Collapse Of Transient Cavities Research Articles

Inertial collapse of liquid rings

AbstractLiquid rings can be generated in the Leidenfrost state using liquid oxygen of low boiling point ($- 183~\textdegree \mathrm{C} $) and high magnetic susceptibility, allowing one to ‘sculpt’ the liquid into a ring shape using an annular magnet. When the magnetic field is turned off, the ring shrinks back into a puddle with a constant acceleration. A potential flow approach accurately describes the dynamics of closure with an equation reminiscent of the Rayleigh–Plesset equation for the collapse of transient cavities.

Deep Drilling into the Chesapeake Bay Impact Structure

Samples from a 1.76-kilometer-deep corehole drilled near the center of the late Eocene Chesapeake Bay impact structure (Virginia, USA) reveal its geologic, hydrologic, and biologic history. We conducted stratigraphic and petrologic analyses of the cores to elucidate the timing and results of impact-melt creation and distribution, transient-cavity collapse, and ocean-water resurge. Comparison of post-impact sedimentary sequences inside and outside the structure indicates that compaction of the crater fill influenced long-term sedimentation patterns in the mid-Atlantic region. Salty connate water of the target remains in the crater fill today, where it poses a potential threat to the regional groundwater resource. Observed depth variations in microbial abundance indicate a complex history of impact-related thermal sterilization and habitat modification, and subsequent post-impact repopulation.

Observations and interpretations at Vredefort, Sudbury, and Chicxulub: Towards an empirical model of terrestrial impact basin formation

Abstract— The structural, topographic and other characteristics of the Vredefort, Sudbury, and Chicxulub impact structures are described. Assuming that the structures originally had the same morphology, the observations/interpretations for each structure are compared and extended to the other structures. This does not result in any major inconsistencies but requires that the observations be scaled spatially. In the case of Vredefort and Sudbury, this is accomplished by scaling the outer limit of particular shock metamorphic features. In the case of Chicxulub, scaling requires a reasoned assumption as to the formation mechanism of an interior peak ring. The observations/interpretations are then used to construct an integrated, empirical kinematic model for a terrestrial peak‐ring basin. The major attributes of the model include: a set of outward‐directed thrusts in the parautochthonous rocks of the outermost environs of the crater floor, some of which are pre‐existing structures that have been reactivated during transient cavity formation; inward‐directed motions along the same outermost structures and along a set of structures, at intermediate radial distances, during transient cavity collapse; structural uplift in the center followed by a final set of radially outward‐directed thrusts at the outer edges of the structural uplift, during uplift collapse. The rock displacements on the intermediate, inward and innermost, outward sets of structures are consistent with the assumption that a peak ring will result from the convergence of the collapse of the transient cavity rim area and the collapse of the structural uplift.

Geology, Mineralization, and Emplacement of the Whistle-Parkin Offset Dike, Sudbury*

The Whistle-Parkin dike is a 12-km-long radial offset dike located in the northeast sector of the 1.85 Ga Sudbury impact structure. The dike is connected to the Sudbury Igneous Complex via a 0.5-km-long, 250-m-wide embayment. The Whistle segment of the dike narrows to a width of approximately 30 m and extends for about 1.5 km from the complex, where it is cut by the Post Creek fault zone. This fault displaces the dike 2 km to the northwest, beyond which it continues as the Parkin portion of the dike. The Parkin segment extends a further 10 km north-northeast. The dike and genetically associated embayment are comprised of numerous rock types. These include the sublayer, radial breccia, mafic sulfide-bearing breccia, inclusion-rich quartz diorite, and inclusion-poor quartz diorite. Economic Ni-Cu-PGE mineralization is found in association with the sublayer, mafic sulfide-bearing breccia, and inclusion-bearing quartz diorite phases. Rare earth element (REE) data reveal that the dike lithologies are representative of bulk Sudbury Igneous Complex melt. We propose a multistage emplacement mechanism for the dike. The breccia units, including the inclusion-bearing quartz diorite phase, were forcefully injected laterally into an impact-generated radial crack during the excavation stage of crater formation. Subsequent early modification processes facilitated the intrusion of inclusion-poor quartz diorite, particularly along the dike margins. Inclusion-poor quartz diorite emplacement may have been gravitationally driven from the overlying impact melt sheet. Later stage modification (i.e., final transient cavity collapse) caused decoupling of the Whistle and Parkin dike segments via faulting. This occurred concurrently with the settling of the sublayer unit and associated economic sulfides into the embayment and main dike segments.

