89,416 research outputs found
Travelling-wave thermoacoustic electricity generator using an ultra-compliant alternator for utilization of low-grade thermal energy
This paper proposes a novel concept of a travelling-wave thermoacoustic electricity generator, which employs a looped-tube travelling-wave thermoacoustic engine to convert thermal energy into acoustic power, an ultra-compliant alternator within the engine loop to extract and convert the engine acoustic power to electricity and an acoustic stub matching technique to match the alternator to the engine. In addition, a carefully designed cold heat exchanger acts as a phase shifting inertance to improve the performance. A simple model has been developed to capture and demonstrate the physics of this new concept, while the whole system has been investigated in detail numerically by using a specialized design tool DeltaEC. Based on the current concept, a prototype has been designed, constructed and tested. It uses atmospheric air as the working fluid, a commercially available audio loudspeaker as the electro-dynamic transducer, and inexpensive standard parts as the acoustic resonator. The experimental results have verified the simplified model and the numerical simulations of the practical build. The small-scale inexpensive prototype generator produced 11.6 W of electrical power, which shows the potential for developing cheap thermoacoustic electricity generators for energy recovery from waste heat sources. It is concluded that such concept could be very attractive provided that inexpensive ultra-compliant alternators based on the standard technology used in audio loudspeakers could be developed. Finally, some guidelines have been discussed and proposed for developing such alternators
Developments in electromagnetic tomography instrumentation.
A new EMT sensor and instrumentation is described which combines the best features of previous systems and has a modular structure to allow for future system expansion and development
A Body-Nonlinear Green's Function Method with Viscous Dissipation Effects for Large-Amplitude Roll of Floating Bodies
A novel time-domain body-nonlinear Greenâs function method is developed for evaluating large-amplitude roll damping of two-dimensional floating bodies with consideration of viscous dissipation effects. In the method, the instantaneous wetted surface of floating bodies is accurately considered, and the viscous dissipation effects are taken into account based on the âfairly perfect fluidâ model. As compared to the method based on the existing inviscid body-nonlinear Greenâs function, the newly proposed method can give a more accurate damping coefficient of floating bodies rolling on the free surface with large amplitudes according to the numerical tests and comparison with experimental data for a few cases related to ship hull sections with bilge keels
Radio Emission from Pulsar Wind Nebulae without Surrounding Supernova Ejecta: Application to FRB 121102
In this paper, we propose a new scenario in which a rapidly-rotating
strongly-magnetized pulsar without any surrounding supernova ejecta produces
fast radio bursts (FRBs) repeatedly via some mechanisms, and meanwhile, an
ultra-relativistic electron/positron pair wind from the pulsar sweeps up its
ambient dense interstellar medium, giving rise to a non-relativistic pulsar
wind nebula (PWN). We show that the synchrotron radio emission from such a PWN
is bright enough to account for the recently-discovered persistent radio source
associated with the repeating FRB 121102 in reasonable ranges of the model
parameters. In addition, our PWN scenario is consistent with the non-evolution
of the dispersion measure inferred from all the repeating bursts observed in
four years.Comment: 6 pages, 1 figure, ApJ Letters in pres
GRB Precursors in the Fallback Collapsar Scenario
Precursor emission has been observed in a non-negligible fraction of
gamma-ray bursts.The time gap between the precursor and the main burst extends
in some case up to hundreds of seconds, such as in GRB041219A, GRB050820A and
GRB060124. Both the origin of the precursor and the large value of the time gap
are controversial. Here we investigate the maximum possible time gaps arising
from the jet propagation inside the progenitor star, in models which assume
that the precursor is produced by the jet bow shock or the cocoon breaking out
of the progenitor. Due to the pressure drop ahead of the jet head after it
reaches the stellar surface, a rarefaction wave propagates back into the jet at
the sound speed, which re-accelerates the jet to a relativistic velocity and
therefore limits the gap period to within about ten seconds. This scenario
therefore cannot explain gaps which are hundreds of seconds long. Instead, we
ascribe such long time gaps to the behavior of the central engine, and suggest
a fallback collapsar scenario for these bursts. In this scenario, the precursor
is produced by a weak jet formed during the initial core collapse, possibly
related to MHD processes associated with a short-lived proto-neutron star,
while the main burst is produced by a stronger jet fed by fallback accretion
onto the black hole resulting from the collapse of the neutron star. We have
examined the propagation times of the weak precursor jet through the stellar
progenitor. We find that the initial weak jet can break out of the progenitor
in a time less than ten seconds (a typical precursor duration) provided that it
has a moderately high relativistic Lorentz factor \Gamma>=10 (abridged).Comment: 8 pages, accepted by ApJ, this version contains significantly
expanded discussion and an additional figure, conclusions unchange
Plasma dispersion of multisubband electron systems over liquid helium
Density-density response functions are evaluated for nondegenerate
multisubband electron systems in the random-phase approximation for arbitrary
wave number and subband index. We consider both quasi-two-dimensional and
quasi-one- dimensional systems for electrons confined to the surface of liquid
helium. The dispersion relations of longitudinal intrasubband and transverse
intersubband modes are calculated at low temperatures and for long wavelengths.
We discuss the effects of screening and two-subband occupancy on the plasmon
spectrum. The characteristic absorption edge of the intersubband modes is
shifted relatively to the single-particle intersubband separation and the
depolarization shift correction can be significant at high electron densities
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