Bubble Cloud Dynamics Research Articles

Shared-Memory Parallelization for Two-Way Coupled Euler–Lagrange Modeling of Cavitating Bubbly Flows

Cavitating and bubbly flows are encountered in many engineering problems involving propellers, pumps, valves, ultrasonic biomedical applications, etc. In this contribution, an openmp parallelized Euler–Lagrange model of two-phase flow problems and cavitation is presented. The two-phase medium is treated as a continuum and solved on an Eulerian grid, while the discrete bubbles are tracked in a Lagrangian fashion with their dynamics computed. The intimate coupling between the two description levels is realized through the local void fraction, which is computed from the instantaneous bubble volumes and locations, and provides the continuum properties. Since, in practice, any such flows will involve large numbers of bubbles, schemes for significant speedup are needed to reduce computation times. We present here a shared-memory parallelization scheme combining domain decomposition for the continuum domain and number decomposition for the bubbles; both selected to realize maximum speedup and good load balance. The Eulerian computational domain is subdivided based on geometry into several subdomains, while for the Lagrangian computations, the bubbles are subdivided based on their indices into several subsets. The number of fluid subdomains and bubble subsets matches with the number of central processing unit (CPU) cores available in a shared-memory system. Computation of the continuum solution and the bubble dynamics proceeds sequentially. During each computation time step, all selected openmp threads are first used to evolve the fluid solution, with each handling one subdomain. Upon completion, the openmp threads selected for the Lagrangian solution are then used to execute the bubble computations. All data exchanges are executed through the shared memory. Extra steps are taken to localize the memory access pattern to minimize nonlocal data fetch latency, since severe performance penalty may occur on a nonuniform memory architecture (NUMA) multiprocessing system where thread access to nonlocal memory is much slower than to local memory. This parallelization scheme is illustrated on a typical nonuniform bubbly flow problem, cloud bubble dynamics near a rigid wall driven by an imposed pressure function (Ma et al., 2013, “Euler–Lagrange Simulations of Bubble Cloud Dynamics Near a Wall,” International Mechanical Engineering Congress and Exposition, San Diego, CA, Nov. 15–21, Paper No. IMECE2013-65191 and Ma et al., 2015, “Euler–Lagrange Simulations of Bubble Cloud Dynamics Near a Wall,” ASME J. Fluids Eng., 137(4), p. 041301).

Hysteresis of inertial cavitation activity induced by fluctuating bubble size distribution

Amongst the variety of complex phenomena encountered in nonlinear physics, a hysteretic effect can be expected on ultrasound cavitation due to the intrinsic nonlinearity of bubble dynamics. When applying successive ultrasound shots for increasing and decreasing acoustic intensities, a hysteretic behaviour is experimentally observed on inertial cavitation activity, with a loop area sensitive to the inertial cavitation threshold. To get a better insight of the phenomena underlying this hysteretic effect, the evolution of the bubble size distribution is studied numerically by implementing rectified diffusion, fragmentation process, rising and dissolution of bubbles from an initial bubble size distribution. When applying increasing and decreasing acoustic intensities, the numerical distribution exhibits asymmetry in bubble number and distribution. The resulting inertial cavitation activity is assessed through the numerical broadband noise of the emitted acoustic radiation of the bubble cloud dynamics. This approach allows obtaining qualitatively the observed hysteretic effect and its interest in terms of control is discussed.

Removal of residual cavitation nuclei to enhance histotripsy erosion of model urinary stones.

