23,611 research outputs found
Comparison of approximate and numerical analyses of nonlinear combustion instability
At the present time, there are three general analytical techniques available to study problems of unsteady motions in rocket motors: linear stability analysis; approximate nonlinear analysis, founded on examining the behavior of coupled normal modes; and numerical calculations based on
the conservation equations for one-dimensional flows. The last two yield the linear results as a limit. It is the main purpose of this paper to check the accuracy of the approximate analysis against the numerical analysis for some special cases. The results provide some justification for using the approximate analysis to study three dimensional problems
Test system accurately determines tensile properties of irradiated metals at cryogenic temperatures
Modified testing system determines tensile properties of irradiated brittle-type metals at cryogenic temperatures. The system includes a lightweight cryostat, split-screw grips, a universal joint, and a special temperature control system
Langmuir probe measurements of double-layers in a pulsed discharge
Langmuir probe measurements were carried out which confirm the occurrence of double-layers in an argon positive column. Pulsing the discharge current permitted probe measurements to be performed in the presence of the double-layer. Supplementary evidence, obtained from DC and pulsed discharges, indicated that the double-layers formed in the two modes of operation were similar. The double-layers observed were weak and stable; their relation to other classes of double-layers are discussed, and directions for future work are suggested
A fluid description of plasma double-layers
The space-charge double-layer that forms between two plasmas with different densities and thermal energies was investigated using three progressively realistic models which are treated by fluid theory, and take into account four species of particles: electrons and ions reflected by the double-layer, and electrons and ions transmitted through it. The two plasmas are assumed to be cold, and the self-consistent potential, electric field and space-charge distributions within the double-layer are determined. The effects of thermal velocities are taken into account for the reflected particles, and the modifications to the cold plasma solutions are established. Further modifications due to thermal velocities of the transmitted particles are examined. The applicability of a one dimensional fluid description, rather than plasma kinetic theory, is discussed. Theoretical predictions are compared with double layer potentials and lengths deduced from laboratory and space plasma experiments
Deformation of crosslinked semiflexible polymer networks
Networks of filamentous proteins play a crucial role in cell mechanics. These
cytoskeletal networks, together with various crosslinking and other associated
proteins largely determine the (visco)elastic response of cells. In this letter
we study a model system of crosslinked, stiff filaments in order to explore the
connection between the microstructure under strain and the macroscopic response
of cytoskeletal networks. We find two distinct regimes as a function primarily
of crosslink density and filament rigidity: one characterized by affine
deformation and one by non-affine deformation. We characterize the crossover
between these two.Comment: Typos fixed and some technical details clarified. To appear in Phys.
Rev. Let
Non-equilibrium mechanics and dynamics of motor activated gels
The mechanics of cells is strongly affected by molecular motors that generate
forces in the cellular cytoskeleton. We develop a model for cytoskeletal
networks driven out of equilibrium by molecular motors exerting transient
contractile stresses. Using this model we show how motor activity can
dramatically increase the network's bulk elastic moduli. We also show how motor
binding kinetics naturally leads to enhanced low-frequency stress fluctuations
that result in non-equilibrium diffusive motion within an elastic network, as
seen in recent \emph{in vitro} and \emph{in vivo} experiments.Comment: 21 pages, 8 figure
Robust Estimates of the New Keynesian Phillips Curve
In this paper, we examine the hybrid specification of the New Keynesian Phillips Curve (NKPC) proposed by Gali and Gertler (1999) by employing recently developed momentconditions inference procedures. These methods provide a more efficient and reliable econometric framework for the analysis of the NKPC. In particular, we address the issue of parameter identification, providing robust estimates and confidence sets for the model’s parameters. Our results show that the NKPC remains a valid and reliable empirical tool to explain inflation dynamics.price regulation, commitment problem, ratchet effect, under-investment
Synthesis of polyethers of hexafluorobenzene and hexafluoropentanediol
Two new polyethers, poly /hexafluoropentamethylene tetrafluoro-p-phenylene ether/ and a completely hydroxyl-terminated polyether, is prepared by reactions of hexafluorobenzene with hexafluoropentanediol. The polyethers can be prepared as low molecular weight oils, as intermediate molecular weight waxes, or as high molecular weight elastomers
Dynamics of viscoelastic membranes
We determine both the in-plane and out-of-plane dynamics of viscoelastic
membranes separating two viscous fluids in order to understand microrheological
studies of such membranes. We demonstrate the general viscoelastic signatures
in the dynamics of shear, bending, and compression modes. We also find a
screening of the otherwise two-dimensional character of the response to point
forces due to the presence of solvent. Finally, we show that there is a linear,
hydrodynamic coupling between the in-plane compression modes of the membrane
and the out-of-plane bending modes in the case where the membrane separates two
different fluids or environments
Discovery of orbital decay in SMC X-1
The results are reported of three observations of the binary X ray pulsar SMC X-1 with the Ginga satellite. Timing analyses of the 0.71 s X ray pulsations yield Doppler delay curves which, in turn, provide the most accurate determination of the SMC X-1 orbital parameters available to date. The orbital phase of the 3.9 day orbit is determined in May 1987, Aug. 1988, and Aug. 1988 with accuracies of 11, 1, and 3.5 s, respectively. These phases are combined with two previous determinations of the orbital phase to yield the rate of change in the orbital period: P sub orb/P sub orb = (-3.34 + or - 0.023) x 10(exp -6)/yr. An interpretation of this measurement and the known decay rate for the orbit of Cen X-3 is made in the context of tidal evolution. Finally, a discussion is presented of the relation among the stellar evolution, orbital decay, and neutron star spinup time scales for the SMC X-1 system
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