5,910 research outputs found
Downwind rotor horizontal axis wind turbine noise prediction
NASA and industry are currently cooperating in the conduct of extensive experimental and analytical studies to understand and predict the noise of large, horizontal axis wind turbines. This effort consists of (1) obtaining high quality noise data under well controlled and documented test conditions, (2) establishing the annoyance criteria for impulse noise of the type generated by horizontal axis wind turbines with rotors downwind of the support tower, (3) defining the wake characteristics downwind of the axial location of the plane of rotation, (4) comparing predictions with measurements made by use of wake data, and (5) comparing predictions with annoyance criteria. The status of work by Hamilton Standard in the above areas which was done in support of the cooperative NASA and industry studies is briefly summarized
Comment on "Mechanical analog of temperature for the description of force distribution in static granular packings"
It has been proposed by Ngan [Phys. Rev. E 68, 011301 (2003)] that the
granular contact force distribution may be analytically derived by minimizing
the analog of a thermodynamic free energy, in this case consisting of the total
potential energy stored in the compressed contacts minus a particular form of
entropy weighted by a parameter. The parameter is identified as a mechanical
temperature. I argue that the particular form of entropy cannot be correct and
as a result the proposed method produces increasingly errant results for
increasing grain rigidity. This trend is evidenced in Ngan's published results
and in other numerical simulations and experiments.Comment: 4 pages, 1 figure, minor editorial correction
Foliations of asymptotically flat manifolds by surfaces of Willmore type
The goal of this paper is to establish the existence of a foliation of the asymptotic region of an asymptotically flat manifold with nonzero mass by surfaces which are critical points of the Willmore functional subject to an area constraint. Equivalently these surfaces are critical points of the Geroch-Hawking mass. Thus our result has applications in the theory of General Relativity
Prop-fan data support study
Updated parametric prop-fan data packages are presented and the rationale used in developing the new prop-fan data is detailed. These data represent Hamilton Standard's projections of prop-fan characteristics for aircraft that are expected to be in-service in the 1985 to 1990 time frame. The basic prop-fan configuration was designed for efficient cruise operation at 0.8 Mach number and 10,668M altitude. The design blade tip speed is 244 mps and the design power loading is 301 KW/M squared
Mercury 2000: Stereoscopic Observations of Gamma Ray Flares
Stereoscopic observations of gamma ray radiation from solar flares would provide
further scientific impetus to recent proposals for a planetary observer mission to Mercury in the late 1990's. The solar monitoring phase of this mission could continue through the
period of maximum flare activity in the years 2002-2006 with a dawn-dusk polar orbit
which would allow continuous solar visibility and minimize solar tracking requirements.
Simultaneous measurements of flare radiation from gamma ray instruments with comparable
solar flux sensitivity in orbits around Mercury and Earth would provide stereoscopic
information on directivity and altitude location in the solar atmosphere of the flare radiation sources and might significantly advance understanding of energy release and particle
acceleration processes in solar flares. The closer proximity of Mercury to the Sun would
allow use of a much smaller gamma ray spectrometer system than required at 1 A.U. and
would also provide the first opportunity for direct detection of solar neutrons at energies
of 1-10 MeV. The Mercury orbiter would also be capable of monitoring 1-500 MeV solar
protons to search for decay protons from solar neutron flares and to provide automatic
early warning of large proton flares which would be a hazard to manned space operations
near Earth and beyond
Performance and noise of a low pressure ratio variable pitch fan designed for general aviation applications
A limited study has been conducted to establish the performance and noise characteristics of a low design tip speed (168 m/s, 550 ft/sec) low pressure ratio (1.04) variable pitch fan which was tested in the Langley 30 X 60 tunnel. This fan was designed for minimum noise when installed in the tail mount location of a twin engine aircraft which normally has both nose and tail mounted propulsors. Measurements showed the fan noise to be very close to predictions made during the design of the fan and extremely low in level (65 dBA at 1000 ft) with no acoustic treatment. This is about 8 dB lower than the unshrouded 2 blade propeller normally used in this installation. On the basis of tests conducted during this program, it appears that this level could be further reduced by 2 dBA if optimized acoustic treatments were installed in the fan duct. Even the best of the shrouded propellers tested previously were 7 dB higher in level than the Q-Fan without acoustic treatment. It was found that the cruise performance of this fan was within 5% of the predicted efficiency of 72%. Evaluation of the performance data indicated that disturbances in the inflow to the fan were the probable cause of the reduced performance
How branching can change the conductance of ballistic semiconductor devices
We demonstrate that branching of the electron flow in semiconductor
nanostructures can strongly affect macroscopic transport quantities and can
significantly change their dependence on external parameters compared to the
ideal ballistic case even when the system size is much smaller than the mean
free path. In a corner-shaped ballistic device based on a GaAs/AlGaAs
two-dimensional electron gas we observe a splitting of the commensurability
peaks in the magnetoresistance curve. We show that a model which includes a
random disorder potential of the two-dimensional electron gas can account for
the random splitting of the peaks that result from the collimation of the
electron beam. The shape of the splitting depends on the particular realization
of the disorder potential. At the same time magnetic focusing peaks are largely
unaffected by the disorder potential.Comment: accepted for publication in Phys. Rev.
