Bulletin of the American Physical Society
70th Annual Meeting of the APS Division of Fluid Dynamics
Volume 62, Number 14
Sunday–Tuesday, November 19–21, 2017; Denver, Colorado
Session F8: Bubbles: Cavitation, Nucleation, Collapse, CoalescenceBubbles
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Chair: Michael L. Calvisi, University of Colorado Colorado Springs Room: 501 |
Monday, November 20, 2017 8:00AM - 8:13AM |
F8.00001: Inertial collapse of bubble pairs near a solid surface Shahaboddin Alahyari Beig, Eric Johnsen Cavitation occurs in a variety of applications ranging from naval structures to biomedical ultrasound. One important consequence is structural damage to neighboring surfaces following repeated inertial collapse of vapor bubbles. Although the mechanical loading produced by the collapse of a single bubble has been widely investigated, less is known about the detailed dynamics of the collapse of multiple bubbles. In such a problem, the bubble-bubble interactions typically affect the dynamics, e.g., by increasing the non-sphericity of the bubbles and amplifying/hindering the collapse intensity depending on the flow parameters. Here, we quantify the effects of bubble-bubble interactions on the bubble dynamics, as well as the pressures/temperatures produced by the collapse of a pair of gas bubbles near a rigid surface. We perform high-resolution simulations of this problem by solving the three-dimensional compressible Navier-Stokes equations for gas/liquid flows. The results are used to investigate the non-spherical bubble dynamics and characterize the pressure and temperature fields based on the relevant parameters entering the problem: stand-off distance, geometrical configuration (angle, relative size, distance), collapse strength. [Preview Abstract] |
Monday, November 20, 2017 8:13AM - 8:26AM |
F8.00002: Bubble coalescence in a Newtonian fluid Vishrut Garg, Osman Basaran Bubble coalescence plays a central role in the hydrodynamics of gas-liquid systems such as bubble column reactors, spargers, and foams. Two bubbles approaching each other at velocity $V$ coalesce when the thin film between them ruptures, which is often the rate-limiting step. Experimental studies of this system are difficult, and recent works provide conflicting results on the effect of $V$ on coalescence times. We simulate the head-on approach of two bubbles of equal radii $R$ in an incompressible Newtonian fluid (density $\rho$ , viscosity $\mu$ , and surface tension $\sigma$) by solving numerically the free boundary problem comprised of the Navier Stokes and continuity equations. Simulations are made challenging by the existence of highly disparate lengthscales, i.e. film thickness and drop radii, which are resolved by using the method of elliptic mesh generation. For a given liquid, the bubbles are shown to coalesce for all velocities below a critical value. The effects of Ohnesorge number $Oh = \mu/ \sqrt{\rho \sigma R}$ on coalescence time and critical velocity are also investigated. [Preview Abstract] |
Monday, November 20, 2017 8:26AM - 8:39AM |
F8.00003: The collapse of a cavitation bubble in a corner Ivo Peters, Yoshiyuki Tagawa The collapse of cavitation bubbles is influenced by the surrounding geometry. A classic example is the collapse of a bubble near a solid wall, where a fast jet is created towards the wall. The addition of a second wall creates a non-axisymmetric flow field, which influences the displacement and jet formation during the collapse of a bubble. In this experimental study we generate mm-sized vapor bubbles using a focused pulsed laser, giving us full control over the position of the bubble. The corner geometry is formed by two glass slides. High-speed imaging reveals the directional motion of the bubble during the collapse. We find that the bubble displacement cannot be fully described by a simple superposition of the bubble dynamics of the two walls individually. Comparison of our experimental results to a model based on potential flow shows a good agreement for the direction of displacement. [Preview Abstract] |
Monday, November 20, 2017 8:39AM - 8:52AM |
