Prediction of far-field acoustic emissions from cavitation clouds during shock wave lithotripsy for development of a clinical device

This study presents the key simulation and decision stage of a multi-disciplinary project to develop a hospital device for monitoring the effectiveness of kidney stone fragmentation by shock wave lithotripsy (SWL). The device analyses, in real time, the pressure fields detected by sensors placed on the patient's torso, fields generated by the interaction of the incident shock wave, cavitation, kidney stone and soft tissue. Earlier free-Lagrange simulations of those interactions were restricted (by limited computational resources) to computational domains within a few centimetres of the stone. Later studies estimated the far-field pressures generated when those interactions involved only single bubbles. This study extends the free-Lagrange method to quantify the bubble–bubble interaction as a function of their separation. This, in turn, allowed identification of the validity of using a model of non-interacting bubbles to obtain estimations of the far-field pressures from 1000 bubbles distributed within the focus of the SWL field. Up to this point in the multi-disciplinary project, the design of the clinical device had been led by the simulations. This study records the decision point when the project's direction had to be led by far more costly clinical trials instead of the relatively inexpensive simulations. <br/

Numerical simulations of non-spherical bubble collapse

A high-order accurate shock- and interface-capturing scheme is used to simulate the collapse of a gas bubble in water. In order to better understand the damage caused by collapsing bubbles, the dynamics of the shock-induced and Rayleigh collapse of a bubble near a planar rigid surface and in a free field are analysed. Collapse times, bubble displacements, interfacial velocities and surface pressures are quantified as a function of the pressure ratio driving the collapse and of the initial bubble stand-off distance from the wall; these quantities are compared to the available theory and experiments and show good agreement with the data for both the bubble dynamics and the propagation of the shock emitted upon the collapse. Non-spherical collapse involves the formation of a re-entrant jet directed towards the wall or in the direction of propagation of the incoming shock. In shock-induced collapse, very high jet velocities can be achieved, and the finite time for shock propagation through the bubble may be non-negligible compared to the collapse time for the pressure ratios of interest. Several types of shock waves are generated during the collapse, including precursor and water-hammer shocks that arise from the re-entrant jet formation and its impact upon the distal side of the bubble, respectively. The water-hammer shock can generate very high pressures on the wall, far exceeding those from the incident shock. The potential damage to the neighbouring surface is quantified by measuring the wall pressure. The range of stand-off distances and the surface area for which amplification of the incident shock due to bubble collapse occurs is determined

Cavitation Induction by Projectile Impacting on a Water Jet

The present paper focuses on the simulation of the high-velocity impact of a projectile impacting on a water-jet, causing the onset, development and collapse of cavitation. The simulation of the fluid motion is carried out using an explicit, compressible, density-based solver developed by the authors using the OpenFOAM library. It employs a barotropic two-phase flow model that simulates the phase-change due to cavitation and considers the co-existence of non-condensable and immiscible air. The projectile is considered to be rigid while its motion through the computational domain is modelled through a direct-forcing Immersed Boundary Method. Model validation is performed against the experiments of Field et al. [Field, J., Camus, J. J., Tinguely, M., Obreschkow, D., Farhat, M., 2012. Cavitation in impacted drops and jets and the effect on erosion damage thresholds. Wear 290–291, 154–160. doi:10.1016/j.wear.2012.03.006. URL http://www.sciencedirect.com/science/article/pii/S0043164812000968 ], who visualised cavity formation and shock propagation in liquid impacts at high velocities. Simulations unveil the shock structures and capture the high-speed jetting forming at the impact location, in addition to the subsequent cavitation induction and vapour formation due to refraction waves. Moreover, model predictions provide quantitative information and a better insight on the flow physics that has not been identified from the reported experimental data, such as shock-wave propagation, vapour formation quantity and induced pressures. Furthermore, evidence of the Richtmyer-Meshkov instability developing on the liquid-air interface are predicted when sufficient dense grid resolution is utilised

