551 research outputs found
An integrated approach to the optimum design of actively controlled composite wings
The importance of interactions among the various disciplines in airplane wing design has been recognized for quite some time. With the introduction of high gain, high authority control systems and the design of thin, flexible, lightweight composite wings, the integrated treatment of control systems, flight mechanics and dynamic aeroelasticity became a necessity. A research program is underway now aimed at extending structural synthesis concepts and methods to the integrated synthesis of lifting surfaces, spanning the disciplines of structures, aerodynamics and control for both analysis and design. Mathematical modeling techniques are carefully selected to be accurate enough for preliminary design purposes of the complicated, built-up lifting surfaces of real aircraft with their multiple design criteria and tight constraints. The presentation opens with some observations on the multidisciplinary nature of wing design. A brief review of some available state of the art practical wing optimization programs and a brief review of current research effort in the field serve to illuminate the motivation and support the direction taken in our research. The goals of this research effort are presented, followed by a description of the analysis and behavior sensitivity techniques used. The presentation concludes with a status report and some forecast of upcoming progress
Delayed Detonation at a Single Point in Exploding White Dwarfs
Delayed detonation in an exploding white dwarf, which propagates from an
off-center transition point, rather than from a spherical transition shell, is
described and simulated. The differences between the results of 2D simulations
and the 1D case are presented and discussed. The two dimensional effects become
significant in transition density below 3.e7 g/cm^3, where the energetics, the
production of Fe group elements and the symmetry of the explosion are all
affected. In the 2D case the explosion is less energetic and less Ni is
produced in the detonation phase of the explosion. For low transition density
the reduction in Ni mass can reach 20-30 percent. The asymmetry in abundances
between regions close to the transition point and regions far from that point
is large, and could be a source to polarization patterns in the emitted light.
We conclude that the spatial and temporal distribution of transition locations,
is an important parameter which must be included in delayed detonation models
for Type Ia supernovae. \Comment: 11 pages, 1 figur
Simple parallel and distributed algorithms for spectral graph sparsification
We describe a simple algorithm for spectral graph sparsification, based on
iterative computations of weighted spanners and uniform sampling. Leveraging
the algorithms of Baswana and Sen for computing spanners, we obtain the first
distributed spectral sparsification algorithm. We also obtain a parallel
algorithm with improved work and time guarantees. Combining this algorithm with
the parallel framework of Peng and Spielman for solving symmetric diagonally
dominant linear systems, we get a parallel solver which is much closer to being
practical and significantly more efficient in terms of the total work.Comment: replaces "A simple parallel and distributed algorithm for spectral
sparsification". Minor change
Neutrino Signatures and the Neutrino-Driven Wind in Binary Neutron Star Mergers
We present VULCAN/2D multigroup flux-limited-diffusion radiation-hydrodynamics simulations of binary neutron star mergers, using the Shen equation of state, covering âł 100 ms, and starting from azimuthal-averaged two-dimensional slices obtained from three-dimensional smooth-particle-hydrodynamics simulations of Rosswog & Price for 1.4Mâ (baryonic) neutron stars with no initial spins, co-rotating spins, or counter-rotating spins. Snapshots are post-processed at 10 ms intervals with a multiangle neutrino-transport solver. We find polar-enhanced neutrino luminosities, dominated by ¯νe and âνΟâ neutrinos at the peak, although νe emission may be stronger at late times. We obtain typical peak neutrino energies for νe, ¯νe, and âνΟâ of âź12, âź16, and âź22 MeV, respectively. The supermassive neutron star (SMNS) formed from the merger has a cooling timescale of âž 1 s. Charge-current neutrino reactions lead to the formation of a thermally driven bipolar wind with (M¡) âź 10^â3 Mâ s^â1 and baryon-loading in the polar regions, preventing any production of a Îł-ray burst prior to black hole formation. The large budget of rotational free energy suggests that magneto-rotational effects could produce a much-greater polar mass loss. We estimate that âž 10^â4 Mâ of material with an electron fraction in the range 0.1â0.2 becomes unbound during this SMNS phase as a result of neutrino heating. We present a new formalism to compute the νi ¯νi annihilation rate based on moments of the neutrino-specific intensity computed with our multiangle solver. Cumulative annihilation rates, which decay as âźt^â1.8, decrease over our 100 ms window from a few Ă1050 to âź 1049 erg sâ1, equivalent to a few Ă10^54 to âź10^53 eâe+ pairs per second
The Breakdown of Linear Elastic Fracture Mechanics near the Tip of a Rapid Crack
We present high resolution measurements of the displacement and strain fields
near the tip of a dynamic (Mode I) crack. The experiments are performed on
polyacrylamide gels, brittle elastomers whose fracture dynamics mirror those of
typical brittle amorphous materials. Over a wide range of propagation
velocities (), we compare linear elastic fracture mechanics (LEFM)
to the measured near-tip fields. We find that, sufficiently near the tip, the
measured stress intensity factor appears to be non-unique, the crack tip
significantly deviates from its predicted parabolic form, and the strains ahead
of the tip are more singular than the divergence predicted by LEFM.
