440 research outputs found
LDR segmented mirror technology assessment study
In the mid-1990s, NASA plans to orbit a giant telescope, whose aperture may be as great as 30 meters, for infrared and sub-millimeter astronomy. Its primary mirror will be deployed or assembled in orbit from a mosaic of possibly hundreds of mirror segments. Each segment must be shaped to precise curvature tolerances so that diffraction-limited performance will be achieved at 30 micron (nominal operating wavelength). All panels must lie within 1 micron on a theoretical surface described by the optical precipitation of the telescope's primary mirror. To attain diffraction-limited performance, the issues of alignment and/or position sensing, position control of micron tolerances, and structural, thermal, and mechanical considerations for stowing, deploying, and erecting the reflector must be resolved. Radius of curvature precision influences panel size, shape, material, and type of construction. Two superior material choices emerged: fused quartz (sufficiently homogeneous with respect to thermal expansivity to permit a thin shell substrate to be drape molded between graphite dies to a precise enough off-axis asphere for optical finishing on the as-received a segment) and a Pyrex or Duran (less expensive than quartz and formable at lower temperatures). The optimal reflector panel size is between 1-1/2 and 2 meters. Making one, two-meter mirror every two weeks requires new approaches to manufacturing off-axis parabolic or aspheric segments (drape molding on precision dies and subsequent finishing on a nonrotationally symmetric dependent machine). Proof-of-concept developmental programs were identified to prove the feasibility of the materials and manufacturing ideas
Breakdown of a conservation law in incommensurate systems
We show that invariance properties of the Lagrangian of an incommensurate
system, as described by the Frenkel Kontorova model, imply the existence of a
generalized angular momentum which is an integral of motion if the system
remains floating. The behavior of this quantity can therefore monitor the
character of the system as floating (when it is conserved) or locked (when it
is not). We find that, during the dynamics, the non-linear couplings of our
model cause parametric phonon excitations which lead to the appearance of
Umklapp terms and to a sudden deviation of the generalized momentum from a
constant value, signalling a dynamical transition from a floating to a pinned
state. We point out that this transition is related but does not coincide with
the onset of sliding friction which can take place when the system is still
floating.Comment: 7 pages, 6 figures, typed with RevTex, submitted to Phys. Rev. E
Replaced 27-03-2001: changes to text, minor revision of figure
Clinical characteristics, treatment patterns and outcomes of Hispanic hypertensive patients
Hispanics are the largest and fastest-growing minority population in the United States, currently comprising about 16.3% (52 million) of the total population. With an increased prevalence of metabolic risk factors in this population, the rate of uncontrolled hypertension (HTN) in Hispanics significantly exceeds the rates observed among non-Hispanic blacks and whites. Unfortunately, data on HTN in Hispanics remains limited due to the under-representation of Hispanics in clinical trials; with most of the data primarily restricted to observational and retrospective subgroup analyses. This article aims to review the available data on prevalence, awareness and control of HTN, risk factors and some of the challenges unique to the Hispanics population. We also discuss treatment strategies derived from large HTN trials that included Hispanics
A class of constant modulus algorithms for uniform linear arrays with a conjugate symmetric constraint
A class of constant modulus algorithms (CMAs) subject to a conjugate symmetric constraint is proposed for blind beamforming based on the uniform linear array structure. The constraint is derived from the beamformer with an optimum output signal-to-interference-plus-noise ratio (SINR). The effect of the additional constraint is equivalent to adding a second step to the original adaptive algorithms. The proposed approach is general and can be applied to both the traditional CMA and its all kinds of variants, such as the linearly constrained CMA (LCCMA) and the least squares CMA (LSCMA) as two examples. With this constraint, the modified CMAs will always generate a weight vector in the desired form for each update and the number of adaptive variables is effectively reduced by half, leading to a much improved overall performance. (C) 2010 Elsevier B.V. All rights reserved
Rubber friction on wet and dry road surfaces: the sealing effect
Rubber friction on wet rough substrates at low velocities is typically 20-30%
smaller than for the corresponding dry surfaces. We show that this cannot be
due to hydrodynamics and propose a novel explanation based on a sealing effect
exerted by rubber on substrate "pools" filled with water. Water effectively
smoothens the substrate, reducing the major friction contribution due to
induced viscoelastic deformations of the rubber by surface asperities. The
theory is illustrated with applications related to tire-road friction.Comment: Format Revtex 4; 8 pages, 11 figures (no color); Published on Phys.
Rev. B (http://link.aps.org/abstract/PRB/v71/e035428); previous work on the
same topic: cond-mat/041204
Effects of Defects on Friction for a Xe Film Sliding on Ag(111)
The effects of a step defect and a random array of point defects (such as
vacancies or substitutional impurities) on the force of friction acting on a
xenon monolayer film as it slides on a silver (111) substrate are studied by
molecular dynamic simulations and compared with the results of lowest order
perturbation theory in the substrate corrugation potential. For the case of a
step, the magnitude and velocity dependence of the friction force are strongly
dependent on the direction of sliding respect to the step and the corrugation
strength. When the applied force F is perpendicular to the step, the film is
pinned forF less than a critical force Fc. Motion of the film along the step,
however, is not pinned. Fluctuations in the sliding velocity in time provide
evidence of both stick-slip motion and thermally activated creep. Simulations
done with a substrate containing a 5 percent concentration of random point
defects for various directions of the applied force show that the film is
pinned for the force below a critical value. The critical force, however, is
still much lower than the effective inertial force exerted on the film by the
oscillations of the substrate in experiments done with a quartz crystal
microbalance (QCM). Lowest order perturbation theory in the substrate potential
is shown to give results consistent with the simulations, and it is used to
give a physical picture of what could be expected for real surfaces which
contain many defects.Comment: 13 pages, 17 figures, latex plus postscript files for figure
Theory of friction: contribution from fluctuating electromagnetic field
We calculate the friction force between two semi-infinite solids in relative
parallel motion (velocity ), and separated by a vacuum gap of width . The
friction force result from coupling via a fluctuating electromagnetic field,
and can be considered as the dissipative part of the van der Waals interaction.
We consider the dependence of the friction force on the temperature , and
present a detailed discussion of the limiting cases of small and large and
.Comment: 15 pages, No figure
Strongly Temperature Dependent Sliding Friction for a Superconducting Interface
A sudden drop in mechanical friction, between an adsorbed nitrogen monolayer
and a lead substrate, occurs when the lead passes through the superconducting
transition temperature. We attribute this effect to a sudden drop at the
superconducting transition temperature of the substrate Ohmic heating. The
Ohmic heating is due to the electronic screening current that results from the
sliding adsorbed film.Comment: Revte
Static and dynamic properties of frictional phenomena in a one-dimensional system with randomness
Static and dynamic frictional phenomena at the interface with random
impurities are investigated in a two-chain model with incommensurate structure.
Static frictional force is caused by the impurity pinning and/or by the pinning
due to the regular potential, which is responsible for the breaking of
analyticity transition for impurity-free cases. It is confirmed that the static
frictional force is always finite in the presence of impurities, in contrast to
the impurity-free system. The nature of impurity pinning is discussed in
connection with that in density waves. The kinetic frictional force of a steady
sliding state is also investigated numerically. The relationship between the
sliding velocity dependence of the kinetic frictional force and the strength of
impurity potential is discussed.Comment: RevTex, 14 pages, 6 PostScript figures, to appear in Phys. Rev.
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