Shock Wave Focusing Research Articles

Focusing of shock waves induced by optical breakdown in water

The focusing of laser-generated shock waves by a truncated ellipsoidal reflector was experimentally and numerically investigated. Pressure waveform and distribution around the first (F(1)) and second foci (F(2)) of the ellipsoidal reflector were measured. A neodymium doped yttrium aluminum garnet laser of 1046 nm wavelength and 5 ns pulse duration was used to create an optical breakdown at F(1), which generates a spherically diverging shock wave with a peak pressure of 2.1-5.9 MPa at 1.1 mm stand-off distance and a pulse width at half maximum of 36-65 ns. Upon reflection, a converging shock wave is produced which, upon arriving at F(2), has a leading compressive wave with a peak pressure of 26 MPa and a zero-crossing pulse duration of 0.1 mus, followed by a trailing tensile wave of -3.3 MPa peak pressure and 0.2 mus pulse duration. The -6 dB beam size of the focused shock wave field is 1.6 x 0.2 mm(2) along and transverse to the shock wave propagation direction. Formation of elongated plasmas at high laser energy levels limits the increase in the peak pressure at F(2). General features in the waveform profile of the converging shock wave are in qualitative agreement with numerical simulations based on the Hamilton model.

Ways to discharge-based soft X-ray lasers with the wavelength λ<15 nm

AbstractTwo basic ways to amplification of spontaneous emission in the soft X-ray region are described. The first is based on the electron-collisional recombination pumping scheme, which uses recombination of fully stripped ions into hydrogen-like ions to create (in the case of sufficiently fast cooling) a population inversion on energy levels belonging to the Balmer-alpha transition. We test this scheme on nitrogen, for which the lasing wavelength is 13.4 nm. The second way to amplification of spontaneous emission is based on the electron-collisional excitation pumping scheme: this uses for creation of population inversion a fast excitation of Ne- or Ni-like ions. However, for wavelength below 15 nm it is necessary to use Ni-like ions of some metal vapors. Feeding metal vapors into a capillary is difficult, and if being fed they deposit on the capillary wall and significantly reduce the capillary lifetime. That is why we prepare metal vapor plasma in a capillary with liquid wall – by wire explosion in water. For slowdown of the plasma-channel expansion a local-water-compression by linearly focused shock wave is being developed.

Numerical simulation of shock wave generation and focusing in shock wave lithotripsy

Shock wave lithotripsy is a procedure where focused shock waves are fired at kidney stones in order to pulverize them. Lithotripters with different source mechanisms and reflector shapes (or lenses) are in clinical use, but prediction of focal region pressure is made difficult by nonlinearity and cavitation. We report on development of a numerical simulation framework aimed at accurate prediction of focal region pressures and bubble dynamics. Shock wave generation and beam focusing are simulated via the Euler equations with MUSCL‐type shock‐capturing scheme and adaptive mesh refinement (AMR). Electrohydraulic, electromagnetic, and piezoelectric‐array lithotripters are modeled with axisymmetric geometries. In the electrohydraulic case, an expanding bubble model simulates spark firing. In the piezoelectric case, a boundary condition prescribing the motion of individual elements is used. Calculated peak pressures and pulse widths agree well with experimental data for the electrohydraulic and electromagnetic ...

Time-Reversal Acoustics in Biomedical Engineering

Time-reversal is a very powerful method for focusing through complex and heterogeneous media and shows very promising results in biomedical applications. In this presentation, we review some of the main applications investigated during the past decade. An iterative implementation of the time-reversal process allows tracking gallstones in real time during lithotripsy treatments. In this application domain, a smart exploitation of the reverberations in solid waveguides permits the focusing of high amplitude ultrasonic shock waves with a small number of transducers. Because time reversal is able to correct the strong distortions induced by the skull bone on ultrasonic propagation, this adaptive focusing technique allows brain ultrasonic therapy through the skull bone. Finally, time-reversal focusing guided by speckle noise can be implemented to correct distortion in medical ultrasonic imaging and recent results will be presented.