A new approach to modelling an unsteady free surface in boundary integral methods with application to bubble-structure interactions

The interaction of transient cavities in fluid flows with rigid boundaries is investigated numerically. The class of fluid motions considered here can be described by a velocity potential and a newly-developed fully three-dimensional boundary integral method is employed to compute the evolution of the free surface. Particular attention is given to the approximation of the surface velocities by radial basis function methods, thus avoiding some of the limitations of polynomial approximation. The efficiency of the algorithm is demonstrated through computed examples which show the interaction of buoyancy with the Bjerknes effect of a rigid boundary for the growth and collapse of transient cavities in a fluid.

A late Triassic age for the Rochechouart impact structure, France

Abstract— Argon‐40/Argon‐39 laser spot fusion dating of pseudotachylyte from the ∼25 km diameter Rochechouart impact structure of western‐central France yields a matrix age of 214 ± 8 Ma (2s̀). Field evidence indicates that the pseudotachylyte was generated during the modification stage of the impact process, probably during transient cavity collapse. This new age is considerably older than the previously accepted age of 186 ± 8 Ma for this structure, which was obtained from hydrothermally‐altered melt sheet samples. The new age is in accordance with earlier paleomagnetic and fission track data, which indicated that Rochechouart was formed during the late Triassic. Moreover, the new determination is in agreement with the regional geological setting and field relations of the structure. The new age of 214 ± 8 Ma falls within the Norian stage of the Triassic system.

An estimate of the Kelvin impulse of a transient cavity

The Lagally theorem is used to obtain an expression for the Bjerknes force acting on a bubble in terms of the singularities of the fluid velocity potential, defined within the bubble by analytic continuation. This expression is applied to transient cavity collapse in the neighbourhood of boundaries, allowing analytical estimates to be made of the Kelvin impulse of the cavity. The known result for collapse near a horizontal rigid boundary is recovered, and the Kelvin impulse of a cavity collapsing in the neighbourhood of a submerged and partially submerged sphere is estimated. A numerical method is developed to deal with more general body shapes and in particular, bodies of revolution. Noting that the direction of the impulse at the end of the collapse phase generally indicates the direction of the liquid jet that may form, the behaviour of transient cavities in these geometries is predicted. In these examples the concept of a zone of attraction is introduced. This is a region around the body, within which the Kelvin impulse at the time of collapse, and consequent jet formation, is expected to be directed towards the body. Outside this zone the converse is true.

The formation of toroidal bubbles upon the collapse of transient cavities

A spectacular feature of transient cavity collapse in the neighbourhood of a rigid boundary is the formation of a high-speed liquid jet that threads the bubble and ultimately impacts upon the side of the bubble nearest to the boundary. The bubble then evolves into some toroidal form, the flow domain being doubly connected. In this work, the motion of the toroidal bubble is computed by connecting the jet tip to the side of the bubble upon which it impacts. This connection is via a cut introduced into the flow domain and across which the potential is discontinuous, the value of this discontinuity being equal to the circulation in the flow. A boundary integral algorithm is developed to account for this geometry and some example computations are presented. Consideration of the pressure field in the fluid has implications for possible damage mechanisms to structures due to nearby cavity collapse.

Sequence and mechanisms of deformation around the Hellas and Isidis Impact Basins on Mars

The distribution and age of early volcanic and tectonic features surrounding the Hellas and Isidis impact basins are shown to fit a four‐stage sequence. First, concentric “canyons” form outside the basin boundary scarp at or near the time of basin formation, followed shortly thereafter by radial troughs extending beyond the boundary scarp. After a hiatus, concentric graben develop inside the basin within the massif ring. Finally, soon after graben formation, widespread volcanic plains are emplaced between the massif ring and the boundary scarp to one side of the basin. Three models of basin‐centered deformation are compared to these observed deformation events: elastic flexure due to loading, elastic flexure due to uplift, and impact fracture. A central load of basin fill can account for concentric fractures by elastic flexure around the load. Formation of the distant concentric canyons, however, requires lithospheric thicknesses a factor of 5 greater than those indicated by planetary thermal history models and an increase in feature width by an order of magnitude relative to flexural graben identified elsewhere on Mars and the Moon. Although inclusion of loading by basin ejecta reduces the required lithospheric thickness to acceptable values, flexure still fails to account for the large canyon widths observed. Impact fracture from a rapid inward flow of asthenosphere during transient cavity collapse, however, is consistent with the three key observational constraints for the distant canyons: their age, width, and distance from the basin. As the basin then undergoes isostatic uplift soon after impact, elastic flexure induces a radial pattern of failure consistent with the observed extent and timing of the radial troughs. Later, after a long period of basin infilling, elastic failure due to loading of the central basin region can account for the ring grabens in the massif ring. The localization of volcanism into a single ridged plains unit on the basin rim indicates a thermal source offset from the basin which is not included in these models and perhaps reflects an additional mechanism unique to Mars.

A mechanism for the generation of cavitation maxima by pulsed ultrasound.

