mover, an externally driven thermoacoustic refrigerator, and a combined prime mover/refrigerator system.Item A note on the effective slip properties for microchannel flows with ultra-hydrophobic surfaces(The Physics of Fluids, 4/23/2007) Sbragaglia, M.; Prosperetti, AndreaA type of superhydrophobic surface consists of a solid plane boundary with an array of grooves which, due to the effect of surface tension, prevent a complete wetting of the wall. The effect is greatest when the grooves are aligned with the flow. The pressure difference between the liquid and the gas in the grooves causes a curvature of the liquid surface resisted by surface tension. The effects of this surface deformation are studied in this paper. The corrections to the effective slip length produced by the curvature are analyzed theoretically and a comparison with available data and related mathematical models is presented.Item A simple analytic approximation to the Rayleigh-B ?enard stability threshold(The Physics of Fluids, 12/7/2011) Prosperetti, AndreaThe Rayleigh-Bénard linear stability problem is solved by means of a Fourier series expansion. It is found that truncating the series to just the first term gives an excellent explicit approximation to the marginal stability relation between the Rayleigh number and the wave number of the perturbation. Where the error can be compared with published exact results, it is found not to exceed a few percent over the entire wave number range. Several cases with no-slip boundaries of equal or unequal thermal conductivities are considered explicitly.Item A Simplified Model for Linear and Nonlinear Processes in Thermoacoustic Prime Movers. Part I. Model and linear theory(The Journal of the Acoustical Society of America, 1997-12) Watanabe, M.; Prosperetti, Andrea; Yuan, H.A simplified quasi-one-dimensional model of thermoacoustic devices is formulated by averaging the conservation equations over the cross section. Heat transfer and drag effects are introduced by means of suitable coefficients. While the primary motivation for this work is the development of a model approximately valid in the nonlinear regime, the focus of this paper is the proper formulation of the transfer coefficients and the analysis of the linear problem. The accuracy of the model is demonstrated by comparison with existing more precise theories and data. Examples of devices with variable cross section demonstrate the flexibility of the approach.Item A Simplified Model for Linear and Nonlinear Processes in Thermoacoustic Prime Movers. Part II. Nonlinear Oscillations(The Journal of the Acoustical Society of America, 1997-12) Yuan, H.; Karpov, Sergey; Prosperetti, AndreaThe simplified quasi-one-dimensional model of thermoacoustic devices formulated in Part I [Watanabe et al., J. Acoust. Soc. Am. 102, 3484–3496 (1997)] is studied in the nonlinear regime. A suitable numerical method is described which is able to deal with the steep waveforms that develop in the system without inducing spurious oscillations, appreciable numerical damping, or numerical diffusion. The results are compared with some experimental ones available in the literature. Several of the observed phenomena are reproduced by the model. Quantitative agreement is also reasonable when allowance is made for likely temperature nonuniformities across the heat exchangers.Item A theoretical study of sonoluminescence(The Journal of the Acoustical Society of America, 1993-07) Kamath, Vinod; Prosperetti, Andrea; Egolfopoulos, F.N.The production of OH radicals by dissociation of water vapor in oscillating argon bubbles is studied theoretically to examine a possible mechanism for the emission of the 310?nm line observed in sonoluminescence experiments. Accurate models are used for the calculation of the temperature field in the gas and for the description of the associated chemical kinetics. Heat transfer between the bubble and the liquid is found to play a dominant role in the process. At the low excitation amplitudes considered, the bubble radius is also an important parameter.Item Active and Passive Acoustic Behavior of Bubble Clouds at the Ocean’s Surface(The Journal of the Acoustical Society of America, 1993-06) Prosperetti, Andrea; Lu, N.Q.; Kim, Hyun S.The emission and scattering of sound from bubble clouds is studied theoretically. It is shown that clouds having a size and air content similar to what might be expected as a consequence of the breaking of ocean waves can oscillate at frequencies as low as 100 Hz and below. Thus cloud oscillations may furnish an explanation of the substantial amount of low?frequency wind?dependent oceanic ambient noise observed experimentally. Detailed results for the backscattering