Histotripsy has been shown to be an effective treatment for model kidney stones, eroding their surface to tiny particulate debris via a cavitational bubble cloud. However, similar to shock wave lithotripsy, histotripsy stone treatments display a rate-dependent efficacy, with pulses applied at a low rate generating more efficient stone erosion in comparison with those applied at a high rate. This is hypothesized to be the result of residual cavitation bubble nuclei generated by bubble cloud collapse. Although the histotripsy bubble cloud only lasts on the order of 100 μs, these microscopic remnant bubbles can persist on the order of 1 s, inducing direct attenuation of subsequent histotripsy pulses and influencing bubble cloud dynamics. In an effort to mitigate these effects, we have developed a novel strategy to actively remove residual cavitation nuclei from the field using low-amplitude ultrasound pulses. Previous work has demonstrated that with selection of the appropriate acoustic parameters these bubble removal pulses can stimulate the aggregation and subsequent coalescence of microscopic bubble nuclei, effectively deleting them from the target volume. Here, we incorporate bubble removal pulses in histotripsy treatment of model kidney stones. It was found that when histotripsy is applied at low rate (1 Hz), bubble removal does not produce a statistically significant change in erosion. At higher pulse rates of 10, 100, and 500 Hz, incorporating bubble removal results in 3.7-, 7.5-, and 2.7-fold increases in stone erosion, respectively. High-speed imaging indicates that the introduction of bubble removal pulses allows bubble cloud dynamics resulting from high pulse rates to more closely approximate those generated at the low rate of 1 Hz. These results corroborate previous work in the field of shock wave lithotripsy regarding the ill effects of residual bubble nuclei, and suggest that high treatment efficiency can be recovered at high pulse rates through appropriate manipulation of the cavitation environment surrounding the stone.

Improvement in microhollow production using bubble cloud cavitation by dual-frequency ultrasonic wave irradiation

Bubble cloud cavitation plays an important role in sonoporation, which produces microhollows on the cell membrane using a microjet. However, it has complicated mechanisms and the optimization of the ultrasonic wave irradiation sequence is difficult. The optimization of an ultrasonic wave irradiation sequence, which allows for effective bubble cloud cavitation, is very important for producing high-quality microhollows. Thus, it is necessary to clarify the mechanism of cavitation for different-frequency ultrasonic waves. In this study, we evaluate bubble cloud cavitation for ultrasonic wave irradiations of different frequencies. The observation of bubble cloud dynamics using a high-speed camera and of microhollows using a confocal laser microscope are performed in this study. From these results, a novel ultrasonic wave irradiation sequence that uses two ultrasonic waves with different frequencies is proposed.

Cavitation mapping by sonochemiluminescence with less bubble displacement induced by acoustic radiation force in a 1.2 MHz HIFU

An acoustic radiation force counterbalanced appliance was employed to map the cavitation distribution in water. The appliance was made up of a focused ultrasound transducer and an aluminum alloy reflector with the exactly same shape. They were centrosymmetry around the focus of the source transducer. Spatial–temporal dynamics of cavitation bubble clouds in the 1.2MHz ultrasonic field within this appliance were observed in water. And they were mapped by sonochemiluminescence (SCL) recordings and high-speed photography. There were significant differences in spatial distribution and temporal evolution between normal group and counterbalanced group. The reflector could avoid bubble directional displacement induced by acoustic radiation force under certain electric power (⩽50W). As a result, the SCL intensity in the pre-focal region was larger than that of normal group. In event of high electric power (⩾70W), most of the bubbles were moving in acoustic streaming. When electric power decreased, bubbles kept stable and showed stripe structure in SCL images. Both stationary bubbles and moving bubbles have been captured, and exhibited analytical potential with respect to bubbles in therapeutic ultrasound.

Shock-controlled bubble cloud dynamics and light emission.

Cavitation bubble collapse can generate intense concentrations of mechanical energy, sufficient to erode even the hardest metals and to generate light emissions visible to the naked eye. In this talk we describe cavitation bubble cloud experiments carried out in spherical resonators at ambient and acoustic pressures up to 30 MPa. Key to our system is the ability to nucleate with temporal and spatial controls, which we achieve using dielectric breakdown in water from pulsed focused laser beams. Our observations show that the cloud dynamics are controlled by the repetitive emission of shock waves, which propagate outward from the inertial cloud collapse, reflect off of the sphere wall, and then converge on the resonator center. Shock convergence phenomena and light emission from compact cloud collapse will be discussed. [Work supported by the Impulse Devices, Inc.]

