13,142 research outputs found
Design, fabrication, and structural testing of a lightweight shadow shield for deep-space application
Two full-scale, lightweight, double-sheeted shadow shields were developed as the primary element of a deep-space thermal protection system for liquid-hydrogen propellant tankage. The thermal and mechanical considerations used in s, the method of fabrication, and the environmental testing results on a prototype shield are discussed. Testing consisted of a transient cooldown period, a prolonged cold soak, and a transient warmup. The mechanical and thermal analyses used in the shield design are sufficient to produce a lightweight rugged shadow shield assembly that is structurally adequate for its intended application
Contractile stresses in cohesive cell layers on finite-thickness substrates
Using a minimal model of cells or cohesive cell layers as continuum active
elastic media, we examine the effect of substrate thickness and stiffness on
traction forces exerted by strongly adhering cells. We obtain a simple
expression for the length scale controlling the spatial variation of stresses
in terms of cell and substrate parameters that describes the crossover between
the thin and thick substrate limits. Our model is an important step towards a
unified theoretical description of the dependence of traction forces on cell or
colony size, acto-myosin contractility, substrate depth and stiffness, and
strength of focal adhesions, and makes experimentally testable predictions.Comment: 5 pages, 3 figure
Quasiperiodic spin-orbit motion and spin tunes in storage rings
We present an in-depth analysis of the concept of spin precession frequency
for integrable orbital motion in storage rings. Spin motion on the periodic
closed orbit of a storage ring can be analyzed in terms of the Floquet theorem
for equations of motion with periodic parameters and a spin precession
frequency emerges in a Floquet exponent as an additional frequency of the
system. To define a spin precession frequency on nonperiodic synchro-betatron
orbits we exploit the important concept of quasiperiodicity. This allows a
generalization of the Floquet theorem so that a spin precession frequency can
be defined in this case too. This frequency appears in a Floquet-like exponent
as an additional frequency in the system in analogy with the case of motion on
the closed orbit. These circumstances lead naturally to the definition of the
uniform precession rate and a definition of spin tune. A spin tune is a uniform
precession rate obtained when certain conditions are fulfilled. Having defined
spin tune we define spin-orbit resonance on synchro--betatron orbits and
examine its consequences. We give conditions for the existence of uniform
precession rates and spin tunes (e.g. where small divisors are controlled by
applying a Diophantine condition) and illustrate the various aspects of our
description with several examples. The formalism also suggests the use of
spectral analysis to ``measure'' spin tune during computer simulations of spin
motion on synchro-betatron orbits.Comment: 62 pages, 1 figure. A slight extension of the published versio
Mechanisms for Stable Sonoluminescence
A gas bubble trapped in water by an oscillating acoustic field is expected to
either shrink or grow on a diffusive timescale, depending on the forcing
strength and the bubble size. At high ambient gas concentration this has long
been observed in experiments. However, recent sonoluminescence experiments show
that in certain circumstances when the ambient gas concentration is low the
bubble can be stable for days. This paper presents mechanisms leading to
stability which predict parameter dependences in agreement with the
sonoluminescence experiments.Comment: 4 pages, 3 figures on request (2 as .ps files
Multiple Thresholds in a Model System of Noisy Ion Channels
Voltage-activated ion channels vary randomly between open and closed states,
influenced by the membrane potential and other factors. Signal transduction is
enhanced by noise in a simple ion channel model. The enhancement occurs in a
finite range of signals; the range can be extended using populations of
channels. The range increases more rapidly in multiple-threshold channel
populations than in single-threshold populations. The diversity of ion channels
may thus be present as a strategy to reduce the metabolic costs of handling a
broad class of electrochemical signals.Comment: REVTeX 4, 5 pages, 4 figures; added paragrap
Bubble Shape Oscillations and the Onset of Sonoluminescence
An air bubble trapped in water by an oscillating acoustic field undergoes
either radial or nonspherical pulsations depending on the strength of the
forcing pressure. Two different instability mechanisms (the Rayleigh--Taylor
instability and parametric instability) cause deviations from sphericity.
Distinguishing these mechanisms allows explanation of many features of recent
experiments on sonoluminescence, and suggests methods for finding
sonoluminescence in different parameter regimes.Comment: Phys. Rev. Lett., in pres
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