837 research outputs found
Breakdown of Hydrodynamics in a Simple One-Dimensional Fluid
We investigate the behavior of a one-dimensional diatomic fluid under a shock
wave excitation. We find that the properties of the resulting shock wave are in
striking contrast with those predicted by hydrodynamic and kinetic approaches,
e.g., the hydrodynamic profiles relax algebraically toward their equilibrium
values. Deviations from local thermodynamic equilibrium are persistent,
decaying as a power law of the distance to the shock layer. Non-equipartition
is observed infinitely far from the shock wave, and the velocity-distribution
moments exhibit multiscaling. These results question the validity of simple
hydrodynamic theories to understand collective behavior in 1d fluids.Comment: 4 pages, 5 figure
Modeling M-Theory Vacua via Gauged S-Duality
We construct a model of M-theory vacua using gauged S-duality and the
Chan-Paton symmetries by introducing an infinite number of open string charges.
In the Bechi-Rouet-Stora-Tyutin formalism, the local description of the gauged
S-duality on its moduli space of vacua is fully determined by one physical
state condition on the vacua. We introduce the string probe of the spatial
degrees of freedom and define the increment of the cosmic time. The
dimensionality of space-time and the gauge group of the low energy effective
theory originate in the symmetries (with or without their breakdown) in our
model. This modeling leads to the derived category formulation of the quantum
mechanical world including gravity and to the concept of a non-linear potential
of gauged and affinized S-duality which specifies the morphism structure of
this derived category.Comment: 31 pages, version reflecting the erratum. arXiv admin note:
substantial text overlap with arXiv:1102.460
Nonresponse of native Cottonwood trees to water additions during summer drought
Studies have demonstrated that some riparian trees may switch their reliance on surface soil water (unsaturated or vadose zone) to groundwater (saturated zone) sources during the growing season in association with changes in moisture availability. A closely related question is: How do these trees respond to pulse increases in water availability in previously dry zones? We tested the whole-tree physiological response of 6 natural Populus genotypes to water additions during the peak of summer drought in northern Utah, USA. We found clear evidence that trees were insensitive to water additions to the surface soil that were twice the magnitude of whole-tree transpiration rates. Our results suggest that some cottonwoods may have little immediate transpiration of leaf conductance response to pulse soil moisture increases. This lack of response may be related to water-use strategy associated with regional climate patterns (i.e. genetic or environmental programming), cavitation recovery, or other physical determinants of water use such as depth to groundwater. Our data suggest that it is important to consider potential nonresponsiveness to changes in soil water availability when evaluating the impact of climate change on these important and productive ecosystems
The Modulation of Multiple Phases Leading to the Modified KdV Equation
This paper seeks to derive the modified KdV (mKdV) equation using a novel
approach from systems generated from abstract Lagrangians that possess a
two-parameter symmetry group. The method to do uses a modified modulation
approach, which results in the mKdV emerging with coefficients related to the
conservation laws possessed by the original Lagrangian system. Alongside this,
an adaptation of the method of Kuramoto is developed, providing a simpler
mechanism to determine the coefficients of the nonlinear term. The theory is
illustrated using two examples of physical interest, one in stratified
hydrodynamics and another using a coupled Nonlinear Schr\"odinger model, to
illustrate how the criterion for the mKdV equation to emerge may be assessed
and its coefficients generated.Comment: 35 pages, 5 figure
A conditional trophic cascade: Birds benefit faster growing trees with strong links between predators and plants
Terrestrial systems are thought to be organized predominantly from the bottom-up, but there is a growing literature documenting top-down trophic cascades under certain ecological conditions. We conducted an experiment to examine how arthropod community structure on a foundation riparian tree mediates the ability of insectivorous birds to influence tree growth. We built whole-tree bird exclosures around 35 mature cottonwood (Populus spp.) trees at two sites in northern Utah, USA, to measure the effect of bird predation on arthropod herbivore and predator species richness, abundance, and biomass, and on tree performance. We maintained bird exclosures over two growing seasons and conducted nondestructive arthropod surveys that recorded 63652 arthropods of 689 morphospecies representing 19 orders. Five major patterns emerged: (1) We found a significant trophic cascade (18% reduction in trunk growth when birds were excluded) only at one site in one year. (2) The significant trophic cascade was associated with higher precipitation, tree growth, and arthropod abundance, richness, and