Mitigation of Damage to Solid Surfaces From the Collapse of Cavitation Bubble Clouds

The collapse of transient bubble clouds near a solid surface was investigated to test a scheme for mitigation of cavitation-induced damage. The target was a porous ceramic disk through which air could be forced. Transient cavitation bubbles were created using a shock-wave lithotripter focused on the surface of the disk. The dynamics of bubble clouds near the ceramic disks were studied for two boundary conditions: no back pressure resulting in surface free of bubbles and 10 psi (0.7 atm) of back pressure, resulting in a surface with a sparse (30% of area) bubble layer. Images of the cavitation near the surface were obtained from a high-speed camera. Additionally, a passive cavitation detector (3.5 MHz focused acoustic transducer) was aligned with the surface. Both the images and the acoustic measurements indicated that bubble clouds near a ceramic face without a bubble layer collapsed onto the boundary, subsequently leading to surface erosion. When a sparse bubble layer was introduced, bubble clouds collapsed away from the surface, thus mitigating cavitation damage. The erosion damage to the ceramic disks after 300 shock waves was quantified using micro-CT imaging. Pitting up to 1 mm deep was measured for the bubble-free surface, and the damage to the bubble surface was too small to be detected.

Introduction of a compliant gas-layer serves to mitigate damage to solid surfaces from the collapse of cavitation bubble clouds

The collapse of transient bubble clouds near a boundary was investigated. Transient cavitation bubbles were created using a shock-wave lithotripter. A porous ceramic disk (flow-pressure 7.5psi) was placed at the lithotripter focus. Air was forced through the disk to alter the boundary condition at the ceramic disk's proximal face. Gas pressure below 7.5psi resulted in a ceramic disk partially filled with fluid (rigid boundary); gas pressure over 7.5psi resulted in active bubbling at the proximal face (compliant boundary). Cavitation dynamics of bubble clouds near ceramic disks were studied for varying gas pressures (0-10psi). Images of the collapse were obtained from a high-speed camera. Additionally, a passive cavitation detector (3.5MHz focused transducer) was aligned with the lithotripter focus. Both the images and the acoustic measurements indicated that bubble clouds near a rigid boundary collapse onto the boundary, forming a re-entrant liquid jet whose impact leads to surface erosion. When a compliant boundary is introduced, bubble clouds collapse away from the surface, thus mitigating cavitation damage. The damage to the ceramic disks was quantified using micro-CT imaging. [Supported by the ORNL Spallation Neutron Source, which is managed by UT-Battelle, LLC, under contract DE-AC05-00OR22725 for the U.S. Department of Energy.]

Evolution of bubble clouds induced by pulsed cavitational ultrasound therapy - Histotripsy

Mechanical tissue fractionation can be achieved using successive, high-intensity ultrasound pulses in a process termed histotripsy. Histotripsy has many potential clinical applications where noninvasive tissue removal is desired. The primary mechanism for histotripsy is believed to be cavitation. Using fast-gated imaging, this paper studies the evolution of a cavitating bubble cloud induced by a histotripsy pulse (10 and 14 cycles) at peak negative pressures exceeding 21MPa. Bubble clouds are generated inside a gelatin phantom and at a tissue-water interface, representing two situations encountered clinically. In both environments, the imaging results show that the bubble clouds share the same evolutionary trend. The bubble cloud and individual bubbles in the cloud were generated by the first cycle of the pulse, grew with each cycle during the pulse, and continued to grow and collapsed several hundred microseconds after the pulse. For example, the bubbles started under 10 microm, grew to 50 microm during the pulse, and continued to grow 100 microm after the pulse. The results also suggest that the bubble clouds generated in the two environments differ in growth and collapse duration, void fraction, shape, and size. This study furthers our understanding of the dynamics of bubble clouds induced by histotripsy.

