Hydrodynamic shock wave propagation after electrical breakdown

Characterization of acoustic shock wave will guarantee efficient tissue differentiation as feedback to reduce the probability of undesirable damaging (i.e. cutting) of tissues in laser surgery applications. We ablated hard (bone) and soft (muscle) tissues using a nanosecond pulsed Nd:YAG laser at 532 nm and a microsecond pulsed Er:YAG laser at 2.94 μm. When the intense short ns-pulsed laser is applied to material, the energy gain causes locally a plasma at the ablated spot that expands and propagates as an acoustic shock wave with a rarefaction wave behind the shock front. However, when using a μs-pulsed Er:YAG laser for material ablation, the acoustic shock wave is generated during the explosion of the ablated material. We measured and compared the emitted acoustic shock wave generated by a ns-pulsed Nd:YAG laser and a μs-pulsed Er:YAG laser measured by a calibrated microphone. As the acoustic shock wave attenuates as it propagates through air, the distance between ablation spots and a calibrated microphone was at 5 cm. We present the measurements on the propagation characteristics of the laser generated acoustic shock wave by measuring the arrival time-of-flight with a calibrated microphone and the energy-dependent evolution of acoustic parameters such as peak-topeak pressure, the ratio of the peak-to-peak pressures for the laser induced breakdown in air, the ablated muscle and the bone, and the spectral energy.

Acoustic shock waves, anew weapon against angina?

The mechanism of anode-initiated breakdown in liquid organic dielectrics with long molecular chains is proposed on the basis of the experimental data on high velocities of the breakdown channel propagation in organosilicon and organofluorine liquids (~107 cm/s), which are comparable to that obtained earlier in crystals in the same conditions. The high velocities of the anode-initiated breakdown channel are satisfactory explained within the model of the cascade Auger transitions, developed for the crystalline materials. According to this model, velocity of the breakdown channel propagation is proportional to the electrical field strength. The time delay in breakdown channel formation relative to the voltage pulse rise time does not exceed ~5·10-10 s within the margin of error Keywords: electrical breakdown in liquid, nanosecond breakdown, breakdown mechanism.

A strongly confined acoustic surface wave is an acoustic eigenstate localized on an interface between an ordinary acoustic medium and an acoustic metamaterial with negative effective dynamical mass density. Here we show that there is a new unconventional acoustic surface wave sustained by a parity-time -symmetric acoustic interface system, in which the negative effective dynamical mass density is not required (but the effective mass densities of the two adjacent acoustic media should fulfill the parity-time symmetry). Such an acoustic parity-time symmetry in the effective mass density can be used to manipulate acoustic wave propagation, e.g., it can exhibit both weak and strong confinement of the unusual -symmetric acoustic surface wave, and can offer mechanisms for designing acoustic metamaterial devices that would have specific functions in controlling and guiding acoustic wave, including acoustic field enhancement and extraordinary acoustic transmission.

We study the reflection of acoustic shock waves grazing at a small angle over a rigid surface. Depending on the incidence angle and the Mach number, the reflection patterns are mainly categorized into two types, namely regular reflection and irregular reflection. In the present work, using the nonlinear KZ equation, this reflection problem is investigated for extremely weak shocks as encountered in acoustics. A critical parameter, defined as the ratio of the sine of the incidence angle and the square root of the acoustic Mach number, is introduced in a natural way. For step shocks, we recover the self-similar (pseudo-steady) nature of the reflection, which is well known from von Neumann's work. Four types of reflection as a function of the critical parameter can be categorized. Thus, we describe the continuous but nonlinear and non-monotonic transition from linear reflection (according to the Snell–Descartes laws) to the weak von-Neumann-type reflection observed for almost perfectly grazing incidence. This last regime is a new, one-shock regime, in contrast with the other, already known, two-shock (regular reflection) or three-shock (von Neumann-type reflection) regimes. Hence, the transition also resolves another paradox on acoustic shock waves addressed by von Neumann in his classical paper. However, step shocks are quite unrealistic in acoustics. Therefore, we investigate the generalization of this transition for N-waves or periodic sawtooth waves, which are more appropriate for acoustics. Our results show an unsteady reflection effect necessarily associated with the energy decay of the incident wave. This effect is the counterpart of step-shock propagation over a concave surface. For a given value of the critical parameter, all the patterns categorized for the step shock may successively appear when the shock is propagating along the surface, starting from weak von-Neumann-type reflection, then gradually turning to von Neumann reflection and finally evolving into nonlinear regular reflection. This last one will asymptotically result in linear regular reflection (Snell–Descartes). The transition back to regular reflection is one of two types, depending on whether a secondary reflected shock is observed. The latter case, here described for the first time, appears to be related to the non-constant state behind the incident shock, which prevents secondary reflection.

