Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session H21: Bubbles: Cavitation, Nucleation, Collapse, Coalescence |
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Chair: Varghese Mathai, UMass Amherst Room: North 221 C |
Monday, November 22, 2021 8:00AM - 8:13AM |
H21.00001: The interaction of an oil droplet and a gas bubble rising in a quiescent liquid Madeline Federle, Roberto Zenit Gas flotation for oily-water mixtures is common practice in many industrial cleanup applications. In practice, gas bubbles (typically air) will come into contact with dispersed oil drops leading to the formation of compound drops (gas-filled oil drops) that rise faster. Although this process is widely used in the industry, the details of the interaction and the capture conditions have not been studied in detail. An experimental setup was designed and built to study the interaction between individual air bubbles and oil droplets ascending in a liquid. The motion of the bubble-droplet pair was filmed with a high-speed camera. Many experiments were conducted considering different bubble and droplet sizes, and viscosities of the surrounding liquid. In most cases, the air bubble and oil drop bounced off of each other rather than coalescing. These observations indicate that the conditions for coalescence fall within a small window of parameters. We conjecture that the capture conditions are determined by the value of the spreading parameter (a generalization of the well-known spreading parameter) and the Stokes number of the interaction. |
Monday, November 22, 2021 8:13AM - 8:26AM |
H21.00002: Bubble Raft Statistics Of Plunging Liquid Jet Aaron Fishbein, Varghese Mathai When a liquid jet impinges on the surface of a deep pool of liquid, theair entrained during jet impact is pushed under and continues to flow with thecurrent. The majority of this air rises to the free surface and forms bubbles which tend to stay together as a raft. Here we study the entrainment phenomenon and the characteristics of the bubble raft formed by a continuous jet of water plunging into a large pool of water. We explore the average radius and size distributions of the bubble raft over a range Weber, Froude, and Reynolds numbers: We ∈ [100,2500], Fr ∈ [1,1500], and Re ∈ [300,3000], respectively.We study the formation and advection statistics of the free-surface bubbles, and the capillary waves generated by their intermittent popping (dissipation) events.An analogy is drawn to other non-equilibrium processes such as the turbulent energy cascade. |
Monday, November 22, 2021 8:26AM - 8:39AM |
H21.00003: Development of a compressible multi-scale model for cavitating flows Aditya Madabhushi, Krishnan Mahesh A compressible multi-scale model is developed to accurately predict the dynamics of both the resolved vapor and the sub-grid bubbles during cavitation inception. Here, the mixture of liquid and resolved vapor is treated as a carrier fluid and the sub-grid bubbles are tracked in a Lagrangian sense by the Rayleigh-Plesset equation. Also, a mixture equation of state is used that accounts for the compressibility of the mixture medium. The differences between the proposed model and the homogeneous mixture model are discussed. A modified Rayleigh-Plesset equation, which can better predict the bubble behavior in the presence of a large number of bubbles, is presented and compared to the traditional Rayleigh-Plesset equation. This model is then applied to complex cases (O(10^3) sub-grid bubbles) where phenomena such as violent bubble collapse, shielding effects etc. are observed. |
Monday, November 22, 2021 8:39AM - 8:52AM |
H21.00004: Volumetric Velocimetry Study of Cavitation Inception in a Stretched Vortex Daniel Knister, Harish Ganesh, Steven L Ceccio Cavitation inception in turbulent shear flows occurs in the weaker stream-wise vortical structures more readily than in the stronger span-wise vortices. Vortex stretching of the weaker (secondary) by the strong primary vortex leads to transient reduction in pressure in the cores of the weaker (secondary) vortical structures, causing inception. This work is an experimental study of this phenomenon of cavitation inception during the interaction of two line vortices experiencing the Crow instability. A pair of initially parallel tip vortices are produced with hydrofoils in a re-circulating water channel. Cavitation inception is studied with high speed video and acoustic measurements. Based on single phase flow velocity measurements with high speed Shake the Box and Tomographic PIV velocimetry measurements, pressures within the vortex cores are estimated. The nuclei distribution of the water channel is measured with a Cavitation Susceptibility Meter (CSM), and the observed inception rate is then compared to that predicted by the measured pressure fields and nuclei content. |
Monday, November 22, 2021 8:52AM - 9:05AM |
H21.00005: shape oscillation dynamics of a single bubble in a cavitation-induced acoustic field Javad Eshraghi, Sayantan Bhattacharya, Lalit K Rajendran, Pavlos P Vlachos The final stable shape taken by a fluid-fluid interface when it experiences a growing instability can be important in determining features as diverse as atmospheric weather patterns and the growth of cells and viruses. A laboratory-scale example is that an air bubble driven by an acoustic field when it becomes shape-unstable through a parametric instability. The excitation of volume/shape oscillations and oscillation-mode transitions in a single bubble system have been investigated. However, the role of absorbed energy by the acoustically excited gas bubble in the shape taken by a fluid-fluid interface and transition from volume oscillation to shape oscillation is missing. In this paper, we focus on the dynamics of a single rising air bubble exposed to an acoustic pressure field induced by a cavitation bubble. The air bubble is injected into the water pool through a needle, and then the laser beam is focused on the vicinity of the air bubble to generate the cavitation bubble. We elucidate the effects of acoustic source intensity, the distance from the acoustic source, and air bubble size on the air bubble final oscillation regime. If the acoustic wavelength is much larger than the bubble size, the initial bubble response is spherical oscillations of the radius. However, the forcing of large amplitude volume oscillations leads to the onset of shape oscillations. Finally, a dimensional analysis indicates the critical threshold for the excitation of the shape oscillation from an energy absorption perspective. |
Monday, November 22, 2021 9:05AM - 9:18AM |
