8,981 research outputs found
On-demand Aerodynamics in Integrally Actuated Membranes with Feedback Control
This paper is a numerical investigation on model reduction and control system design of integrally actuated membrane wings. A high-fidelity electro-aeromechanical model is used for the simulation of the dynamic fluid-structure interaction between a low-Reynolds-number flow and a dielectric elastomeric wing. Two reduced-order models with different levels of complexity are then derived. They are based on the projection of the fullorder discretisation of fluid and structure on modal shapes obtained from eigenvalue analysis and Proper Orthogonal Decomposition. The low-order systems are then used for the design of Proportional-Integral-Derivative and Linear Quadratic Gaussian feedback schemes to control wing lift. When implemented in the full-order model, closed-loop dynamics are in very good agreement with the reduced-order model for both tracking and gust rejection, demonstrating the suitability of the approach. The control laws selected in this work were found to be effective only for low-frequency disturbances due to the large phase delay introduced by the fluid convective time-scales, but results demonstrate the potential for the aerodynamic control of membrane wings in outdoor flight using dielectric elastomers
Impact of internal gravity waves on the rotation profile inside pre-main sequence low-mass stars
We study the impact of internal gravity waves (IGW), meridional circulation,
shear turbulence, and stellar contraction on the internal rotation profile and
surface velocity evolution of solar metallicity low-mass pre-main sequence
stars. We compute a grid of rotating stellar evolution models with masses
between 0.6 and 2.0Msun taking these processes into account for the transport
of angular momentum, as soon as the radiative core appears and assuming no more
disk-locking from that moment on.IGW generation along the PMS is computed
taking Reynolds-stress and buoyancy into account in the bulk of the stellar
convective envelope and convective core (when present). Redistribution of
angular momentum within the radiative layers accounts for damping of prograde
and retrograde IGW by thermal diffusivity and viscosity in corotation
resonance. Over the whole mass range considered, IGW are found to be
efficiently generated by the convective envelope and to slow down the stellar
core early on the PMS. In stars more massive than ~ 1.6Msun, IGW produced by
the convective core also contribute to angular momentum redistribution close to
the ZAMS. Overall, IGW are found to significantly change the internal rotation
profile of PMS low-mass stars.Comment: Accepted for publication in A&A (15 pages
Orbital eigenchannel analysis for ab-initio quantum transport calculations
We show how to extract the orbital contribution to the transport
eigenchannels from a first-principles quantum transport calculation in a
nanoscopic conductor. This is achieved by calculating and diagonalizing the
first-principles transmission matrix reduced to selected scattering
cross-sections. As an example, the orbital nature of the eigenchannels in the
case of Ni nanocontacts is explored, stressing the difficulties inherent to the
use of non-orthogonal basis sets and first-principles Hamiltonians.Comment: 5 pages, 5 figurs; replaced with final version, introduction revised;
to be published in PR
Evolution of small-scale magnetic elements in the vicinity of granular-size swirl convective motions
Advances in solar instrumentation have led to a widespread usage of time
series to study the dynamics of solar features, specially at small spatial
scales and at very fast cadences. Physical processes at such scales are
determinant as building blocks for many others occurring from the lower to the
upper layers of the solar atmosphere and beyond, ultimately for understanding
the bigger picture of solar activity. Ground-based (SST) and space-borne
(Hinode) high-resolution solar data are analyzed in a quiet Sun region
displaying negative polarity small-scale magnetic concentrations and a cluster
of bright points observed in G-band and Ca II H images. The studied region is
characterized by the presence of two small-scale convective vortex-type plasma
motions, one of which appears to be affecting the dynamics of both, magnetic
features and bright points in its vicinity and therefore the main target of our
investigations. We followed the evolution of bright points, intensity
variations at different atmospheric heights and magnetic evolution for a set of
interesting selected regions. A description of the evolution of the
photospheric plasma motions in the region nearby the convective vortex is
shown, as well as some plausible cases for convective collapse detected in
Stokes profiles.Comment: 9 figure
Metastability and paramagnetism in superconducting mesoscopic disks
A projected order parameter is used to calculate, not only local minima of
the Ginzburg-Landau energy functional, but also saddle points or energy
barriers responsible for the metastabilities observed in superconducting
mesoscopic disks (Geim et al. Nature {\bf 396}, 144 (1998)). We calculate the
local minima magnetization and find the energetic instability points between
vortex configurations with different vorticity. We also find that, for any
vorticity, the supercurrent can reverse its flow direction on decreasing the
magnetic field before one vortex can escape.Comment: Modified version as to appear in Phys. Rev. Let
Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars at various metallicities
The availability of asteroseismic constraints for a large sample of stars
from the missions CoRoT and Kepler paves the way for various statistical
studies of the seismic properties of stellar populations. In this paper, we
evaluate the impact of rotation-induced mixing and thermohaline instability on
the global asteroseismic parameters at different stages of the stellar
evolution from the Zero Age Main Sequence to the Thermally Pulsating Asymptotic
Giant Branch to distinguish stellar populations. We present a grid of stellar
evolutionary models for four metallicities (Z = 0.0001, 0.002, 0.004, and
0.014) in the mass range between 0.85 to 6.0 Msun. The models are computed
either with standard prescriptions or including both thermohaline convection
and rotation-induced mixing. For the whole grid we provide the usual stellar
parameters (luminosity, effective temperature, lifetimes, ...), together with
the global seismic parameters, i.e. the large frequency separation and
asymptotic relations, the frequency corresponding to the maximum oscillation
power {\nu}_{max}, the maximal amplitude A_{max}, the asymptotic period spacing
of g-modes, and different acoustic radii. We discuss the signature of
rotation-induced mixing on the global asteroseismic quantities, that can be
detected observationally. Thermohaline mixing whose effects can be identified
by spectroscopic studies cannot be caracterized with the global seismic
parameters studied here. But it is not excluded that individual mode
frequencies or other well chosen asteroseismic quantities might help
constraining this mixing.Comment: 15 pages, 11 figures, accepted for publication in A&
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