13,987 research outputs found
Acoustic and aerodynamic performance of a variable-pitch 1.83-meter-(6-ft) diameter 1.20-pressure-ratio fan stage (QF-9)
Far field noise data and related aerodynamic performance are presented for a variable pitch fan stage having characteristics suitable for low noise, STOL engine application. However, no acoustic suppression material was used in the flow passages. The fan was externally driven by an electric motor. Tests were made at several forward thrust rotor blade pitch angles and one for reverse thrust. Fan speed was varied from 60 to 120 percent of takeoff (design) speed, and exhaust nozzles having areas 92 to 105 percent of design were tested. The fan noise level was at a minimum at the design rotor blade pitch angles of 64 deg for takeoff thrust and at 57 deg for approach (50 percent takeoff thrust). Perceived noise along a 152.4-m sideline reached 100.1 PNdb for the takeoff (design) configuration for a stage pressure ratio of 1.17 and thrust of 57,600 N. For reverse thrust the PNL values were 4 to 5 PNdb above the takeoff values at comparable fan speeds
Low flight speed acoustic results for a supersonic inlet with auxiliary inlet doors
A model supersonic inlet with auxiliary inlet doors and bounday layer bleeds was acoustically tested in simulated low speed flight up to Mach 0.2 in the NASA Lewis 9x15 Anechoic Wind Tunnel and statically in the NASA Lewis Anechoic Chamber. A JT8D refan model was used as the noise source. Data were also taken for a CTOL inlet and for an annular inlet with simulated centerbody support struts. Inlet operation with open auxiliary doors increased the blade passage tone by about 10 dB relative to the closed door configuration although noise radiation was primarily through the main inlet rather than the doors. Numerous strong spikes in the noise spectra were associated with the bleed system, and were strongly affected by the centerbody location. The supersonic inlet appeared to suppress multiple pure tone (MPT) generation at the fan source. Inlet length and the presence of support struts were shown not to cause this MPT suppression
Spitzer observations of the Hyades: Circumstellar debris disks at 625 Myr of age
We use the Spitzer Space Telescope to search for infrared excess at 24, 70,
and 160 micron due to debris disks around a sample of 45 FGK-type members of
the Hyades cluster. We supplement our observations with archival 24 and 70
micron Spitzer data of an additional 22 FGK-type and 11 A-type Hyades members
in order to provide robust statistics on the incidence of debris disks at 625
Myr of age an era corresponding to the late heavy bombardment in the Solar
System. We find that none of the 67 FGK-type stars in our sample show evidence
for a debris disk, while 2 out of the 11 A-type stars do so. This difference in
debris disk detection rate is likely to be due to a sensitivity bias in favor
of early-type stars. The fractional disk luminosity, L_dust/L*, of the disks
around the two A-type stars is ~4.0E-5, a level that is below the sensitivity
of our observations toward the FGK-type stars. However, our sensitivity limits
for FGK-type stars are able to exclude, at the 2-sigma level, frequencies
higher than 12% and 5% of disks with L_dust/L* > 1.0E-4 and L_dust/L* > 5.0E-4,
respectively. We also use our sensitivity limits and debris disk models to
constrain the maximum mass of dust, as a function of distance from the stars,
that could remain undetected around our targets.Comment: 33 pages, 11 figures, accepted by Ap
The clock paradox in a static homogeneous gravitational field
The gedanken experiment of the clock paradox is solved exactly using the
general relativistic equations for a static homogeneous gravitational field. We
demonstrate that the general and special relativistic clock paradox solutions
are identical and in particular that they are identical for finite
acceleration. Practical expressions are obtained for proper time and coordinate
time by using the destination distance as the key observable parameter. This
solution provides a formal demonstration of the identity between the special
and general relativistic clock paradox with finite acceleration and where
proper time is assumed to be the same in both formalisms. By solving the
equations of motion for a freely falling clock in a static homogeneous field
elapsed times are calculated for realistic journeys to the stars.Comment: Revision: Posted with the caption included with the figure
Study of an automatic trajectory following control system
It is shown that the estimator part of the Modified Partitioned Adaptive Controller, (MPAC) developed for nonlinear aircraft dynamics of a small jet transport can adapt to sensor failures. In addition, an investigation is made into the potential usefulness of the configuration detection technique used in the MPAC and the failure detection filter is developed that determines how a noise plant output is associated with a line or plane characteristic of a failure. It is shown by computer simulation that the estimator part and the configuration detection part of the MPAC can readily adapt to actuator and sensor failures and that the failure detection filter technique cannot detect actuator or sensor failures accurately for this type of system because of the plant modeling errors. In addition, it is shown that the decision technique, developed for the failure detection filter, can accurately determine that the plant output is related to the characteristic line or plane in the presence of sensor noise
Near-Infrared Imaging Polarimetry of Young Stellar Objects in rho-Ophiuchi
The results of a near-infrared (J H K LP) imaging linear polarimetry survey
of 20 young stellar objects (YSOs) in rho Ophiuchi are presented. The majority
of the sources are unresolved, with K-band polarizations, P_K < 6 per cent.
