22,888 research outputs found
Calibration of the 13- by 13-inch adaptive wall test section for the Langley 0.3-meter transonic cryogenic tunnel
A 13 by 13 inch adaptive wall test section was installed in the 0.3 Meter Transonic Cryogenic Tunnel circuit. This new test section is configured for 2-D airfoil testing. It has four solid walls. The top and bottom walls are flexible and movable whereas the sidewalls are rigid and fixed. The wall adaptation strategy employed requires the test section wall shapes associated with uniform test section Mach number distributions. Calibration tests with the test section empty were conducted with the top and bottom walls linearly diverged to approach a uniform Mach number distribution. Pressure distributions were measured in the contraction cone, the test section, and the high speed diffuser at Mach numbers from 0.20 to 0.95 and Reynolds numbers from 10 to 100 x 10 (exp 6)/per foot
Crossover from Endogenous to Exogenous Activity in Open-Source Software Development
We have investigated the origin of fluctuations in the aggregated behaviour
of an open-source software community. In a recent series of papers, de Menezes
and co-workers have shown how to separate internal dynamics from external
fluctuations by capturing the simultaneous activity of many system's
components. In spite of software development being a planned activity, the
analysis of fluctuations reveals how external driving forces can be only
observed at weekly and higher time scales. Hourly and higher change frequencies
mostly relate to internal maintenance activities. There is a crossover from
endogenous to exogenous activity depending on the average number of file
changes. This new evidence suggests that software development is a
non-homogeneous design activity where stronger efforts focus in a few project
files. The crossover can be explained with a Langevin equation associated to
the cascading process, where changes to any file trigger additional changes to
its neighbours in the software network. In addition, analysis of fluctuations
enables us to detect whether a software system can be decomposed in several
subsystems with different development dynamics.Comment: 7 pages, 4 figures, submitted to Europhysics Letter
Seeing Galaxies Through Thick & Thin. III. HST Imaging of the Dust in Backlit Spiral Galaxies
We present analysis of WFPC2 imaging of two spiral galaxies partially backlit
by E/S0 systems in the pairs AM1316-241 and AM0500-620, and the spiral
foreground system in NGC 1275. Images in B and I are used to determine the
reddening curve of in these systems. The spiral component of AM1316-241 shows
dust strongly concentrated in discrete arms, with a reddening law very close to
the Milky Way mean. The dust distribution is scale-free between about 100 pc
and the arm scale. The spiral in AM0500-620 shows dust concentrated in arms and
interarm spurs, with measurable interarm extinction as well. Although its dust
properties are less well-determined, we find evidence for a steeper extinction
law here. The shape of the reddening law suggests that, at least in AM1316-241,
we have resolved most of the dust structure. In AM0500-620, the slope of the
fractal perimeter-scale relation steepens systematically from low to high
extinction. In AM1316-241, we cannot determine a unique fractal dimension from
the defining area-perimeter relation, so the projected dust distribution is
best defined as fractal-like. In neither galaxy do we see regions even on
single-pixel scales in spiral arms with AB > 2.5. The measurements in NGC 1275
are compromised by our lack of independent knowledge of the foreground system's
light distribution, but masked sampling of the absorption suggests an effective
reddening curve much flatter than the Milky Way mean (perhaps indicating that
the foreground system has been affected by immersion in the hot intracluster
gas).Comment: Astronomical Journal, in press. 13 figures. Full-size PostScript
figures available at http://www.astr.ua.edu/preprints/kee
Mapping compositional and particle size variations across Silver Lake Playa: Relevance to analyses of Mars TIR data
The high spectral and spatial resolution thermal infrared (TIR) data to be acquired from the upcoming Mars Observer-Thermal Emission Spectra (TES) mission will map the composition and texture of the Martian sediments. To prepare for these data, portions of two remote sensing experiments were conducted to test procedures for extracting surface property information from TIR data. Reported here is the continuing analysis of Thermal Infrared Multispectral Scanner (TIMS) data, field emission spectra, laboratory Fourier Transform Infrared (FTIR) reflectance spectra, and field observations with respect to the physical characteristics (composition, emissivity, etc.) of Silver Lake playa in southern California
Switching of the magnetic order in CeRhInSn in the vicinity of its quantum critical point
We report neutron diffraction experiments performed in the tetragonal
antiferromagnetic heavy fermion system CeRhInSn in its (, )
phase diagram up to the vicinity of the critical concentration
0.40, where long range magnetic order is suppressed. The propagation vector of
the magnetic structure is found to be =(1/2, 1/2, ) with
increasing from =0.298 to =0.410 when increases from =0
to =0.26. Surprisingly, for =0.30, the order has changed drastically and
a commensurate antiferromagnetism with =(1/2, 1/2, 0) is found.