H21.00006: The chemical production of a single bubble acoustically collapsing at audible frequencies Prashant Valluri, Davide Masiello, Ignacio Tudela-Montes, Stephen J Shaw, Rama Govindarajan The vast majority of the studies concerned with the chemical response of collapsing bubbles has been carried out at frequencies larger than 20 kHz. However, solid evidence of the inefficacy of low frequency sound waves in driving bubbles to chemically active collapse have never been produced. In this work, we have investigated gas/vapor micro-bubbles in water acoustically forced by sound in the audible spectrum (i.e. < 20 kHz). Particular focus is given to both mass and heat transfer phenomena, which have revealed to be of dramatic importance in inertially collapsing bubbles driven at very low frequencies. The chemical production is investigated by considering the case study of the production of gaseous ammonia (NH3) in bubbles initially containing molecular nitrogen (N2). Our study suggests that, although the larger extent of vapor segregation cushions the collapse at very low frequencies, the bubble response in terms of peak temperatures and pressures is comparable to that obtained with higher frequencies. Production of ammonia appears to be possible only above a certain amplitude threshold which is found to increase with frequency. The results suggest that at low enough power, the production of ammonia should be observable only at low frequencies. |
Monday, November 22, 2021 9:18AM - 9:31AM |
H21.00007: Laser-induced cavitation near absolute zero Kenneth Langley, Nathan B Speirs, Tariq Alghamdi, Matthew Wallace, Peter Taborek, Sigurdur T Thoroddsen When a short duration pulsed laser is focused into a liquid, the liquid will break down into a plasma once the laser-power density exceeds some threshold. After the plasma is extinguished, the remaining hot gas expands forming a bubble whose ultimate size depends on the thermo-physical and transport properties of the fluids and the total energy absorbed. Herein, we use ultra-high-speed imaging at frame rates up to 5 million frames per second to experimentally investigate laser-induced cavitation bubble dynamics in liquid helium between 1.25 K and the liquid-vapor critical point at 5.19 K. This small temperature range results in a wide variation of fluid properties including the density, viscosity, vapor pressure and thermal conductivity. A 532 nm, pulsed Nd:YAG laser with duration of approximately 6 ns and variable pulse energy up to 100 mJ is focused into the volume of interest using a parabolic mirror submerged in the liquid with a minimum beam width at the point of cavitation of 150 µm. We present the bubble growth and collapse dynamics and its dependence on temperature and pressure. We compare our results with well known bubble dynamic theory such as the Rayleigh-Plesset equation. We also present the measured cavitation threshold for our entire experimental range. |
Monday, November 22, 2021 9:31AM - 9:44AM |
H21.00008: Experimental study of bubble growth by non-condensible disolved gas diffusion in super-saturated low-pressure water. OMRI RAM, Joseph Katz, Kuanyu Li A recent study found that residual microbubbles that linger in locations of cavitation inception on curved surfaces act as nuclei for the re-inception of attached cavities. To better predict the re-inception process, this study experimentally measures the growth rate of a bubble by non-condensable gas diffusion in supersaturated low-pressure water, simulating the conditions close to regions of attached cavitation inception. Bubbles with an initial diameter of about 0.1 mm are released into a vertical 2 mm ×10 mm glass test section of a small-scale, pressure-controlled water tunnel. The flow in the test section is directed downwards with the speed required to counter the rising of the bubble. With this apparatus, the bubble remains stationary in the laboratory frame, and its growth process is filmed. The flow around the small bubbles is slow enough to keep it in the Stokes flow regime, which naturally allows for simplified analysis of the diffusive and convective contributions to the bubble growth. The experiments are performed using purified water while monitoring its temperature and dissolved gas level. The presentation will cover the experimental technique and present initial results showing the dependency of bubble growth rate on ambient pressures. |
Monday, November 22, 2021 9:44AM - 9:57AM |
H21.00009: How do bubbles collapse and displace near porous plates? Elijah Andrews, David Fernández Rivas, Ivo R Peters The collapse of a gas or vapour bubble near a solid boundary produces a jet directed towards the boundary. High surface pressure and shear stress induced by this jet can damage, or clean, the surface. A porous boundary, such as a filter, would act similarly to a solid boundary but with reduced effect. Prior research has measured the cleaning effect of bubbles on filters using ultrasonic cleaning, but how the bubble dynamics are fundamentally affected by the porosity of the surface is not known. We address this question experimentally by investigating how both the porosity and pore size of the boundary affects two collapse parameters: bubble displacement and bubble rebound strength. We measure these parameters for various porosity values, pore sizes, hole shapes, plate materials, and bubble positions. These experiments demonstrate a clear dependence of both collapse parameters on the porosity of the boundary but indicate limited sensitivity at high and low porosity values. Surprisingly, the position of the bubble, over either a hole or a solid part of the boundary, does not significantly influence the displacement or rebound strength. |
Monday, November 22, 2021 9:57AM - 10:10AM |
H21.00010: SIngularity influences jet direction during the collapse of bubbles in contact with walls Mandeep Saini, Daniel Fuster Bubble dynamics in contact with walls have applications in diverse areas of industrial cleaning, biomedical sciences and underwater explosions among many others. In this work we theoretically and numerically study the collapse of a single bubble sitting at a wall after an impulsive change of the ambient pressure. The flow is decomposed as sum of potential and a viscous field. The potential field, obtained from the solution of the Laplace equation, exhibits singular solutions at the contact point for contact angles larger than 90 degrees that are shown to describe well the overall dynamics of the bubble collapse process obtained from DNS. In particular, we describe the appearance of a jet developing parallel to the solid wall that ultimaly leads to a jetting process in the opposite direction to the solid boundary. Viscosity is shown to regularize the flow near the singular point and contol the finite jet velocities. |
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