S. J. Shaw

The role of 'splashing' in the collapse of a laser-generated cavity near a rigid boundary

Vapour cavities in liquid flows have long been associated with cavitation damage to nearby solid surfaces and it is thought that the final stage of collapse, when a high- speed liquid jet threads the cavity, plays a vital role in this process. The present study investigates this aspect of the motion of laser-generated cavities in a quiescent liquid when the distance (or stand-off) of the point of inception from a rigid boundary is between 0.8 and 1.2 times the maximum radius of the cavity. Numerical simulations using a boundary integral method with an incompressible liquid impact model provide a framework for the interpretation of the experimental results. It is observed that, within the given interval of the stand-off parameter, the peak pressures measured on the boundary at the first collapse of a cavity attain a local minimum, while at the same time there is an increase in the duration of the pressure pulse. This contrasts with a monotonic increase in the peak pressures as the stand-off is reduced, when the cavity inception point is outside the stated interval. This phenomenon is shown to be due to a splash effect which follows the impact of the liquid jet. Three cases are chosen to typify the splash interaction with the free surface of the collapsing cavity: (i) surface reconnection around the liquid jet; (ii) splash impact at the base of the liquid jet; (iii) thin film splash. Hydrodynamic pressures generated following splash impact are found to be much greater than those produced by the jet impact. The combination of splash impact and the emission of shock waves, together with the subsequent re-expansion, drives the flow around the toroidal cavity producing a distinctive double pressure peak.

Experimental observations of the interaction of a laser generated cavitation bubble with a flexible membrane

In this paper we present new observations of a laser generated cavitation bubble interacting with an inertial boundary. Employing schlieren photography and Mach–Zehnder interferometry techniques, we present photographic sequences of a cavity interacting with a flexible membrane. During expansion the membrane is deformed away from the bubble centroid but during the collapse phase, instead of moving with the expected fluid motion caused by the contracting bubble, the flexible membrane introduces a certain asymmetry into the problem which in turn results in novel fluid motions around the bubble.

THE INTERACTION OF A SINGLE LASER-GENERATED CAVITY IN WATER WITH A SOLID SURFACE

This paper reexamines the collapse of a cavity in water and its role in possible damage processes. Laser‐generated cavities were formed near to a water–solid boundary and simultaneous observations made of the collapse processes and stresses on the solid surface. Using schlieren photography and a thin film transducer on the solid, the importance of fluid flow to induced material stress was determined. The stresses in the solid were also observed using dynamic photoelasticity. The cavity deformation in the collapse phase was observed with high spatial and temporal resolution. Two distinct surface stresses were observed associated with bubble collapse near the boundary. The results indicate that one was due to a shock wave. It is postulated that the second stress was associated with fluid flow through the bubble, set up by the so‐called ‘‘liquid jet’’ as opposed to jet impact on its own.

Translation and oscillation of a bubble under axisymmetric deformation

In this work the nonlinear interactions between the axisymmetric shape distortions, the axial translational motion, and the volume oscillations of a gas bubble in an inviscid, incompressible liquid are considered. Representing the surface deformation by a complete set of Legendre polynomials and assuming that both this deformation and the translational motion are small, a Lagrangian energy formulation is used to derive a system of equations valid to third order in these interaction terms. The effects of surface tension and pressure are also accounted for to this order. No restriction is placed on the size of the volume oscillations. Examination of the translational motion equation indicates that while at second order the interaction of two neighboring shape modes can cause the bubble to move, at third order any combination of three odd shape modes or one odd and two even shape modes can result in the bubble moving. Third order interactions between any three even shape modes or one even and two odd modes w...

A study of the interaction of a laser-generated cavity with a nearby solid boundary

In this work we consider the interaction of a laser-generated cavitation bubble with a solid boundary for the case of a cavity created very close to the wall. Using a combination of a thin-film transducer placed on the surface of the boundary and schlieren photography techniques we observe the induced pressure stresses on the solid boundary and in the surrounding fluid. By studying bubble shapes and identifying the formation of a liquid jet and shock wave emission, we speculate on the dominant pressures stresses induced by the bubble around the time of its first minimum volume for this chosen creation site of the cavity.

