Bulletin of the American Physical Society
73rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 65, Number 13
Sunday–Tuesday, November 22–24, 2020; Virtual, CT (Chicago time)
Session F02: Bubbles: Cavitation, Nucleation, Collapse, Coalescence (3:55pm  4:40pm CST)Interactive On Demand

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F02.00001: Inertial collapse of a gasvapor bubble in a viscoelastic medium Kazuya Murakami, Eric Johnsen Inertial collapse of cavitation bubbles is a key phenomenon in a high strainrate rheometry for soft material and medical applications. The bubble contents in water are dominated by vapor, while a finite amount of noncondensable gas as well as vapor exist inside cavitation bubbles in soft matter. It is thus important to understand gasvapor mixture transport on bubble dynamics in a viscoelastic medium. We use a RayleighPlessettype equation in which we account for variable specific heats ratios for the gas and vapor. During inertial collapse, a large amount of vapor is trapped inside the bubble because a shell of noncondensible gas is formed near the bubble wall and prevents the vapor from condensing. As a result, the bubble does not collapse to as small a minimum bubble radius and rebounds to a larger because of less energy is lost to acoustic radiation near collapse. Furthermore, the maximum temperature is induced near the bubble wall in the gasdominated region rather than at the center of the bubble in the vapordominated region. The assumption of constant specific heats ratio causes some discrepancies when the inertial collapse of gasvapor bubble is more violent. [Preview Abstract] 

F02.00002: A data assimilation method for analysis of cavitation bubble dynamics. Javad Eshraghi, Arezoo Ardekani, Pavlos Vlachos The original or modified RayleighPlesset equation (RPE) is often used for the analysis of cavitation bubble dynamics. The accuracy of the bubble's characteristics predictions by these equations is governed by initial values of the physical parameters.~Currently, this initialization is predominantly userdefined, where through trial and error, the initial values are determined. Here, we present a novel state observerbased dataassimilation technique, designed for the assimilation of measured cavitation bubble diameter with RayleighPlessettype models to enhance the spatiotemporal predictions by these models. This fully autonomous method places an observer in the original/modified RPE and uses a proportionalintegralderivative (PID) control law on the difference between the predicted and measured cavitation bubble diameter. The dataassimilated modeling using an observer in the RPtype equations most accurately estimates the bubble diameter and farfield pressure at the probed locations and is not sensitive to the initial values of the physical parameters. [Preview Abstract] 

F02.00003: Analysis of different cavitation models in a laminar flow. Andrea SAVIO, Marta Cianferra, Vincenzo Armenio We compare the performance of different cavitation models in terms of vaporization and condensation rates for the correct simulation of the process. We compare homogeneous mixture models, that study the cavitation through the vapor fraction solving its transport equation and defining the formulations for the source terms. We consider 4 models: Kunz, Merkle, SchnerrSauer, and Saito, and evaluate their performance in the case of laminar flow around a circular cylinder at {\$}Re$=$200{\$}, already analyzed by [Gnanaskandan, Mahesh, JFM 2016]; The numerical tests are carried out adopting the interPhaseChangeFoam solver and cavitation models already present in the OpenFOAM library. The Saito model was implemented by scratch. We analyze some fundamental aspects of the cavitating flow, such as the pressure distribution over the body surface, vortex shedding frequency, the length of the attached cavity downstream the body, and the time oscillations of the entire cavity volume. The results are found to be strongly dependent on the cavitation model adopted and on the magnitude of the empirical parameters contained in the cavitation models, namely the vaporization and condensation coefficients. [Preview Abstract] 

