Abstract
The dynamics of the medium within a collapsing and rebounding cavitation bubble is investigated by means of molecular dynamics (MD) simulations adopting a hard sphere model for the species inside the bubble. The dynamics of the surrounding liquid (water) is modelled using a Rayleigh–Plesset (RP)-type equation coupled to the bubble interior by the gas pressure at the wall obtained from the MD calculations. Water vapour and vapour chemistry are included in the RP-MD model as well as mass and energy transfer through the bubble wall. The calculations reveal the evolution of temperature, density and pressure within a bubble at conditions typical of single-bubble sonoluminescence and predict how the particle numbers and densities of different vapour dissociation and reaction products in the bubble develop in space and time. Among the parameters varied are the sound pressure amplitude of a sonoluminescence bubble in water, the noble gas mixture in the bubble and the accommodation coefficients for mass and energy exchange through the bubble wall. Simulation particle numbers up to 10 million are used; most calculations, however, are performed with one million particles to save computer run time. Validation of the MD code was done by comparing MD results with solutions obtained by continuum mechanics calculations for the Euler equations.
Highlights
The dynamics of the medium within a collapsing and rebounding cavitation bubble is investigated by means of molecular dynamics (MD) simulations adopting a hard sphere model for the species inside the bubble
The liquid confining the bubble may be modelled by particles, for instance as a Lennard–Jones (LJ) fluid, i.e. by particles interacting via the LJ pair potential, or else in a hybrid model, as a continuum with prescribed liquid motion or, interactively, with the MD calculations coupled to a corresponding continuum equation
Twodimensional (2D) MD simulations with hard spheres and again a constantly contracting cavity have been compared with continuum solutions based on Euler’s equations and Navier–Stokes equations [37]
Summary
Deutsches Zentrum fur Luft- und Raumfahrt eV, Institut fur Aerodynamik und Stromungstechnik, Bunsenstrasse 10, 37073 Gottingen, Germany. 2 Author to whom any correspondence should be addressed. The main results are that, for a typical sonoluminescence bubble, there are strong compression waves running to the centre upon collapse even when water vapour, its dissociation and chemical reactions are included; that water vapour is trapped at the centre of the bubble and counteracts the temperature increase [9]; that mixture segregation occurs as predicted by continuum mechanics calculations [25]; and that the energy focusing mechanism is quite robust to perturbations of the collapsing surface, as might be expected from experiments [50] These investigations were extended and refined by Schanz [51] to times beyond the collapse ([42], figure 10; [52], figure 29; [53]), to the oscillatory rebound phase of a sonoluminescing bubble ([52], figure 30; [17], figure 98) and to laser-induced bubbles for comparing the RP-MD model ([52], figure 31) with bubbles inserted into a sound field at different phases [54]. Water vapour and its chemistry are included in the investigation
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