43,921 research outputs found
Multigrid waveform relaxation for the time-fractional heat equation
In this work, we propose an efficient and robust multigrid method for solving
the time-fractional heat equation. Due to the nonlocal property of fractional
differential operators, numerical methods usually generate systems of equations
for which the coefficient matrix is dense. Therefore, the design of efficient
solvers for the numerical simulation of these problems is a difficult task. We
develop a parallel-in-time multigrid algorithm based on the waveform relaxation
approach, whose application to time-fractional problems seems very natural due
to the fact that the fractional derivative at each spatial point depends on the
values of the function at this point at all earlier times. Exploiting the
Toeplitz-like structure of the coefficient matrix, the proposed multigrid
waveform relaxation method has a computational cost of
operations, where is the number of time steps and is the number of
spatial grid points. A semi-algebraic mode analysis is also developed to
theoretically confirm the good results obtained. Several numerical experiments,
including examples with non-smooth solutions and a nonlinear problem with
applications in porous media, are presented
Quasinormal ringing of acoustic black holes in Laval nozzles: Numerical simulations
Quasinormal ringing of acoustic black holes in Laval nozzles is discussed.
The equation for sounds in a transonic flow is written into a
Schr\"{o}dinger-type equation with a potential barrier, and the quasinormal
frequencies are calculated semianalytically. From the results of numerical
simulations, it is shown that the quasinormal modes are actually excited when
the transonic flow is formed or slightly perturbed, as well as in the real
black hole case. In an actual experiment, however, the purely-outgoing boundary
condition will not be satisfied at late times due to the wave reflection at the
end of the apparatus, and a late-time ringing will be expressed as a
superposition of "boxed" quasinormal modes. It is shown that the late-time
ringing damps more slowly than the ordinary quasinormal ringing, while its
central frequency is not greatly different from that of the ordinary one. Using
this fact, an efficient way for experimentally detecting the quasinormal
ringing of an acoustic black hole is discussed.Comment: 9 pages, 8 figures, accepted for publication in Physical Review
Compressive Parameter Estimation for Sparse Translation-Invariant Signals Using Polar Interpolation
We propose new compressive parameter estimation algorithms that make use of
polar interpolation to improve the estimator precision. Our work extends
previous approaches involving polar interpolation for compressive parameter
estimation in two aspects: (i) we extend the formulation from real non-negative
amplitude parameters to arbitrary complex ones, and (ii) we allow for mismatch
between the manifold described by the parameters and its polar approximation.
To quantify the improvements afforded by the proposed extensions, we evaluate
six algorithms for estimation of parameters in sparse translation-invariant
signals, exemplified with the time delay estimation problem. The evaluation is
based on three performance metrics: estimator precision, sampling rate and
computational complexity. We use compressive sensing with all the algorithms to
lower the necessary sampling rate and show that it is still possible to attain
good estimation precision and keep the computational complexity low. Our
numerical experiments show that the proposed algorithms outperform existing
approaches that either leverage polynomial interpolation or are based on a
conversion to a frequency-estimation problem followed by a super-resolution
algorithm. The algorithms studied here provide various tradeoffs between
computational complexity, estimation precision, and necessary sampling rate.
The work shows that compressive sensing for the class of sparse
translation-invariant signals allows for a decrease in sampling rate and that
the use of polar interpolation increases the estimation precision.Comment: 13 pages, 5 figures, to appear in IEEE Transactions on Signal
Processing; minor edits and correction
Detectability of gravitational wave events by spherical resonant-mass antennas
We have calculated signal-to-noise ratios for eight spherical resonant-mass
antennas interacting with gravitational radiation from inspiralling and
coalescing binary neutron stars and from the dynamical and secular bar-mode
instability of a rapidly rotating star. We find that by using technology that
could be available in the next several years, spherical antennas can detect
neutron star inspiral and coalescence at a distance of 15 Mpc and the dynamical
bar-mode instability at a distance of 2 Mpc.Comment: 39 pages, 4 EPS Figures, some additional SNRs for secular
instabilities, some changes to LIGO SNRs, Appendix added on the asymptotic
expansion of energy sensitivity, corrected supernova rates. Results available
at http://www.physics.umd.edu/rgroups/gen_rel_exp/snr.html Submitted to Phys.
Rev.
Gravitational wave bursts from cusps and kinks on cosmic strings
The strong beams of high-frequency gravitational waves (GW) emitted by cusps
and kinks of cosmic strings are studied in detail. As a consequence of these
beams, the stochastic ensemble of GW's generated by a cosmological network of
oscillating loops is strongly non Gaussian, and includes occasional sharp
bursts that stand above the ``confusion'' GW noise made of many smaller
overlapping bursts. Even if only 10% of all string loops have cusps these
bursts might be detectable by the planned GW detectors LIGO/VIRGO and LISA for
string tensions as small as . In the implausible case
where the average cusp number per loop oscillation is extremely small, the
smaller bursts emitted by the ubiquitous kinks will be detectable by LISA for
string tensions as small as . We show that the strongly
non Gaussian nature of the stochastic GW's generated by strings modifies the
usual derivation of constraints on from pulsar timing experiments. In
particular the usually considered ``rms GW background'' is, when G \mu \gaq
10^{-7}, an overestimate of the more relevant confusion GW noise because it
includes rare, intense bursts. The consideration of the confusion GW noise
suggests that a Grand Unified Theory (GUT) value is
compatible with existing pulsar data, and that a modest improvement in pulsar
timing accuracy could detect the confusion noise coming from a network of cuspy
string loops down to . The GW bursts discussed here might
be accompanied by Gamma Ray Bursts.Comment: 24 pages, 3 figures, Revtex, submitted to Phys. Rev.
Optimisation of Quantum Trajectories Driven by Strong-field Waveforms
Quasi-free field-driven electron trajectories are a key element of
strong-field dynamics. Upon recollision with the parent ion, the energy
transferred from the field to the electron may be released as attosecond
duration XUV emission in the process of high harmonic generation (HHG). The
conventional sinusoidal driver fields set limitations on the maximum value of
this energy transfer, and it has been predicted that this limit can be
significantly exceeded by an appropriately ramped-up cycleshape. Here, we
present an experimental realization of such cycle-shaped waveforms and
demonstrate control of the HHG process on the single-atom quantum level via
attosecond steering of the electron trajectories. With our optimized optical
cycles, we boost the field-ionization launching the electron trajectories,
increase the subsequent field-to-electron energy transfer, and reduce the
trajectory duration. We demonstrate, in realistic experimental conditions, two
orders of magnitude enhancement of the generated XUV flux together with an
increased spectral cutoff. This application, which is only one example of what
can be achieved with cycle-shaped high-field light-waves, has farreaching
implications for attosecond spectroscopy and molecular self-probing
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