1032 レーザー誘起水中集束衝撃波の圧力波形に及ぼすレーザー強度分布の影響(S04-3 実験力学における計測・解析法の新展開(3)高速・衝撃現象,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

1032 レーザー誘起水中集束衝撃波の圧力波形に及ぼすレーザー強度分布の影響(S04-3 実験力学における計測・解析法の新展開(3)高速・衝撃現象,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Conditions of detonation initiation by focusing shock waves in a combustible gas mixture

Based on experiments on focusing shock waves in hydrogen-air mixtures and available publications, the critical shock-wave Mach number at which detonation is initiated near the apex of a concave reflector is analyzed as a function of the reflector size and reactivity of the mixture. The effect of the reflector shape and size on the value of this Mach number is studied.

Light emission during shock wave focusing in air and argon

The light emission from a converging shock wave was investigated experimentally. Results show that the shape of the shock wave close to the center of convergence has a large influence on the amount of emitted light. It was found that a symmetrical polygonal shock front produced more light than an asymmetrical shape. The light emission appears as the shock wave collapses. The full width at half maximum of the light pulse is about 200ns for all geometrical shapes. It was also found that argon as a test gas produces more light than air. Numerical simulations showed good agreement with experimental results regarding the shape of the shock and the flow field behind the shock. The temperature field from the numerical simulations was investigated and shows that the triple points behind the shock front are hot spots that increase the temperature at the center as they arrive there.

Magnetic imaging of the Vredefort impact crater, South Africa

While most impact craters are characterised by negative magnetic anomalies over their central regions, aeromagnetic surveys over the Vredefort meteorite impact crater reveal multiple concentric magnetic patterns with no significant anomaly at its centre. We performed ground magnetic surveys across a portion of a prominent negative magnetic anomaly that extends in a broad semicircular belt about half way into the basement floor of the crater. Magnetic anomalies defined by our data are most often negative and occur over a wide range of wavelengths. The longest wavelength negative anomaly coincides well with aeromagnetic data. We find that this feature is centred over the amphibolite to granulite metamorphic facies transition exposed in the basement floor. The transition zone is analogous to the Conrad discontinuity, observed at depths of about 20 km elsewhere in the Kaapvaal craton. Petrographic studies show a marked increase in the intensity of the impact-related thermal and shock metamorphism at this transition, which we explain by the focusing and defocusing of shock waves at a rheologic interface during impact. We therefore suggest that the magnetic signature at this boundary is caused by a combination of both thermal and shock effects related to the impact event. A numerical model of the long wavelength anomaly suggests that it is underlain by a body of coherently magnetised rock whose direction and intensity are similar to those found in pseudotachylites and impact melts that formed during impact. On the other hand, negative anomalies occurring over smaller (100 to 20 m) wavelengths often do not coincide with the surface geology. These features cannot be modeled using the same criteria as that for the long wavelength anomaly.

Flow features resulting from shock wave impact on a cylindrical cavity

The complex flow features that arise from the impact of a shock wave on a concave cavity are determined by means of high-speed video photography. Besides additional information on features that have previously been encountered in specific studies, such as those relating to shock wave reflection from a cylindrical wall and those associated with shock wave focusing, a number of new features become apparent when the interaction is studied over longer times using time-resolved imaging. The most notable of these new features occurs when two strong shear layers meet that have been generated earlier in the motion. Two jets can be formed, one facing forward and the other backward, with the first one folding back on itself. The shear layers themselves develop a Kelvin–Helmholtz instability which can be triggered by interaction with weak shear layers developed earlier in the motion. Movies are available with the online version of the paper.

