39,445 research outputs found
An analysis of turbulent diffusion flame in axisymmetric jet
The kinetic theory of turbulent flow was employed to study the mixing limited combustion of hydrogen in axisymmetric jets. The integro-differential equations in two spatial and three velocity coordinates describing the combustion were reduced to a set of hyperbolic partial differential equations in the two spatial coordinates by a binodal approximation. The MacCormick's finite difference method was then employed for solution. The flame length was longer than that predicted by the flame-sheet analysis, and was found to be in general agreement with a recent experimental result. Increase of the turbulence energy and scale resulted in an enhancement of the combustion rate and, hence, in a shorter flame length. Details of the numerical method as well as of the physical findings are discussed
Upper Bounds for the Critical Car Densities in Traffic Flow Problems
In most models of traffic flow, the car density is the only free
parameter in determining the average car velocity . The
critical car density , which is defined to be the car density separating
the jamming phase (with ) and the moving phase (with
), is an important physical quantity to investigate. By
means of simple statistical argument, we show that for the
Biham-Middleton-Levine model of traffic flow in two or higher spatial
dimensions. In particular, we show that in 2 dimension and
in () dimensions.Comment: REVTEX 3.0, 5 pages with 1 figure appended at the back, Minor
revision, to be published in the Sept issue of J.Phys.Soc.Japa
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Regulation of Wages and Hours Prior to 1938
Direct numerical simulations are performed to investigate the transient upstream propagation (flashback) of premixed hydrogen–air flames in the boundary layer of a fully developed turbulent channel flow. Results show that the well-known near-wall velocity fluctuations pattern found in turbulent boundary layers triggers wrinkling of the initially flat flame sheet as it starts propagating against the main flow direction, and that the structure of the characteristic streaks of the turbulent boundary layer ultimately has an important impact on the resulting flame shape and on its propagation mechanism. It is observed that the leading edges of the upstream-propagating premixed flame are always located in the near-wall region of the channel and assume the shape of several smooth, curved bulges propagating upstream side by side in the spanwise direction and convex towards the reactant side of the flame. These leading-edge flame bulges are separated by thin regions of spiky flame cusps pointing towards the product side at the trailing edges of the flame. Analysis of the instantaneous velocity fields clearly reveals the existence, on the reactant side of the flame sheet, of backflow pockets that extend well above the wall-quenching distance. There is a strong correspondence between each of the backflow pockets and a leading edge convex flame bulge. Likewise, high-speed streaks of fast flowing fluid are found to be always colocated with the spiky flame cusps pointing towards the product side of the flame. It is suggested that the origin of the formation of the backflow pockets, along with the subsequent mutual feedback mechanism, is due to the interaction of the approaching streaky turbulent flow pattern with the Darrieus–Landau hydrodynamic instability and pressure fluctuations triggered by the flame sheet. Moreover, the presence of the backflow pockets, coupled with the associated hydrodynamic instability and pressure–flow field interaction, greatly facilitate flame propagation in turbulent boundary layers and ultimately results in high flashback velocities that increase proportionately with pressure
Adjacency labeling schemes and induced-universal graphs
We describe a way of assigning labels to the vertices of any undirected graph
on up to vertices, each composed of bits, such that given the
labels of two vertices, and no other information regarding the graph, it is
possible to decide whether or not the vertices are adjacent in the graph. This
is optimal, up to an additive constant, and constitutes the first improvement
in almost 50 years of an bound of Moon. As a consequence, we
obtain an induced-universal graph for -vertex graphs containing only
vertices, which is optimal up to a multiplicative constant,
solving an open problem of Vizing from 1968. We obtain similar tight results
for directed graphs, tournaments and bipartite graphs
Solutions of Conformal Turbulence on a Half Plane
Exact solutions of conformal turbulence restricted on a upper half plane are
obtained. We show that the inertial range of homogeneous and isotropic
turbulence with constant enstrophy flux develops in a distant region from the
boundary. Thus in the presence of an anisotropic boundary, these exact
solutions of turbulence generalize Kolmogorov's solution consistently and
differ from the Polyakov's bulk case which requires a fine tunning of
coefficients. The simplest solution in our case is given by the minimal model
of and moreover we find a fixed point of solutions when
become large.Comment: 10pages, KHTP-93-07, SNUCTP-93-3
Erratum: Dynamics and scaling in a quantum spin chain material with bond randomness
Follow-up neutron measurements, performed on a sample much larger than the
one used in the original study, show that in the energy range 0.5-45 meV the
magnetic excitations in BaCu2SiGeO7 are indistinguishable from those in
conventional (disorder-free) quantum S=1/2 chains. Scrutinizing the previous
data, we found that the analysis was affected by a poorly identified structured
background and an additional technical mistake in the data reduction.Comment: This is a complete withdrawal of the original paper, also published
as in Phys. Rev. Lett 93, 077206 (2004). One page, one figur
Non-monotonic temperature dependent transport in graphene grown by Chemical Vapor Deposition
Temperature-dependent resistivity of graphene grown by chemical vapor
deposition (CVD) is investigated. We observe in low mobility CVD graphene
device a strong insulating behavior at low temperatures and a metallic behavior
at high temperatures manifesting a non-monotonic in the temperature dependent
resistivity.This feature is strongly affected by carrier density modulation. To
understand this anomalous temperature dependence, we introduce thermal
activation of charge carriers in electron-hole puddles induced by randomly
distributed charged impurities. Observed temperature evolution of resistivity
is then understood from the competition among thermal activation of charge
carriers, temperature-dependent screening and phonon scattering effects. Our
results imply that the transport property of transferred CVD-grown graphene is
strongly influenced by the details of the environmentComment: 7 pages, 3 figure
Inverse Avalanches On Abelian Sandpiles
A simple and computationally efficient way of finding inverse avalanches for
Abelian sandpiles, called the inverse particle addition operator, is presented.
In addition, the method is shown to be optimal in the sense that it requires
the minimum amount of computation among methods of the same kind. The method is
also conceptually nice because avalanche and inverse avalanche are placed in
the same footing.Comment: 5 pages with no figure IASSNS-HEP-94/7
Cuscuton: A Causal Field Theory with an Infinite Speed of Sound
We introduce a model of scalar field dark energy, Cuscuton, which can be
realized as the incompressible (or infinite speed of sound) limit of a scalar
field theory with a non-canonical kinetic term (or k-essence). Even though
perturbations of Cuscuton propagate superluminally, we show that they have a
locally degenerate phase space volume (or zero entropy), implying that they
cannot carry any microscopic information, and thus the theory is causal. Even
coupling to ordinary scalar fields cannot lead to superluminal signal
propagation. Furthermore, we show that the family of constant field
hypersurfaces are the family of Constant Mean Curvature (CMC) hypersurfaces,
which are the analogs of soap films (or soap bubbles) in a Euclidian space.
This enables us to find the most general solution in 1+1 dimensions, whose
properties motivate conjectures for global degeneracy of the phase space in
higher dimensions. Finally, we show that the Cuscuton action can model the
continuum limit of the evolution of a field with discrete degrees of freedom
and argue why it is protected against quantum corrections at low energies.
While this paper mainly focuses on interesting features of Cuscuton in a
Minkowski spacetime, a companion paper (astro-ph/0702002) examines cosmology
with Cuscuton dark energy.Comment: 11 pages, 1 figure, added discussion of "coupled cuscuton", matches
the published version in PR
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