2,007 research outputs found
On acoustic cavitation of slightly subcritical bubbles
The classical Blake threshold indicates the onset of quasistatic evolution
leading to cavitation for gas bubbles in liquids. When the mean pressure in the
liquid is reduced to a value below the vapor pressure, the Blake analysis
identifies a critical radius which separates quasistatically stable bubbles
from those which would cavitate. In this work, we analyze the cavitation
threshold for radially symmetric bubbles whose radii are slightly less than the
Blake critical radius, in the presence of time-periodic acoustic pressure
fields. A distinguished limit equation is derived that predicts the threshold
for cavitation for a wide range of liquid viscosities and forcing frequencies.
This equation also yields frequency-amplitude response curves. Moreover, for
fixed liquid viscosity, our study identifies the frequency that yields the
minimal forcing amplitude sufficient to initiate cavitation. Numerical
simulations of the full Rayleigh-Plesset equation confirm the accuracy of these
predictions. Finally, the implications of these findings for acoustic pressure
fields that consist of two frequencies will be discussed.Comment: 14 pages, Presented at APS/DFD conference in Philadelphia 199
Transport in Transitory Dynamical Systems
We introduce the concept of a "transitory" dynamical system---one whose
time-dependence is confined to a compact interval---and show how to quantify
transport between two-dimensional Lagrangian coherent structures for the
Hamiltonian case. This requires knowing only the "action" of relevant
heteroclinic orbits at the intersection of invariant manifolds of "forward" and
"backward" hyperbolic orbits. These manifolds can be easily computed by
leveraging the autonomous nature of the vector fields on either side of the
time-dependent transition. As illustrative examples we consider a
two-dimensional fluid flow in a rotating double-gyre configuration and a simple
one-and-a-half degree of freedom model of a resonant particle accelerator. We
compare our results to those obtained using finite-time Lyapunov exponents and
to adiabatic theory, discussing the benefits and limitations of each method.Comment: Updated and corrected version. LaTeX, 29 pages, 21 figure
Deformations of Gabor Frames
The quantum mechanical harmonic oscillator Hamiltonian generates a
one-parameter unitary group W(\theta) in L^2(R) which rotates the
time-frequency plane. In particular, W(\pi/2) is the Fourier transform. When
W(\theta) is applied to any frame of Gabor wavelets, the result is another such
frame with identical frame bounds. Thus each Gabor frame gives rise to a
one-parameter family of frames, which we call a deformation of the original.
For example, beginning with the usual tight frame F of Gabor wavelets generated
by a compactly supported window g(t) and parameterized by a regular lattice in
the time-frequency plane, one obtains a family of frames F_\theta generated by
the non-compactly supported windows g_\theta=W(theta)g, parameterized by
rotated versions of the original lattice. This gives a method for constructing
tight frames of Gabor wavelets for which neither the window nor its Fourier
transform have compact support. When \theta=\pi/2, we obtain the well-known
Gabor frame generated by a window with compactly supported Fourier transform.
The family F_\theta therefore interpolates these two familiar examples.Comment: 8 pages in Plain Te
Search for the magnetic field of the O7.5 III star xi Persei
Cyclical wind variability is an ubiquitous but as yet unexplained feature
among OB stars. The O7.5 III(n)((f)) star xi Persei is the brightest
representative of this class on the Northern hemisphere. As its prominent
cyclical wind properties vary on a rotational time scale (2 or 4 days) the star
has been already for a long time a serious magnetic candidate. As the cause of
this enigmatic behavior non-radial pulsations and/or a surface magnetic field
are suggested. We present a preliminary report on our attempts to detect a
magnetic field in this star with high-resolution measurements obtained with the
spectropolarimeter Narval at TBL, France during 2 observing runs of 5 nights in
2006 and 5 nights in 2007. Only upper limits could be obtained, even with the
longest possible exposure times. If the star hosts a magnetic field, its
surface strength should be less than about 300 G. This would still be enough to
disturb the stellar wind significantly. From our new data it seems that the
amplitude of the known non-radial pulsations has changed within less than a
year, which needs further investigation.Comment: 2 pages, 6 figures, contributed poster at IAU Symposium 259 "Cosmic
Magnetic Fields: from Planets, to Stars and Galaxies", Tenerife, Spain,
November 3-7, 200
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