Optical and acoustic monitoring of bubble cloud dynamics at a tissue-fluid interface in ultrasound tissue erosion

Short, high-intensity ultrasound pulses have the ability to achieve localized, clearly demarcated erosion in soft tissue at a tissue-fluid interface. The primary mechanism for ultrasound tissue erosion is believed to be acoustic cavitation. To monitor the cavitating bubble cloud generated at a tissue-fluid interface, an optical attenuation method was used to record the intensity loss of transmitted light through bubbles. Optical attenuation was only detected when a bubble cloud was seen using high speed imaging. The light attenuation signals correlated well with a temporally changing acoustic backscatter which is an excellent indicator for tissue erosion. This correlation provides additional evidence that the cavitating bubble cloud is essential for ultrasound tissue erosion. The bubble cloud collapse cycle and bubble dissolution time were studied using the optical attenuation signals. The collapse cycle of the bubble cloud generated by a high intensity ultrasound pulse of 4-14 micros was approximately 40-300 micros depending on the acoustic parameters. The dissolution time of the residual bubbles was tens of ms long. This study of bubble dynamics may provide further insight into previous ultrasound tissue erosion results.

Numerical investigation of interactions between ultrasound propagation and cavitation bubbles in shock wave therapy

Acoustic cavitation in shock wave applications is responsible for several therapeutic effects as well as for undesired side effects. Using a two‐phase continuum approach for bubbly liquids, a numerical model is presented to simulate the interactions between nonlinear ultrasound propagation and cavitation bubbles. Numerical results are presented, showing in what manner the presence of cavitation bubbles alters the pressure pulse propagation and how the bubble dynamics can be specifically affected by changing the shock pressure waveform. An increased bubble density leads to significant changes in the tensile part of the pressure waveform. This effect is caused by the changes in medium compressibility due to the oscillating cavitation bubbles. Calculations for the bubble cloud dynamics induced by three pressure waveforms with identical energy but different temporal forms of the tensile part are compared. Simulation results demonstrate that tensile parts with low amplitude but long duration increase the cavit...

Modeling of interaction between therapeutic ultrasound propagation and cavitation bubbles

In medical applications of high intense focused ultrasound the mechanism of interaction between ultrasound waves and cavitation bubbles is responsible for several therapeutic effects as well as for undesired side effects. Based on a two-phase continuum approach for bubbly liquids, in this paper a numerical model is presented to simulate these interactions. The numerical results demonstrate the influence of the cavitation bubble cloud on ultrasound propagation. In the case of a lithotripter pulse an increased bubble density leads to significant changes in the tensile part of the pressure waveform. The calculations are verified by measurements with a fiber optical hydrophone and by experimental results of the bubble cloud dynamics.

Interpreting passive cavitation detection signals during high-intensity focused ultrasound (HIFU)

Inertial cavitation is known to be associated with significantly enhanced rates of heating during cancer treatment by high-intensity focused ultrasound (HIFU). However, during prolonged exposure, the cavitation region gradually shifts towards the HIFU transducer, causing shielding of the original focus. It is hoped that controlling the HIFU field to constrain cavitation to the focal region will lead to sustained enhanced focal heating, thus reducing treatment times and improving treatment resolution. Monitoring of cavitation can be achieved during insonation by using a focussed passive cavitation detector (PCD) to listen for broadband noise emissions resulting from microbubble collapse. The ability to effectively control the cavitation field relies on accurate interpretation of the PCD data. During HIFU exposure of a tissue-mimicking material, it was noted that the frequency content of the PCD signal shifted downwards with increasing phantom propagation path length to the PCD, as a result of depth-dependent attenuation. This has significant implications for the effective sensing volume of the PCD. In addition, it was noted that separate analysis of the peak and rms time-domain PCD values provides an incomplete description of inertial cavitation activity, but that combining these two parameters enables a better understanding of bubble cloud dynamics.