Inflow/Outflow conditions are formulated for time-harmonic waves in a duct governed by the Euler equations. These conditions are used to compute the propagation of acoustic and vortical disturbances and the scattering of vortical waves into acoustic waves by an annular cascade. The outflow condition is expressed in terms of the pressure, thus avoiding the velocity discontinuity across any vortex sheets. The numerical solutions are compared with the analytical solutions for acoustic and vortical wave propagation with and without the presence of vortex sheets. Grid resolution studies are also carried out to discern the truncation error of the numerical scheme from the error associated with numerical reflections at the boundary. It is observed that even with the use of exponentially accurate boundary conditions, the dispersive characteristics of the numerical scheme may result in small reflections from the boundary that slow convergence. Finally, the three-dimensional interaction of a wake with a flat plate cascade is computed and the aerodynamic and aeroacoustic results are compared with those of lifting surface methods.

In recent results, several second-order finite difference schemes were adapted to obtain shocked solutions for sound propagation in varying area hard-wall ducts with transonic flow [N. J. Walkington and W. Eversman, J. Sound Vib. 104, 81–107 (1986)]. The second-order MacCormack scheme with a fourth viscous term was then recommended by the authors. But its numerical error still seemed to be a bit large. In this paper, the fourth-order MacCormack scheme with a fourth viscous term is used to improve the solution. It is shown from three sample calculations with acoustic shock waves that the new method is much better than the second-order MacCormack method. Its numerical error, on an average, is only one fifth of that in the latter method. Here the new method is depicted in detail, considering especially the treatment of boundary conditions, the selection of viscous coefficient, the stability condition, and the computation correctness criterion. This method needs less grid points and is more stable. It is also time-saving and storage-saving. Maybe it is a good method for calculating shocked solutions of sound propagation in ducts.

Cadmium sulfide (CdS) is a promising semiconductor with a narrower bandgap, making it highly suitable for optoelectronic and energy storage applications. However, its response to extreme conditions, such as acoustic shock waves, remains unexplored. In this study, CdS samples were subjected to varying shock pulses such as 100, 200, 300, and 400 at a transient pressure of 0.59 MPa and a temperature of 520 K, with a Mach number of 1.5, to investigate structural, optical, and morphological modifications. Analytical techniques, including X-ray diffraction (XRD), Raman spectroscopy, UV-Vis Diffuse Reflectance Spectroscopy (DRS), Photoluminescence (PL) spectroscopy, and Field-Emission Scanning Electron Microscopy (FE-SEM), were employed for comprehensive analysis. XRD and Raman results confirm that the structure remains stable up to 400 shock pulses, with only a minor peak shift observed at 300 shock pulses and, at 400 shock pulses which subsequently reverts to its original position. Optical studies reveal a reversible bandgap shift from 2.37 eV to 2.24 eV, while the PL emission peak shifts from 518 nm to 528 nm and reverts to its original position at 400 shock pulses. Most interestingly, FE-SEM analysis reveals a morphological transition, and the rod morphology evolves into a cube morphology at 300 shock pulses due to shock wave-induced dynamic recrystallization. Remarkably, at 400 shock pulses, the morphology reverts to its original rod morphology, highlighting the reversibility of morphology. These results highlight the critical role of dynamic recrystallization in enabling morphology modulation without structural phase alteration. This study establishes acoustic shock waves as a promising pathway to tune both optical properties and morphology of materials while preserving their structural integrity, offering new directions for functional material design and processing.

