48,989 research outputs found
A filamentation instability for streaming cosmic-rays
We demonstrate that cosmic rays form filamentary structures in the precursors
of supernova remnant shocks due to their self-generated magnetic fields. The
cosmic-ray filamentation results in the growth of a long wavelength
instability, and naturally couples the rapid non-linear amplification on small
scales to larger length scales. Hybrid magnetohydrodynamics--particle
simulations are performed to confirm the effect. The resulting large scale
magnetic field may facilitate the scattering of high energy cosmic rays as
required to accelerate protons beyond the knee in the cosmic-ray spectrum at
supernova remnant shocks. Filamentation far upstream of the shock may also
assist in the escape of cosmic rays from the accelerator.Comment: Accepted for publication in MNRA
Calibration of the CH and CN Variations Among Main Sequence Stars in M71 and in M13
An analysis of the CN and CH band strengths measured in a large sample of M71
and M13 main sequence stars by Cohen (1999a,b) is undertaken using synthetic
spectra to quantify the underlying C and N abundances. In the case of M71 it is
found that the observed CN and CH band strengths are best matched by the
{\it{identical}} C/N/O abundances which fit the bright giants, implying: 1)
little if any mixing is taking place during red giant branch ascent in M71, and
2) a substantial component of the C and N abundance inhomogeneities is in place
before the main sequence turn-off. The unlikelihood of mixing while on the main
sequence requires an explanation for the abundance variations which lies
outside the present stars (primordial inhomogeneities or intra-cluster self
enrichment). For M13 it is shown that the 3883\AA CN bands are too weak to be
measured in the spectra for any reasonable set of expected compositions. A
similar situation exists for CH as well. However, two of the more luminous
program stars do appear to have C abundances considerably greater than those
found among the bright giants thereby suggesting deep mixing has taken place on
the M13 red giant branch.Comment: 14 pages, 4 figures, accepted for publication by A
Computer program to determine the irrotational nozzle admittance
Irrotational nozzle admittance is the boundary condition that must be satisfied by combustor flow oscillations at nozzle entrance. Defined as the ratio of axial velocity perturbation to the pressure perturbation at nozzle entrance, nozzle admittance can also be used to determine whether wave motion in nozzle under consideration adds or removes energy from combustor oscillations
Does a Flexible Industry Wage Structure Increase Employment?: The U.S. Experience
This paper examines the flexibility of wages across industries inthe U.S. and seeks to determine the potential impact which changes in the industrial wage structure may have for employment. With regard to the flexibility of wages across industries, we find that the U.S., alone among the major OECD countries, has experienced substantial changes in the industry wage structure since 1970, with the variation of log wages among industries increasing dramatically, particularly in the 1970s. This represents a widening of the gap between wages in the high and low wage sectors. In order to evaluate these changes, we estimate equations linking changes in industry wages over an extended period of time to a variety of potential wage determining characteristics. We find that industrial wages are positively correlated with value productivity per worker, even after controlling for institutional and supply side factors which may have contributed to the increased dispersion of wages in the 1970s. Our results are not consistent with the standard competitive model of industry labor markets, in which wages and productivity are uncorrelated across sectors and wages depend on aggregate, rather than sectoral conditions.With regard to the impact of a flexible industry wage structure on employment, we evaluate the circumstances under which flexible wages among industries may be employment enhancing, and the set of circumstances under which flexible wages are likely to be employment reducing. For the U.S.economy in the 1970s we find that the data support the latter set of circumstances. The bottom line of the U.S. experience is that flexible wages by industry have not contributed to employment growth.
Modelling shared space users via rule-based social force model
The promotion of space sharing in order to raise the quality of community living and safety of street surroundings is increasingly accepted feature of modern urban design. In this context, the development of a shared space simulation tool is essential in helping determine whether particular shared space schemes are suitable alternatives to traditional street layouts. A simulation tool that enables urban designers to visualise pedestrians and cars trajectories, extract flow and density relation in a new shared space design and achieve solutions for optimal design features before implementation. This paper presents a three-layered microscopic mathematical model which is capable of representing the behaviour of pedestrians and vehicles in shared space layouts and it is implemented in a traffic simulation tool. The top layer calculates route maps based on static obstacles in the environment. It plans the shortest path towards agents' respective destinations by generating one or more intermediate targets. In the second layer, the Social Force Model (SFM) is modified and extended for mixed traffic to produce feasible trajectories. Since vehicle movements are not as flexible as pedestrian movements, velocity angle constraints are included for vehicles. The conflicts described in the third layer are resolved by rule-based constraints for shared space users. An optimisation algorithm is applied to determine the interaction parameters of the force-based model for shared space users using empirical data. This new three-layer microscopic model can be used to simulate shared space environments and assess, for example, new street designs
Towards the Distributed Burning Regime in Turbulent Premixed Flames
Three-dimensional numerical simulations of canonical statistically-steady
statistically-planar turbulent flames have been used in an attempt to produce
distributed burning in lean methane and hydrogen flames. Dilatation across the
flame means that extremely large Karlovitz numbers are required; even at the
extreme levels of turbulence studied (up to a Karlovitz number of 8767)
distributed burning was only achieved in the hydrogen case. In this case,
turbulence was found to broaden the reaction zone visually by around an order
of magnitude, and thermodiffusive effects (typically present for lean hydrogen
flames) were not observed. In the preheat zone, the species compositions differ
considerably from those of one-dimensional flames based a number of different
transport models (mixture-averaged, unity Lewis number, and a turbulent eddy
viscosity model). The behaviour is a characteristic of turbulence dominating
non-unity Lewis number species transport, and the distinct limit is again
attributed to dilatation and its effect on the turbulence. Peak local reaction
rates are found to be lower in the distributed case than in the lower Karlovitz
cases but higher than in the laminar flame, which is attributed to effects that
arise from the modified fuel-temperature distribution that results from
turbulent mixing dominating low Lewis number thermodiffusive effects. Finally,
approaches to achieve distributed burning at realisable conditions are
discussed; factors that increase the likelihood of realising distributed
burning are higher pressure, lower equivalence ratio, higher Lewis number, and
lower reactant temperature
Diffusive Transport Enhanced by Thermal Velocity Fluctuations
We study the contribution of advection by thermal velocity fluctuations to
the effective diffusion coefficient in a mixture of two indistinguishable
fluids. The enhancement of the diffusive transport depends on the system size L
and grows as \ln(L/L_0) in quasi two-dimensional systems, while in three
dimensions it scales as L_0^{-1}-L^{-1}, where L_0 is a reference length. The
predictions of a simple fluctuating hydrodynamics theory are compared to
results from particle simulations and a finite-volume solver and excellent
agreement is observed. Our results conclusively demonstrate that the nonlinear
advective terms need to be retained in the equations of fluctuating
hydrodynamics when modeling transport in small-scale finite systems.Comment: To appear in Phys. Rev. Lett., 201
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