A Computational Study of the Inertial Collapse of Gas Bubbles Near a Rigid Surface

Cavitation research is essential to a variety of applications ranging from naval hydrodynamics to medicine and energy sciences. Vapor cavities can grow from sub-micron-sized nuclei to millimeter-sized bubbles, and collapse violently in an inertial fashion. This implosion, which concentrates energy into a small volume, can produce high pressures and temperatures, generate strong shock waves, and even emit visible light. One of the main consequences of cavitation is structural damage to neighboring surfaces due to bubble collapse. The propagation of shock and rarefaction waves in a multiphase medium results in a complicated multiscale and multiphysics problem. Laboratory experiments of such flows are challenging due to the wide range of spatial and temporal scales, difficult optical access, and limitations of measurement devices. To better understand these flows, we use highly resolved numerical simulations of the inertial collapse of individual vapor bubbles near a rigid surface. For this purpose, we developed a novel numerical multiphase model combined with high-performance computing techniques to perform accurate and efficient simulations of the three-dimensional compressible Navier-Stokes equations for a binary, gas-liquid system. We present the detailed dynamics of the Rayleigh collapse of a single vapor bubble near a rigid wall for different geometrical configurations and driving pressures. We explain that the presence of a rigid boundary breaks the symmetry of the collapse and hinders the energy concentration. As a result, a liquid re-entrant jet directed toward the wall forms, ultimately giving rise to lower pressure and temperatures produced upon collapse. We characterize the collapse non-sphericity, and show that this quantity, which strongly depends on the initial stand-off distance of the bubble from the wall, significantly affects the overall dynamics. We further show that bubbles initially close to the wall or attached to the surface are responsible not only for the high pressure loads along the wall, but also the elevated temperatures on the solid surface. In fact, for certain soft materials, instantaneous temperatures greater than the melting point may be achieved on the surface, thus confirming that thermal damage is a potential threat to such materials exposed to cavitating flows. Furthermore, the development of scalings for important collapse properties (jet velocity, shock pressure, wall pressures/temperatures), in terms of the initial stand-off distance and driving pressure, not only illustrates universality of non-spherical bubble dynamics but also provides means to predict these phenomena. Since real flows involve many bubbles, we also investigate the inertial collapse of a pair of vapor bubbles near a rigid surface. We explain that the presence of a second bubble in the vicinity of the original (primary) bubble leads to far more complicated dynamics and completely changes the single-bubble scalings. Strong interactions between the bubbles and the boundary drastically increase the collapse non-sphericity and amplify/hinder the pressures and temperatures produced by the collapse. Our simulations show that the re-entrant jets in both bubbles form at distorted angles, and for certain configurations, ``double jetting'', occurs, in which two jets penetrate the primary bubble. The results indicate that bubble-bubble interactions and their effects on collapse dynamics near a wall are non-negligible. Furthermore, given the complexity of even this simple problem and the large number of parameters, the value of extending such high-resolution simulations to develop scalings for the collapse of many bubbles is debatable at the present time; it may be worth considering alternative modeling approaches.PHDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/144079/1/alahyari_1.pd

Controlled translation and oscillation of micro-bubbles near a surface in an acoustic standing wave field

The removal of contamination particles from silicon wafers is critical in the semiconductor industry. Traditional cleaning techniques encounter difficulties in cleaning micro and nanometer-sized particles. A promising method that uses acoustically-driven micro-bubbles to clean contaminated surfaces has been reported. However, little is understood about the microscopic interaction between the micro-bubble and particle. This thesis explores the mechanism underlying the ultrasonic cleaning using micro-bubbles at the micrometer scale. The investigation was carried out from the perspective of bubble dynamics near a surface and bubble-particle interaction. Prior to contributing to the particle removal, micro-bubbles normally need to be transported to a target surface. The motion of a bubble was analyzed based on a force balance model for single and multi-bubble translations respectively. A good agreement is found between the observed bubble movement trajectories and the theoretical predictions. After arriving on a surface, a micro-bubble starts to disturb the flow field near the boundary through its oscillation. The characteristics of the flow field are closely related to the bubble oscillation modes. The influence of a wall on the change of bubble oscillation mode during its translation toward the boundary was studied. The relationship between bubble oscillation modes and the corresponding microstreaming around the bubble was established. The experimental results of bubble oscillation modes and the flow motion are quantitatively in good agreement with the simulation results. From a mechanic point of view, a possible ultrasonic cleaning mechanism is explained by exploring the relationship between different torques that are exerted on micro and sub-micrometer-sized particles. This estimation provides a qualitative insight into the ultrasonic cleaning process at a moderate pressure amplitude. The experimental investigation of the complicated particle detachment process requires improved test equipment to be developed in the future

