128 research outputs found
Asymptotic behaviour of the Rayleigh--Taylor instability
We investigate long time numerical simulations of the inviscid
Rayleigh-Taylor instability at Atwood number one using a boundary integral
method. We are able to attain the asymptotic behavior for the spikes predicted
by Clavin & Williams\cite{clavin} for which we give a simplified demonstration.
In particular we observe that the spike's curvature evolves like while
the overshoot in acceleration shows a good agreement with the suggested
law. Moreover, we obtain consistent results for the prefactor coefficients of
the asymptotic laws. Eventually we exhibit the self-similar behavior of the
interface profile near the spike.Comment: 4 pages, 6 figure
Linear relaxation to planar Travelling Waves in Inertial Confinement Fusion
We study linear stability of planar travelling waves for a scalar
reaction-diffusion equation with non-linear anisotropic diffusion. The
mathematical model is derived from the full thermo-hydrodynamical model
describing the process of Inertial Confinement Fusion. We show that solutions
of the Cauchy problem with physically relevant initial data become planar
exponentially fast with rate s(\eps',k)>0, where
\eps'=\frac{T_{min}}{T_{max}}\ll 1 is a small temperature ratio and
the transversal wrinkling wavenumber of perturbations. We rigorously recover in
some particular limit (\eps',k)\rightarrow (0,+\infty) a dispersion relation
s(\eps',k)\sim \gamma_0 k^{\alpha} previously computed heuristically and
numerically in some physical models of Inertial Confinement Fusion
Model Equation for the Dynamics of Wrinkled Shockwaves: Comparison with DNS and Experiments
International audienceA model equation for the dynamics and the geometry of the wrinkled front of shock waves, obtained for strong shocks in the Newtonian limit, is tested by comparison with direct numerical simulations and a shock tube experiment
A Variational Principle Based Study of KPP Minimal Front Speeds in Random Shears
Variational principle for Kolmogorov-Petrovsky-Piskunov (KPP) minimal front
speeds provides an efficient tool for statistical speed analysis, as well as a
fast and accurate method for speed computation. A variational principle based
analysis is carried out on the ensemble of KPP speeds through spatially
stationary random shear flows inside infinite channel domains. In the regime of
small root mean square (rms) shear amplitude, the enhancement of the ensemble
averaged KPP front speeds is proved to obey the quadratic law under certain
shear moment conditions. Similarly, in the large rms amplitude regime, the
enhancement follows the linear law. In particular, both laws hold for the
Ornstein-Uhlenbeck process in case of two dimensional channels. An asymptotic
ensemble averaged speed formula is derived in the small rms regime and is
explicit in case of the Ornstein-Uhlenbeck process of the shear. Variational
principle based computation agrees with these analytical findings, and allows
further study on the speed enhancement distributions as well as the dependence
of enhancement on the shear covariance. Direct simulations in the small rms
regime suggest quadratic speed enhancement law for non-KPP nonlinearities.Comment: 28 pages, 14 figures update: fixed typos, refined estimates in
section
Nonsteady condensation and evaporation waves
We study motion of a phase transition front at a constant temperature between
stable and metastable states in fluids with the universal Van der Waals
equation of state (which is valid sufficiently close to the fluid's critical
point). We focus on a case of relatively large metastability and low viscosity,
when it can be shown analytically that no steadily moving phase-transition
front exists. Numerically simulating a system of the one-dimensional
Navier-Stokes and continuity equations, we find that, in this case, the
nonsteady phase-transition front emits acoustic shocks in forward and backward
directions. Through this mechanism, the front drops its velocity and eventually
comes to a halt. The acoustic shock wave may shuttle, bouncing elastically from
the system's edge and strongly inelastically from the phase transition front.
