9,609 research outputs found
Development of configurational forces during the injection of an elastic rod
When an inextensible elastic rod is 'injected' through a sliding sleeve
against a fixed constraint, configurational forces are developed, deeply
influencing the mechanical response. This effect, which is a consequence of the
change in length of the portion of the rod included between the sliding sleeve
and the fixed constraint, is theoretically demonstrated (via integration of the
elastica) and experimentally validated on a proof-of-concept structure
(displaying an interesting force reversal in the load/deflection diagram), to
provide conclusive evidence to mechanical phenomena relevant in several
technologies, including guide wire for artery catheterization, or wellbore
insertion of a steel pipe.Comment: 10 pages, 4 figures, Extreme Mechanics Letters (2015
Operational considerations for laminar flow aircraft
Considerable progress has been made in the development of laminar flow technology for commercial transports during the NASA Aircraft Energy Efficiency (ACEE) laminar flow program. Practical, operational laminar flow control (LFC) systems have been designed, fabricated, and are undergoing flight testing. New materials, fabrication methods, analysis techniques, and design concepts were developed and show much promise. The laminar flow control systems now being flight tested on the NASA Jetstar aircraft are complemented by natural laminar flow flight tests to be accomplished with the F-14 variable-sweep transition flight experiment. An overview of some operational aspects of this exciting program is given
A model for the quasi-static growth of brittle fractures based on local minimization
We study a variant of the variational model for the quasi-static growth of
brittle fractures proposed by Francfort and Marigo. The main feature of our
model is that, in the discrete-time formulation, in each step we do not
consider absolute minimizers of the energy, but, in a sense, we look for local
minimizers which are sufficiently close to the approximate solution obtained in
the previous step. This is done by introducing in the variational problem an
additional term which penalizes the -distance between the approximate
solutions at two consecutive times. We study the continuous-time version of
this model, obtained by passing to the limit as the time step tends to zero,
and show that it satisfies (for almost every time) some minimality conditions
which are slightly different from those considered in Francfort and Marigo and
in our previous paper, but are still enough to prove (under suitable regularity
assumptions on the crack path) that the classical Griffith's criterion holds at
the crack tips. We prove also that, if no initial crack is present and if the
data of the problem are sufficiently smooth, no crack will develop in this
model, provided the penalization term is large enough.Comment: 20 page
Performance of laminar-flow leading-edge test articles in cloud encounters
An extensive data bank of concurrent measurements of laminar flow (LF), particle concentration, and aircraft charging state was gathered for the first time. From this data bank, 13 flights in the simulated airline service (SAS) portion were analyzed to date. A total of 6.86 hours of data at one-second resolution were analyzed. An extensive statistical analysis, for both leading-edge test articles, shows that there is a significant effect of cloud and haze particles on the extent of laminar flow obtained. Approximately 93 percent of data points simulating LFC flight were obtained in clear air conditions; approximately 7 percent were obtained in cloud and haze. These percentages are consistent with earlier USAF and NASA estimates and results. The Hall laminar flow loss criteria was verified qualitatively. Larger particles and higher particle concentrations have a more marked effect on LF than do small particles. A particle spectrometer of a charging patch are both acceptable as diagnostic indicators of the presence of particles detrimental to laminar flow
Electromagnetic Chirps from Neutron Star-Black Hole Mergers
We calculate the electromagnetic signal of a gamma-ray flare coming from the
surface of a neutron star shortly before merger with a black hole companion.
Using a new version of the Monte Carlo radiation transport code Pandurata that
incorporates dynamic spacetimes, we integrate photon geodesics from the neutron
star surface until they reach a distant observer or are captured by the black
hole. The gamma-ray light curve is modulated by a number of relativistic
effects, including Doppler beaming and gravitational lensing. Because the
photons originate from the inspiraling neutron star, the light curve closely
resembles the corresponding gravitational waveform: a chirp signal
characterized by a steadily increasing frequency and amplitude. We propose to
search for these electromagnetic chirps using matched filtering algorithms
similar to those used in LIGO data analysis.Comment: 13 pages, 5 figures, submitted to Ap
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