Bubble nucleation and jetting inside a millimetric droplet

In this work, we present experiments and simulations on the nucleation and successive dynamics of laser-induced bubbles inside liquid droplets in free-fall motion, i.e. a case where the bubbles are subjected to the influence of a free boundary in all directions. The droplets of a millimetric size are released from a height of around 20\,cm and acquire a velocity of around 2\,m/s at the moment the bubble is nucleated. Within this droplet, we have investigated the nucleation of secondary bubbles induced by the rarefaction wave that is produced when the shock wave emitted by the laser-induced plasma reflects at the drop surface. Interestingly, three-dimensional clusters of cavitation bubbles are observed. Their shape is compared with the negative pressure distribution computed with a CFD model and allows us to estimate a cavitation threshold value. High-speed recordings of the drop/bubble dynamics are complemented by the velocity and pressure fields simulated for the same initial conditions. The effect of the proximity of a curved free surface on the jetting dynamics of the bubbles was qualitatively assessed by classifying the cavitation events using a non-dimensional stand-off parameter which depends on the drop size, the bubble maximum radius and the relative position of the bubble inside the drop. Here, we found that the curvature of the free surface does not play a determinant role on the jet dynamics, being the distance to the surface the dominant parameter. The oscillation of the laser-induced bubbles promote the onset of Rayleigh-Taylor and Rayleigh-Plateau instabilities, observed on the drop's surface. The specific mechanisms leading to the destabilisation of the droplet surface were identified through a careful inspection of the high speed images

Sound propagation and scattering in bubbly liquids

In the ocean, natural and artificial processes generate clouds of bubbles which scatter and attenuate sound. Measurements have shown that at the individual bubble resonance frequency, sound propagation in this medium is highly attenuated and dispersive. Theory to explain this behavior exists in the literature, and is adequate away from resonance. However, due to excessive attenuation near resonance, little experimental data exists for comparison. An impedance tube was developed specifically for exploring this regime. Using the instrument, unique phase speed and attenuation measurements were made for void fractions ranging from 6.2 × 10^−5 to 2.7 × 10^−3 and bubble sizes centered around 0.62 mm in radius. Improved measurement speed, accuracy and precision is possible with the new instrument, and both instantaneous and time-averaged measurements were obtained. Behavior at resonance was observed to be sensitive to the bubble population statistics and agreed with existing theory, within the uncertainty of the bubble population parameters. Scattering from acoustically compact bubble clouds can be predicted from classical scattering theory by using an effective medium description of the bubbly fluid interior. Experimental verification was previously obtained up to the lowest resonance frequency. A novel bubble production technique has been employed to obtain unique scattering measurements with a bubbly-liquid-filled latex tube in a large indoor tank. The effective scattering model described these measurements up to three times the lowest resonance frequency of the structure.United States Navy Office of Naval Research Ocean Acoustics Progra

Tools for developing continuous-flow micro-mixer : numerical simulation of transitional flow in micro geometries and a quantitative technique for extracting dynamic information from micro-bubble images

Recent advance in the microfluidics including its fabrication technologies has led to many novel applications in micro-scale flows. Among them is the continuous-flow micromixer that utilizes the advantages associated with turbulent flows for rapid mixing, achieving the detection of fast kinetic reaction as short as tens of microseconds. However, for developing a high performance continuous-flow micromixer there are certain fundamental issues need to be solved. One of them is an universal simulation approach capable of calculating the flow field across entire passage for entire regime from very low Reynolds number laminar flow through transition to fully turbulent flow. Though the direct numerical simulation is potentially possible solution but its extremely high computing time stops itself from practical applications. The second major issue is the inevitable occurrence of cavitation bubbles in this rapid flow apparatus. This phenomenon has opposite effects: (a) deteriorating performance and damaging the micromixer; (b) playing a catalyst role in enhancing mixing. A fully understanding of these micro bubbles will provide a sound theoretical base for guiding the design of micromixer in order to explore the advantage to maximum while minimizing its disadvantages. Therefore, the objectives of this PhD programme is to study the tools that will effectively advance our fundamental understandings on these key issues while in short term fulfil the requires from the joint experimental PhD programme held in the life science faculty for designing a prototype experimental device. During this PhD study, an existing numerical approach suitable for predicting the possibly entire flow regime including the turbulence transition is proposed for simulating the microscale flows in the microchannel and micromixer. The simulation results are validated against the transitional micro-channel experiments and this numerical method is then further applied for the micromixer simulation. This provides the researcher a realistic and feasible CFD tool to establish guidelines for designing high-efficiency and cost-effective micromixers by utilizing various possible measures which may cause very different flows simultaneously in micromixer. In order to study microscale cavitation bubbles and their effects on micromixers, an innovative experimental setup is purposely designed and constructed that can generate laser-induced micro-bubbles at desired position and size for testing. Experiments withvarious micro-scale bubbles have been performed successfully by using an ultra high-speed camera up to 1 million frame rate per second. A novel technique for tracking the contours of micro-scale cavitation bubble dynamically has been developed by using active contour method. By using this technique, for the first time, various geometric and dynamic data of cavitation bubble have been obtained to quantitatively analyze the global behaviours of bubbles thoroughly. This powerful tool will greatly benefit the study of bubble dynamics and similar demands in other fields for fast and accurate image treatments as well

