Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session L36: Bubbles: Cavitation and Coalescence |
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Chair: Michael Kinzel, The Pennsylvania State University - Applied Research Laboratory Room: Ballroom C |
Monday, November 23, 2015 4:05PM - 4:18PM |
L36.00001: A Numerical Investigation of a Gaseous Jet Interacting with a Supercavity Michael Kinzel, Michael Moeny, Michael Krane, Ivan Kirschner In this work, the interaction between a ventilated supercavity and a jet are examined using computational fluid dynamics (CFD). In this context, supercavities are large gaseous cavities that surround a vehicle for drag reduction. Its interaction with a gaseous jet is not well understood, and CFD is used to help understand the physical interactions. A validated CFD model is used, indicating that the CFD qualitatively captures a wide range of interaction regimes. More importantly, for the context of developing physical insight, the CFD seems to capture the correct qualitative trend in the bulk cavity behavior. Using these validated models, a number of novel insights into the physical characteristics of the interaction are developed. These interactions are described by: (1) the jet gas and ventilation gas poorly mix within the cavity, (2) the jet appears to cause additional gas leakage by transitioning the cavity from a recirculating flow to an axial flow, (3) the jet has the ability to lengthen the cavity, and (4) the jet invokes wake instabilities that drive cavity pulsation. These phenomena are to be presented and discussed within the presentation. [Preview Abstract] |
Monday, November 23, 2015 4:18PM - 4:31PM |
L36.00002: A cavitation bubble bursts near a particle Stephane Poulain, Gabriel Guenoun, Sean Gart, William Crowe, Sunghwan Jung Cavitation bubbles induce impulsive forces on surrounding substrates, particles, or surfaces. Even though cavitation is a traditional topic in fluid mechanics, current understanding and studies do not capture the effect of cavitation on suspended objects in fluids. In the present work, the dynamics of a spherical particle due to a cavitation bubble is experimentally characterized and compared with an analytical model. Three phases are observed: the growth of the bubble where the particle is pushed away, its collapse where the particle approaches the bubble, and a longer time scale postcollapse where the particle continues to move toward the collapsed bubble. The particle motion in the longer time scale presumably results from the asymmetric cavitation evolution at an earlier time. Our theory considering the asymmetric bubble dynamics shows that the particle velocity strongly depends on the distance from the bubble as an inverse-fourthpower law, which is in good agreement with our experimentation. This study sheds light on how small free particles respond to cavitation bubbles in fluids. [Preview Abstract] |
Monday, November 23, 2015 4:31PM - 4:44PM |
L36.00003: The Dynamics of Partial Cavities and Effect of Non-Condensable Gas Simo A. Makiharju, Harish Ganesh, Steven L. Ceccio Partial cavitation is encountered in a variety of common applications, from fuel injectors to lifting surfaces, and in general it has detrimental effects on the system wear and performance. Partial cavities undergoing auto-oscillation can cause large pressure oscillations, unsteady hydrodynamic loading, and significant noise. In the present study, experiments were conducted focusing on the dynamics of shedding cavities forming in a canonical geometry (downstream of a wedge apex). The inlet cavitation number was fixed at 2.0 and the Reynolds number based on the hydraulic diameter was 6x10$^{\mathrm{5}}$. The effects of dissolved gas content and of non-condensable gas injection into the cavity were carefully studied utilizing dynamic pressure transducers and x-ray densitometry. Gas was injected either immediately downstream of the wedge's apex or further downstream into mid-cavity. The gas injected near the wedge apex was found to end up in the separated shear layer, and relatively miniscule amounts of gas were enough to significantly reduce the vapor production rate and dampen the cavity's auto-oscillations. In addition, the results suggest that non-condensable gas injection can cause the shedding mechanism to switch from one dominated by condensation shock to one dominated by re-entrant liquid jet. [Preview Abstract] |
Monday, November 23, 2015 4:44PM - 4:57PM |
L36.00004: Cavitation structures formed during the collision of a sphere with an ultra-viscous wetted surface Mohammad Mansoor, Jeremy Marston, Jamal Uddin, Sigurdur Thoroddsen We investigate the inception of cavitation and its structures when a sphere collides with a solid surface covered with a layer of non-Newtonian liquid having kinematic viscosities of up to $\nu_{0}$ = 20,000,000 cSt. Liquids with high visco-elastic properties are shown to enable sphere rebound without any prior contact with the solid wall. Cavitation by depressurization (i.e. during rebound) in such non-contact cases is observed to onset after a noticeable delay from when the minimum gap distance is reached and originate from \textit{remnant} bubbles (remains of the obliterated primary bubble entrapped initially by the lubrication pressure of air during film entry). Contact-cases produced a cylindrical structure attached to the wall having undulations along the cavity interface which were further investigated using high-speed particle image velocimetry (PIV) techniques. We show the existence of shear-stress-induced cavitation during sphere approach towards the base wall (i.e. the pressurization stage) in ultra-viscous films. A theoretical model based on the lubrication assumption is solved for the squeeze flow in the regime identified for shear-induced cavity events to investigate the criterion for cavity inception in further detail. [Preview Abstract] |
