Tetsuro Obara

Liquid-jet impact on liquid and solid surfaces

Abstract Experiments were conducted to investigate the mechanisms of the liquid-jet impact on liquid and solid surfaces associated with cavitation damage and rain erosion. In this work, a liquid jet of 3 mm diameter, generated using a single impact jet apparatus, was impacted at ca. 600 m s −1 on the surface of water and polymethyl-methacrylate (PMMA) placed 15 mm below the nozzle exit. The various phenomena which occurred were photographed using an image-converter high-speed camera. These include (i) the shock wave generated at the moment of the liquid-jet impact on the surface, (ii) shock waves reflected at the liquid/air or solid/air interfaces, as release waves, (iii) the damage to the PMMA, (iv) the overlap of release waves on the central axis, causing central damage in PMMA and cavitation in liquids. The cavitation behaviour and the process of the damage is directly related to the behaviour of the compressive wave caused by the impact of the liquid jet.

High-speed photography and stress gauge studies of jet impact upon surfaces

Experiments were conducted to investigate the mechanisms of damage to brittle materials by liquid–jet impact as seen in cavitation or simulated rain erosion. In this work, a liquid–jet of 3 mm in diameter and speed ca. 600 m s-1, produced using the single–impact jet apparatus (SIJA), was impacted on a polymethylmethacrylate (PMMA) target, chosen as a representative brittle solid. Damage was produced by the liquid jet both near the surface of, and within the bulk of the target. These failure phenomena, which proceeded in several stages, were observed in detail by means of a high–speed camera and with the aid of schlieren visualization techniques. The damage introduced resulted from the interactions of stress waves (compressive, tensile and shear) within the target. A surface ring crack was produced behind a propagating release wave. A central crack along the axis of the jet was also produced by interaction of these waves and, in the case of targets of finite thickness, spall damage was observed at the rear surface. A further crack was opened by the interaction of a shear wave with a reflected release. All the failure mechanisms resulted from wave interactions induced in the target and occurred within a few microseconds of impact.

Diffusion flame stabilized on a porous plate in a parallel airstream

Effects of an obstacle on the structure and stability of a laminar diffusion e ame established on a porous plate in a parallel airstream have been investigated experimentally. The obstacle, a backward-facing step or a rectangular cylinder, is located upstream of the porous plate through which gaseous methane is injected uniformly. Structures of the e ame are elucidated by the direct and schlieren photography. Flame shapes are described and stability diagrams are plotted for the freestream velocity and the fuel injection velocity, which are discussed with e ow structures.

Underwater Shock Wave Propagation and Focusing in Inhomogeneous Media

It is important to clarify the mechanism of human tissue damage which occurs associated with Extracorporeal Shock Wave Lithotripsy (ESWL) clinical treatments. Human tissue being non-homogeneous, shock wave propagation in it involves complicated wave interactions, such as reflection, refraction and diffraction. Therefore, in order to understand the dynamic behavior of human tissue exposed to shock waves, it is necessary to determine experimentally the equation of state (EOS) of human tissue. This paper reports the experimentally obtained Hugo- niot curve of human blood and other model tissue. Using these experimental results, a numerical simulation of shock propagation in inhomogeneous model tissue was carried out using a TVD finite difference scheme.

The construction and calibration of an inexpensive PVDF stress gauge for fast pressure measurements

Commercial polyvinylidene difluoride (PVDF) piezoelectric film was used to produce an inexpensive stress sensor with good spatial accuracy to measure the stress state behind a shock wave. The PVDF gauge was found to respond within approximately 100 ns to fast-rising pulses. Since the gauge is piezoelectric no power supply or amplifier was required for the gauge. In order to measure fast signals the gauge was found to perform best in the current mode in which the derivative of the stress was measured; the signal was then numerically integrated. Signals were recorded on a fast (100 MS s-1) storage oscilloscope. The gauges were calibrated against thin-foil manganin stress gauges to approximately 2%. An application in which the gauges were used to measure the response of an alumina to shock loading is described.

A high-speed photographic study of the transition from deflagration to detonation wave

Experiments were conducted to investigate the DDT process of the oxyhydrogen gas in the rectangular detonation tube of 3 m long. The repeated obstacle was installed near the ignition plug and the effects of the obstacle on the DDT process were investigated. The behaviour of the combustion and detonation wave were visualized utilizing Imacon high-speed camera with the aid of Schlieren optics.As a result, DDT process was visualized, i.e. (i) multiple shock waves were induced by the expanding combustion wave, because the combustion flame played a role as a piston and compressed the unburned gases. (ii) The acceleration of the combustion wave was occurred and the distance between the shock wave and the combustion flame became shorter. (iii) Eventually, the local explosion was occurred and cause overdriven detonation wave to propagate at the velocity of about 3 kms−1.

The Impact and Penetration of a Water Surface by a Liquid Jet

It is well known, that a high velocity jet threads a spherical bubble during asymmetric collapse. The early phases of jet formation and travel have been investigated by several workers. The final stage of collapse, in which a microjet impacts on the rear wall of the cavity and penetrates it are less well studied and difficult to model accurately. A series of experiments were thus conducted in which liquid jets of varying velocity were fired at semi-infinite water targets. The penetration was monitored using high-speed streak and framing photography. An unexpected feature of the penetration was found to be the appearance of an axial cavitation cloud ahead of the jet. This cloud was subsequently collapsed by what is believed to be an entrapped air bubble collapsing at the jet-target interface. The subsequent motion of this interface showed periodic acceleration and deceleration. The results have interest to a range of practical situations involving cavities and their collapse.

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 holographic interferometric studies of underwater shock-wave focusing to medicine

Holographic interferometric flow visualization was successfully applied to underwater shock wave focusing and its application to extracorporeal shock wave lithotripsy (ESWL). Real time diffuse holograms revealed the shock wave focusing process in an ellipsoidal reflector made from PMMA and double exposure holographic interferometry also clarified quantitatively the shock focusing process. Disintegration of urinary tract stones and gallbladder stones was observed by high speed photogrammetry. Tissue damage associated with the ESWL treatment is discussed in some detail.

A study on DDT processes in a narrow channel

One of the fundamental problems to be studied on a Pulse Detonation Engine (PDE) is the deflagration to detonation transition (DDT). For the development of the PDE, it is essential to shorten a distance of detonation transition that is called a detonation induction distance (DID). We carried out an experimental study of DDT in a narrow channel with height of 1-5mm by using pressure and soot track records in oxyhydrogen mixtures. Detonation limits was discussed according to height of tube and equivalence ratio. According to pressure history and soot track record, detonation velocity, DDT process and DID was discussed. Over driven detonation and attenuated detonation was observed in the narrow channel. DID that measured by soot track record applied to experimental formula for oxygen and hydrogen system.