AN IMPEDANCE TUBE FOR THE IN-SITU CLASSIFICATION OF BUBBLY LIQUIDS

It is well documented that the presence of even a few air bubbles in water can signifi- cantly alter the propagation and scattering of sound. Air bubbles are both naturally and artificially generated in all marine environments, especially near the sea surface. The abil- ity to measure the acoustic propagation parameters of bubbly liquids in situ has long been a goal of the underwater acoustics community. One promising solution is a submersible, thick-walled, liquid-filled impedance tube. Recent water-filled impedance tube work was successful at characterizing low void fraction bubbly liquids in the laboratory [1]. This work details the modifications made to the existing impedance tube design to allow for submersed deployment in a controlled environment, such as a large tank or a test pond. As well as being submersible, the useable frequency range of the device is increased from 5 - 9 kHz to 1 - 16 kHz and it does not require any form of calibration. The opening of the new impedance tube is fitted with a large stainless steel flange to better define the boundary condition on the plane of the tube opening. The new device was validated against the classic theoretical result for the complex reflection coefficient of a tube opening fitted with an infinite flange. The complex reflection coefficient was then measured with a bubbly liquid (order 250 micron radius and 0.1 - 0.5 % void fraction) outside the tube opening. Results from the bubbly liquid experiments were inconsistent with flanged tube theory using current bubbly liquid models. The results were more closely matched to unflanged tube theory, suggesting that the high attenuation and phase speeds in the bubbly liquid made the tube opening appear as if it were radiating into free space.US Navy Office of Naval Research

Vocal fold vibratory and acoustic features in fatigued Karaoke singers

Session 3aMU - Musical Acoustics and Speech Communication: Singing Voice in Asian CulturesKaraoke is a popular singing entertainment particularly in Asia and is gaining more popularity in the rest of world. In Karaoke, an amateur singer sings with the background music and video (usually guided by the lyric captions on the video screen) played by Karaoke machine, using a microphone and an amplification system. As the Karaoke singers usually have no formal training, they may be more vulnerable to vocal fatigue as they may overuse and/or misuse their voices in the intensive and extensive singing activities. It is unclear whether vocal fatigue is accompanied by any vibration pattern or physiological changes of vocal folds. In this study, 20 participants aged from 18 to 23 years with normal voice were recruited to participate in an prolonged singing task, which induced vocal fatigue. High speed laryngscopic imaging and acoustic signals were recorded before and after the singing task. Images of /i/ phonation were quantitatively analyzed using the High Speed Video Processing (HSVP) program (Yiu, et al. 2010). It was found that the glottis became relatively narrower following fatigue, while the acoustic signals were not sensitive to measure change following fatigue. © 2012 Acoustical Society of Americapublished_or_final_versio

Long-term physical evolution of an elastomeric ultrasound contrast microbubble

Hypothesis: One of the main assets of crosslinked polymer-shelled microbubbles (MBs) as ultrasound-active theranostic agents is the robustness of the shells, combined with the chemical versatility in modifying the surface with ligands and/or drugs. Despite the long shelf-life, subtle modifications occur in the MB shells involving shifts in acoustic, mechanical and structural properties. Experiments: We carried out a long-term morphological and acoustic evolution analysis on elastomeric polyvinyl-alcohol (PVA)-shelled MBs, a novel platform accomplishing good acoustic and surface performances in one agent. Confocal laser scanning microscopy, acoustic spectroscopy and AFM nanomechanics were integrated to understand the mechanism of PVA MBs ageing. The changes in the MB acoustic properties were framed in terms of shell thickness and viscoelasticity using a linearised oscillation theory, and compared to MB morphology and to nanomechanical analysis. Findings: We enlightened a novel, intriguing ageing time evolution of the PVA MBs with double behaviour with respect to a crossover time of ∼50 days. Before, significant changes occur in MB stiffness and shell thickness, mainly due to a massive release of entangled PVA chains. Then, the MB resonance frequency increases together with shell thickening and softening. Our benchmark study is of general interest for emerging viscoelastomeric bubbles towards personalised medicine