Underwater Shock Wave Focusing Research Articles

A prediction model for two-dimensional pressure distribution from underwater shock wave focusing by an ellipsoidal reflector.

Underwater shock wave focusing by ellipsoidal reflector is an important method for medical treatment, detection, and acoustic warfare. However, its pressure field is difficult to predict due to complicated physics. In this study, the pressure by focusing is modeled based on theories of shock wave propagation, nonlinear reflection, and nonlinear focusing, and the calculation domain is determined by approximate equations of wave fronts and lines. The pressure field during the whole process is described by combining direct and focusing pressures in the time and space domains. On this basis, the focusing behavior is simulated, and obtained pressure profiles are compared with experimental results, and the influence of reflector length on focusing performance is also discussed. The results indicate that although there are some rough assumptions, this model can simulate the underwater focusing in some detail and does a good job of predicting the pressure distribution, especially for the positive peak pressure, with an error below 10%; as the reflector length increases, the dynamic focus tends to move linearly forward to the other geometric focus, and the pressure gain increases continuously but the growth rate decreases.

Time-Resolved High-Speed Visualization and Analysis of Underwater Shock Wave Focusing Generated by a Magnetic Pulse Compression Unit

Paper reports on visualization and analysis of underwater shock waves generation, focusing, and propagation. Recently, shock waves have been attractive with rising attention in medical field. Extracorporeal shock wave lithotripter has been one of its very successful applications. However, the effects of shock waves on living tissue are not sufficiently understood, due to unknown parameters involving the shock wave interaction. For better understanding of affecting points, a precise and controllable shock wave source is required. In this paper, in order to study shock wave effects, pulsed electric discharges produced by magnetic pulse compression circuit were used to generate underwater shock waves. A half-ellipsoidal reflector was used for shock wave focusing. The whole sequences of the shock wave generation, propagation, focusing, and cavitation bubbles, which appeared after shock wave passage, were visualized by time-resolved high-speed shadowgraph method. The peak pressure of shock wave focusing at second focal point reached 180 MPa. A cylindrical test section was used to observe phenomena around the electrode from the pulse discharge to bubble collapse. The first cavitation bubble's collapse occurred at about 750 μs. In the experiment, the two shock waves, one generated by expansion of plasma and another generated by collapse of the cavitation bubble were studied. Using high-speed visualization and image analysis, the velocity and pressure of the two shock waves were evaluated. The peak pressure of shock wave generated by bubble collapse reached 157 MPa.

High strain rate forming using an underwater shock wave focusing technique

This paper proposes an effective method for controlling the morphology of impulsive pressure waves in hydro-spark forming, making use of the technique of underwater shock wave focusing. Free forming experiments using ellipsoidal reflectors with two kinds of minor-to-major axis ratios b / a =0.6 and 0.8 are performed under several blank-setting conditions. The height of the formed product depends solely on the impulse of the shock wave, regardless of its focusing behavior, while the shape of the product is controlled by distribution of the applied pressure wave. It is demonstrated that the reflector having b / a =0.8 yields a wide range of pressure distribution, which can be estimated from the shock wave front distribution calculated considering the effect of non-linear reflection.

Cavitation phenomena in extracorporeal microexplosion lithotripsy

An experimental investigation was made of cavitation phenomena induced by underwater shock wave focusing applied to the extracorporeal microexplosion lithotripsy (microexplosion ESWL). Firstly an underwater microexplosion generated by detonation of a 10 mg silver azide pellet was studied and secondly underwater shock focusing and its induced cavitation phenomena were investgated. Underwater shock wave was focused by using a semi-ellipsoidal reflector in which a shock wave generated at the first focal point of the reflector was reflected and focused at the second focal point. It is found that an explosion product gas bubble did not produce any distinct rebound shocks. Meantime cavitation appeared after shock focusing at the second focal point where expansion waves originated at the exit of the reflector were simultaneously collected. A shock/bubble interaction is found to contribute not only to urinary tract stone disintegration but also tissue damage. The cavitation effect associated with the microexplosion ESWL was weaker in comparison with a spark discharge ESWL. The microexplosion ESWL is an effective method which can minimize the number of shock exposures hence decreasing tissue damage by conducting precise positioning of urinary tract stones.

Application of Underwater Shock Wave Focusing to the Development of Extracorporeal Shock Wave Lithotripsy

This paper describes a summary of a research project for the development of extracorporeal shock wave lithotripsy (ESWL), which has been carried out, under close collaboration between the Shock Wave Research Center of Tohoku University and the School of Medicine, Tohoku University. The ESWL is a noninvasive clinical treatment of disintegrating human calculi and one of the most peaceful applications of shock waves. Underwater spherical shock waves were generated by explosion of microexplosives. Characteristics of the underwater shock waves and of ultrasound focusing were studied by means of holographic interferometric flow visualization and polyvinyliden-difluoride (PVDF) pressure transducers. These focused pressures, when applied to clinical treatments, could effectively and noninvasively disintegrate urinary tract stones or gallbladder stones. However, despite clincal success, tissue damage occurs during ESWL treatments, and the possible mechanism of tissue damage is briefly described.

Underwater Shock Wave Focusing by Microexplosions, a Medical Application.

This is a preliminary report of an application of underwater shock wave focusing to the so-called extracorporeal shock wave lithotripsy, ESWL. Underwater spherical shock waves were initiated by microexplosions at one of two focus points of an ellipsoidal cavity. Consequently, the reflected shock waves were focused at the second focus point outside the ellipsoidal reflector, and high pressure was generated for a few microseconds. To the shock wave propagation, human tissue has a character very similar to that of water, and we can generate high pressure inside the human body by shock wave focusing sufficient to disintegrate human calculi. This paper reports an experimental and numerical approach of determining the most effective shape of ellipsoidal reflector for the lithotriptor and clarifying the mechanism of focused high-pressure generation by microexplosions. It was found that a half-ellipsoidal shape having a major and minor axis ratio of about 1.4∼1.5 was very suitable for the lithotriptor.

16 水中衝撃波フォーカッシングの可視化と数値シミュレーション

Recently, underwater shock wave focusing has been applied to the non-invasive disintegration of kidny stones. However, due to the highly non-linearity, the shock wave focusing phenomenon is stil unresolved. Therefore, it is crucial to clarify this complicated phenomenon. In this paper, the experimental and computational study of the underwater shock wave focusing is presented. The underwater shock wave was generated by using a 5mg lead azide (Pb(N3)2) pellet and a 10mg silver azide (AgN3) pellet, and reflected by an ellipsoidal cavity, and focuses at 2nd focal point. The focusing process was visualized by using holographic interferometry. The peak pressure along the axis of the reflector were also measured by Kstler 601H pressure transducer. The interferometric results were compared with the numerical simulation conducted by using a Total Variation Diminishing (TVD) finite difference scheme. A good agreement was obtained between the computation and experiment.

水中衝撃波フォーカシングによる非観血的結石破砕について

水中衝撃波フォーカシングによる非観血的結石破砕について