from bubble clouds—particularly at low grazing angles—are also presented and shown to be largely compatible with oceanic data. Although the cloud model used here is idealized (a uniform hemispherical cloud under a plane water free?surface), it is shown that the results are relatively robust in terms of bubble size, distribution, and total air content. A similar insensitivity to cloud shape is found in a companion paper [Sarkar and Prosperetti, J. Acoust. Soc. Am. 93, XXX (1993)].Item An Investigation of the Collective Oscillations of a Bubble Cloud(The Journal of the Acoustical Society of America, 1991-02) Yoon, S.W.; Crum, Lawrence A.; Prosperetti, Andrea; Lu, N.Q.It is well known that ocean ambient noise levels in the frequency range from a few hundred hertz to several tens of kilohertz are well correlated with wind speed. A physical mechanism that could account for some of this sound generation is the production of bubble clouds by breaking waves. A simple laboratory study of the sound generated by a column of bubbles is reported here. From measurements of the various characteristics of this column, good evidence is obtained that the bubbles within the column are vibrating in a collective mode of oscillation. Based upon an assumption of collective oscillations, analytical calculations of the predicted frequency of vibration of this column as well as the dependence of this frequency on such parameters as bubble population and column geometry agree closely with the measured values. These results give evidence that the bubble plumes generated by breaking waves can be a strong source of relatively low frequency (< 1 kHz) ambient noise.Item Application of the Subharmonic Threshold to the Measurement of the Damping of Oscillating Gas Bubbles(Journal of the Acoustical Society of America, 1977-01) Prosperetti, AndreaIt is suggested that the damping affecting the forced radial oscillations of gas bubbles can be determined very accurately through the measurement of the subharmonic threshold for the oscillations. The advantage of the method is that, while the subharmonic oscillations have a large amplitude and can be easily detected, the theory can be accurately based on a perturbation approach. The theoretical basis for the subharmonic generation is also reviewed and its results illustrated for the case of an air bubble in water.Item Asymmetry-induced particle drift in a rotating flow(The Physics of Fluids, 7/11/2005) Bluemink, Johanna J.; van Nierop, Ernst A.; Luther, S.; Deen, N. G.; Magnaudet, Jacques; Prosperetti, Andrea; Lohse, DetlefWe report on an intriguing phenomenon taking place in a liquid rotating around a fixed horizontal axis. Under suitable conditions, bubbles and particles are observed to drift along the axis of rotation maintaining a constant distance from it and a constant angle of elevation above the horizontal. Absence of fore-aft symmetry of the bubble or particle shape is a prerequisite for this phenomenon. For bubbles, this requires a volume sufficiently large for surface-tension effects to be small and large deformations to be possible. Particle image velocimetry and flow visualization suggest that the wake does not play a role. The dependence on bubble radius, particle shape, liquid viscosity, and speed of rotation is investigated.Item Backscattering of Underwater Noise by Bubble Clouds(The Journal of the Acoustical Society of America, 1993-06) Sarkar, Kausik; Prosperetti, AndreaThis paper is a continuation of an earlier one [Prosperetti et al., J. Acoust. Soc. Am. 93, XXX (1993)] in which the low?frequency backscattering of sound by hemispherical bubble clouds at the ocean’s surface was studied. Here, clouds of various geometrical shapes (spheroids, spherical segments, cones, cylinders, ellipsoids) are considered and results in substantial agreement with the earlier ones and with the experiments of Chapman and Harris [J. Acoust. Soc. Am. 34, 1592–1597 (1962)] are found. The implication is that the backscattering levels are not strongly dependent on the shape of the clouds, which strengthens the earlier conclusion that bubble clouds produced by breaking waves can very well be responsible for the unexpectedly high backscattering levels observed experimentally. The accuracy of the Born approximation used by others for similar problems is also examined in the light of the exact results. Significant differences are found for gas concentrations by volume of the order of 0.01% or higher. Finally, shallow nonaxisymmetric plumes are briefly considered.Item Bubble Oscillations in the Nearly Adiabatic Limit(The Journal of the Acoustical Society of America, 10/1/1992) Kamath, Vinod; Oguz, H.N.; Prosperetti, AndreaMiksis and Ting [J. Acoust. Soc. Am. 