384 research outputs found
Emitted vibration measurement device and method
This invention is directed to a method and apparatus for measuring emitted vibrational forces produced by a reaction wheel assembly due to imbalances, misalignment, bearing defects and the like. The apparatus includes a low mass carriage supported on a large mass base. The carriage is in the form of an octagonal frame having an opening which is adapted for receiving the reaction wheel assembly supported thereon by means of a mounting ring. The carriage is supported on the base by means of air bearings which support the carriage in a generally frictionless manner when supplied with compressed air from a source. A plurality of carriage brackets and a plurality of base blocks provided for physical coupling of the base and carriage. The sensing axes of the load cells are arranged generally parallel to the base and connected between the base and carriage such that all of the vibrational forces emitted by the reaction wheel assembly are effectively transmitted through the sensing axes of the load cells. In this manner, a highly reliable and accurate measurment of the vibrational forces of the reaction wheel assembly can be had. The output signals from the load cells are subjected to a dynamical analyzer which analyzes and identifies the rotor and spin bearing components which are causing the vibrational forces
Verification and Validation of the RAGE Hydrocode in Preparation for Investigation of Impacts into a Volatile-rich Target
Before a hydrocode is used to investigate a question of scientific interest, it should be tested against analogous laboratory experiments and problems with analytical solutions. The Radiation Adaptive Grid Eulerian (RAGE) hydrocode[1], developed by Los Alamos National Laboratory (LANL) and Science Applications International Corporation (SAIC)[2,3] has been subjected to many tests during its development.[4,5] We extend and review this work, emphasizing tests relevant to impact cratering into volatile-rich targets
One-Bead Microrheology with Rotating Particles
We lay the theoretical basis for one-bead microrheology with rotating
particles, i.e, a method where colloids are used to probe the mechanical
properties of viscoelastic media. Based on a two-fluid model, we calculate the
compliance and discuss it for two cases. We first assume that the elastic and
fluid component exhibit both stick boundary conditions at the particle surface.
Then, the compliance fulfills a generalized Stokes law with a complex shear
modulus whose validity is only limited by inertial effects, in contrast to
translational motion. Secondly, we find that the validity of the Stokes regime
is reduced when the elastic network is not coupled to the particleComment: 7 pages, 5 figures, submitted to Europhys. Let
Mechanically induced helix-coil transition in biopolymer networks
The quasi-equilibrium evolution of the helical fraction occurring in a
biopolymer network (gelatin gel) under an applied stress has been investigated
by observing modulation in its optical activity. Its variation with the imposed
chain extension is distinctly non-monotonic and corresponds to the transition
of initially coiled strands to induced left-handed helices. The experimental
results are in qualitative agreement with theoretical predictions of helices
induced on chain extension. This new effect of mechanically stimulated
helix-coil transition has been studied further as a function of the elastic
properties of the polymer network: crosslink density and network aging
The Influence of Environment on the Star Formation Rates of Galaxies
We have used a sample of 15749 galaxies taken from the Las Campanas Redshift
Survey to investigate the effects of environment on the rate of star formation
(SFR) in galaxies. The size and homogeneity of this data set allows us to
sample, for the first time, the entire range of galactic environment, from the
voids to the clusters, in a uniform manner, thus, we could decouple the local
galaxy density from the membership in associations. This decoupling is very
crucial for constraining the physical processes responsible for the
environmental dependencies of SFR. On the other hand, the use of an
automatically-measured concentration index (C), rather than Hubble type, allows
us to cleanly separate the morphological component from the SFR vs. environment
relationship. We find that cluster galaxies exhibit lower SFR for the same C
than field galaxies, while a further division of clusters by `richness' reveals
a new possible excitation of `starbursts' in poor clusters. Meanwhile, a more
general environmental investigation reveals that the SFR of a given C shows a
continuous correlation with the local density. Interestingly, this trend is
also observed both inside and outside of clusters, implying that physical
processes responsible for this correlation might not be intrinsic to the
cluster environment. On the other hand, galaxies with differing levels of SFR
appear to respond differently to the local density. Low levels of SFR are more
sensitive to environment inside than outside of clusters. In contrast, high
levels of SFR, identified as ``starbursts'', are as sensitive to local density
in the field as in clusters. We conclude that at least two separate processes
are responsible for the environmental sensitivity of the SFR.Comment: 25 pages, 10 figures, submitted to Ap
Spatial variations of the Sr i 4607 {\AA} scattering polarization peak
Context. The scattering polarization signal observed in the photospheric Sr i
4607 {\AA} line is expected to vary at granular spatial scales. This variation
can be due to changes in the magnetic field intensity and orientation (Hanle
effect), but also to spatial and temporal variations in the plasma properties.
Measuring the spatial variation of such polarization signal would allow us to
study the properties of the magnetic fields at subgranular scales, but
observations are challenging since both high spatial resolution and high
spectropolarimetric sensitivity are required.
Aims. We aim to provide observational evidence of the polarization peak
spatial variations, and to analyze the correlation they might have with
granulation.
Methods. Observations conjugating high spatial resolution and high
spectropolarimetric precision were performed with the Zurich IMaging
POLarimeter, ZIMPOL, at the GREGOR solar telescope, taking advantage of the
adaptive optics system and the newly installed image derotator.
Results. Spatial variations of the scattering polarization in the Sr i 4607
{\AA} line are clearly observed. The spatial scale of these variations is
comparable with the granular size. Small correlations between the polarization
signal amplitude and the continuum intensity indicate that the polarization is
higher at the center of granules than in the intergranular lanes.Comment: 5 pages, 4 figure
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