Several objects are associated with extended reflection nebulae. These objects
have centrosymmetric vector patterns with polarization discs over their cores;
maximum polarizations of P_K > 20 per cent are seen over their envelopes.
Correlations are observed between the degree of core polarization and the
evolutionary status inferred from the spectral energy distribution. K-band core
polarizations >6 per cent are only observed in Class I YSOs. A 3D Monte Carlo
model with oblate grains aligned with a magnetic field is used to investigate
the flux distributions and polarization structures of three of the rho Oph YSOs
with extended nebulae. A rho proportional to r^(-1.5) power law for the density
is applied throughout the envelopes. The large-scale centrosymmetric
polarization structures are due to scattering. However, the polarization
structure in the bright core of the nebula appears to require dichroic
extinction by aligned non-spherical dust grains. The position angle indicates a
toroidal magnetic field in the inner part of the envelope. Since the measured
polarizations attributed to dichroic extinction are usually <10 per cent, the
grains must either be nearly spherical or very weakly aligned. The higher
polarizations observed in the outer parts of the reflection nebulae require
that the dust grains responsible for scattering have maximum grain sizes <=1.05
microns.Comment: 26 pages. Accepted by MNRAS. Available as online early versio
2013 Morley Nelson Snake River Birds of Prey National Conservation Area RapidEye 7m Landcover Classification
Boise State University conducted an area-wide vegetation classification of the Orchard Combat Training Center (OCTC) for the Idaho National Guard/IDARNG and expanded the classification to cover areas of the Morley Nelson Snake River Birds of Prey National Conservation Area for the Bureau of Land Management. This report documents the field data collection and processing, image acquisition and processing, and image classification. Work was performed between January 2012 – October 2015
An optical spectroscopic H-R diagram for low-mass stars and brown dwarfs in Orion
The masses and temperatures of young low mass stars and brown dwarfs in star-
forming regions are not yet well established because of uncertainties in the
age of individual objects and the spectral type vs. temperature scale
appropriate for objects with ages of only a few Myr. Using multi-object optical
spectroscopy, 45 low-mass stars and brown dwarfs in the Trapezium Cluster in
Orion have been classified and 44 of these confirmed as bona fide cluster
members. The spectral types obtained have been converted to effective
temperatures using a temperature scale intermediate between those of dwarfs and
giants, which is suitable for young pre-main sequence objects. The objects have
been placed on an H-R diagram overlaid with theoretical isochrones. The low
mass stars and the higher mass substellar objects are found to be clustered
around the 1 Myr isochrone, while many of the lower mass substellar objects are
located well above this isochrone. An average age of 1 Myr is found for the
majority of the objects. Assuming coevality of the sources and an average age
of 1 Myr, the masses of the objects have been estimated and range from 0.018 to
0.44Msun. The spectra also allow an investigation of the surface gravity of the
objects by measurement of the sodium doublet equivalent width. With one
possible exception, all objects have low gravities, in line with young ages,
and the Na indices for the Trapezium objects lie systematically below those of
young stars and brown dwarfs in Chamaeleon, suggesting that the 820 nm Na index
may provide a sensitive means of estimating ages in young clusters.Comment: 19 pages, accepted by MNRA
Drying and cracking mechanisms in a starch slurry
Starch-water slurries are commonly used to study fracture dynamics. Drying
starch-cakes benefit from being simple, economical, and reproducible systems,
and have been used to model desiccation fracture in soils, thin film fracture
in paint, and columnar joints in lava. In this paper, the physical properties
of starch-water mixtures are studied, and used to interpret and develop a
multiphase transport model of drying. Starch-cakes are observed to have a
nonlinear elastic modulus, and a desiccation strain that is comparable to that
generated by their maximum achievable capillary pressure. It is shown that a
large material porosity is divided between pore spaces between starch grains,
and pores within starch grains. This division of pore space leads to two
distinct drying regimes, controlled by liquid and vapor transport of water,
respectively. The relatively unique ability for drying starch to generate
columnar fracture patterns is shown to be linked to the unusually strong
separation of these two transport mechanisms.Comment: 9 pages, 8 figures [revised in response to reviewer comments
Drying and cracking mechanisms in a starch slurry
Starch-water slurries are commonly used to study fracture dynamics. Drying
starch-cakes benefit from being simple, economical, and reproducible systems,
and have been used to model desiccation fracture in soils, thin film fracture
in paint, and columnar joints in lava. In this paper, the physical properties
of starch-water mixtures are studied, and used to interpret and develop a
multiphase transport model of drying. Starch-cakes are observed to have a
nonlinear elastic modulus, and a desiccation strain that is comparable to that
generated by their maximum achievable capillary pressure. It is shown that a
large material porosity is divided between pore spaces between starch grains,
and pores within starch grains. This division of pore space leads to two
distinct drying regimes, controlled by liquid and vapor transport of water,
respectively. The relatively unique ability for drying starch to generate
columnar fracture patterns is shown to be linked to the unusually strong
separation of these two transport mechanisms.Comment: 9 pages, 8 figures [revised in response to reviewer comments
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