F02.00004: Surface Bubble Coalescence Daniel Shaw, Luc Deike Bubble coalescence at a free surface occurs in our daily lives with drinks and on a global scale at the surface of the ocean. We present an experimental study of bubble coalescence at an airwater interface, and characterize the evolution of both the underwater neck and the surface bridge. We explore a wide range of Bond number, which compares gravity and capillary forces and is a dimensionless measure of the free surface's effect on bubble geometry. The nearly spherical $Bo \ll 1$ bubbles exhibit the same inertialcapillary growth of the classic underwater dynamics, with limited upper surface displacement. For $Bo > 1$, the bubbles are nonspherical  residing predominantly above the free surface  and while an inertialcapillary scaling for the underwater neck growth is still observed, the controlling length scale is defined by the curvature of the bubbles near their contact region. With it, an inertialcapillary scaling collapses the neck contours across all Bond number to a universal shape. Finally, we characterize the upper surface with a simple oscillatory model which balances capillary forces and the inertia of liquid trapped at the center of the liquidfilm surface. [Preview Abstract] 

F02.00005: The effects of geometry on flow in a venturi formedby fastclosing jaws Jose Valdes Herrera, Daniel Aguirre, Francisco Godinez We designed a mechanical apparatus inspired by the snappingclaw of pistol shrimps. The device consists of a pair of jaws, each with an inner contour that conforms to a halfventuri geometry. One of the jaws is fixed and the other rotates around a pivot. When the jaws suddenly close a venturi tubelike is formed with the subsequent generation of a high speed cavitation flow. Geometrical parameters as the ratio of the throat section length to diameter, the ratio of the diameters of the throat section to the contraction section and the diffusion angle were varied to experimentally evaluate their influence on the flow dynamics. Conventional 2D PIV was used to visualize the flow structure features and highspeed imaging was performed to study the dynamics of bubble clouds induced by pressure drop at the end of the closing process. The experiments were conducted with tap water under laboratory conditions using ten different geometries. We were able to determine which geometry induces more bubbles; as well as their location and the intensity with which they collapse inside the tube. These aspects are key to developing venturi reactors. [Preview Abstract] 

F02.00006: Jet Direction in Bubble Collapse Within Rectangular and Triangular Channels Lebo Molefe, Ivo Peters A vapor bubble collapsing near a solid boundary in a liquid produces a liquid jet that points toward the boundary. The direction of this jet has been studied for boundaries such as flat planes and parallel walls enclosing a channel. Extending these investigations to enclosed polygonal boundaries, we experimentally measure jet direction for collapsing bubbles inside a square and an equilateral triangular channel. Following the method of Tagawa and Peters [Phys. Rev. Fluids 3, 081601 (2018)] for predicting the jet direction in corners, we model the bubble as a sink in a potential flow and demonstrate by experiment that analytical solutions accurately predict jet direction within an equilateral triangle and square. We further use the method to develop predictions for several other polygons, specifically, a rectangle, an isosceles right triangle, and a $30^{\circ}60^{\circ}90^{\circ}$ right triangle. [Preview Abstract] 

F02.00007: A Study of Homogeneous and Heterogeneous Bubble Nucleation Inside Nanopores through Resistive Pulse Sensing Soumyadeep Paul, WeiLun Hsu, Mirco Magnini, Lachlan Mason, YaLun Ho, Omar Matar, Hirofumi Daiguji The explosive growth of nanoconfined bubbles is inherently difficult to capture using traditional imaging techniques due to high surface tension dominance. In this study, we employ Joule heating in nanopores filled with a 3M aqueous solution of NaCl, to raise liquid temperatures and detect the nucleation point at nanosecond time resolution. This is achieved by measuring bubble induced ionflow blockage using a high bandwidth oscilloscope. Through continuum simulations we obtain temperature distributions at nucleation points, based on which we are able to ascertain whether bubbles nucleated heterogeneously on the pore walls or homogeneously at the pore center. Joule heating results in a positive temperature difference ($\Delta $T$_{\mathrm{p}})$ between the pore center and the pore walls, allowing homogenous bubble clusters to incur a lower freeenergy cost. Compared to a larger pore (525 nm), a smaller pore (280 nm) has higher values of $\Delta $T$_{\mathrm{p}}$ and pinning freeenergy barrier for heterogeneous nucleation. This suppresses heterogeneous nucleation and homogenous nucleation emerges as the dominant mode for bubble nucleation inside smalldiameter pores. [Preview Abstract] 

