archives@tulane.edu / Reactive Cavitation Erosion (RCE), a new materials processing technique for the production of functionalized nanomaterials in which acoustic cavitation erosion is performed in a reactive medium, is described herein. Background material on acoustic cavitation erosion in the form of a literature review is presented. The effects of fluid properties and ambient pressure on the bubble dynamics at the high acoustic pressures commensurate with RCE are studied. The solutions to the Rayleigh-Plesset equation (RPE) and Keller-Miksis equation (KME) are compared. It is shown that to a first approximation, the RPE and KME give similar results. Analyses of the RPE solutions for real-world fluids reveal that many fluids result in cavitation intensity comparable to or greater than that of water. The groundwork for future modelling of RCE was established through the development of the Hemispherical Pit Model (HPM). The HPM is based upon a simple geometrical model of the volume loss process and contains parameters that may be more directly related to material properties and experimental parameters. Formation of functionalized clinoatacamite nanoparticles is achieved through Reactive Cavitation Erosion of copper discs in a 1 M guanidine hydrochloride solution. From analyses, the mechanism for formation of the clinoatacamite proceeded from ablation of metallic copper from the disc surface followed by subsequent reactions in solution. / 1 / Jeremy William Wright

The deposition of ultrasonic energy in tissue can cause tissue damage due to local heating. For pressures above a critical threshold, cavitation will occur in tissue and bubbles will be created. These oscillating bubbles can induce a much larger thermal energy deposition in the local region. Traditionally, clinicians and researchers have not exploited this bubble-enhanced heating since cavitation behavior is erratic and very difficult to control. The present work is an attempt to control and utilize this bubble-enhanced heating. First, by applying appropriate bubble dynamic models, limits on the asymptotic bubble size distribution are obtained for different driving pressures at 1 MHz. The size distributions are bounded by two thresholds: the bubble shape instability threshold and the rectified diffusion threshold. The growth rate of bubbles in this region is also given, and the resulting time evolution of the heating in a given insonation scenario is modeled. In addition, some experimental results have been obtained to investigate the bubble-enhanced heating in an agar and graphite based tissue- mimicking material. Heating as a function of dissolved gas concentrations in the tissue phantom is investigated. Bubble-based contrast agents are introduced to investigate the effect on the bubble-enhanced heating, and to control the initial bubble size distribution. The mechanisms of cavitation-related bubble heating are investigated, and a heating model is established using our understanding of the bubble dynamics. By fitting appropriate bubble densities in the ultrasound field, the peak temperature changes are simulated. The results for required bubble density are given. Finally, a simple bubbly liquid model is presented to estimate the shielding effects which may be important even for low void fraction during high intensity focused ultrasound (HIFU) treatment.

A complete understanding of high-intensity focused ultrasound-induced temperature changes in tissue requires insight into all potential mechanisms for heat deposition. Applications of therapeutic ultrasound often utilize acoustic pressures capable of producing cavitation activity. Recognizing the ability of bubbles to transfer acoustic energy into heat generation, a study of the role bubbles play in tissue hyperthermia becomes necessary. These bubbles are typically less than 50μm. This dissertation examines the contribution of bubbles and their motion to an enhanced heating effect observed in a tissue-mimicking phantom. A series of experiments established a relationship between bubble activity and an enhanced temperature rise in the phantom by simultaneously measuring both the temperature change and acoustic emissions from bubbles. It was found that a strong correlation exists between the onset of the enhanced heating effect and observable cavitation activity. In addition, the likelihood of observing the enhanced heating effect was largely unaffected by the insonation duration for all but the shortest of insonation times, 0.1 seconds. Numerical simulations were used investigate the relative importance of two candidate mechanisms for heat deposition from bubbles as a means to quantify the number of bubbles required to produce the enhanced temperature rise. The energy deposition from viscous dissipation and the absorption of radiated sound from bubbles were considered as a function of the bubble size and the viscosity of the surrounding medium. Although both mechanisms were capable of producing the level of energy required for the enhanced heating effect, it was found that inertial cavitation, associated with high acoustic radiation and low viscous dissipation, coincided with the the nature of the cavitation best detected by the experimental system. The number of bubbles required to account for the enhanced heating effect was determined through the numerical study to be on the order of 150 or less.

No description available.

