Akira Shima

Interaction of cavitation bubbles with a free surface

The motion of single- and two-cavitation bubbles generated by laser beams directly beneath a free surface is studied experimentally, using high-speed photography, and theoretically using the highly accurate boundary integral method. Favorable comparisons of bubble shape history and centroid motion are observed while the numerical calculations provide information on the pressure field surrounding the bubbles. A range of responses, including the null impulse state, is obtained for the two bubbles depending on the bubble size ratio and the interbubble and bubble-free surface distances, although in all cases reported in this article, the bubble nearest the free surface yields a high-speed liquid jet directed away from the free surface. It is also found that when the free-surface–bubble interaction is strong, a fast free-surface spike is formed for both the single- and two-bubble cases.

The growth and collapse of cavitation bubbles near composite surfaces

An experimental study has been made of the growth and collapse of a bubble near a composite surface consisting of two viscoelastic materials. The migratory characteristics of the bubble are examined by means of streak photography. The bubble migration depends not only on the properties of the composite surface but also on bubble size and distance from the surface. Both the surface stiffness and surface inertia are considered to be effective parameters contributing to the bubble migration: the former seeming to be a particularly important factor. The state of neutral bubble collapse, with no migration towards or away from a surface, can be made to occur with an appropriate combination of the surface stiffness and inertia.

Numerical analysis of a gas bubble near a rigid boundary in an oscillatory pressure field

The behavior of a gas bubble near a rigid boundary in an oscillatory pressure field is numerically investigated by means of two kinds of methods. The boundary integral method is utilized to simulate the bubble profiles at various times, and the image theory is also applied for solving the differential equations describing the nonlinear oscillations and migrations of a spherical gas bubble. Both calculations are performed, assuming that the liquid around a bubble is inviscid, incompressible, and irrotational and that the gas inside the bubble follows a polytropic gas law. The influence of the boundary proximity on the bubble motion is characterized by the bubble migration and the deformation of the bubble shape, especially by the liquid microjet formation. It is found that the period and the amplitude of the bubble oscillation are related not only to the nature of the time‐dependent pressure in the liquid but also to the bubble location from a rigid wall. Furthermore, the influence of the boundary proximit...

Interaction of two cavitation bubbles with a rigid boundary

The motion of two cavitation bubbles near a rigid boundary is observed experimentally using a high-speed camera and compared against numerical solutions obtained using a boundary integral method. The comparisons are favourable with regard to both bubble shape history and centroid motion. The bubbles show a range of responses depending on the experimental configuration. Elongated bubbles, jets directed towards or away from the rigid boundary and bubble splitting phenomena are all observed and predicted for the given parameters. It is clear that nearby bubbles are equally as important as the presence of a rigid boundary in determining the behaviour of bubbles.

Dynamic behavior of two-laser-induced bubbles in water

Two bubbles are simultaneously generated in water by focusing a Q‐switched ruby laser. The dynamic behavior of the induced bubbles is investigated by means of high‐speed photography. Consequently it is found that the bubble–bubble interaction is significantly influenced not only by the relative size of bubbles but also by the mutual distance between them. When two bubbles are axisymmetrically produced one behind the other near a rigid wall, we can observe a very interesting phenomenon, bubble splitting followed by bubble pinching.

Collapse of multiple gas bubbles by a shock wave and induced impulsive pressure

The problem of bubble‐bubble interaction is studied experimentally. The motions of multiple gas bubbles attached to a solid wall by a shock wave are observed by using a high speed camera, and the induced impulsive pressures are measured. On the basis of these results, the effects of number and configuration of gas bubbles on the collapsing process and the impulsive pressure are clarified. The maximum impulsive pressure quickly decreases with reducing the interval beteween bubbles due to significant interaction. The direction of a liquid jet formed within a bubble is determined as a resultant of effects such as shock direction, shock strength, and interactions between bubbles and between a bubble and a solid wall.

Secondary cavitation due to interaction of a collapsing bubble with a rising free surface

An experimental investigation was made of the motion of a cavitation bubble in the vicinity of a free surface in order to study an induced secondary cavitation during the bubble rebound. A bubble was produced by focusing a ruby laser into water, and its subsequent behavior was observed with a high‐speed camera. The deformable nature of both a bubble and a free surface becomes significant as the mutual distance between them is decreased. Immediately after bubble rebound, a secondary cavitation occurs at around zero dynamic pressure region which is developed in water between a rising free surface and a collapsing bubble, due to the local pressure reduction mainly caused by the interaction of expansion waves originated from the surface.

The Collapse of a Gas Bubble near a Solid Wall by a Shock Wave and the Induced Impulsive Pressure

An experimental study concerning the shock wave—bubble interaction was conducted in order to obtain a unified consideration of the mechanism of the impulsive pressure generation induced by the cavitation bubble collapse. It was found that the relation between the maximum impulsive pressure, pG, max, and the relative distance, lc/Re, is closely similar to the known result obtained from a single spark-generated bubble, and that a gas bubble within the region of lc/Re ≤ 7 behaves as a source capable of generating more intensive impulsive pressure than the impact pressure induced by a shock wave impinging directly on a solid wall without the presence of a gas bubble.

The collapse of minute gas bubbles in a dilute polymer solution

Collapse times of minute gas bubbles in both water and a dilute polymer solution were measured precisely by means of the streak schlieren method. It was found that when the bubble becomes smaller in size, the collapse time in a polymer solution is shortened in comparsion with it in water due to the relaxation effect.

Effect of polymer additives on the generation of subharmonic and harmonic bubble oscillations in an ultrasonically irradiated liquid

A study of non-linear oscillation of bubbles in polymer solutions subjected to a pulsating pressure has been performed. The equation of the bubble oscillation, the equation of the natural frequency of the bubble, and the pressure equation are derived. The frequency response curves and the maximum pressure in a Polyox solution, a PAM solution, a CMC solution, and water are found. The effects of polymer concentration on onset curves for the subharmonic and harmonic resonances are clarified.