Planar and oblique shock wave interaction with a droplet seeded gas cylinder

We present an experimental study of a shock interaction with an initially diffuse heavy-gas cylinder seeded with submicron-scale glycol droplets. Unlike most earlier studies, the investigation covers not just a quasi-two-dimensional geometry, where the axis of the cylinder is parallel to the plane of the shock, but also the oblique interaction at an angle of 15\u25e6 between the cylinder axis and the plane of the shock wave. Our experimental data cover the range of Mach numbers from 1.2 to 2.4. The heavy gas cylinder is produced by injecting sulfur hexafluoride pre-mixed with glycol vapor into the test section of a tiltable shock tube through a co-flowing nozzle, with the gravity-driven flow of the heavy gas stabilized by an annular flow of air in the downward direction. Droplets in the gas cylinder are visualized via Mie scattering of diffuse white light. Two views of the flow—side and top— are simultaneously captured by a high-speed gated and intensified CCD camera, producing a spatially and temporally resolved description of the evolution of the gas cylinder upon impul- sive acceleration. While the observations for the planar interaction reveal that the large-scale flow structure remains largely two-dimensional, confirming the assump- tions of earlier studies, during the oblique shock interaction, we observe evidence of flow evolution in three dimensions, including asymmetric interaction of the gas cylin- der with the boundary layers forming on the walls of the shock tube, and rotation of this cylinder in the vertical plane parallel to the streamwise direction

Rapid evaporation at the superheat limit

In an experimental investigation of the transient processes that occur when a single droplet of butane at the superheat limit vaporizes explosively, short-exposure photographs and fast-response pressure measurements have been used to construct a description of the complete explosion process. It is observed that only a single bubble forms within the drop during each explosion, and that the growth proceeds on a microsecond time scale. An interfacial instability driven by rapid evaporation has been observed on the surface of the bubbles. It is suggested that the Landau mechanism of instability, originally described in connection with the instability of laminar flames, also applies to rapid evaporation at the superheat limit. The photographic evidence and the pressure data are used to estimate the evaporative mass flux across the liquid-vapour interface after the onset of instability. The ;ate of evaporation is shown to be two orders of magnitude greater than would be predicted by conventional bubble-growth theories that do not account for the effects of instability. An estimate of the mean density within the bubbles during the evaporative stage indicates that it is more than one half of the critical density of butane. Additional interesting dynamical effects that are observed include a series of toroidal waves that form on the interface between the butane vapour and the external host liquid in the bubble column apparatus after the bubble has grown large enough to contact the outer edge of the drop, and violent oscillations of the bubble that occur on a millisecond time scale, after evaporation of the liquid butane is complete, that cause the disintegration of the bubble into a cloud of tiny bubbles by Rayleigh-Taylor instability

High-speed imaging in fluids

High-speed imaging is in popular demand for a broad range of experiments in fluids. It allows for a detailed visualization of the event under study by acquiring a series of image frames captured at high temporal and spatial resolution. This review covers high-speed imaging basics, by defining criteria for high-speed imaging experiments in fluids and to give rule-of-thumbs for a series of cases. It also considers stroboscopic imaging, triggering and illumination, and scaling issues. It provides guidelines for testing and calibration. Ultra high-speed imaging at frame rates exceeding 1 million frames per second is reviewed, and the combination of conventional experiments in fluids techniques with high-speed imaging techniques are discussed. The review is concluded with a high-speed imaging chart, which summarizes criteria for temporal scale and spatial scale and which facilitates the selection of a high-speed imaging system for the applicatio

Study of the self noise generated by supercavitating vehicles

This study investigates the self noise from a ventilated supercavitating vehicle. A ventilated supercavity is a gaseous envelope surrounding an underwater vehicle that significantly reduces the drag felt by the vehicle. But the hydrodynamic noise generated by the creation of the supercavity could impact the successful deployment of the vehicle. A principal source of self noise for these types of vehicles is sound created by the ventilating gas jets impinging on the air-water interface. Analytical models of the radiated sound through the interface have been developed. Sometimes jets impinging on the interface entrain bubbles beneath the surface. This thesis outlines a theory to predict the influence of bubbles near the interface. Experimental measurements were made at the Naval Undersea Warfare Center (NUWC) in Newport, RI to test the accuracy of the model. These measurements include the unsteady force spectrum of a gas jet impinging on a rigid wall. The acoustic pressure spectrum of a gas jet striking the air-water interface was also recorded. The experimental results were compared to theoretical models for validation

Incepting cavitation acoustic emissions due to vortex stretching

The acoustic signature of the vortex cavitation bubbles can be characterized during inception, growth, and collapse. Growing and collapsing bubbles produced a sharp, broadband, popping sound. However, some elongated cavitation bubbles produce a short tone burst, or chirp, with frequencies on the order of 1 to 6 kHz. The frequency content of the acoustic signal during bubble inception and growth were related to the volumetric oscillations of the bubble and vortex dynamics coupling. A relationship was also found between the frequency of the oscillations and the flow and water quality conditions.http://deepblue.lib.umich.edu/bitstream/2027.42/84225/1/CAV2009-final183.pd