Monday, November 23, 2015 4:57PM - 5:10PM |
L36.00005: Bubble coalescence at any Reynolds number James Munro, Christopher Anthony, Osman Basaran, John Lister When two bubbles touch, a hole is formed in the fluid sheet between them, and surface tension drives a radial flow which quickly pulls the hole wider. The singular shape and velocity of the initial configuration make experimental imaging or numerical simulation of the very early stages of coalescence challenging. Here we present detailed similarity solutions for the thickness of the fluid sheet and the velocity profile, and show that the radius of the hole increases as $r_{\! {\scriptscriptstyle E}}\propto t^{1/2}$ for any Reynolds (Ohnesorge) number. Remarkably, the initially quadratic profile of the sheet allows for an exact solution in which inertia and viscosity have the same scalings with time and remain in fixed proportion. Solution of a third-order set of ordinary differential equations determines the prefactors and profiles. In addition, asymptotic analysis of the compressional boundary layer structure in the inviscid limit formally justifies and brings new insight to earlier ad hoc `blob' models. Comparison can be made between our similarity solutions, full Navier--Stokes simulations and experimental data from Paulsen et al., Nat. Commun., vol. 5, 2014. [Preview Abstract] |
Monday, November 23, 2015 5:10PM - 5:23PM |
L36.00006: Coalescence of Bubbles in a Newtonian Fluid Christopher Anthony, Sumeet Thete, James Munro, John Lister, Michael Harris, Osman Basaran Bubble coalescence plays a central role in industry and nature. While considerable work has been done in the past decade to analyze the coalescence of drops in a passive outer fluid, it is only quite recently that the problem of bubble coalescence has begun to receive comparable interest. During bubble coalescence, two bubbles touch and create a gas bridge that grows from microscopic to macroscopic scales. We use high-accuracy simulation to analyze the dynamics in the vicinity of the space-time singularity created by the merging of two bubbles immersed in an outer Newtonian fluid of non-negligible density and viscosity while treating the inner gas as dynamically passive. This problem has recently been studied experimentally by Nagel and coworkers (2014) and theoretically by Munro and coworkers (2015) by asymptotic analysis. While both studies agree on power law scaling of the variation of the minimum neck radius with time, there is a discrepancy in the proposed/observed prefactors. In order to reconcile these differences, simulations are used to access earlier times than it has been possible in experiments. Extremely small length scales are also attained in the simulations through the use of a truncated domain approach. [Preview Abstract] |
Monday, November 23, 2015 5:23PM - 5:36PM |
L36.00007: Bubble coalescence in a power-law fluid Pritish Kamat, Sumeet Thete, Osman Basaran As two spherical gas bubbles in a liquid are slowly brought together, the liquid film or sheet between them drains and ultimately ruptures, forming a circular hole that connects them. The high curvature near the edge of the liquid sheet drives flow radially outward, causing the film to retract and the radius of the hole to increase with time. Recent experimental and theoretical work in this area has uncovered self-similarity and universal scaling regimes when two bubbles coalesce in a Newtonian fluid. Motivated by applications such as polymer and composites processing, food and drug manufacture, and aeration/deaeration systems where the liquids often exhibit deformation-rate thinning rheology, we extend the recent Newtonian studies to bubble coalescence in power-law fluids. In our work, we use a combination of thin-film theory and full 3D, axisymmetric computations to probe the dynamics in the aftermath of the singularity. [Preview Abstract] |
Monday, November 23, 2015 5:36PM - 5:49PM |
L36.00008: Numerical simulations of the translation of collapsing bubbles Elena Igualada-Villodre, Daniel Fuster, Javier Rodriguez-Rodriguez In this work we present a numerical method developed to solve the collapse of single non-spherical bubbles in an incompressible liquid. The Gerris software is used to solve for the 3D conservation equations in both phases in a system where the total volume changes in the gas are imposed. The numerical results are used to discriminate various bubble collapse regimes as a function of the collapse intensity and the strength of a non-symmetrical force (e.g. gravity). At low Weber numbers and non-zero Froude numbers, the bubble remains approximately spherical. In this regime the solution numerically obtained is shown to converge in the inviscid case to the theoretical solution. For large Weber numbers, a fast jet breaks the bubble dissipating an important part of energy during the collapse. Interestingly, it is possible to identify regimes for moderate Weber numbers where the initiation of jet formation influences its translational motion without breaking the bubble. In accordance with numerical results, experiments with bubbles generated by water electrolysis subjected to shock waves show that bubbles suffer non-spherical interface deformations. The results of this study may help to further develop medical applications using bubbles as drug-carriers. [Preview Abstract] |