81, 1331 (1987)] reported examples of a marked increase of the radius of an oscillating gas bubble as predicted by their nearly adiabatic model. They attributed this phenomenon to a process of rectified heat transfer into the bubble. By comparison with a more complete model which contains the nearly adiabatic one as an approximation, it is shown that the real cause of this result is instead the error inherent in the approximation. This error arises primarily from the failure of the approximation to capture the complex behavior of the gas temperature and manifests itself in a spurious growth of the mass of gas contained in the bubble. In addition to being more accurate, the more complete model is also found to be less computationally demanding than the approximate one.Item Bubble Oscillations in the Vicinity of a Nearly Plane Free Surface(The Journal of the Acoustical Society of America, 1990-05) Oguz, H.N.; Prosperetti, AndreaThe linear oscillation frequency of a bubble in the vicinity of a distorted plane free surface is calculated by a perturbation method. The approximate expression found is compared with numerical results valid for surface deformations of arbitrary magnitude. It is found that the approximate analytical result is quite good, provided that the deformation is small compared with the depth of immersion of the bubble. It is also shown that, unless the deformation of the free surface extends to distances at least of the order of an acoustic wavelength, the ‘‘image’’ bubble has the same source strength of the real bubble so that a dipolar acoustic emission can be expected in spite of the deformation of the surface.Item Bubble plumes in a stratified environment: Source parameters, intrusion height and neutral height(Physical Review Fluids, 10/27/2017) Chu, Shigan; Prosperetti, AndreaA cross-sectionally averaged model is used to study several aspects of the physics of a bubble plume rising in a stratified quiescent liquid. Scaling analyses for the peel height, at which the plume momentum vanishes, and the neutral height, at which its average density equals the ambient density, are presented. Contrary to a widespread practice in the literature, it is argued that the neutral height cannot be identified with the experimentally reported intrusion height. Recognizing this difference provides an explanation of the reason why the intrusion height is found so frequently to be much above predictions and brings the theoretical results in line with observations. The mathematical model depends on three dimensionless parameters, some of which are related to the inlet conditions at the plume source. Their influence on the peel and neutral heights is illustrated by means of physical considerations, scaling analyses, and numerical results.Item Bubble-Related Ambient Noise in the Ocean(The Journal of the Acoustical Society of America, 1988-09) Prosperetti, AndreaAn analysis is presented of the mechanisms by which bubbles can generate ambient noise in the ocean and the resulting noise levels are estimated. Bubbles can be extremely efficient amplifiers of water turbulence noise up to 100–200 Hz. At higher frequencies, the Lagrangian spectral intensity of the turbulence is too poor for this mechanism to contribute. Above 1–2 kHz, however, the oscillations by which newly formed bubbles dispose of their initial energy is shown to lead to substantial noise levels. This same process cannot account for the noise in the frequency range intermediate between these two because it would require unrealistically large bubbles, with a diameter of 1 cm or more. A possible mechanism active in this intermediate range, in which relatively large levels of ambient noise are observed, is that of collective oscillations of bubble clouds. In all cases the results obtained by the formal derivations (which are based on an adaptation of Lighthill’s theory of aerodynamic noise) are substantiated by simple physical arguments. Other possible noise mechanisms in which bubbles are involved are also briefly considered.Item Bubbles(The Physics of Fluids, 4/28/2004) Prosperetti, AndreaVanitas vanitatum et omnia vanitas: bubbles are emptiness, non-liquid, a tiny cloud shielding a mathematical singularity. Born from chance, a violent and brief life ending in the union with the (nearly) infinite. But a wealth of phenomena spring forth from this nothingness: underwater noise, sonoluminescence, boiling, and many others. Some recent results on a “blinking bubble” micropump and vapor bubbles in sound fields are outlined. The last section describes Leonardo da Vinci’s observation of the non-rectlinear ascent of buoyant bubbles and justifies the name Leonardo’s paradox recently attributed to this phenomenon.