124,271 research outputs found
Astrophysical Accretion as an Analogue Gravity Phenomena
In recent years, strong analogies have been established between the physics
of acoustic perturbations in an inhomogeneous dynamical fluid system, and some
kinematic features of space-time in general relativity. An effective metric,
referred to as the `acoustic metric', which describes the geometry of the
manifold in which acoustic perturbations propagate, can be constructed. This
effective geometry can capture the properties of curved space-time in general
relativity. Physical models constructed utilizing such analogies are called
`analogue gravity models'. Classical analogue gravity effect may be observed
when acoustic perturbations propagate through a inhomogeneous transonic
classical fluid. The acoustic horizon, which resembles a black hole event
horizon in many ways, is generated at the transonic point in the fluid flow.
The acoustic horizon is essentially a null hyper surface, generators of which
are the acoustic null geodesics, i.e. the phonons. The acoustic horizon emits
acoustic radiation with quasi thermal phonon spectra, which is analogous to the
actual Hawking radiation. The temperature of the radiation emitted from the
acoustic horizon is referred to as the analogue Hawking temperature.
It has been demonstrated that, in general, the transonic accretion in
astrophysics can be considered as an example of the classical analogue gravity
model naturally found in the Universe.Comment: 56 pages, 11 figures, revtex4. Send email request to the author for
high resolution version of the manuscrip
An analog fluid model for some tachyonic effects in field theory
We consider the sound radiation from an acoustic point-like source moving
along a supersonic ("space-like") trajectory in a fluid at rest. We call it an
acoustic "tachyonic" source. We describe the radiation emitted by this
supersonic source. After quantizing the acoustic perturbations, we present the
distribution of phonons generated by this classical tachyonic source and the
classical wave interference pattern.Comment: 14 pages, 5 figures, accepted for publication in Mod. Phys. Lett.
Acoustic radiation- and streaming-induced microparticle velocities determined by micro-PIV in an ultrasound symmetry plane
We present micro-PIV measurements of suspended microparticles of diameters
from 0.6 um to 10 um undergoing acoustophoresis in an ultrasound symmetry plane
in a microchannel. The motion of the smallest particles are dominated by the
Stokes drag from the induced acoustic streaming flow, while the motion of the
largest particles are dominated by the acoustic radiation force. For all
particle sizes we predict theoretically how much of the particle velocity is
due to radiation and streaming, respectively. These predictions include
corrections for particle-wall interactions and ultrasonic thermoviscous
effects, and they match our measurements within the experimental uncertainty.
Finally, we predict theoretically and confirm experimentally that the ratio
between the acoustic radiation- and streaming-induced particle velocities is
proportional to the square of the particle size, the actuation frequency and
the acoustic contrast factor, while it is inversely proportional to the
kinematic viscosity.Comment: 11 pages, 9 figures, RevTex 4-
Acoustic radiation stress measurement
Ultrasonic radio frequency tone-bursts are launched into a sample of material tested. The amplitude of the tone-bursts and the slope of the resulting static displacement pulses are measured. These measurements are used to calculate the nonlinearities of the materials
Acoustic radiation from the end of two-dimensional duct, effects of uniform flow and duct lining
A study is presented of the radiation of acoustic modes from the end of a duct immersed in a uniformly moving medium. It is shown that the uniform flow has roughly the same effect as an increase in frequency at constant mode number: the number of lobes of the radiation pattern increases, and the radiation maximum is slightly displaced towards the duct axis. When the mode is near cut-off the forward radiation for an inlet is enhanced. The acoustic radiation characteristics of ducts with soft or absorbing walls and hard, perfectly-reflecting walls are then compared. It is shown, and this is of technological interest, that the side radiation from the end of an acoustically soft duct is greatly reduced for lower-order modes
Total energy losses due to the radiation in an acoustically based undulator: the undulator and the channeling radiation included
This paper is devoted to the investigation of the radiation energy losses of
an ultra-relativistic charged particle channeling along a crystal plane which
is periodically bent by a transverse acoustic wave. In such a system there are
two essential mechanisms leading to the photon emission. The first one is the
ordinary channeling radiation. This radiation is generated as a result of the
transverse oscillatory motion of the particle in the channel. The second one is
the acoustically induced radiation. This radiation is emitted because of the
periodic bending of the particle's trajectory created by the acoustic wave. The
general formalism described in our work is applicable for the calculation of
the total radiative losses accounting for the contributions of both radiation
mechanisms. We analyze the relative importance of the two mechanisms at various
amplitudes and lengths of the acoustic wave and the energy of the projectile
particle. We establish the ranges of projectile particle energies, in which
total energy loss is small for the LiH, C, Si, Ge, Fe and W crystals. This
result is important for the determination of the projectile particle energy
region, in which acoustically induced radiation of the undulator type and also
the stimulated photon emission can be effectively generated. The latter effects
have been described in our previous works
Investigation of two-dimensional acoustic resonant modes in a particle separator
Within an acoustic standing wave particles experience acoustic radiation forces, a phenomenon which is exploited in particle or cell manipulation devices. When developing such devices, one-dimensional acoustic characteristics corresponding to the transducer(s) are typically of most importance and determine the primary radiation forces acting on the particles. However, radiation forces have also been observed to act in the lateral direction, perpendicular to the primary radiation force, forming striated patterns. These lateral forces are due to lateral variations in the acoustic field influenced by the geometry and materials used in the resonator. The ability to control them would present an advantage where their effect is either detrimental or beneficial to the particle manipulation process.The two-dimensional characteristics of an ultrasonic separator device have been modelled within a finite element analysis (FEA) package. The fluid chamber of the device, within which the standing wave is produced, has a width to height ratio of approximately 30:1 and it is across the height that a half-wavelength standing wave is produced to control particle movement. Two-dimensional modal analyses have calculated resonant frequencies which agree well with both the one-dimensional modelling of the device and experimentally measured frequencies. However, these two-dimensional analyses also reveal that these modes exhibit distinctive periodic variations in the acoustic pressure field across the width of the fluid chamber. Such variations lead to lateral radiation forces forming particle bands (striations) and are indicative of enclosure modes.The striation spacings predicted by the FEA simulations for several modes compare well with those measured experimentally for the ultrasonic particle separator device. It is also shown that device geometry and materials control enclosure modes and therefore the strength and characteristics of lateral radiation forces, suggesting the potential use of FEA in designing for the control of enclosure modes in similar particle manipulator devices
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