Cavitation inside an automotive torque converter running at various pump speeds was simulated by using the Computational Fluid Dynamics (CFD) commercial package ANSYS-CFX 10.0/11.0. The numerical solution obtained for the case with no cavitation was used as an initial condition for the case of flow with cavitation to accelerate convergence. The converter was initially modeled using several grid sizes to evaluate the effect of grid density on the numerical solution and to select the optimum grid size for subsequent simulations. Comparison of CFD to actual test results demonstrates that the cavitation model built in the commercial code, which was developed by Zwart et. al. (2004) based on the simplified Rayleigh-Plesset equation of bubble dynamics, does not capture the full effect of cavitation inside the converter. Modifications to this model have been investigated in this study. The effect of the variation of the automotive transmission oil vapor pressure due to the rise in temperature during normal operating conditions was also investigated and found not to cause any significant change to the area of vapor formation, and hence did not have a significant effect on the converter performance. Values of the empirical coefficients of the cavitation model had to be modified in order for the model to capture the full effect of cavitation on the performance of the converter operating at high pump speeds. Results showed a much larger area of vapor over the converter stator and traces of vapor appeared inside the pump, and turbine blades. With these modifications, the model produced results in better agreement with the available experimental data. Moreover, simulations have been carried out in both steady and transient states using various turbulence models available in CFX10.0/11.0 in order to evaluate the effect of the choice of turbulence models on cavitation prediction. / Thesis / Master of Applied Science (MASc)

Histotripsy is a noninvasive mechanical ablation method that uses focused ultrasound to disintegrate target tissues into acellular homogenate through the generation of acoustic cavitation and is currently being developed for numerous clinical applications. Histotripsy uses high-pressure (>10 MPa), short-duration (<15 cycles) pulses to cause the rapid expansion and collapse of nuclei at the focus resulting in large applied stress and strain in the adjacent tissue. At a sufficiently high pressure above the target medium's intrinsic cavitation threshold and an adequate number of applied pulses, cavitation "bubble clouds" create precise lesions with high fidelity to the region of the focus. Despite advances in histotripsy, additional research is still needed to better understand the acoustic cavitation nucleation process and its effects on therapies using focused ultrasound. This understanding is critical to better predict and control pulse dose for more rapid and efficient ablation procedures, to reduce off-target cavitation events for safer focused ultrasound therapies, and to localize ablation for high-precision procedures near critical structures or treatments without active imaging guidance. In this dissertation, I investigate the nucleation and dynamics of ultrasonically generated acoustic cavitation for novel applications of focused ultrasound. My Ph.D. thesis focuses on (1) investigating the effect of histotripsy pulsing parameters on bubble cloud cavitation nucleation, bubble dynamics, and ablation efficiency, (2) investigating the effect of nuclei characteristics on the threshold for cavitation nucleation and resulting bubble dynamics for therapeutic applications, and (3) developing methods alter select characteristics and dynamics of acoustic cavitation by adjusting pulsing parameters to optimize ablation efficiency in conventional and nanoparticle-mediated histotripsy. The culmination of this thesis will advance our understanding of the nucleation and behavior of acoustic cavitation from pulsed focused ultrasound and develop innovative systems to improve the efficacy, efficiency, and safety of clinical focused ultrasound therapies. / Doctor of Philosophy / Histotripsy is a noninvasive focused ultrasound method that precisely destroys target tissues such as tumors through the acoustic generation of cavitation and is currently being developed for numerous clinical applications. Histotripsy uses high-pressure, short-duration pulsed soundwaves to cause the bubbles to rapidly expand and collapse within a precise region called the focus. This rapid cavitation results in large mechanical strain in the targeted tissue. With increasingly higher pressure, numerous bubbles form in the shape of cavitation "bubble clouds" that create lesions, closely matching their shape, in the target tissue after a sufficient number of pulses have been applied. Despite advances in histotripsy, additional research is still needed to better understand the initiation of the acoustic cavitation process in histotripsy and its effects on focused ultrasound therapies. This understanding is critical to better predict and control ablation procedures, improve procedure efficiency, reduce off-target cavitation events for safer focused ultrasound therapies, and further increase ablation precision for procedures near critical structures or treatments without active image guidance. In this dissertation, I investigate the initiation, growth, and collapse of ultrasonically generated acoustic cavitation for novel applications of focused ultrasound. My Ph.D. thesis focuses on (1) investigating the effect of histotripsy pulsing parameters on bubble cloud cavitation initiation, bubble growth and collapse, and treatment efficiency, (2) investigating the effect of particle characteristics on the threshold for cavitation initiation and resulting bubble behavior for therapeutic applications, and (3) adjusting pulsing parameters to optimize ablation efficiency in conventional and particle mediated histotripsy. The culmination of this thesis will advance our understanding of the initiation and behavior of acoustic cavitation from pulsed focused ultrasound and develop innovative systems to improve the efficacy, efficiency, and safety of clinically focused ultrasound therapies.

