175 research outputs found
New formulation for flexible beams undergoing large overall plane motion
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/76279/1/AIAA-21161-979.pd
A NEW FLEXIBLE BODY DYNAMIC FORMULATION FOR BEAM STRUCTURES UNDERGOING LARGE OVERALL MOTION ITHE THREE-DIMENSIONAL CASE
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77304/1/AIAA-1993-1435-369.pd
A new flexible body dynamic formulation for beam structures undergoing large overall motion
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/77331/1/AIAA-1992-2261-829.pd
Electronic states and Landau levels in graphene stacks
We analyze, within a minimal model that allows analytical calculations, the
electronic structure and Landau levels of graphene multi-layers with different
stacking orders. We find, among other results, that electrostatic effects can
induce a strongly divergent density of states in bi- and tri-layers,
reminiscent of one-dimensional systems. The density of states at the surface of
semi-infinite stacks, on the other hand, may vanish at low energies, or show a
band of surface states, depending on the stacking order
Flight Demonstration Of Low Overpressure N-Wave Sonic Booms And Evanescent Waves
The recent flight demonstration of shaped sonic booms shows the potential for quiet overland supersonic flight, which could revolutionize air transport. To successfully design quiet supersonic aircraft, the upper limit of an acceptable noise level must be determined through quantitative recording and subjective human response measurements. Past efforts have concentrated on the use of sonic boom simulators to assess human response, but simulators often cannot reproduce a realistic sonic boom sound. Until now, molecular relaxation effects on low overpressure rise time had never been compared with flight data. Supersonic flight slower than the cutoff Mach number, which generates evanescent waves, also prevents loud sonic booms from impacting the ground. The loudness of these evanescent waves can be computed, but flight measurement validation is needed. A novel flight demonstration technique that generates low overpressure N-waves using conventional military aircraft is outlined, in addition to initial quantitative flight data. As part of this demonstration, evanescent waves also will be recorded
Initial Results from the Variable Intensity Sonic Boom Propagation Database
An extensive sonic boom propagation database with low- to normal-intensity booms (overpressures of 0.08 lbf/sq ft to 2.20 lbf/sq ft) was collected for propagation code validation, and initial results and flight research techniques are presented. Several arrays of microphones were used, including a 10 m tall tower to measure shock wave directionality and the effect of height above ground on acoustic level. A sailplane was employed to measure sonic booms above and within the atmospheric turbulent boundary layer, and the sailplane was positioned to intercept the shock waves between the supersonic airplane and the ground sensors. Sailplane and ground-level sonic boom recordings were used to generate atmospheric turbulence filter functions showing excellent agreement with ground measurements. The sonic boom prediction software PCBoom4 was employed as a preflight planning tool using preflight weather data. The measured data of shock wave directionality, arrival time, and overpressure gave excellent agreement with the PCBoom4-calculated results using the measured aircraft and atmospheric data as inputs. C-weighted acoustic levels generally decreased with increasing height above the ground. A-weighted and perceived levels usually were at a minimum for a height where the elevated microphone pressure rise time history was the straightest, which is a result of incident and ground-reflected shock waves interacting
A Flight Research Overview of WSPR, a Pilot Project for Sonic Boom Community Response
In support of NASAs ongoing effort to bring supersonic commercial travel to the public, NASA Dryden Flight Research Center and NASA Langley Research Center, in cooperation with other industry organizations, conducted a flight research experiment to identify the methods, tools, and best practices for a large-scale quiet (or low) sonic boom community human response test. The name of the effort was Waveforms and Sonic boom Perception and Response. Such tests will go towards building a dataset that governing agencies like the Federal Aviation Administration and International Civil Aviation Organization will use to establish regulations for acceptable sound levels of overland sonic booms. Until WSPR, there had never been an effort that studied the response of people in their own homes and performing daily activities to non-traditional, low sonic booms.WSPR was a NASA collaborative effort with several industry partners, in response to a NASA Aeronautics Research Mission Directorate Research Opportunities in Aeronautics. The primary contractor was Wyle. Other partners included Gulfstream Aerospace Corporation, Pennsylvania State University, Tetra Tech, and Fidell Associates, Inc.A major objective of the effort included exposing a community with the sonic boom magnitudes and occurrences expected in high-air traffic regions with a network of supersonic commercial aircraft in place. Low-level sonic booms designed to simulate those produced by the next generation of commercial supersonic aircraft were generated over a small residential community. The sonic boom footprint was recorded with an autonomous wireless microphone array that spanned the entire community. Human response data was collected using multiple survey methods. The research focused on essential elements of community response testing including subject recruitment, survey methods, instrumentation systems, flight planning and operations, and data analysis methods.This paper focuses on NASAs role in the efforts logistics and operations including human response subject recruitment, the operational processes involved in implementing the surveys throughout the community, instrumentation systems, logistics, flight planning, and flight operations. Findings discussed in this paper include critical lessons learned in all of those areas. The paper also discusses flight operations results. Analysis of the accuracy and repeatability of planning and executing the unique aircraft maneuver used to generate low sonic booms concluded that the sonic booms had overpressures within 0.15 pounds-per-square-feet of the planned values for 76 of t he attempts. Similarly, 90 of the attempts to generate low sonic booms within the community were successful
Probing Mechanical Properties of Graphene with Raman Spectroscopy
The use of Raman scattering techniques to study the mechanical properties of
graphene films is reviewed here. The determination of Gruneisen parameters of
suspended graphene sheets under uni- and bi-axial strain is discussed and the
values are compared to theoretical predictions. The effects of the
graphene-substrate interaction on strain and to the temperature evolution of
the graphene Raman spectra are discussed. Finally, the relation between
mechanical and thermal properties is presented along with the characterization
of thermal properties of graphene with Raman spectroscopy.Comment: To appear in the Journal of Materials Scienc
The spectral energy distribution of the central parsecs of the nearest AGN
Spectral energy distributions (SEDs) of the central few tens of parsec region
of some of the nearest, most well studied, active galactic nuclei (AGN) are
presented. These genuine AGN-core SEDs, mostly from Seyfert galaxies, are
characterised by two main features: an IR bump with the maximum in the 2-10
micron range, and an increasing X-ray spectrum in the 1 to ~200 keV region.
These dominant features are common to Seyfert type 1 and 2 objects alike. Type
2 AGN exhibit a sharp drop shortward of 2 micron, with the optical to UV region
being fully absorbed, while type 1s show instead a gentle 2 micron drop ensued
by a secondary, partially-absorbed optical to UV emission bump. Assuming the
bulk of optical to UV photons generated in these AGN are reprocessed by dust
and re-emitted in the IR in an isotropic manner, the IR bump luminosity
represents >70% of the total energy output in these objects while the high
energies above 20 keV are the second energetically important contribution.
Galaxies selected by their warm IR colours, i.e. presenting a relatively-flat
flux distribution in the 12 to 60 micron range have often being classified as
AGN. The results from these high spatial resolution SEDs question this
criterion as a general rule. It is found that the intrinsic shape of the IR SED
of an AGN and inferred bolometric luminosity largely depart from those derived
from large aperture data. AGN luminosities can be overestimated by up to two
orders of magnitude if relying on IR satellite data. We find these differences
to be critical for AGN luminosities below or about 10^{44} erg/s. Above this
limit, AGNs tend to dominate the light of their host galaxy regardless of the
aperture size used. We tentatively mark this luminosity as a threshold to
identify galaxy-light- vs AGN- dominated objects.Comment: 50 pages, 14 figures. Accepted for publication in MNRA
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