3,826 research outputs found
Study of impact on helicopter blade
This article presents a study of damage in structures that are similar to helicopter blade sections, subjected to an impact. These complex composite structures were impacted by a steel ball of 125 g at impact speed ranging from 30 to 130 m/s. This led to properly highlight the kinematics of the impact and to define the sequence of the damage’s mechanisms. An explicit FE model is also presented. The damage modelling of the roving is performed through a scale change. It allows a good representation of observed experimental behaviour. As the mesh density is low, it can be used for the modelling of a real structure
The flow of plasma in the solar terrestrial environment
The overall goal of our NASA Theory Program was to study the coupling, time delays, and feedback mechanisms between the various regions of the solar-terrestrial system in a self-consistent, quantitative manner. To accomplish this goal, it will eventually be necessary to have time-dependent macroscopic models of the different regions of the solar-terrestrial system and we are continually working toward this goal. However, with the funding from this NASA program, we concentrated on the near-earth plasma environment, including the ionosphere, the plasmasphere, and the polar wind. In this area, we developed unique global models that allowed us to study the coupling between the different regions. These results are highlighted in the next section. Another important aspect of our NASA Theory Program concerned the effect that localized 'structure' had on the macroscopic flow in the ionosphere, plasmasphere, thermosphere, and polar wind. The localized structure can be created by structured magnetospheric inputs (i.e., structured plasma convection, particle precipitation or Birkland current patterns) or time variations in these input due to storms and substorms. Also, some of the plasma flows that we predicted with our macroscopic models could be unstable, and another one of our goals was to examine the stability of our predicted flows. Because time-dependent, three-dimensional numerical models of the solar-terrestrial environment generally require extensive computer resources, they are usually based on relatively simple mathematical formulations (i.e., simple MHD or hydrodynamic formulations). Therefore, another goal of our NASA Theory Program was to study the conditions under which various mathematical formulations can be applied to specific solar-terrestrial regions. This could involve a detailed comparison of kinetic, semi-kinetic, and hydrodynamic predictions for a given polar wind scenario or it could involve the comparison of a small-scale particle-in-cell (PIC) simulation of a plasma expansion event with a similar macroscopic expansion event. The different mathematical formulations have different strengths and weaknesses and a careful comparison of model predictions for similar geophysical situations provides insight into when the various models can be used with confidence
Probing Turbulence with Infrared Observations in OMC1
A statistical analysis is presented of the turbulent velocity structure in
the Orion Molecular Cloud at scales ranging from 70 AU to 30000 AU. Results are
based on IR Fabry-Perot interferometric observations of shock and
photon-excited H2 in the K-band S(1) v=1-0 line at 2.121micron and refer to the
dynamical characteristics of warm perturbed gas. Observations establish that
the Larson size-linewidth relation is obeyed to the smallest scales studied
here extending the range of validity of this relationship by nearly 2 orders of
magnitude. The velocity probability distribution function (PDF) is constructed
showing extended exponential wings, providing evidence of intermittency,
further supported by the skewness and kurtosis of the velocity distribution.
Variance and kurtosis of the PDF of velocity differences are constructed as a
function of lag. The variance shows an approximate power law dependence on lag,
with exponent significantly lower than the Kolmogorov value, and with
deviations below 2000AU which are attributed to outflows and possibly disk
structures associated with low mass star formation within OMC1. The kurtosis
shows strong deviation from a gaussian velocity field, providing evidence of
velocity correlations at small lags. Results agree accurately with
semi-empirical simulations in Eggers & Wang (1998).