Nonsteady rarefaction shock waves appear in the shuttle process, despite the
fact that the model does not admit steady rarefaction waves propagating between
stationary states. If the viscosity is below a certain threshold, an
instability sets in, driving the system into a turbulent state. This work was
supported by the Japan Society for Promotion of Science.Comment: revtex text file and four eps files with figures. Physical Review
Letters, in pres
The Thermonuclear Explosion Of Chandrasekhar Mass White Dwarfs
The flame born in the deep interior of a white dwarf that becomes a Type Ia
supernova is subject to several instabilities. We briefly review these
instabilities and the corresponding flame acceleration. We discuss the
conditions necessary for each of the currently proposed explosion mechanisms
and the attendant uncertainties. A grid of critical masses for detonation in
the range - g cm is calculated and its
sensitivity to composition explored. Prompt detonations are physically
improbable and appear unlikely on observational grounds. Simple deflagrations
require some means of boosting the flame speed beyond what currently exists in
the literature. ``Active turbulent combustion'' and multi-point ignition are
presented as two plausible ways of doing this. A deflagration that moves at the
``Sharp-Wheeler'' speed, , is calculated in one dimension
and shows that a healthy explosion is possible in a simple deflagration if the
front moves with the speed of the fastest floating bubbles. The relevance of
the transition to the ``distributed burning regime'' is discussed for delayed
detonations. No model emerges without difficulties, but detonation in the
distributed regime is plausible, will produce intermediate mass elements, and
warrants further study.Comment: 28 pages, 4 figures included, uses aaspp4.sty. Submitted to Ap
Finite size effects near the onset of the oscillatory instability
A system of two complex Ginzburg - Landau equations is considered that applies at the onset of the oscillatory instability in spatial domains whose size is large (but finite) in one direction; the dependent variables are the slowly modulated complex amplitudes of two counterpropagating wavetrains. In order to obtain a well posed problem, four boundary conditions must be imposed at the boundaries. Two of them were already known, and the other two are first derived in this paper. In the generic case when the group velocity is of order unity, the resulting problem has terms that are not of the same order of magnitude. This fact allows us to consider two distinguished limits and to derive two associated (simpler) sub-models, that are briefly discussed. Our results predict quite a rich variety of complex dynamics that is due to both the modulational instability and finite size effects
An Exploratory Study into the Factors Impeding Ethical Consumption
Although consumers are increasingly engaged with ethical factors when forming opinions about products and making purchase decisions, recent studies have highlighted significant differences between consumersâ intentions to consume ethically, and their actual purchase behaviour. This article contributes to an understanding of this âethical purchasing gapâ through a review of existing literature, and the inductive analysis of focus group discussions. A model is suggested which includes exogenous variables such as moral maturity and age which have been well covered in the literature, together with further impeding factors identified from the focus group discussions. For some consumers, inertia in purchasing behaviour was such that the decision-making process was devoid of ethical considerations. Several manifested their ethical views through post-purchase dissonance and retrospective feelings of guilt. Others displayed a reluctance to consume ethically due to personal constraints, a perceived negative impact on image or quality, or an outright negation of responsibility. Those who expressed a desire to consume ethically often seemed deterred by cynicism, which caused them to question the impact they, as an individual, could achieve. These findings enhance the understanding of ethical consumption decisions and provide a platform for future research in this area
Conservation efficiency and nutritive value of silages made from grass-red clover and multi-species swards compared with grass monocultures
peer reviewedBinary grass-clover and multi-species swards can increase herbage yields or facilitate reduced inputs of inorganic
fertiliser nitrogen (N) compared with perennial ryegrass monocultures. However, the efficiency of the ensilage
process and the nutritive value of silage produced from multi-species swards has not been documented. Replicate
samples from grass-red clover binary mixture and multi-species mixture swards were ensiled in laboratory silos to
assess the ensilability, fermentation characteristics, conservation losses and silage nutritive value compared with
grass monocultures produced using inorganic N fertiliser. The results suggest that assessment of the ensilability
and subsequent ensilage characteristics of binary and multi-species mixtures should be based on direct sampling
from such mixtures rather than being predicted from values obtained from monocultures of constituent species.
Under favourable ensiling conditions, unwilted binary mixtures and multi-species mixtures are satisfactorily
preserved as silage, comparable to a perennial ryegrass monoculture receiving inorganic N fertiliser. However,
when ensiled under more challenging crop conditions the mixtures exhibited a greater requirement for their
preservation to be aided, compared with the perennial ryegrass monoculture. Despite the application of inorganic N
reducing the legume content of multi-species mixture swards, it had relatively little effect on herbage ensilability or
silage preservation. For all species treatments, silage nutritive values were primarily dependent on the pre-ensiling
values, although herbage digestibility values declined during ensilage where the ensilage process was inefficient.
The current study suggests that in order to be satisfactorily preserved as silage, binary grass-clover and multispecies
swards have a greater requirement for an adequate rapid field wilt and/or effective preservative application
compared with perennial ryegrass produced using inorganic fertiliser N
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