97,818 research outputs found
Performance limitations of subband adaptive filters
In this paper, we evaluate the performance limitations of subband adaptive filters in terms of achievable final error terms. The limiting factors are the aliasing level in the subbands, which poses a distortion and thus presents a lower bound for the minimum mean squared error in each subband, and the distortion function of the overall filter bank, which in a system identification setup restricts the accuracy of the equivalent fullband model. Using a generalized DFT modulated filter bank for the subband decomposition, both errors can be stated in terms of the underlying prototype filter. If a source model for coloured input signals is available, it is also possible to calculate the power spectral densities in both subbands and reconstructed fullband. The predicted limits of error quantities compare favourably with simulations presented
Instabilities of Shercliffe and Stewartson layers in spherical Couette flow
We explore numerically the flow induced in a spherical shell by differentially rotating the inner and outer spheres. The fluid is also taken to be electrically conducting (in the low magnetic Reynolds number limit), and a magnetic field is imposed parallel to the axis of rotation. If the outer sphere is stationary, the magnetic field induces a Shercliffe layer on the tangent cylinder, the cylinder just touching the inner sphere and parallel to the field. If the magnetic field is absent, but a strong overall rotation is present, Coriolis effects induce a Stewartson layer on the tangent cylinder. The nonaxisymmetric instabilities of both types of layer separately have been studied before; here, we consider the two cases side by side, as well as the mixed case, and investigate how magnetic and rotational effects interact. We find that if the differential rotation and the overall rotation are in the same direction, the overall rotation may have a destabilizing influence, whereas if the differential rotation and the overall rotation are in the opposite direction, the overall rotation always has a stabilizing influence
Double Coronal Hard and Soft X-ray Source Observed by RHESSI: Evidence for Magnetic Reconnection and Particle Acceleration in Solar Flares
We present data analysis and interpretation of an M1.4-class flare observed
with the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) on April
30, 2002. This event, with its footpoints occulted by the solar limb, exhibits
a rarely observed, but theoretically expected, double-source structure in the
corona. The two coronal sources, observed over the 6-30 keV range, appear at
different altitudes and show energy-dependent structures with the higher-energy
emission being closer together. Spectral analysis implies that the emission at
higher energies in the inner region between the two sources is mainly
nonthermal, while the emission at lower energies in the outer region is
primarily thermal. The two sources are both visible for about 12 minutes and
have similar light curves and power-law spectra above about 20 keV. These
observations suggest that the magnetic reconnection site lies between the two
sources. Bi-directional outflows of the released energy in the form of
turbulence and/or particles from the reconnection site can be the source of the
observed radiation. The spatially resolved thermal emission below about 15 keV,
on the other hand, indicates that the lower source has a larger emission
measure but a lower temperature than the upper source. This is likely the
result of the differences in the magnetic field and plasma density of the two
sources.Comment: Accepted by ApJ (12/06/2007), scheduled for the 03/20/2008 Vol. 676
No. 1 Issue, 13 pages, 9 figure
Optimal design application on the advanced aeroelastic rotor blade
The vibration and performance optimization procedure using regression analysis was successfully applied to an advanced aeroelastic blade design study. The major advantage of this regression technique is that multiple optimizations can be performed to evaluate the effects of various objective functions and constraint functions. The data bases obtained from the rotorcraft flight simulation program C81 and Myklestad mode shape program are analytically determined as a function of each design variable. This approach has been verified for various blade radial ballast weight locations and blade planforms. This method can also be utilized to ascertain the effect of a particular cost function which is composed of several objective functions with different weighting factors for various mission requirements without any additional effort
Dipolar order by disorder in the classical Heisenberg antiferromagnet on the kagome lattice
Ever since the experiments which founded the field of highly frustrated