Shock emission from collapsing gas bubbles

The origin and the resultant properties of the strong pulses or shocks emitted by collapsing gas bubbles into a surrounding liquid are investigated numerically. The compressible flow in both phases is resolved. Results are presented for micron- and millimetre-sized bubbles and for bubble collapse triggered either by an acoustic driving or by an initially imposed spherical shock in the liquid. The origin of the diverging shocks is investigated, and the results of a parametric study for the acoustically driven collapse reveal a predominant linear dependence of the shock strength and width on the maximum bubble radius. The results compare favourably with experimental data and agree well with acoustic theory in the limit of weak forcing.

Experimental observations of the stress experienced by a solid surface when a laser-created bubble oscillates in its vicinity

In an attempt to shed more light on the complex process by which cavities interact destructively with rigid boundaries we consider experimentally the interaction of a laser-generated bubble with a nearby solid boundary. To determine the stresses the surface of the boundary experiences, particularly during the final stages of the first collapse phase, we present a series of pressure transducer traces observed when the laser-created cavity is produced close to the solid boundary. Through careful control of experimental parameters we record pressure increases for cavities created between 0.56 and 1.5 times the maximum bubble radius from the solid boundary. Previous studies have shown this to be a complex process which is particularly sensitive to the working parameters of the problem, which is borne out by our observations.

The stability of a bubble in a weakly viscous liquid subject to an acoustic traveling wave

The volume oscillations, translation, and axisymmetric deformation of a bubble in an acoustic traveling wave are considered. Assuming the bubble translation and deformation is small, but placing no restriction on the volume oscillations, a combination of the Rayleigh dissipation function and perturbation analysis is employed to account for the effects of viscosity in the absence of vorticity to third order in the small interaction terms. Contributions from the acoustic field are also determined to this order, while the free oscillation terms are drawn from a previously derived model correct to the same order of analysis. To permit the study of large amplitude acoustic forcing, appropriate compressibility terms are phenomenologically added to the volume pulsation equation. Stability maps of driving pressure versus driving frequency and driving pressure versus the equilibrium bubble radius are presented. A predominant number of results are for micron-sized bubbles driven in the ultrasonic regime, but the be...

Acoustical and optical tracking of the collapse of a laser-generated cavitation bubble near a solid boundary

A pulse from a Nd:YAG laser is used to generate a cavitation bubble near a plane solid boundary in deionised water. The different stages in the collapse of the bubble are followed using a high speed photographic technique involving sensitive Schlieren imaging and an ultra short pulse from a nitrogen laser pumped dye laser. The first collapse phase of the cavitation bubble is recorded with high spatial and temporal resolution. A thin film PVDF pressure transducer attached to the surface of the solid was used to study the development of the pressure on the surface during the bubble collapse and fluid flow was also directly observed. Both the optical and acoustical analysis techniques confirm the formation of a liquid jet and a shock wave which we believe are responsible for damage and erosion associated with cavitation near solid surfaces.

The inviscid stability of supersonic flow past heated or cooled axisymmetric bodies

The inviscid, linear, nonaxisymmetric, temporal stability of the boundary layer associated with the supersonic flow past axisymmetric bodies (with particular emphasis on long thin, straight circular cylinders), subject to heated or cooled wall conditions is investigated. The eigenvalue problem is computed in some detail for Mach numbers 2.8 and 3.8, revealing that curvature and choice of wall conditions both have a significant effect on the stability of the flow. The asymptotic, large azimuthal wave‐number solution is obtained for the inviscid stability of the flow and compared with numerical results. Additionally, asymptotic analyses valid for large radii of curvature with cooled/heated wall conditions are presented.