In addition, 170 individual H2 emitting clumps have been analysed with sizes
between 500 and 2200 AU. These show considerable diversity with regard to PDFs
and variance functions. Our analysis constitutes the first characterization of
the turbulent velocity field at the scale of star formation and provide a
dataset which models of star-forming regions should aim to reproduce.Comment: 17 pages, 11 figures, to appear in A&A, typos correcte
Generalized Forward-Backward Splitting
This paper introduces the generalized forward-backward splitting algorithm
for minimizing convex functions of the form , where
has a Lipschitz-continuous gradient and the 's are simple in the sense
that their Moreau proximity operators are easy to compute. While the
forward-backward algorithm cannot deal with more than non-smooth
function, our method generalizes it to the case of arbitrary . Our method
makes an explicit use of the regularity of in the forward step, and the
proximity operators of the 's are applied in parallel in the backward
step. This allows the generalized forward backward to efficiently address an
important class of convex problems. We prove its convergence in infinite
dimension, and its robustness to errors on the computation of the proximity
operators and of the gradient of . Examples on inverse problems in imaging
demonstrate the advantage of the proposed methods in comparison to other
splitting algorithms.Comment: 24 pages, 4 figure
The Excitation of Extended Red Emission: New Constraints on its Carrier From HST Observations of NGC 7023
The carrier of the dust-associated photoluminescence process causing the
extended red emission (ERE) in many dusty interstellar environments remains
unidentified. Several competing models are more or less able to match the
observed broad, unstructured ERE band. We now constrain the character of the
ERE carrier further by determining the wavelengths of the radiation that
initiates the ERE. Using the imaging capabilities of the Hubble Space
Telescope, we have resolved the width of narrow ERE filaments appearing on the
surfaces of externally illuminated molecular clouds in the bright reflection
nebula NGC 7023 and compared them with the depth of penetration of radiation of
known wavelengths into the same cloud surfaces. We identify photons with
wavelengths shortward of 118 nm as the source of ERE initiation, not to be
confused with ERE excitation, however. There are strong indications from the
well-studied ERE in the Red Rectangle nebula and in the high-|b| Galactic
cirrus that the photon flux with wavelengths shortward of 118 nm is too small
to actually excite the observed ERE, even with 100% quantum efficiency. We
conclude, therefore, that ERE excitation results from a two-step process. While
none of the previously proposed ERE models can match these new constraints, we
note that under interstellar conditions most polycyclic aromatic hydrocarbon
(PAH) molecules are ionized to the di-cation stage by photons with E > 10.5 eV
and that the electronic energy level structure of PAH di-cations is consistent
with fluorescence in the wavelength band of the ERE. Therefore, PAH di-cations
deserve further study as potential carriers of the ERE. (abridged)Comment: Accepted for Publication in the Ap
Apparent yield stress in rigid fibre suspensions: the role of attractive colloidal interactions
International audienceThis work is focused on the modelling of the shear and normal stresses in fibre suspensions that are subjected to a simple shear flow in the presence of short-range lubrication forces, van der Waals and electrostatic forces, as well as solid friction forces between fibres. All these forces are weighed by the contact probability. The theory is developed for attractive fibres with van der Waals interaction dominating over electrostatic repulsion. The model predicts a simple Bingham law for both the shear stress and the first normal stress difference with the apparent shear and normal yield stresses proportional, respectively, to the second and the third power of particle volume fraction. The model is applied to the experimental data of Rakatekar et al. Adv. Mater 21, 874-878 (2009) and Natale et al. AIChE J. 60, 1476-1487 (2014) on the suspensions of carbon nanotubes dispersed in a Newtonian epoxy resin. It reproduces well the quadratic dependency of the apparent yield stress on particle volume fraction (σ Y ∝φ^2) for average particle aspect ratios of r=160 and 1200, while it underpredicts the power-law exponent for rD80 (always predictingφ^2 behaviour instead of φ^3.2
Temperature dependent characterization of optical fibres for distributed temperature sensing in hot geothermal wells
This study was performed in order to select a proper fibre for the
application of a distributed temperature sensing system within a hot geothermal
well in Iceland. Commercially available high temperature graded index fibres
have been tested under in-situ temperature conditions. Experiments have been
performed with four different polyimide coated fibres, a fibre with an aluminum
coating and a fibre with a gold coating. To select a fibre, the relationship
between attenuation, temperature, and time has been analyzed together with SEM
micrographs. On the basis of these experiments, polyimide fibres have been
chosen for utilisation. Further tests in ambient and inert atmosphere have been
conducted with two polyimide coated fibres to set an operating temperature
limit for these fibres. SEM micrographs, together with coating colour changes
have been used to characterize the high temperature performance of the fibres.
A novel cable design has been developed, a deployment strategy has been worked
out and a suitable well for deployment has been selected.Comment: PACS: 42.81.Pa, 93.85.Fg, 47.80.Fg, 91.35.Dc, 07.20.Dt, 07.60.V
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