Monday, November 23, 2015 5:49PM - 6:02PM |
L36.00009: Measurements of planing forces and cavity shapes on cylindrical afterbodies Aren Hellum, Jesse Belden, David Beal, Stephen Huyer, Charles Henoch, Dana Hrubes Supercavitation is a drag reduction technique by which an underwater body is enclosed over a significant portion of its length in a bubble of gas. Hydrodynamic forces act on the body only through contact with the nose and a planing section at the rear. Models of the planing forces typically assume that the body is placed into a cavity which is unchanged by the presence of the body, and the present study was designed to test the validity of this assumption. Measurements were taken of the planing forces for five afterbody lengths over a range of angles concurrently with photographs showing the size and shape of the cavity produced. These observations reveal that the cavity form and growth rate are significantly affected by both the length and angle of attack of the body; the length of the cavity shrinks at the same angle of attack as the body length is reduced past a critical threshold, suggesting a hydrodynamic interaction between the afterbody trailing edge and the cavity. Additionally, the planing forces demonstrate a non-monotonic dependence on attack angle that is not readily explained by existing models, specifically a “lift crisis” for short bodies in which the planing lift goes to zero over a range from -1 to -3 degrees. [Preview Abstract] |
Monday, November 23, 2015 6:02PM - 6:15PM |
L36.00010: Experimental study on the onset of cavitation induced by an impact Akihito Kiyama, Chihiro Kurihara, Yoshiyuki Tagawa We study reasonable expression for predicting the onset of cavitation induced by an impact experimentally. A liquid-filled test tube is dropped and impacts a floor, followed by the emergence of cavitation bubbles inside a liquid. As floor materials, a metal and a resin are chosen. As a wetting liquid, gas-saturated silicone oil was used. Experiments are conducted at room temperature. The condition for cavitation occurrence for a resin floor cannot be described by the typical velocity measured by high-speed imaging, temporal resolution and spatial resolution of which are respectively O(10) $\mu $s and O(100) $\mu $m. We investigate sudden acceleration at the impact using an accelerometer. Its temporal resolution is O(1) ns, much smaller than that of high-speed imaging. The time history of acceleration for the resin floor is more moderate and peak acceleration is smaller than that for the metal floor. Based on these findings, we discuss the reasonable description of the criterion for the onset of cavitation bubbles, applicable for various floors. [Preview Abstract] |
Monday, November 23, 2015 6:15PM - 6:28PM |
L36.00011: High temperatures produced by bubble collapse near a rigid wall Shahaboddin Alahyari Beig, Bahman Aboulhasanzadeh, Eric Johnsen The collapse of a cavitation bubble is known to have damaging effects on its surroundings. Although numerous investigations have been conducted to predict the pressures produced by this process, fewer have been devoted to determine the heating produced by the bubble collapse. Such heating of the surrounding medium may be important for materials whose mechanical properties depend on temperature (e.g., polymeric coatings). A newly developed computational method to solve the compressible Navier-Stokes equations for gas/liquid flows is used to investigate the dynamics of non-spherical collapse of gas bubbles near rigid surfaces. The subsequent temperature fields are characterized based on the relevant non-dimensional parameters entering the problem, and a model is developed to determine the temperature of the wall based on the temperature of the flow in contact with the wall. We demonstrate that significant wall temperatures may be achieved, depending on the initial location of the collapsing bubble and the heat diffusivity of the material. [Preview Abstract] |
Monday, November 23, 2015 6:28PM - 6:41PM |
L36.00012: Investigation of cavitating flows by X-ray and optical imaging Olivier Coutier-Delgosha, Sylvie Fuzier, Ilyass Khlifa, Kamel Fezzaa Hydrodynamic cavitation is the partial vaporization of high speed liquid flows. The turbulent, compressible and unsteady character of these flows makes their study unusually complex and challenging. Instabilities generated by the occurrence of cavitation have been investigated in the last years in the LML laboratory by various non-intrusive measurements including X-ray imaging (to obtain the fields of void fraction and velocity in both phases), and PIV with fluorescent particles (to obtain the velocity fields in both phases). It has been shown that cavitation is characterized by significant slip velocities between liquid and vapor, especially in the re-entrant jet area and the cavity wake. This results suggests some possible improvements in the numerical models currently used for CFD of cavitating flows. [Preview Abstract] |
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