These results show how LEFM breaks down as the crack tip is approached.Comment: 4 pages, 4 figures, first of a two-paper series (experiments); no
change in content, minor textual revision
Abnormal Fees and Timely Loss Recognition - A Long-Term Perspective
This is the author accepted manuscript. The final version is available from the American Accounting Association via the DOI in this recordWe examine the relation between timely loss recognition and abnormal audit, non-audit, and total fees over a long period (2001â2007 and 2010â2015). We use positive abnormal audit fees as a measure of abnormal audit effort, and positive abnormal non-audit fees as a measure of economic bond between the auditor and the auditee. Using the Ball and Shivakumar (2006) model, we report some evidence suggesting audit effort is associated with slower loss recognition in accruals before the SarbanesâOxley Act (SOX) became effective. However, we find stronger evidence that audit effort is associated with slower loss recognition post-SOX when clients raise substantial external funds or when the auditor is not an industry specialist. Using C_Score, we find a negative association between changes in abnormal audit fees and total fees, and changes in C_Score post-SOX, but not pre-SOX. We find no sample-wide evidence that abnormal non-audit fees are associated with the speed of loss recognition. Collectively, the results suggest post-SOX auditors exert more effort when losses are delayed and that non-audit services do not compromise auditor independence
A New Mechanism for Gravitational-Wave Emission in Core-Collapse Supernovae
We present a new theory for the gravitational wave signatures of core-collapse supernovae. Previous studies identified axisymmetric rotating core collapse, core bounce, postbounce convection, and anisotropic neutrino emission as the primary processes and phases for the radiation of gravitational waves. Our results, which are based on axisymmetric, Newtonian radiation-hydrodynamics supernova simulations (Burrows et al. 2006), indicate that the dominant emission process of gravitational waves in core-collapse supernovae may be the oscillations of the protoneutron star core. The oscillations are predominantly of g-mode character, are excited hundreds of milliseconds after bounce, and typically last for several hundred milliseconds. Our results suggest that even nonrotating core-collapse supernovae should be visible to current LIGO-class detectors throughout the Galaxy, and depending on progenitor structure, possibly out to Megaparsec distances
Multi-Dimensional Explorations in Supernova Theory
In this paper, we bring together various of our published and unpublished findings from our recent 2D multi-group, flux-limited radiation hydrodynamic simulations of the collapse and explosion of the cores of massive stars. Aided by 2D and 3D graphical renditions, we motivate the acoustic mechanism of core-collapse supernova explosions and explain, as best we currently can, the phases and phenomena that attend this mechanism. Two major foci of our presentation are the outer shock instability and the inner core g-mode oscillations. The former sets the stage for the latter, which damp by the generation of sound. This sound propagates outward to energize the explosion and is relevant only if the core has not exploded earlier by some other means. Hence, it is a more delayed mechanism than the traditional neutrino mechanism that has been studied for the last twenty years since it was championed by Bethe and Wilson. We discuss protoneutron star convection, accretion-induced-collapse, gravitational wave emissions, pulsar kicks, the angular anisotropy of the neutrino emissions, a subset of numerical issues, and a new code we are designing that should supercede our current supernova code VULCAN/2D. Whatever ideas last from this current generation of numerical results, and whatever the eventual mechanism(s), we conclude that the breaking of spherical symmetry will survive as one of the crucial keys to the supernova puzzle
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