F02.00008: Reynolds number effects on pressure fluctuations and cavitation inception in a turbulent shear layer Karuna Agarwal, Omri Ram, Jin Wang, Yuhui Lu, Joseph Katz Cavitation inception in turbulent shear layers occurs in quasi streamwise vortices located between the primary spanwise structures, and the cavitation inception indices increase with the Reynolds number. To investigate these phenomena, we measure the pressure distributions in these vortices and the flow mechanisms involved. The experiments are performed behind a backward facing step at Reynolds numbers based on step height and freestream velocity of 1.6x10$^{\mathrm{4}}$ and 5.8x10$^{\mathrm{4}}$. Tomographic imaging and 3D particle tracking in a volume of 12.5x7.5x4.5 mm$^{\mathrm{3}}$ provides the 3D timeresolved velocity distributions. Data are interpolated by constrained cost minimization to obtain divergence free velocity and curlfree material acceleration fields at a resolution of 200$\mu $m. The pressure is calculated by spatially integrating the material acceleration. The appearance of the intermittent quasistreamwise structures agrees with that of the cavitation. The minimum pressure coefficients are lower than 0.3, and they last longer than the cavity growth times scales. Statistical analysis shows that with increasing Reynolds number, the pressure minima are more preferentially located within the quasi streamwise vortices, persist for longer durations, and are more strongly correlated with vortex stretching. [Preview Abstract] 

F02.00009: Visualization of atmospheric shockwave from underwater explosion Shihori Jinnouchi, Keita Ando In the context of underwater explosions, much attention has been put on acoustic phenomena (including shockwaves and cavitation) in the water. However, to the authors' knowledge, little is studied regarding shockwave transmission from underwater explosions to the atmosphere. The transmitted shock dynamics will be of importance when it comes to examining ignition in the case of flammable gases over the water interface. Here, we perform visualization of shock transmission from laserinduced underwater explosions to the atmosphere. Nanosecond pulse laser at 1064 nm is focused into water to create a spherical shockwave. The shock interaction with an airwater interface is recorded by a highspeed camera with pulse laser backlighting. The evolution of the transmitted shockwave in the air is examined with the aid of the RankingHugoniot relation and Euler flow simulation. [Preview Abstract] 

F02.00010: Cavitation inception during vortex pair interaction Aditya Madabhushi, Krishnan Mahesh Cavitation inception during vortex pair interaction is commonly observed in the wakes of propeller blades, in jets and other shear layer flows. Here, DNS is employed to study the interaction between two counterrotating vortices of unequal strength at Re = 200000. The growth of the Crow instability on the weaker filament results in its stretching and wrapping around the stronger filament. The axial stretching causes a significant drop in the weaker filamentâ€™s core size, and hence its core pressure, for most of the evolution. This axial stretching is found to be nonuniform and its effect on the core size reduction is discussed. As the filaments approach very close to each other, the cores start to break up due to large mutual strain. During this breakup process, a combination of axial jet and local axial stretching results in a further drop in the core pressure of both the filaments, possibly leading to inception. Also, the effect of initial secondary core size on the minimum pressure attained in both the filaments is analyzed. A compressible EulerLagrangian model to accurately predict the dynamics of the subgrid bubbles during inception is discussed. A modified RayleighPlesset equation is presented and compared to the traditional RayleighPlesset equation, for this purpose. [Preview Abstract] 