Column flotation cells have become the most popular machine designed for industrial applications that require the separation and concentration of wanted or unwanted minerals from the rest material associated in a pulp. To achieve this process separation an air sparging device, which is required to produce bubbles in the flotation cell is required. In column flotation operations, Sparger sparging devices are employed in column flotation operations to generate small bubbles into the cell with the aim to carry the the desired mineral to the surface for later be recovered and proceeded. However, field studies suggest that air injector sparging systems are not always optimized. Two of the reasonsReasons that contributinge to the lack of optimization areis unfavorable state are: (i) ineffective internal design of the sparging system, and (ii) poor operation techniques employed inby the industrial processing industrial plants. The present project intends to better understand sparging performance into the column cell and how to optimize sparging systems more effectively. To achieve this end, With this in mind, data of for gas-water injection rate, froth addition, and inlet-pressure have been collected and analyzed. The This data not only will facilitate an insight of to better operational practices that plant operators can employ to improve column performance, but it also will make it possible to correct flaws in the design of the sparging systems currently used in column flotation operations. / Master of Science

Gaseous cavitation is examined from a practical and theoretical standpoint. Classical cavitation experiments which disregard dissolved gas are not directly relevant to natural water systems and require a redefined cavitation inception number which considers dissolved gases. In a pressurized water distribution system, classical cavitation is only expected to occur at extreme negative pressure caused by water hammer or at certain valves. Classical theory does not describe some practical phenomena including noisy pipes, necessity of air release valves, faulty instrument readings due to bubbles, and reports of premature pipe failure; inclusion of gaseous cavitation phenomena can better explain these events. Gaseous cavitation can be expected to influence corrosion in water distribution pipes. Bubbles can form within the water distribution system by a mechanism known as gaseous cavitation. A small scale apparatus was constructed to track gaseous cavitation as it could occur in buildings. Four independent measurements including visual observation of bubbles, an inline turbidimeter, an ultrasonic flow meter, and an inline total dissolved gas probe were used to track the phenomenon. All four measurements confirmed that gaseous cavitation was occurring within the experimental distribution system, even at pressures up to 40 psi. Gaseous cavitation was more likely at higher initial dissolved gas content, higher temperature, higher velocity and lower pressure. Certain changes in pH, conductivity, and surfactant concentration also tended to increase the likelihood of cavitation. For example, compared to the control at pH 5.0 and 30 psig, the turbidity increased 295% at pH 9.9. The formation of bubbles reduced the pump's operating efficiency, and in the above example, the velocity was decreased by 17% at pH 9.9 versus pH 5.0. / Master of Science

Avoiding cavitation and especially cavitation erosion are tasks, which have to be considered when working with hydraulics. State of the art is the assessment of the risk of erosion by component testing or to completely avoid cavitation by means of CFD. Another reliable method to assess the risk of cavitation erosion is until now not available. This paper deals with this problem and delivers comparative values for a later method development. In a first step the cavitation of a poppet valve, which controls a methanol flow, is visualized. The resulting three cavitation appearances are deeply examined. After that the results of long-term tests at different operation conditions are presented. A poppet surface analysis following each experiment has shown different types of surface attacks. As a result of this work it is shown that both cavitation appearance and surface attack are strongly influenced by the temperature dependent air solubility of the liquid.

Efforts to improve diesel fuel sprays have led to a significant increase in fuel injection pressures and a reduction in nozzle-hole diameters. Under these conditions, the likelihood for the internal nozzle flow to cavitate is increased, which potentially affects spray breakup and atomisation, but also increases the risk of causing cavitation damage to the injector. This thesis describes the study of cavitating flow phenomena in various single and multi-hole optical nozzle geometries. It includes the design and development of a high-pressure optical fuel injector test facility with which the cavitating flows were observed. Experiments were undertaken using real-scale optical diesel injector nozzles at fuel injection pressures up to 2050 bar, observing for the first time the characteristics of the internal nozzle-flow under realistic fuel injection conditions. High-speed video and high resolution photography, using laser illumination sources, were used to capture the cavitating flow in the nozzle-holes and sac volume of the optical nozzles, which contained holes ranging in size from 110 micrometers to 300 micrometers. Geometric cavitation in the nozzle-holes and string cavitation formation in the nozzle-holes and sac volume were both observed using transient and steady-state injection conditions; injecting into gaseous and liquid back pressures up to 150 bar. Results obtained have shown that cavitation strings observed at realistic fuel injection pressures exhibit the same physical characteristics as those observed at lower pressures. The formation of string cavitation was observed in the 300 micrometers multi-hole nozzle geometries, exhibiting a mutual dependence on nozzle flow-rate and the geometry of the nozzle-holes. Pressure changes, caused by localised turbulent perturbations in the sac volume and transient fuel injection characteristics, independently affected the geometric and string cavitation formation in each of the holes. String cavitation formation of was shown to occur when free-stream vapour was entrained into the low pressure core of a sufficiently intense coherent vortex. Hole diameters less than or equal to 160 micrometers were found to suppress string cavitation formation, with this effect a result of the reduced nozzle flow rate and vortex intensity. Using different hole spacing geometries, it was demonstrated that the formation of cavitation strings in a particular geometry became independent of fuel injection and back pressure once a threshold pressure drop across the nozzle had been reached.