Spatial–temporal dynamics of cavitation bubble clouds in 1.2 MHz focused ultrasound field

Cavitation bubbles have been recognized as being essential to many applications of ultrasound. Temporal evolution and spatial distribution of cavitation bubble clouds induced by a focused ultrasound transducer of 1.2 MHz center frequency are investigated by high-speed photography. It is revealed that at a total acoustic power of 72 W the cavitation bubble cloud first emerges in the focal region where cavitation bubbles are observed to generate, grow, merge and collapse during the initial 600 μs. The bubble cloud then grows upward to the post-focal region, and finally becomes visible in the pre-focal region. The structure of the final bubble cloud is characterized by regional distribution of cavitation bubbles in the ultrasound field. The cavitation bubble cloud structure remains stable when the acoustic power is increased from 25 W to 107 W, but it changes to a more violent form when the acoustic power is further increased to 175 W.

Medical ultrasound with microbubbles

In the medical ultrasound applications, microbubbles are closely tied to the diagnostic/therapeutic uses. For diagnostic applications, their sound scattering properties yield improved imaging, when the microbubbles are used as contrast agents. The harmonics and subharmonics responses from the bubbles assist in distinguishing the acoustic scattering of blood from that of the surrounding tissue. The therapeutic use of microbubbles has recently been the subject of much interest. In a HIFU treatment, the heat generated by the bubble motion contributes an enhanced localized heating effect from the ultrasound. In the lithotripsy of renal calculi, the acoustic cloud cavitation contributes to the comminution of the renal stones. In all these applications, it is essential to understand the microbubbles and bubble cloud dynamics. The bubble motion and bubble cloud behavior are strongly influenced by the internal phenomena of the bubbles, such as thermal diffusion, mist formation, mass diffusion and heat and mass transfer through the bubble wall. In our research, bubble and bubble cloud dynamics in the medical ultrasound field have been examined. In this paper numerical results and the new techniques of medical ultrasound applications with microbubbles, microbubble enhanced HIFU and Cavitation Control Lithotripsy (CCL), are highlighted.

Automated identification of thoracic cage region from X-ray CT images

In the medical ultrasound applications, microbubbles are closely tied to the diagnostic/therapeutic uses. For diagnostic applications, their sound scattering properties yield improved imaging, when the microbubbles are used as contrast agents. The harmonics and subharmonics responses from the bubbles assist in distinguishing the acoustic scattering of blood from that of the surrounding tissue. The therapeutic use of microbubbles has recently been the subject of much interest. In a HIFU treatment, the heat generated by the bubble motion contributes an enhanced localized heating effect from the ultrasound. In the lithotripsy of renal calculi, the acoustic cloud cavitation contributes to the comminution of the renal stones. In all these applications, it is essential to understand the microbubbles and bubble cloud dynamics. The bubble motion and bubble cloud behavior are strongly influenced by the internal phenomena of the bubbles, such as thermal diffusion, mist formation, mass diffusion and heat and mass transfer through the bubble wall. In our research, bubble and bubble cloud dynamics in the medical ultrasound field have been examined. In this paper numerical results and the new techniques of medical ultrasound applications with microbubbles, microbubble enhanced HIFU and Cavitation Control Lithotripsy (CCL), are highlighted.

1206 超音波キャビテーションの制御

It is known that cavitation erosion cause to break the parts of fluid machinery or damage the tissue of the body during medical ultrasound, for example, ESWL : Extracorporeal Shock Wave Lithotripsy. But if cavitation behavior can be analyzed and controlled actively, we are able to utilize cavitation as a transducer, which can concentrate energy in a very narrow area. And this cavitation control has the possibilities to be applied in engineering and medical field. In order to analyze cavitation behavior in ultrasound field, we conduct experiments and numerically simulate the dynamics of bubble cloud in detail, and we obtain the following conclusions. We build the method to concentrate cavitation cloud only in a small region with high frequency ultrasound and oscillate and crush the cloud with low frequency one near the natural frequency of it. And it is possible to break an object in a narrow area by this method.