F02.00011: Satellite Bubble Formation From A Retracting Air Cone Fan Yang, Ziqiang Yang, Yuansi Tian, Sigurdur T. Thoroddsen \sout{\textsc{THE FORMATION OF SMALL DROPLETS OR BUBBLES IS IMPORTANT IN MANY INDUSTRIAL PROCESSES WHERE THEY ARE OFTEN UNDESIRABLE, LIKE WATERDROPS IN CRUDE OIL OR AIRBUBBLES IN GLASSFURNACES. IN OTHER SITUATIONS MICROBUBBLES ARE BENEFICIAL, FROM THE AROMA OF CHAMPAGNE TO GASTRANSPORT THROUGH THE OCEAN SURFACE. TWO SUCH MECHANISMS ARE PARTIAL COALESCENCE1AND THE COLLAPSE OF IMPACT CRATERS2. PARTIAL COALESCENCE OF DROPS AND BUBBLES APPEAR SIMILAR BUT DIFFER IN FUNDAMENTAL WAYS, WITH MUCH SMALLER SATELLITE BUBBLES THAN DROPLETS. THE PINCHOFF OF A DIMPLE AT THE BOTTOM OF A REBOUNDING CRATER PORTRAYS PURE INERTIAL DYNAMICS, DIFFERENT FROM THE CAPILLARYINERTIAL PINCHOFF OF A DROP2. RECENT WORK BY }}\sout{\textsc{BRASZ ET AL. (2018)}}\sout{\textsc{INVESTIGATES THE DROPLET FORMATION FROM THE TIP OF A RETRACTING LIQUID CONE USING A SELFSIMILAR FORMALISM. MOTIVATED BY THIS STUDY AND THE ABOVE CONSIDERATIONS WE INVESTIGATE HEREIN HOW THE DYNAMICS IS AFFECTED BY SWITCHING THE FLUIDS, LOOKING AT THE RETRACTION OF AN AIRCONE, OVER A RANGE OF DIFFERENT CONEANGLES AND INITIAL PERTURBATION LEVELS, USING THE VOLUMEOFFLUIDS PROGRAM GERRIS. WE SEE CLEAR DIFFERENCES WITH EARLIER PINCHOFF OF BUBBLES THAN OBSERVED FOR THE LIQUID CASE3.}} [Preview Abstract] 

F02.00012: Optical ForceDeposited Nanoparticles the Necessity for Photothermal Surface Bubble Nucleation in Nanoparticle Suspension Qiushi Zhang, Ruiyang Li, Eungkyu Lee, Tengfei Luo We study the mechanism of surface bubble nucleation in nanoparticle (NP) suspension upon laser irradiation. Our experiments find the bubble nucleation thresholds differ depending on if the surface is forward or backwardfacing the light propagation direction. High speed videography reveals that the optical dispersive forcedriven NP deposition on the surface is a prerequisite for bubble nucleation. Optical pulling force is needed to deposit NPs when the surface is backwardfacing, and this is achieved only when the laser optical fluence is sufficiently high to generate a supercavitating nanobubble around the NP. When the surface is forwardfacing, optical pushing force, which exists on both bare or supercavitating NPs, makes the NP deposition easier and thus lowers the surface bubble nucleation threshold. The results reveal interesting physics leading to photothermal surface bubble generation in NP suspensions. [Preview Abstract] 

F02.00013: Experimental Study of Incipient Cavitation Due to a Pair of Interacting Line Vortices Daniel Knister, Elizabeth Callison, Harish Ganesh, Steven L. Ceccio Incipient cavitation in turbulent shear flows generally occurs in weaker streamwise vortical structures that are stretched by stronger spanwise vortices. The rapid stretching leads to a substantial reduction in core pressure in the secondary vortices, which can lead to cavitation inception. Occurrence of cavitation inception depends on the vortex stretching process, the accompanying pressure drop, and size of captured nuclei. An experiment to study this phenomenon has been conducted, with the two parallel tip vortices of a pair of hydrofoils in a recirculating water tunnel, similar to that of Chang et al. (2012). Cavitation inception and developed cavitation in the interacting vortices are studied with high speed video. In addition, averaged vortex properties of noncavitating line vortices are measured with Stereo Particle Image Velocimetry (SPIV). Observed vortex interaction in the experiments compares favorably to the dynamics predicted using a linear stability framework following Crow (1970) based on the measured averaged properties. The effect of nuclei size on is explored by measuring the nuclei distribution using a new optical Cavitation Susceptibility Meter. [Preview Abstract] 