Acoustic Field of a Ballistic Shock Wave Therapy Device

Shock wave therapy (SWT) refers to the use of focused shock waves for treatment of musculoskeletal indications including plantar fascitis and dystrophic mineralization of tendons and joint capsules. Measurements were made of a SWT device that uses a ballistic source. The ballistic source consists of a handpiece within which compressed air (1–4 bar) is used to fire a projectile that strikes a metal applicator placed on the skin. The projectile generates stress waves in the applicator that transmit as pressure waves into tissue. The acoustic fields from two applicators were measured: one applicator was 15 mm in diameter and the surface slightly convex and the second was 12 mm in diameter the surface was concave. Measurements were made in a water tank and both applicators generated a similar pressure pulse consisting of a rectangular positive phase (4 μs duration and up to 8 MPa peak pressure) followed by a predominantly negative tail (duration of 20 μs and peak negative pressure of −6 MPa), with many oscillations. The rise times of the waveforms were around 1 μs and were shown to be too long for the pulses to be considered shock waves. Measurements of the field indicated that region of high pressure was restricted to the near-field (20–40 mm) of the source and was consistent with the Rayleigh distance. The measured acoustic field did not display focusing supported by calculations, which demonstrated that the radius of curvature of the concave surface was too large to effect a focusing gain. Other SWT devices use electrohydraulic, electromagnetic and piezoelectric sources that do result in focused shock waves. This difference in the acoustic fields means there is potentially a significant mechanistic difference between a ballistic source and other SWT devices. (E-mail: robinc@bu.edu)

EVALUATION OF FOCUSED AND UNFOCUSED SHOCK WAVE PROPAGATION IN POLYACRYLAMIDE GEL AND PORCINE TISSUE.

Background Extracorporeal shock wave therapy (ESWT) has been shown to increase perfusion in ischemic cardiac tissue and increase autograft healing in both animal models and clinical studies. To help clarify the mechanisms by which ESWT induces tissue healing, we studied the propagation of focused and unfocused shock wave sources in animal tissue by comparing the cavitation pattern in polyacrylamide gel and porcine tissue. Study Design and Method Fifteen percent polyacrylamide blocks, marble-embedded polyacrylamide, and porcine thigh were treated with a focused shock wave source with an OssatronTM machine and unfocused shock wave source with a StorzTM device. Cavitation patterns were visualized using a MicroMaxxTM ultrasound machine immediately after a treatment. Results Focused shock wave treatment of polyacrylamide gel with the OssatronTM produced a focal ellipsoidal cavitation field with a 2 cm depth. An increase in treatment energy or intensity resulted in a more intense cavitation field with a larger diameter. Focused treatment of the marble-embedded gel produced a disturbance of the marble-gel interface with increased cavitation intensity at increased treatment energy. Unfocused shockwave treatment of the polyacrylamide gel produced a focal cavitation field superficially at the site of treatment. However, the cavitation field displayed a radiating pattern with deeper propagation into the gel. Increasing the treatment intensity led to a corresponding increase in the intensity of the cavitation field. Unfocused shock wave treatment of marble-embedded gel showed no observable change in the marble-gel interface. Treatment of porcine thigh with both focused and unfocused shock wave at low and high energy and intensity did not produce any observable changes. Conclusion The result of ESWT in polyacrylamide gel indicates that deep tissue stimulation with an unfocused shock wave source may not yield any benefits because high tissue impedance may reduce shock wave propagation. However, focused shock wave may be more favorable for deep tissue stimulation.

OS8-2-1 Generation of underwater focusing shock wave by pulsed laser irradiation

OS8-2-1 Generation of underwater focusing shock wave by pulsed laser irradiation

Experimental study of micro shock wave focusing for precise medical applications

For applying shock waves to sensitive medical procedures like cranioplasty in a close vicinity of the brain or treatment of myocardial dysfunction, generation of micro underwater shock waves plays an important role. Such delicate applications make limits on usage of conventional shock wave (SW) sources. In the present research a half‐ellipsoidal cavity with 20.0‐mm minor diameter and the ratio of major to minor diameters of 1.41 was designed as a compact extracorporeal SW source. Silver azide AgN3 pellets ranging from 1.0 to 20 μg with their energy ranging from 1.9 to 38 mJ were used to generate shock waves at the first focal point F1 inside the reflector. Irradiation of a Q‐switched Nd:YAG laser beam through a 400‐μm optical fiber was used to ignite the pellet. The whole sequences of the shock wave generation, propagation, and focusing were visualized by quantitative double pulse holographic interferometry and time‐resolved high speed shadowgraph methods. Pressure histories were measured at the shock wav...

