1,528 research outputs found
Spinning jets
A fluid jet with a finite angular velocity is subject to centripetal forces
in addition to surface tension forces. At fixed angular momentum, centripetal
forces become large when the radius of the jet goes to zero. We study the
possible importance of this observation for the pinching of a jet within a
slender jet model. A linear stability analysis shows the model to break down at
low viscosities. Numerical simulations indicate that angular momentum is
expelled from the pinch region so fast that it becomes asymptotically
irrelevant in the limit of the neck radius going to zero
On the stabilization of ion sputtered surfaces
The classical theory of ion beam sputtering predicts the instability of a
flat surface to uniform ion irradiation at any incidence angle. We relax the
assumption of the classical theory that the average surface erosion rate is
determined by a Gaussian response function representing the effect of the
collision cascade and consider the surface dynamics for other
physically-motivated response functions. We show that although instability of
flat surfaces at any beam angle results from all Gaussian and a wide class of
non-Gaussian erosive response functions, there exist classes of modifications
to the response that can have a dramatic effect. In contrast to the classical
theory, these types of response render the flat surface linearly stable, while
imperceptibly modifying the predicted sputter yield vs. incidence angle. We
discuss the possibility that such corrections underlie recent reports of a
``window of stability'' of ion-bombarded surfaces at a range of beam angles for
certain ion and surface types, and describe some characteristic aspects of
pattern evolution near the transition from unstable to stable dynamics. We
point out that careful analysis of the transition regime may provide valuable
tests for the consistency of any theory of pattern formation on ion sputtered
surfaces
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Cavitation in Linear Bubbles
Recent work has developed a beautiful model system for studying the energy focusing and heating power of collapsing bubbles. The bubble is effectively one-dimensional and the collapse and heating can be quantitatively measured. Thermal effects are shown to play an essential role in the time-dependent dynamics.Engineering and Applied SciencesMolecular and Cellular Biolog
Analysis of Rayleigh-Plesset dynamics for sonoluminescing bubbles
Recent work on single bubble sonoluminescence (SBSL) has shown that many
features of this phenomenon, especially the dependence of SBSL intensity and
stability on experimental parameters, can be explained within a hydrodynamic
approach. More specifically, many important properties can already be derived
from an analysis of bubble wall dynamics. This dynamics is conveniently
described by the Rayleigh-Plesset (RP) equation. In this work we derive
analytical approximations for RP dynamics and subsequent analytical laws for
parameter dependences. These results include (i) an expression for the onset
threshold of SL, (ii) an analytical explanation of the transition from
diffusively unstable to stable equilibria for the bubble ambient radius
(unstable and stable sonoluminescence), and (iii) a detailed understanding of
the resonance structure of the RP equation. It is found that the threshold for
SL emission is shifted to larger bubble radii and larger driving pressures if
surface tension is enlarged, whereas even a considerable change in liquid
viscosity leaves this threshold virtually unaltered. As an enhanced viscosity
stabilizes the bubbles against surface oscillations, we conclude that the ideal
liquid for violently collapsing, surface stable SL bubbles should have small
surface tension and large viscosity, although too large viscosity (>40 times
the viscosity of water) will again preclude collapses.Comment: 41 pages, 21 eps and ps figures; LaTeX stylefiles replaced because
the PostScript file produced at the archive had misplaced and misscaled
figure
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