Using a laser for cutting bones instead of the traditional saws has been shown to improve a patient’s healing process. Additionally, the laser has the potential to reduce the collateral damage to the surrounding tissue if appropriately applied. This can be achieved by building additional sensing elements besides the laser itself into an endoscope. To this end, we use a microsecond pulsed Erbium-doped Yttrium Aluminium Garnet (Er:YAG) laser to cut bones. During ablation, each pulse emits an acoustic shock wave that is captured by an air-coupled transducer. In our research, we use the data from these acoustic waves to predict the depth of the cut during the ablation process.We use a Neural Network (NN) to approximate the depth, where we use one or multiple consecutive measurements of acoustic waves. The NN outperforms the base-line method that assumes a constant ablation rate with each pulse to predict the depth. The results are evaluated and compared against the ground-truth depth measurements from Optical Coherence Tomography (OCT) images that measure the depth in real-time during the ablation process.

Examples are presented which show that the velocity, amplitude, attenuation and apparent frequency of several acoustic waves can be recorded in the borehole. Examination of such recordings, termed "character" logs, indicates that the wave types observed include a refracted compressional wave and a wave which travels with formation shear velocity.Laboratory data are used to show that compressional and shear wave velocities are dependent on porosity, effective stress and lithology; but that the change in reciprocal velocity per unit change in porosity is larger for shear waves than for compressional waves. We, therefore, conclude that the accuracy of porosity determinations can sometimes be improved by use of shear wave velocities, provided that the shear wave amplitudes are large enough to delineate the shear arrival from the preceding compressional arrival on the character log. Borehole data are presented which show that the difference between shear wave and compressional wave reciprocal velocities can be used to predict porosities. This is a refinement which may allow the prediction of porosities from single-receiver acoustic logs without introduction of errors from borehole fluid travel-times. Laboratory and field data are presented to show that the relationship between compressional and shear wave velocities can be used to indicate lithology.An example is presented to show that fractures usually cause a greater reduction in borehole shear wave amplitudes than in compressional wave amplitudes, an effect which may offer a more reliable means of detecting fractures.The complexity of the borehole acoustic wave train can make presently available cement bond logs highly sensitive to the gate and bias settings used. The character log offers a means to circumvent possible misinterpretations by recording all amplitudes, from which the interpreter can select the appropriate data for evaluating the cement bond.Character logs may also be used as a quality control for open-hole transit-time logs when existence of small compressional wave amplitudes interferes with the proper functioning of bias-controlled timing devices.Evaluation of the potential uses of character log data is not complete; but a character log presented in a form convenient for routine use would be a desirable addition to currently available logs. To summarize, possible applications for such a log in formation evaluation include the followingquality control of transit-time logs,refinement of porosity predictions,determination of lithology,improvement of fracture detection andimprovement of cement bond evaluation. Suggestions are made regarding the requirements for a sufficient but practical character log for routine use. Introduction Acoustic logs have become a widely used porosity tool in formation evaluation. In addition, there is a growing application of acoustic logs in cement bond evaluation and fracture detection. These applications have mainly involved the use of logs of first-arrival transit times and amplitudes and have not included detailed studies of the complete signal.The purpose of this paper is to show that significant benefits in formation evaluation can be gained by a more complete use of the acoustic wave train generated in the borehole by an acoustic logging tool. We hope that this discussion will also stimulate further development of logs suitable for routine use so that these benefits may be realized.Examples of acoustic wave train logs, termed "character" logs, are presented to show that several identifiable acoustic waves are present in the borehole. The measurable characteristics of these acoustic waves and some of their relations to formation properties of interest are also discussed. The more obvious potential uses of character logs are listed, and some suggestions are made regarding the requirements for a sufficient but practical character log for routine use. CHARACTER LOGS Some 10 years ago, Vogel and Summers and Broding noted that the signals received uphole from an acoustic logging tool located in a borehole had a number of interesting characteristics. The logging tool consisted of two or more pressure transducers spaced on an acoustically insulated body (Fig. 1a). One of the pressure transducers was used as a transmitter to generate pressure waves in the borehole fluid. The other transducer served as a receiver to detect any pressure waves reaching it in the borehole. The receiver then converted these pressure waves to electrical signals which were transmitted to the surface and displayed on an oscilloscope as a record of time vs receiver-signal amplitude. Fig. 1b is a schematic representation of a typical record. JPT P. 639^

