1,616 research outputs found
Caging Mechanism for a drag-free satellite position sensor
A disturbance compensation system for satellites based on the drag-free concept was mechanized and flown, using a spherical proof mass and a cam-guided caging mechanism. The caging mechanism controls the location of the proof mass for testing and constrains it during launch. Design requirements, design details, and hardware are described
The Effects of Clumping and Substructure on ICM Mass Measurements
We examine an ensemble of 48 simulated clusters to determine the effects of
small-scale density fluctuations and large-scale substructure on X-ray
measurements of the intracluster medium (ICM) mass. We measure RMS density
fluctuations in the ICM which can be characterized by a mean mass-weighted
clumping factor C = /^2 between 1.3 and 1.4 within a density
contrast of 500 times the critical density. These fluctuations arise from the
cluster history of accretion shocks and major mergers, and their presence
enhances the cluster's luminosity relative to the smooth case. We expect,
therefore, that ICM mass measurements utilizing models which assume uniform
density at a given radius carry a bias of order sqrt(C) = 1.16. We verify this
result by performing ICM mass measurements on X-ray images of the simulations
and finding the expected level of bias.
The varied cluster morphologies in our ensemble also allow us to investigate
the effects of departures from spherical symmetry on our measurements. We find
that the presence of large-scale substructure does not further bias the
resulting gas mass unless it is pronounced enough to produce a second peak in
the image of at least 1% the maximum surface brightness. We analyze the subset
of images with no secondary peaks and find a bias of 9% and a Gaussian random
error of 4% in the derived mass.Comment: To appear in ApJ
Heavy oil production with energy effective steam-assisted gravity drainage
In reservoirs with extra heavy oil and bitumen, thermal methods are used to reduce the viscosity, in order to extract the oil. Steam-assisted gravity drainage (SAGD) is a thermal method where continuous steam injection is used. In this method, two horizontal wells are placed in parallel. The upper well injects steam and the lower well produces oil and condensed water. The continuous steam injection creates a chamber with uniform temperature. Heavy oil and bitumen reserves in Western Canada, which exceed 175 billion barrels, are becoming increasingly important petroleum sources due to the technical success of the SAGD processes. This study includes Computational fluid dynamics (CFD) modelling and simulations of a horizontal oil well with SAGD. The simulations are performed with inflow control devices (ICD) and autonomous inflow control valves (AICV) completion. In the SAGD processes, it is important that the residence time for steam in the reservoir is high enough to ensure that all the injected steam condenses in the reservoir to reduce the amount of steam injection and thereby making the SAGD process more energy effective. The simulations are carried out with ICD completion to delay the steam breakthrough and with AICV completion to prevent breakthrough of steam and water to the well. The numerical results showed that a most of the steam was produced together with the oil when ICD completion was used. AICV was able to close for steam and water, and the steam was thereby forced to condense in the reservoir, resulting in better utilization of the condensation energy
The stenotic carotid artery plaque : prevalence, risk factors and relations to clinical disease : the Tromsø study
Stroke is the second leading cause of death
in the world and is responsible for a high
percentage of major disability, requiring
substantial resources spent on care and
rehabilitation. Atherosclerosis due to lipid
accumulation in the vessel wall with
formation of stenotic atheromatous plaques
in the carotid bifurcation and/or the internal
carotid artery is an important cause of
stroke. In 1991, two large, multi-center
trials reported that carotid endarterectomy
was of benefit to patients with a degree of
stenosis above 70%, and thus showed that
the degree of stenosis was a major risk
factor for ipsilateral stroke. However, it
is well known that many high-grade
stenoses remain stable and never cause
cerebrovascular events, while others
develop rapidly and produce serious,
potentially life-threatening disease. While
the majority of patients presenting with
transient ischemic attack (TIA) and stroke
has an ipsilateral carotid lesion, only about
half of them have a hemodynamically
significant carotid stenosis. Only 5-15%
of strokes are heralded by a TIA. This has
led to a search for additional risk factors
which might help identify the individuals
with a high risk for stroke
Constraints on \Omega_0 and Cluster Evolution Using the ROSAT LogN-LogS
We examine the likelihoods of different cosmological models and cluster
evolutionary histories by comparing semi-analytical predictions of X-ray
cluster number counts to observational data from the ROSAT satellite. We model
cluster abundance as a function of mass and redshift using a Press-Schechter
distribution, and assume the temperature T(M,z) and bolometric luminosity
L_X(M,z) scale as power laws in mass and epoch, in order to construct expected
counts as a function of X-ray flux. The L_X-M scaling is fixed using the local
luminosity function while the degree of evolution in the X-ray luminosity with
redshift L_X \propto (1+z)^s is left open, with s an interesting free parameter
which we investigate. We examine open and flat cosmologies with initial,
scale-free fluctuation spectra having indices n = 0, -1 and -2. An independent
constraint arising from the slope of the luminosity-temperature relation
strongly favors the n = -2 spectrum.
The expected counts demonstrate a strong dependence on \Omega_0 and s, with
lesser dependence on \lambda_0 and n. Comparison with the observed counts
reveals a "ridge" of acceptable models in the \Omega_0 - s plane, roughly
following the relation s = 6 \Omega_0 and spanning low-density models with a
small degree of evolution to \Omega = 1 models with strong evolution. Models
with moderate evolution are revealed to have a strong lower limit of \Omega_0
\gtrsim 0.3, and low-evolution models imply that \Omega_0 < 1 at a very high
confidence level. We suggest observational tests for breaking the degeneracy
along this ridge, and discuss implications for evolutionary histories of the
intracluster medium.Comment: MNRAS LaTeX style format, submitted to MNRAS 3/26/97. Thirteen pages,
eleven postscript figures. Uses epsf macros to include figure
Branching Instabilities in Rapid Fracture: Dynamics and Geometry
We propose a theoretical model for branching instabilities in 2-dimensional
fracture, offering predictions for when crack branching occurs, how multiple
cracks develop, and what is the geometry of multiple branches. The model is
based on equations of motion for crack tips which depend only on the time
dependent stress intensity factors. The latter are obtained by invoking an
approximate relation between static and dynamic stress intensity factors,
together with an essentially exact calculation of the static ones. The results
of this model are in good agreement with a sizeable quantity of experimental
data.Comment: 9 pages, 11 figure
Roughening of Fracture Surfaces: the Role of Plastic Deformations
Post mortem analysis of fracture surfaces of ductile and brittle materials on
the m-mm and the nm scales respectively, reveal self affine graphs with an
anomalous scaling exponent . Attempts to use elasticity
theory to explain this result failed, yielding exponent up
to logarithms. We show that when the cracks propagate via plastic void
formations in front of the tip, followed by void coalescence, the voids
positions are positively correlated to yield exponents higher than 0.5.Comment: 4 pages, 6 figure
Dose-related beneficial and harmful effects of gabapentin in postoperative pain management:post hoc analyses from a systematic review with meta-analyses and trial sequential analyses
Tip Splittings and Phase Transitions in the Dielectric Breakdown Model: Mapping to the DLA Model
We show that the fractal growth described by the dielectric breakdown model
exhibits a phase transition in the multifractal spectrum of the growth measure.
The transition takes place because the tip-splitting of branches forms a fixed
angle. This angle is eta dependent but it can be rescaled onto an
``effectively'' universal angle of the DLA branching process. We derive an
analytic rescaling relation which is in agreement with numerical simulations.
The dimension of the clusters decreases linearly with the angle and the growth
becomes non-fractal at an angle close to 74 degrees (which corresponds to eta=
4.0 +- 0.3).Comment: 4 pages, REVTex, 3 figure
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