biomass than other site–year combinations. (3) The trophic cascade was weak or not evident when tree growth and insect populations were low apparently due to drought. (4) Concurrent with the stronger trophic cascade, bird predation significantly reduced total arthropod abundance, richness, and biomass. Arthropod biomass was 67% greater on trees without bird predation. This pattern was driven largely by two herbivore groups (folivores and non-aphid sap-feeders) suggesting that birds targeted these groups. (5) Three species of folivores (Orthoptera: Melanoplus spp.) were strong links between birds and trees and were only present in the site and the year in which the stronger trophic cascade occurred. Our results suggest that this trophic system is predominately bottom-up driven, but under certain conditions the influence of top predators can stimulate whole tree growth. When the most limiting factor for tree growth switched from water availability to herbivory, the avian predators gained the potential to reduce herbivory. This potential could be realized when strong links between the birds and plant, i.e., species that were both abundant herbivores and preferred prey, were present
Thermodynamic phase transitions and shock singularities
We show that under rather general assumptions on the form of the entropy
function, the energy balance equation for a system in thermodynamic equilibrium
is equivalent to a set of nonlinear equations of hydrodynamic type. This set of
equations is integrable via the method of the characteristics and it provides
the equation of state for the gas. The shock wave catastrophe set identifies
the phase transition. A family of explicitly solvable models of
non-hydrodynamic type such as the classical plasma and the ideal Bose gas are
also discussed.Comment: revised version, 18 pages, 6 figure
Towards a continuum theory of clustering in a freely cooling inelastic gas
We performed molecular dynamics simulations to investigate the clustering
instability of a freely cooling dilute gas of inelastically colliding disks in
a quasi-one-dimensional setting. We observe that, as the gas cools, the shear
stress becomes negligibly small, and the gas flows by inertia only. Finite-time
singularities, intrinsic in such a flow, are arrested only when close-packed
clusters are formed. We observe that the late-time dynamics of this system are
describable by the Burgers equation with vanishing viscosity, and predict the
long-time coarsening behavior.Comment: 7 pages, 5 eps figures, to appear in Europhys. Let
Attempted density blowup in a freely cooling dilute granular gas: hydrodynamics versus molecular dynamics
It has been recently shown (Fouxon et al. 2007) that, in the framework of
ideal granular hydrodynamics (IGHD), an initially smooth hydrodynamic flow of a
granular gas can produce an infinite gas density in a finite time. Exact
solutions that exhibit this property have been derived. Close to the
singularity, the granular gas pressure is finite and almost constant. This work
reports molecular dynamics (MD) simulations of a freely cooling gas of nearly
elastically colliding hard disks, aimed at identifying the "attempted" density
blowup regime. The initial conditions of the simulated flow mimic those of one
particular solution of the IGHD equations that exhibits the density blowup. We
measure the hydrodynamic fields in the MD simulations and compare them with
predictions from the ideal theory. We find a remarkable quantitative agreement
between the two over an extended time interval, proving the existence of the
attempted blowup regime. As the attempted singularity is approached, the
hydrodynamic fields, as observed in the MD simulations, deviate from the
predictions of the ideal solution. To investigate the mechanism of breakdown of
the ideal theory near the singularity, we extend the hydrodynamic theory by
accounting separately for the gradient-dependent transport and for finite
density corrections.Comment: 11 pages, 9 figures, accepted for publication on Physical Review
Kink Solution in a Fluid Model of Traffic Flows
Traffic jam in a fluid model of traffic flows proposed by Kerner and
Konh\"auser (B. S. Kerner and P. Konh\"auser, Phys. Rev. E 52 (1995), 5574.) is
analyzed. An analytic scaling solution is presented near the critical point of
the hetero-clinic bifurcation. The validity of the solution has been confirmed
from the comparison with the simulation of the model.Comment: RevTeX v3.1, 6 pages, and 2 figure
Can Naked Singularities Yield Gamma Ray Bursts?
Gamma-ray bursts are believed to be the most luminous objects in the
Universe. There has been some suggestion that these arise from quantum
processes around naked singularities. The main problem with this suggestion is
that all known examples of naked singularities are massless and hence there is
effectively no source of energy. It is argued that a globally naked singularity
coupled with quantum processes operating within a distance of the order of
Planck length of the singularity will probably yield energy burst of the order
of M_pc^2\approx2\times 10^{16} ergs, where M_p is the Planck mass.Comment: 4 pages, TeX, no figure
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