Spatio-temporal dynamics of cavitation bubble clouds in a low frequency reactor: comparison between theoretical and experimental results

The propagation of ultrasound through a liquid induces the growth of inceptions and germs into bubbles. In a low frequency reactor, fragmentary transient bubbles emerge due to the acoustic driving. They violently collapse in one cycle and fragment into many smaller bubbles than in turn cavitate. This violent collapse is responsible for the mechanical effects of ultrasounds effects. The latter bubbles gather in a ball-shaped cloud and migrate to pressure antinodes. During their migration, their nonexplosive collapses mainly contribute to activate chemical reactions by producing OH • radicals. Mathematical modelling is performed as a new approach to predict the bubbles field. Through numerical simulation, we determinate emergence sites of mechanically active cavitation bubbles. Calculus are compared with aluminium foil degradation. The modelling of bubble migration allow us to have an insight on the privileged sites of the chemical reactions. Validation of the modelling is made through direct comparison with chemiluminescence photo. All experiments and computations are made in a 28.2 kHz sonoreactor.

Low-Frequency Resonant Scattering of Bubble Clouds*

Abstract The acoustic properties of water can be drastically modified by a small amount of air content in the fluid. The dynamics of bubble clouds and their collected oscillation mechanisms are among topics of active research in underwater acoustics. In the ocean, bubble clouds are long lasting, and most of the time can be considered as passive scatterers. Solutions of the key parameters such as resonance frequencies, sound speed, and target strength can be derived from treating the bubble cloud as a homogeneous medium with proper effective bulk properties. In this paper, the resonance frequency and target strength are derived from computations based on a classical solution for acoustic scattering of elastic spheres. The range of void fractions covers four orders of magnitude and includes single air bubbles as its asymptotic condition of void fraction equals unity. Based on these computations, it is found that the isothermal condition is approached only at very low void fraction levels (<10−4). At high vo...

A dual passive cavitation detector for localized detection of lithotripsy-induced cavitation in vitro.

A passive cavitation detector (PCD) identifies cavitation events by sensing acoustic emissions generated by the collapse of bubbles. In this work, a dual passive cavitation detector (dual PCD), consisting of a pair of orthogonal confocal receivers, is described for use in shock wave lithotripsy. Cavitation events are detected by both receivers and can be localized to within 5 mm by the nature of the small intersecting volume of the focal areas of the two receivers in association with a coincidence detection algorithm. A calibration technique, based on the impulse response of the transducer, was employed to estimate radiated pressures at collapse near the bubble. Results are presented for the in vitro cavitation fields of both a clinical and a research electrohydraulic lithotripter. The measured lifetime of the primary growth-and-collapse of the cavitation bubbles increased from 180 to 420 microseconds as the power setting was increased from 12 to 24 kV. The measured lifetime compared well with calculations based on the Gilmore-Akulichev formulation for bubble dynamics. The radiated acoustic pressure 10 mm from the collapsing cavitation bubble was measured to vary from 4 to 16 MPa with increasing power setting; although the trends agreed with calculations, the predicted values were four times larger than measured values. The axial length of the cavitation field correlated well with the 6-dB region of the acoustic field. However, the width of the cavitation field (10 mm) was significantly narrower than the acoustic field (25 mm) as bubbles appeared to be drawn to the acoustic axis during the collapse. The dual PCD also detected signals from "rebounds," secondary and tertiary growth-and-collapse cycles. The measured rebound time did not agree with calculations from the single-bubble model. The rebounds could be fitted to a Rayleigh collapse model by considering the entire bubble cloud as an effective single bubble. The results from the dual PCD agreed well with images from high-speed photography. The results indicate that single-bubble theory is sufficient to model lithotripsy cavitation dynamics up to time of the main collapse, but that upon collapse bubble cloud dynamics becomes important.