F8.00004: Cavitation onset of an accelerating liquid Akihito Kiyama, Zhao Pan, Yoshiyuki Tagawa, David Daily Jesse, Scott Thomson, Randy Hurd, Tadd Truscott Accelerating a liquid-filled container can shatter the bottom. High-speed imaging reveals cavitation bubbles collapse near the bottom just before this fracture event. To avoid the damage caused by cavitation, accurate prediction of cavitation onset is crucial. However, the conventional cavitation number, as a function of the mean velocity of the flow does not correctly predict cavitation onset in an accelerating liquid. This study derives an alternative cavitation number from the equation of motion, predicting cavitation as a function of acceleration (cf. Pan \& Kiyama et al., Proc. Natl. Acad. Sci., 2017). We have conducted two separate series of experiments with a broad set of varied parameters. All the experimental results agreed with this theory, indicating that the proposed alternative cavitation number describes the universal threshold of cavitation onset within an accelerating liquid. [Preview Abstract] |
Monday, November 20, 2017 8:52AM - 9:05AM |
F8.00005: Long-time cavitation threshold of silica water mixture under acoustic drive Adrien Bussonnière, Qingxia Liu, Peichun Amy Tsai The low cavitation threshold of water observed experimentally has been attributed to the presence of pre-existing tiny bubbles stabilized by impurities. However, the origin and stability of these cavitation nuclei remain unresolved. We therefore investigate the long-time cavitation evolution of water seeded with micron-sized silica particles under the influences of several parameters. Experimentally, cavitation is induced by a High Intensity Focused Ultrasound and subsequently detected by monitoring the backscattered sound. Degassed or aerated solutions of different concentrations are subjected to acoustic pulses (with the amplitude ranging from 0.1 to 1.7 MPa and a fixed repetition frequency between 0.1 and 6.5 Hz). The cavitation threshold was measured by fitting the cavitation probability curve, averaged over 1000 pulses. Surprisingly, our results shown that the cavitation threshold stabilizes at a reproducible value after a few thousand pulses. Moreover, this long-time threshold was found to decrease with increasing particle concentration, pulse period, and initial oxygen level. In contrast to the depletion of nuclei expected under long acoustic cavitation, the results suggest stabilized nuclei population depending on concentration, oxygen level, and driving period. [Preview Abstract] |
Monday, November 20, 2017 9:05AM - 9:18AM |
F8.00006: Molecular Dynamics Investigation of Each Bubble Behavior in Coarsening of Cavitation Bubbles in a Finite Space Shin-ichi Tsuda, Yuta Nakano, Satoshi Watanabe Recently, several studies using Molecular Dynamics (MD) simulation have been conducted for investigation of Ostwald ripening of cavitation bubbles in a finite space. The previous studies focused a characteristic length of bubbles as one of the spatially-averaged quantities, but each bubble behavior was not been investigated in detail. The objective of this study is clarification of the characteristics of each bubble behavior in Ostwald ripening, and we conducted MD simulation of a Lennard-Jones fluid in a semi-confined space. As a result, the time dependency of the characteristic length of bubbles as a spatially-averaged quantity suggested that the driving force of the Ostwald ripening is Evaporation/Condensation (EC) across liquid-vapor surface, which is the same result as the previous works. The radius change of the relatively larger bubbles also showed the same tendency to a classical EC model. However, the sufficiently smaller bubbles than the critical size, e.g., the bubbles just before collapsing, showed a different characteristic from the classical EC model. Those smaller bubbles has a tendency to be limited by mechanical non-equilibrium in which viscosity of liquid is dominant rather than by EC across liquid-vapor surface. [Preview Abstract] |
Monday, November 20, 2017 9:18AM - 9:31AM |
F8.00007: Numerical modeling of ultrasonic cavitation in ionic liquids Michael L. Calvisi, Ross M. Elder Ionic liquids have favorable properties for sonochemistry applications in which the high temperatures and pressures achieved by cavitation bubbles are important drivers of chemical processes. Two different numerical models are presented to simulate ultrasonic cavitation in ionic liquids, each with different capabilities and physical assumptions. A model based on a compressible form of the Rayleigh-Plesset equation (RPE) simulates ultrasonic cavitation of a spherical bubble with a homogeneous interior, incorporating evaporation and condensation at the bubble surface, and temperature-varying thermodynamic properties in the interior. A second, more computationally intensive model of a spherical bubble uses the finite element method (FEM) and accounts for spatial