A New Integrated Ultrasound System for Shockwave Lithotripsy

Inline ultrasound monitoring requires good image quality for accurate stone localization, as well as low shockwave shadowing and a robust transducer. In general, conventional transducers designed for another purpose, such as abdominal scanning, are employed. The distance between the transducer and the SWL focus can be varied by a mechanical drive. The drawback is reduced fragmentation at short distances and poor imaging at long distances. This paper introduces a new approach using a specially designed transducer without a mechanical drive. A transducer prototype with optimized beam focusing (B-K Medical, Herlev, Denmark) was integrated into a modified Compact Delta II therapy head (Dornier MedTech, Wessling, Germany). Image quality was tested at two clinical sites, where 40 kidney and 14 ureteral stones were treated. The shockwave was characterized by model stone tests and fiberoptic hydrophone measurements. Both kidney and ureteral stone treatments could be monitored reliably. Despite the long distance to the SWL focus, the transducer could be operated with relatively high frequencies (3.5-6 MHz), so that high image resolution was obtained. Model stone tests yielded the same fragmentation as the standard Compact Delta II without a transducer. This study shows that the concept of an integrated transducer distant from the shockwave focus is feasible. Transducer elevation, which is accompanied by shockwave shadowing and early transducer failure, is avoided by employing a dedicated transducer design.

Numerical study of the collapse of a bubble subjected to a lithotripter pulse

In shock wave lithotripsy, the combined effect of focused shock waves and cavitation pulverizes kidney stones. Although cavitation has been shown to play an important role in the stone comminution process, the underlying mechanism has yet to be fully understood. The goal of the present study is to quantify the potential damage caused by the collapse of a single bubble near a solid surface. Using a high‐order accurate quasiconservative, shock‐ and interface‐capturing scheme [E. Johnsen and T. Colonius, J. Comput. Phys., in press (2006)], the response of an air bubble subjected to a lithotripter pulse is considered. In particular, quantities important in cavitation erosion, such as wall stresses and reentrant jet velocity, are measured as functions of the properties of the pulse (amplitude, wavelength) and the geometry of the problem (stand‐off distance from the wall, presence of neighboring bubbles). Preliminary two‐dimensional results for a ‘‘cylindrical bubble’’ show that a large water‐hammer pressure is...

Shock‐wave energy deflection due to the presence of bone

The physical interaction of shock waves with musculoskeletal tissues is inherently different from other high‐pressure acoustic therapies. Whereas high‐intensity focused ultrasound and lithotripsy focus their energy in regions of soft tissue, musculoskeletal shock‐wave therapy (SWT) involves focusing shock waves (SWs) near or at bones. The presence of bones will cause reflection, refraction, and diffraction of acoustic energy. In our analysis of cavitation generated during clinical SWT treatment, we noticed that the cavitation was occurring away from the SW focus. We confirmed this by performing an in‐vitro experiment on a bone embedded in gel. We also quantified the deflection angle as a function position near a bone in water: A talus was manufactured using rapid prototyping. It was placed near the focus of an electrohydraulic SW device. To visualize the pressure field, a high‐speed camera imaged the cavitation field generated around the focus. As the bone was moved closer to the focus, the cavitation field (and thus, the pressure field) deflected away from the bone. The deflection angle was measured as a function of relative distance between the bone and focus. Numerical simulations were performed to model the deflection of energy away from the bone.

