53,627 research outputs found
Spring-damper equivalents of the fractional, poroelastic, and poroviscoelastic models for elastography
In MR elastography it is common to use an elastic model for the tissue's
response in order to properly interpret the results. More complex models such
as viscoelastic, fractional viscoelastic, poroelastic, or poroviscoelastic ones
are also used. These models appear at first sight to be very different, but
here it is shown that they all may be expressed in terms of elementary
viscoelastic models.
For a medium expressed with fractional models, many elementary spring-damper
combinations are added, each of them weighted according to a long-tailed
distribution, hinting at a fractional distribution of time constants or
relaxation frequencies. This may open up for a more physical interpretation of
the fractional models.
The shear wave component of the poroelastic model is shown to be modeled
exactly by a three-component Zener model. The extended poroviscoelastic model
is found to be equivalent to what is called a non-standard four-parameter
model. Accordingly, the large number of parameters in the porous models can be
reduced to the same number as in their viscoelastic equivalents. As long as the
individual displacements from the solid and fluid parts cannot be measured
individually the main use of the poro(visco)elastic models is therefore as a
physics based method for determining parameters in a viscoelastic model.Comment: 11 pages, 7 figures. Changed inconsistent notation in Eqs 1, 5, 8, 10
and corrected mistakes in Eqs 2, 4, 12, 3
Analysis of double-parallel amplified recirculating optical-delay lines
A novel method of analysis of double-parallel amplified recirculating optical-delay lines (DPAROD) is presented. The location of the maxima and the minima of the transfer function for this configuration is calculated and experimentally demonstrated. The influence of different parameters, such as the coupling coefficients, gains, lengths of the fiber loops and fractional losses of the directional couplers, on the shape of the transfer function are analyzed. Different measurements have been taken to verify this model. The potential application of these interconnected delay loops as filters is a reason for developing this method.Publicad
Measurability of the tidal polarizability of neutron stars in late-inspiral gravitational-wave signals
The gravitational wave signal from a binary neutron star inspiral contains
information on the nuclear equation of state. This information is contained in
a combination of the tidal polarizability parameters of the two neutron stars
and is clearest in the late inspiral, just before merger. We use the recently
defined tidal extension of the effective one-body formalism to construct a
controlled analytical description of the frequency-domain phasing of neutron
star inspirals up to merger. Exploiting this analytical description we find
that the tidal polarizability parameters of neutron stars can be measured by
the advanced LIGO-Virgo detector network from gravitational wave signals having
a reasonable signal-to-noise ratio of . This measurability result
seems to hold for all the nuclear equations of state leading to a maximum mass
larger than . We also propose a promising new way of extracting
information on the nuclear equation of state from a coherent analysis of an
ensemble of gravitational wave observations of separate binary merger events.Comment: 28 pages, 6 figures. Submitted to Phys. Rev.
Fractional robust control with iso-damping property
This article deals with the problem of the reduction of structural vibrations with isodamping property. The proposed methodology is based on:
- a contour defined in the Nichols plane and significant of the damping ratio of the closed-loop response
- a robust control method that uses fractional order integration. The methodology is applied to an aircraft wing model made with a beam and a tank whose different levels of fillings are considered as uncertainties
Ages and fundamental properties of Kepler exoplanet host stars from asteroseismology
We present a study of 33 {\it Kepler} planet-candidate host stars for which
asteroseismic observations have sufficiently high signal-to-noise ratio to
allow extraction of individual pulsation frequencies. We implement a new
Bayesian scheme that is flexible in its input to process individual oscillation
frequencies, combinations of them, and average asteroseismic parameters, and
derive robust fundamental properties for these targets. Applying this scheme to
grids of evolutionary models yields stellar properties with median statistical
uncertainties of 1.2\% (radius), 1.7\% (density), 3.3\% (mass), 4.4\%
(distance), and 14\% (age), making this the exoplanet host-star sample with the
most precise and uniformly determined fundamental parameters to date. We assess
the systematics from changes in the solar abundances and mixing-length
parameter, showing that they are smaller than the statistical errors. We also
determine the stellar properties with three other fitting algorithms and
explore the systematics arising from using different evolution and pulsation
codes, resulting in 1\% in density and radius, and 2\% and 7\% in mass and age,
respectively. We confirm previous findings of the initial helium abundance
being a source of systematics comparable to our statistical uncertainties, and
discuss future prospects for constraining this parameter by combining
asteroseismology and data from space missions. Finally we compare our derived
properties with those obtained using the global average asteroseismic
observables along with effective temperature and metallicity, finding an
excellent level of agreement. Owing to selection effects, our results show that
the majority of the high signal-to-noise ratio asteroseismic {\it Kepler} host
stars are older than the Sun.Comment: 25 pages, 17 figures, MNRAS accepte
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