This concentration is located in the proximity of the quantum critical point
where superconductivity is expected.Comment: 5 pages, 5 figures, submitted to Phys. Rev.
The effect of rotation and tidal heating on the thermal lightcurves of Super Mercuries
Short period (<50 days) low-mass (<10Mearth) exoplanets are abundant and the
few of them whose radius and mass have been measured already reveal a diversity
in composition. Some of these exoplanets are found on eccentric orbits and are
subjected to strong tides affecting their rotation and resulting in significant
tidal heating. Within this population, some planets are likely to be depleted
in volatiles and have no atmosphere. We model the thermal emission of these
"Super Mercuries" to study the signatures of rotation and tidal dissipation on
their infrared light curve. We compute the time-dependent temperature map at
the surface and in the subsurface of the planet and the resulting
disk-integrated emission spectrum received by a distant observer for any
observation geometry. We calculate the illumination of the planetary surface
for any Keplerian orbit and rotation. We include the internal tidal heat flow,
vertical heat diffusion in the subsurface and generate synthetic light curves.
We show that the different rotation periods predicted by tidal models
(spin-orbit resonances, pseudo-synchronization) produce different photometric
signatures, which are observable provided that the thermal inertia of the
surface is high, like that of solid or melted rocks (but not regolith). Tidal
dissipation can also directly affect the light curves and make the inference of
the rotation more difficult or easier depending on the existence of hot spots
on the surface. Infrared light curve measurement with the James Webb Space
Telescope and EChO can be used to infer exoplanets' rotation periods and
dissipation rates and thus to test tidal models. This data will also constrain
the nature of the (sub)surface by constraining the thermal inertia.Comment: 15 pages, 13 figures, accepted for publication in Astronomy &
Astrophysic
Suppression of hidden order in URu2Si2 under pressure and restoration in magnetic field
We describe here recent inelastic neutron scattering experiments on the heavy
fermion compound URu2Si2 realized in order to clarify the nature of the hidden
order (HO) phase which occurs below T_0 = 17.5 K at ambient pressure. The
choice was to measure at a given pressure P where the system will go, by
lowering the temperature, successively from paramagnetic (PM) to HO and then to
antiferromagnetic phase (AF). Furthermore, in order to verify the selection of
the pressure, a macroscopic detection of the phase transitions was also
achieved in situ via its thermal expansion response detected by a strain gauge
glued on the crystal. Just above P_x = 0.5 GPa, where the ground state switches
from HO to AF, the Q_0 = (1, 0, 0) excitation disappears while the excitation
at the incommensurate wavevector Q_1 = (1.4, 0, 0) remains. Thus, the Q_0 = (1,
0, 0) excitation is intrinsic only in the HO phase. This result is reinforced
by studies where now pressure and magnetic field can be used as tuning
variable. Above P_x, the AF phase at low temperature is destroyed by a magnetic
field larger than H_AF (collapse of the AF Q_0 = (1, 0, 0) Bragg reflection).
The field reentrance of the HO phase is demonstrated by the reappearance of its
characteristic Q_0 = (1, 0, 0) excitation. The recovery of a PM phase will only
be achieved far above H_AF at H_M approx 35 T. To determine the P-H-T phase
diagram of URu2Si2, macroscopic measurements of the thermal expansion were
realized with a strain gauge. The reentrant magnetic field increases strongly
with pressure. Finally, to investigate the interplay between superconductivity
(SC) and spin dynamics, new inelastic neutron scattering experiments are
reported down to 0.4 K, far below the superconducting critical temperature T_SC
approx 1.3 K as measured on our crystal by diamagnetic shielding.Comment: 5 pages, 7 figures, ICN 2009 conference proceeding
- …