variations in pressure and temperature throughout the flow domain. This model provides insight into heat transfer across the bubble surface and throughout the bubble interior and exterior. Parametric studies are presented for sonochemistry applications involving ionic liquids as a solvent, examining a range of realistic ionic liquid properties and initial conditions to determine their effect on temperature and pressure. Results from the two models are presented for parametric variations including viscosity, thermal conductivity, water content of the ionic liquid solvent, acoustic frequency, and initial bubble pressure. An additional study performed with the FEM model examines thermal penetration into the surrounding ionic liquid during bubble oscillation. The results suggest the prospect of tuning ionic liquid properties for specific applications. [Preview Abstract] |
Monday, November 20, 2017 9:31AM - 9:44AM |
F8.00008: Effect of viscosity on the dynamics of a spark-generated non-equilibrium bubble in free-field and near a free-surface Y S Kannan, Saravanan Balusamy, Badarinath Karri, KIRTI CHANDRA SAHU The effect of viscosity on the behaviour of a spark-generated non-equilibrium bubble is investigated experimentally. In specific, the dynamics of the bubble in two scenarios, namely, when the bubble is generated in the bulk of the fluid (``free-field'' bubble) and when the bubble is generated near a free-surface (``free-surface'' bubble) are investigated. The bubble is created using a low-voltage spark circuit and its dynamics is captured using a high speed camera with back-lit illumination. The viscosity of the surrounding medium is varied by using different grades of silicone oil. It is observed that for a ``free-field'' bubble, the bubble oscillates radially and with an increase in the viscosity of the liquid, both the number of oscillations as well as time period of each oscillation are increased. For ``free-surface'' bubbles, our experiments reveal a variety of distinctive bubble and re-entrant jet behaviours as the initial distance of the bubble from the free-surface and the viscosity of the surrounding fluid are varied. It is observed that beyond a certain initial distance of the bubble from the free surface, the bubble behaves as a ``free-field'' bubble.This limiting initial distance is observed to decrease as the viscosity increases. [Preview Abstract] |
Monday, November 20, 2017 9:44AM - 9:57AM |
F8.00009: Shock waves from non-spherically collapsing cavitation bubbles Outi Supponen, Danail Obreschkow, Mohamed Farhat Combining simultaneous high-speed imaging and hydrophone measurements, we uncover details of the multiple shock wave emission from laser-induced cavitation bubbles collapsing in a non-spherical way. For strongly deformed bubbles collapsing near a free surface, we identify the distinct shock waves caused by the jet impact onto the opposite bubble wall and by the individual collapses of the remaining bubble segments. The energy carried by each of these shocks depends on the level of bubble deformation, quantified by the anisotropy parameter $\zeta$, the dimensionless equivalent of the Kelvin impulse. For jetting bubbles, at $\zeta<0.01$, the jet impact hammer pressure is found to be the most energetic shock. Through statistical analysis of the experimental data and theoretical derivations, and by comparing bubbles deformed by different sources (variable gravity achieved on parabolic flights, and neighboring free and rigid surfaces), we find that the shock peak pressure may be approximated as the jet impact-induced water hammer as $p_{h} = 0.45\left(\rho c^{2}\Delta p\right)^{1/2}\zeta^{-1}$. [Preview Abstract] |
Monday, November 20, 2017 9:57AM - 10:10AM |
F8.00010: Time-resolved observations of shock waves in cavitating jet Nobuyuki Fujisawa, Yasuaki Fujita, Kei Fujisawa, Takayuki Fujisawa The mechanism of erosion in a cavitating jet is studied experimentally using the time-resolved observations of the shock wave formation. The experiments were carried out by the time-resolved laser schlieren combined with shadowgraph. The results indicated that the cavitation cloud collapsed in a periodic growth of cloud near the wall, which is followed by the generation of shock waves and the pits on the wall. Most of the shock waves were generated near the wall and they induced the pits on the wall. The occurrence points of shock waves were in close agreement with the pits pattern on the wall. [Preview Abstract] |
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