This study is focused on exploring the feasibility of an all-optic surface scanning method in determining the size and position of a submerged, laser generated, optoacoustic (OA) source. The optoacoustic effect in this case was generated when the absorption of a short electromagnetic pulse in matter caused a dielectric breakdown, a plasma emission flash and a subsequent acoustic wave. In the experiment, a laser pulse with λ = 1064 nm and 12 ns pulse length was aimed at a volume of deionized water. When the laser beam was focused by a f = 16 mm lens, a single dielectric breakdown spot occurred. When a f = 40 mm was used several breakdowns in a row were induced. The breakdowns were photographed using a double shutter camera. The acoustic wave generated by the dielectric breakdowns were detected at a point on the water surface using a laser Doppler vibrometer (LDV). First, the LDV signal was used to calculate the speed of sound with an accuracy of 10 m/s. Secondly, the location and length of the dielectric breakdown was calculated with an accuracy of 1 mm. The calculated position matched the breakdown location recorded by a camera. The results show that it is possible to use LDV surface measurements from a single spot to determine both the position and length of the OA source as well as the speed of sound in the medium. Furthermore, the LDV measurements also show a secondary peak that originates from the OA source. To unravel the origin and properties of this interesting feature, further investigations are necessary

The acoustic wave velocity varies with fluid saturation and pore-fluid distribution. We use a P-wave source and the staggered grid finite-difference method, with second-order accuracy in time and eighth-order accuracy in space, to simulate the acoustic wave field in a fractured medium that is saturated with a two-phase pore fluid (gas & water). Further, we analyze the variation of acoustic wave velocity with saturation for different pore-fluid distribution modes. The numerical simulation method is simple and yields accurate results.

On the influence of a flexural biasing state on the velocity of piezoelectric surface waves

We aimed to develop a system for controlled exposure of biological samples to conditions they experience when lightning strikes their habitats. We based the generator on a capacitor charged via a bridge rectifier and a dc-dc converter, and discharged via a relay, delivering arcs similar to natural lightning strokes in electric current waveform and similarly accompanied by acoustic shock waves. We coupled the generator to our exposure chamber described previously, measured electrical and acoustic properties of arc discharges delivered, and assessed their ability to inactivate bacterial spores. Submicrosecond discharges descended vertically from the conical emitting electrode across the air gap, entering the sample centrally and dissipating radially toward the ring-shaped receiving electrode. In contrast, longer discharges tended to short-circuit the electrodes. Recording at 341 000 FPS with Vision Research Phantom v2010 camera revealed that initial arc descent was still vertical, but became accompanied by arcs leaning increasingly sideways; after 8-12 μs, as the first of these arcs formed direct contact with the receiving electrode, it evolved into a channel of plasmified air and short-circuited the electrodes. We eliminated this artefact by incorporating an insulating cylinder concentrically between the electrodes, precluding short-circuiting between them. While bacterial spores are highly resistant to electric pulses delivered through direct contact, we showed that with arc discharges accompanied by an acoustic shock wave, spore inactivation is readily obtained. The presented system allows scientific investigation of effects of arc discharges on biological samples. This system will allow realistic experimental studies of lightning-triggered horizontal gene transfer and assessment of its role in evolution.

When a frequency of gas oscillation in an acoustic resonator is sufficiently close to one of resonant frequencies of the resonator, the amplitude of gas oscillation becomes large and hence the nonlinear effect manifests itself. Then, if the dissipation effects due to viscosity and thermal conductivity of the gas are sufficiently small, the gas oscillation may evolve into the acoustic shock wave, in the so-called consonant resonators. At the shock front, the kinetic energy of gas oscillation is converted into heat by the dissipation process inside the shock layer, and therefore the temperature of the gas in the resonator rises. Since the acoustic shock wave travels in the resonator repeatedly over and over again, the temperature rise becomes noticeable in due course of time even if the shock wave is weak. We numerically study the gas oscillation with shock wave in a resonator of square cross section by solving the initial and boundary value problem of the system of three-dimensional Navier-Stokes equations with a finite difference method. In this case, the heat conduction across the boundary layer on the wall of resonator causes a spatially heterogeneous distribution of mean (time-averaged) gas temperature.