F02.00014: Numerical Simulation of Cavitation Bubble Dynamics in a Rankine Vortex Pranav Mohan, Sadegh Dabiri High circulation velocity in a vortex can suppress the pressure at the core causing any nucleation site to cavitate. This cavitation bubble grows in a cylindrical shape and collapses very violently. Compressible NavierStokes equations are numerically solved in three dimensions to capture the cylindrical gas bubble dynamics in a Rankine vortex in water and the reentrant jets formed during the final stage of the collapse. The bubble dynamics and the vortex flow fields are strongly coupled. The effects of nucleation sites, Reynolds number, vortex core size and cavitation number are thoroughly examined. We found that the size of the nucleation site does not play a role in growth dynamics. A combination of other initial conditions lead to an elongation of a single bubble into multiple interconnected cylindrical bubbles aligned along the vortex core, as observed in experiments. Some turbulent structures can also be found inside the bubble. At the final stage, the bubbles collapse towards the center to generate reentrant jets with velocity magnitude of several thousand meters per second. Such a strong collapse fragments the bubble and causes severe damage to the materials in the vicinity of the vortex or the bubble. [Preview Abstract] 

F02.00015: A narrowband variational level set methodology for predicting cavitating surface nucleation sites Karim Alame, Krishnan Mahesh A robust narrowband implementation of the Gibbs free energy minimization algorithm using a variational level set methodology is presented. The method relies on the distance regularized level set equations (DRLSE) which avoids the standard reinitialization. The application of the method is to obtain a liquidgas interface over a wedge and arbitrary rough surfaces to predict nucleation sites for different external pressure conditions. The ability of the method to predict accurately, quickly and efficiently is demonstrated. [Preview Abstract] 

F02.00016: The dynamics of a single cavitation bubble in a rigid externally driven microconfinement Konstantin Leonov, Iskander Akhatov Cavitation in microscopic confinement received some attention recently in the attempt to model free oscillations of the bubbles that preexist or appear by nucleation in the trapped liquid. In our study, the dynamics of a spherical bubble in the confined externally driven liquid cell is considered. It is shown that volume confinement strongly affects the manifestation of the classical cavitation Blake threshold. At relatively large liquid cell exposed to a tension exceeding cavitation Blake threshold, the cavitation bubble abruptly expands to a finite radius in contrast with explosive infinite bubble growth in bulk liquid. At liquid cell size smaller than some critical one, the cavitation is completely suppressed by volumetric confinement. The generalized RayleighPlesset equation for the confined bubble is derived in which the liquid cell size is used as a driving parameter. Three possible regimes of bubble dynamics in confined liquid at different types of driving is studied. First, a simple nonlinear growth initiated by relatively weak liquid cell expansion when the cavitation Blake threshold is not reached yet. Second, an abrupt cavitation expansion with oscillatory transient when cavitation Blake threshold is reached. Third, multiple cavitation inception followed by cavitation vanishing at the periodic liquid cell expansion and contraction. In this case, it is also found that for high driving frequency bubble dynamics in the confined liquid resembles the bubble dynamics in an unbounded liquid. [Preview Abstract] 

F02.00017: Shock wave interactions in singlebubble collapse near a corner William White, Shahaboddin Alahyari Beig, Eric Johnsen Erosion damage to neighboring surfaces due to the repeated collapse of a vapor bubble is one of the most consequential results of cavitating flows, which are found in a number of hydraulic systems. Numerous studies exist on the collapse of a bubble near a single surface. However, the dynamics and shocks produced by bubble collapse in a corner has yet to be investigated numerically. In this study, we quantify the topological parameters of the collapse as well as the pressures, temperatures, and velocities in the flow field to extend present knowledge of collapse near a single wall. For this purpose, we use an inhouse, highorder accurate shock and interfacecapturing method to solve the 3D compressible NavierStokes equations for gas/liquid flows. We demonstrate that bubbleboundary interactions amplify/reduce pressures and temperatures produced during the collapse and increase the collapse time and the nonlinearity of the bubble displacement, depending on geometric parameters. [Preview Abstract] 
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