magnetism, the kagome Heisenberg antiferromagnet has been the archetypical
setting for the study of fluctuation induced exotic ordering. To this day the
nature of its classical low-temperature state has remained a mystery: the
non-linear nature of the fluctuations around the exponentially numerous
harmonically degenerate ground states has not permitted a controlled theory,
while its complex energy landscape has precluded numerical simulations at low
temperature. Here we present an efficient Monte Carlo algorithm which removes
the latter obstacle. Our simulations detect a low-temperature regime in which
correlations saturate at a remarkably small value. Feeding these results into
an effective model and analyzing the results in the framework of an appropriate
field theory implies the presence of long-range dipolar spin order with a
tripled unit cell.Comment: 5 pages, 4 figure
Comment on ``Quantum Phase of Induced Dipoles Moving in a Magnetic Field''
It has recently been suggested that an Aharonov-Bohm phase should be capable
of detection using beams of neutral polarizable particles. A more careful
analysis of the proposed experiment suffices to show, however, that it cannot
be performed regardless of the strength of the external electric and magnetic
fields.Comment: 2 pages, latex file, no figure
Unified Picture for Magnetic Correlations in Iron-Based Superconductors
The varying metallic antiferromagnetic correlations observed in iron-based
superconductors are unified in a model consisting of both itinerant electrons
and localized spins. The decisive factor is found to be the sensitive
competition between the superexchange antiferromagnetism and the
orbital-degenerate double-exchange ferromagnetism. Our results reveal the
crucial role of Hund's rule coupling for the strongly correlated nature of the
system and suggest that the iron-based superconductors are closer kin to
manganites than cuprates in terms of their diverse magnetism and incoherent
normal-state electron transport. This unified picture would be instrumental for
exploring other exotic properties and the mechanism of superconductivity in
this new class of superconductors.Comment: Revised for publication. 3 figure
On the theory of the CO+OH reaction, including H and C kinetic isotope effects
The effect of pressure, temperature, H/D isotopes, and C isotopes on the kinetics of the OH+CO reaction are investigated using Rice-Ramsperger-Kassel-Marcus theory. Pressure effects are treated with a step-ladder plus steady-state model and tunneling effects are included. New features include a treatment of the C isotope effect and a proposed nonstatistical effect in the reaction. The latter was prompted by existing kinetic results and molecular-beam data of Simons and co-workers [J. Phys. Chem. A 102, 9559 (1998); J. Chem. Phys. 112, 4557 (2000); 113, 3173 (2000)] on incomplete intramolecular energy transfer to the highest vibrational frequency mode in HOCO*. In treating the many kinetic properties two small customary vertical adjustments of the barriers of the two transition states were made. The resulting calculations show reasonable agreement with the experimental data on (1) the pressure and temperature dependence of the H/D effect, (2) the pressure-dependent 12C/13C isotope effect, (3) the strong non-Arrhenius behavior observed at low temperatures, (4) the high-temperature data, and (5) the pressure dependence of rate constants in various bath gases. The kinetic carbon isotopic effect is usually less than 10 per mil. A striking consequence of the nonstatistical assumption is the removal of a major discrepancy in a plot of the kOH+CO/kOD+CO ratio versus pressure. A prediction is made for the temperature dependence of the OD+CO reaction in the low-pressure limit at low temperatures
Nonlinear Dirac Equations
We construct nonlinear extensions of Dirac's relativistic electron equation
that preserve its other desirable properties such as locality, separability,
conservation of probability and Poincar\'e invariance. We determine the
constraints that the nonlinear term must obey and classify the resultant
non-polynomial nonlinearities in a double expansion in the degree of
nonlinearity and number of derivatives. We give explicit examples of such
nonlinear equations, studying their discrete symmetries and other properties.
Motivated by some previously suggested applications we then consider nonlinear
terms that simultaneously violate Lorentz covariance and again study various
explicit examples. We contrast our equations and construction procedure with
others in the literature and also show that our equations are not gauge
equivalent to the linear Dirac equation. Finally we outline various physical
applications for these equations
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