Lattice-gas model for alkali-metal fullerides: face-centered-cubic structure
A lattice-gas model is suggested for describing the ordering phenomena in
alkali-metal fullerides of face-centered-cubic structure assuming the electric
charge of alkali ions residing in either octahedral or tetrahedral interstitial
sites is completely screened by the first-neighbor C_60 molecules. This
approximation allows us to derive an effective ion-ion interaction. The van der
Waals interaction between the ion and C_60 molecule is characterized by
introducing an additional energy at the tetrahedral sites. This model is
investigated by using a three-sublattice mean-field approximation and a simple
cluster-variation method. The analysis shows a large variety of phase diagrams
when changing the site energy parameter.Comment: 10 twocolumn pages (REVTEX) including 12 PS figure
Compton Echoes from Gamma-ray Bursts
Recent observations of gamma-ray bursts (GRBs) have provided growing evidence
for collimated outflows and emission, and strengthened the connection between
GRBs and supernovae. If massive stars are the progenitors of GRBs, the hard
photon pulse will propagate in the pre-burst, dense environment. Circumstellar
material will Compton scatter the prompt GRB radiation and give rise to a
reflection echo. We calculate luminosities, spectra, and light curves of such
Compton echoes in a variety of emission geometries and ambient gas
distributions, and show that the delayed hard X-ray flash from a pulse
propagating into a red supergiant wind could be detectable by Swift out to
z~0.2. Independently of the gamma-ray spectrum of the prompt burst, reflection
echoes will typically show a high-energy cutoff between m_ec^2/2 and m_ec^2
because of Compton downscattering. At fixed burst energy per steradian, the
luminosity of the reflected echo is proportional to the beaming solid angle,
Omega_b, of the prompt pulse, while the number of bright echoes detectable in
the sky above a fixed limiting flux increases as Omega_b^{1/2}, i.e. it is
smaller in the case of more collimated jets. The lack of an X-ray echo at one
month delay from the explosion poses severe constraints on the possible
existence of a lateral GRB jet in SN 1987A. The late r-band afterglow observed
in GRB990123 is fainter than the optical echo expected in a dense red
supergiant environment from a isotropic prompt optical flash. Significant MeV
delayed emission may be produced through the bulk Compton (or Compton drag)
effect resulting from the interaction of the decelerating fireball with the
scattered X-ray radiation.Comment: LaTeX, 18 pages, 4 figures, revised version accepted for publication
in the Ap
Ileocecal reservoir reconstruction after total mesorectal excision: functional results of the long-term follow-up
Background: The aim of this study is to obtain functional results of the long-term follow-up after TME and ileocecal interposition as rectal replacement. Methods: The study included patients operated on between March 1993 and August 1997 who received an ileocecal interposition as rectal replacement. Follow-up was carried out 3 and 5 years postoperatively. For statistical analysis, the paired t-test, rank test (Wilcoxon), and chi-square or Fisher's exact test were applied; level of significance, P<0.05. Results: Forty-four patients were included in the studies. Of these, five were not available and four patients could not be evaluated (dementia 1, radiation proctitis 1, fistula 1, pouchitis 1). Seventeen patients died during the observation period; 12 died of the disease. Recurrence of the disorder occurred in 2 of 35 patients (5.7%); 26 and 18 patients, 3 and 5 years postoperatively, respectively remained in the study. At 5 years, 78% of the patients were continent; mean stool frequency was 2.5±1.6 per day. Conclusions: Functional results and subjective assessment of ileocecal interposition were constant at 3 and 5 years postoperatively. If construction of a colonic J-pouch is not possible due to lack of colonic length, especially after prior colonic resections, the ileocecal interpositional reservoir may offer an alternative to rectal replacemen
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