A train of 1-MHz pulses can generate maxima of cavitation activity [V. Ciaravino, H. G. Flynn, and M. W. Miller, Ultrasound Med. Biol. 7, 159-166 (1981)] at pulse lengths of 6 and 60 ms and at pressure amplitudes, PA, between 5.4 and 9.4 bars (or intensities between 10 and 30 W/cm2). Generation of maxima at PA between these limits on pressure amplitude implies that the increase in cavitation activity originates from gas nuclei with radii lying in a critical size range centered at about 0.08 micron. The mechanism proposed for this phenomenon suggests that nuclei in this critical range are unstabilized nuclei generated in one pulse and surviving to the next with an appreciable fraction of the survivors lying in the critical range. Transient cavities that grow from such small nuclei are shown to behave as isolated mechanical systems that on reaching maximum size collapse as imploding spheres. The maximum pressures reached in such imploding cavities would then approximate those calculated for the spherical collapse of cavities. The occurrence of the observed maxima is ascribed to the spherical collapse of transient cavities.

Evidence for free radical production by ultrasonic cavitation in biological media

The statement is often made that free radicals will be produced during the violent collapse of transient cavities in liquids, that may result from the passage of ultrasonic waves in the low megahertz frequency range. Evidence for such free radicals is mostly indirect. We have insonated some argon-saturated aqueous solutions of biological molecules (trypsin, Dulbecco's Minimum Essential Medium, fetal bovine serum, and human blood plasma) and whole human blood, all containing a spin trapping compound, at 1 MHz, continuous wave, 6.6 W/cm 2 (SPTA) for 15 min at 4–8°C. Electron spin resonance spectra of the samples were recorded 10 and 30 min. after cessation of insonation. The spectra included those of spin adducts formed with ·OH and ·H radicals, which were absent in the cases of the sham treated controls. Thus, we have demonstrated the formation of the highly reactive ·OH and ·H radicals in the insonated media, even in the presence of natural radical scavengers in the mammalian derived products.

Optical studies of prebreakdown events in liquid dielectrics

A combination of time-resolved Schlieren and scattered-light optical techniques has been used to examine the prebreakdown events in dielectric liquids. When a negative high-voltage pulse is applied to the point of a nonuniform gap, a disturbance, associated with discrete shock waves, propagates outwards from the point. For the opposite polarity, the disturbance moves at supersonic velocity, and is characteristically different in appearance and self luminosity. In both cases, the disturbance is identified as a region of closely packed microbubbles which ultimately fragments into isolated bubbles if breakdown does not intervene. The events recorded are explained in terms of cavitation nucleated by transient cavity collapse, and the polarity differences are attributed to field emission. The mechanism is further supported by experimental findings using prestressing techniques.

Cavitation Dynamics

A program of research in acoustic cavitation that was started at the suggestion of F. V. Hunt and continued through his cooperation has resulted in an extensive study of the motions of small bubbles under the influence of acoustic pressure fields. The principal tool in this investigation has been a mathematical formulation made up of a set of integral equations, differential equations, and algebraic equations. Solutions of this formulation obtained through use of a digital computer have so far given considerable insight into the free pulsations of cavities, the thresholds for generation of transient cavities, and the temperatures and pressures that might be reached in the collapse of transient cavities. A brief survey of these results will be given. [This work was supported by Acoustics Programs, Office of Naval Research.]

An experimental study of the forces generated by the collapse of transient cavities in water

The paper describes an experimental investigation of the pressures developed at the seat of collapse of cavities in water. Single transient cavities, generated by a spark discharge, are allowed to collapse on the end of a piezoelectric pressure-bar gauge which measures the variation of thrust with time. It is shown that both the peak force and duration of the cavity collapse pulse are functions of the cavity lifetime. From an estimate of the minimum radius attained by the cavity and the peak force, the peak pressure on collapse is found to be at least 10,000 atm. Streak schlieren photographs of the collapse process show that a shock wave is radiated into the water at the moment of collapse and that the cavity rebounds. At the collapse of the rebound cavities the pressures developed are comparable with those developed by the collapse of the initial cavity, and these probably contribute materially to cavitational surface damage.

Ultrasonic Spectrum of Cavitation Noise in Water

The acoustic noise spectrum associated with the collapse of transient cavities produced by a stirring rod in water was measured in the frequency range 1–3500 kc. While the spectrum, in general, goes as f−2 above 1 kc, flat regions are found between 20 kc and 100 kc, and again above 500 kc to at least 3500 kc. If the flat regions are due to discrete random impulses, two time constants, τ1≃10−6 second and τ2<10−8 second, may be obtained by using the criterion 2πfτ≃1, where f is the −3 db point beyond the flat region. With intense cavitation, the acoustic level fails off and becomes erratic in the 600-kc region. Transmission measurements made through the cavitating volume show strong sound absorption, which appears to be caused by a cloud of small, stable bubbles.