Focusing of Strong Shocks in an Annular Shock Tube

Focusing of strong shock waves in a gas-filled thin convergence chamber with various forms of the reflector boundary is investigated experimentally and numerically. The convergence chamber is mounted at the end of the horizontal co-axial shock tube. The construction of the convergence chamber allows the assembly of the outer chamber boundaries of various shapes. Boundaries with three different shapes have been used in the present investigation—a circle, an octagon and a smooth pentagon. The shock tube in the current study was able to produce annular shocks with the initial Mach number in the range M s = 2.3 − 3.6. The influence of the shape of the boundary on the shape and properties of the converging and reflected shock waves in the chamber has then been investigated both experimentally and numerically. It was found that the form of the converging shock is initially governed by the shape of the reflector and the nonlinear interaction between the shape of the shock and velocity of shock propagation. Very close to the center of convergence the shock obtains a square-like form in case of a circular and octagonal reflector boundary. This is believed to stem from the instability of the converging shock front triggered by the disturbances in the flow field. The outgoing, reflected shocks were also observed to be influenced by the shape of the boundary through the flow ahead as created by the converging shocks.

Velocity measurement of induced flow by a laser focusing shock wave

The objective of this study is to apply the shock wave for control in a micro channel. The shock wave was generated by a laser focusing of pulsed laser beam in the channel. Using a pulse laser to generate a shock wave, a non-stationary flow was induced in the small space between the parallel plates. The spherical and cylindrical shock propagations were observed with schlieren method. The shock Mach number decreases with time and approaches to unity. As reported in the previous investigations, the shock speed was attenuated in a short distance and time. In the present experiment, It was not found a remarkable difference in the shock speed between the spherical and cylindrical shock experiments. Subsequently, the flow induced by the cylindrical shock wave was studied using PIV technique. A smoke tracer was used in the experiment and its velocity was measured within 100 μs. A numerical simulation was carried out to investigate the momentum relaxation between the gas and smoke particle. A suitable shock initiation model was introduced in the simulation. The experimental results show that a wide acceleration and deceleration zone exist behind the shock wave. Also, the relaxation distance in the experimental data is much longer than that in numerical simulation.

Computational study of an incident shock wave into a Helmholtz resonator

The behavior of an incident shock wave into a Helmholtz resonator is very important from the acoustical point of view as well as the fundamental researches of shock wave dynamics. When a shock wave propagates into a Helmholtz resonator, complicated wave phenomena are formed both inside and outside the resonator. Shock wave reflections, shock wave focusing phenomena, and shock–vortex interactions cause strong pressure fluctuations inside the resonator, consequently leading to powerful sound emission. The wave phenomena inside the resonator are influenced by detailed configuration of the resonator. It is well known that the gas inside the resonator strongly oscillates at a resonance frequency, as the incident wavelength is larger, compared with the geometrical length scale of the resonator, but there are only a few works regarding a shock wave that has an extremely short wavelength. Meanwhile, the discharge process of the incident shock wave from the resonator is another interest with regard to an impulse wave generation that is a source of serious noise and vibration problems of the resonator. In the present study, the wave phenomena inside and outside the Helmholtz resonator are, in detail, investigated with a help of a computational fluid dynamics method. The incident shock Mach number is varied below 2.0, and many different types of the resonators are explored to investigate the influence of the resonator geometry on the wave phenomena. A total variation diminishing (TVD) scheme is employed to solve two-dimensional, unsteady, compressible Euler equations. The computational results are compared with existing experimental data to ensure that the present computations are valid to predict the resonator wave phenomena. Based upon the results obtained, the shock wave focusing and discharge processes, which are important in determining the resonator flow characteristics, are discussed in detail.

Numerical investigation of toroidal shock wave focusing in a cylindrical chamber

In this paper, focusing of a toroidal shock wave propagating from an annular shock tube into a cylindrical chamber was investigated numerically with the dispersion controlled dissipation (DCD) scheme. The first case for an incident Mach number of 1.5 was conducted and compared with experiments for validation. Then, several cases were calculated for higher incident Mach numbers varying from 2.0 to 5.0, and complicated flow structures were observed. The numerical study was mainly focused on two aspects: focusing process and flow structures. The process, including diffraction, focusing, and reflection, is displayed to reveal the focusing mechanism, and the flow structures at different incident. Mach numbers are used to demonstrate shock reflection styles and focusing characteristics.