Browsing by Author "Oguz, H.N."
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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 Examples of Air-Entraining Flows(The Physics of Fluids, 1992-04) Oguz, H.N.; Prosperetti, Andrea; Lezzi, AdrianoFour examples of air?entraining flows at the free surface of a liquid are briefly considered: (a) the transient impact of a jet, (b) the application of an excess pressure, (c) two counter?rotating vortices below the surface, and (d) a disturbance on a vortex sheet.Item Mechanism of air entrainment by a disturbed liquid jet(The Physics of Fluids, 2000-07) Ohl, C.D.; Oguz, H.N.; Prosperetti, AndreaIt was shown in recent work that the crests of surface disturbances on a falling jet are a powerful agent for air entrainment at the free surface of a liquid pool. The paper explores the opposite case in which the jet is disturbed so as to form an axisymmetric trough, rather than a crest. It is found that no air is entrained in this case. The paper concludes with some considerations on the validity of a recently proposed model for air entrainment.Item Sound Emissions by a Laboratory Bubble Cloud(The Journal of the Acoustical Society of America, 1994-06) Nicholas, M.; Roy, R.A.; Crum, Lawrence A.; Oguz, H.N.; Prosperetti, AndreaThis paper presents the results obtained from a detailed study of the sound field within and around a cylindrical column of bubbles generated at the center of an experimental water tank. The bubbles were produced by forcing air through a circular array of hypodermic needles. As they separated from the needles the ‘‘birthing wails’’ produced were found to excite the column into normal modes of oscillation whose spatial pressure?amplitude distribution could be tracked in the vertical and horizontal directions. The frequencies of vibration were predicted from theoretical calculations based on a collective oscillation model and showed close agreement with the experimentally measured values. On the basis of a model of the column excitation, absolute sound levels were analytically calculated with results again in agreement with the measured values. These findings provide considerable new evidence to support the notion that bubble plumes can be a major source of underwater sound around frequencies of a few hundred hertz.Item The Action of Pressure-Radiation Forces on Pulsating Vapor Bubbles(The Physics of Fluids, 2001-05) Hao, Yue; Oguz, H.N.; Prosperetti, AndreaThe action of pressure-radiation (or Bjerknes) forces on gas bubbles is well understood. This paper studies the analogous phenomenon for vapor bubbles, about which much less is known. A possible practical application is the removal of boiling bubbles from the neighborhood of a heated surface in the case of a downward facing surface or in the absence of gravity. For this reason, the case of a bubble near a plane rigid surface is considered in detail. It is shown that, when the acoustic wave fronts are parallel to the surface, the bubble remains trapped due to secondary Bjerknes force caused by an “image bubble.” When the wave fronts are perpendicular to the surface, on the other hand, the bubble can be made to slide laterally.Item The hydrodynamic interaction of two slowly evaporating spheres(The Physics of Fluids, 1989-10) Oguz, H.N.; Prosperetti, Andrea; Antonelli, DarioThe Stokes flow induced by the slow evaporation or condensation of two spheres is studied. The phase?change velocity is prescribed and uniform over the surfaces of the spheres. Exact expressions are obtained for the streamfunction and the drag forces. Simpler expressions applicable to a variety of limit cases (distant spheres, a source and a sphere, and a sphere and a plane) are presented. When only one sphere is evaporating, depending on the distance from the other sphere, the flow may exhibit a variety of interesting behaviors such as smooth?boundary separation, closed recirculating eddies, and infinite open eddies.Item The natural frequency of oscillation of gas bubbles in tubes(The Journal of the Acoustical Society of America, 1998-06) Oguz, H.N.; Prosperetti, AndreaA numerical study is presented of the natural frequency of the volume oscillations of gas bubbles in a liquid contained in a finite-length tube, when the bubble is not small with respect to the tube diameter. Tubes rigidly terminated at one end, or open at both ends, are considered. The open ends may be open to the atmosphere or in contact with a large mass of liquid. The numerical results are compared with a simple approximation in which the bubble consists of a cylindrical mass of gas filling up the cross section of the tube. It is found that this approximation is very good except when the bubble radius is much smaller than that of the tube. An alternative approximate solution is developed for this case. The viscous energy dissipation in the tube is also estimated and found generally small compared with the thermal damping of the bubble. This work is motivated by the possibility of using gas bubbles as actuators in fluid-handling microdevices.Item The oscillations of a small floating bubble(The Physics of Fluids, 1989-02) Lu, N.Q.; Oguz, H.N.; Prosperetti, AndreaA simple model of a small bubble floating at the surface of a liquid before bursting is considered. The oscillations of this system are studied by means of a Lagrangian method. It is found that two fundamentally different modes exist. The surface mode has low frequency and does not change appreciably the volume of the immersed part of the bubble: As a consequence, its efficiency as a source of sound in the water is very limited. The volume mode has a much higher frequency and is a more efficient radiator in the water, although it may be hard to excite. Both modes behave as monopole sources in the air. It is therefore predicted that an oscillating floating bubble is a much more intense source of sound in the air than in the liquid. This conclusion seems to be supported by experimental observations.Item The Oscillations of Gas Bubbles in Tubes: Experimental Results(The Journal of the Acoustical Society of America, 1999-08) Geng, X.; Yuan, H.; Oguz, H.N.; Prosperetti, AndreaAn experimental study is presented of the frequency dependence and damping of the forced volume oscillations of gas bubbles in liquid-filled tubes. The bubbles occupy the entire section of the tube and are driven by a needle attached to a loudspeaker cone. The liquids used were water, a water–surfactant solution, and silicon oil, and the tube diameters were 1 and 3 mm. The results are in excellent agreement with the theory developed in two earlier papers. This work is motivated by the possibility of using gas bubbles as actuators in fluid-handling microdevices.