431 research outputs found
Numerical Modeling of Turbulent Combustion
The work in numerical modeling is focused on the use of the random vortex method to treat turbulent flow fields associated with combustion while flame fronts are considered as interfaces between reactants and products, propagating with the flow and at the same time advancing in the direction normal to themselves at a prescribed burning speed. The latter is associated with the generation of specific volume (the flame front acting, in effect, as the locus of volumetric sources) to account for the expansion of the flow field due to the exothermicity of the combustion process. The model was applied to the flow in a channel equipped with a rearward facing step. The results obtained revealed the mechanism of the formation of large scale turbulent structure in the wake of the step, while it showed the flame to stabilize on the outer edges of these eddies
Vortex spectrum in superfluid turbulence: interpretation of a recent experiment
We discuss a recent experiment in which the spectrum of the vortex line
density fluctuations has been measured in superfluid turbulence. The observed
frequency dependence of the spectrum, , disagrees with classical
vorticity spectra if, following the literature, the vortex line density is
interpreted as a measure of the vorticity or enstrophy. We argue that the
disagrement is solved if the vortex line density field is decomposed into a
polarised field (which carries most of the energy) and an isotropic field
(which is responsible for the spectrum).Comment: Submitted for publication
http://crtbt.grenoble.cnrs.fr/helio/GROUP/infa.html
http://www.mas.ncl.ac.uk/~ncfb
Determination of force platform parameters during sit to stand movement in elderly: A preliminary study
Stochastic Perturbations in Vortex Tube Dynamics
A dual lattice vortex formulation of homogeneous turbulence is developed,
within the Martin-Siggia-Rose field theoretical approach. It consists of a
generalization of the usual dipole version of the Navier-Stokes equations,
known to hold in the limit of vanishing external forcing. We investigate, as a
straightforward application of our formalism, the dynamics of closed vortex
tubes, randomly stirred at large length scales by gaussian stochastic forces.
We find that besides the usual self-induced propagation, the vortex tube
evolution may be effectively modeled through the introduction of an additional
white-noise correlated velocity field background. The resulting
phenomenological picture is closely related to observations previously reported
from a wavelet decomposition analysis of turbulent flow configurations.Comment: 16 pages + 2 eps figures, REVTeX
Heterogeneous dynamics of the three dimensional Coulomb glass out of equilibrium
The non-equilibrium relaxational properties of a three dimensional Coulomb
glass model are investigated by kinetic Monte Carlo simulations. Our results
suggest a transition from stationary to non-stationary dynamics at the
equilibrium glass transition temperature of the system. Below the transition
the dynamic correlation functions loose time translation invariance and
electron diffusion is anomalous. Two groups of carriers can be identified at
each time scale, electrons whose motion is diffusive within a selected time
window and electrons that during the same time interval remain confined in
small regions in space. During the relaxation that follows a temperature quench
an exchange of electrons between these two groups takes place and the
non-equilibrium excess of diffusive electrons initially present decreases
logarithmically with time as the system relaxes. This bimodal dynamical
heterogeneity persists at higher temperatures when time translation invariance
is restored and electron diffusion is normal. The occupancy of the two
dynamical modes is then stationary and its temperature dependence reflects a
crossover between a low-temperature regime with a high concentration of
electrons forming fluctuating dipoles and a high-temperature regime in which
the concentration of diffusive electrons is high.Comment: 10 pages, 9 figure
Formation and evolution of density singularities in hydrodynamics of inelastic gases
We use ideal hydrodynamics to investigate clustering in a gas of
inelastically colliding spheres. The hydrodynamic equations exhibit a new type
of finite-time density blowup, where the gas pressure remains finite. The
density blowups signal formation of close-packed clusters. The blowup dynamics
are universal and describable by exact analytic solutions continuable beyond
the blowup time. These solutions show that dilute hydrodynamic equations yield
a powerful effective description of a granular gas flow with close-packed
clusters, described as finite-mass point-like singularities of the density.
This description is similar in spirit to the description of shocks in ordinary
ideal gas dynamics.Comment: 4 pages, 3 figures, final versio
Roughness-induced critical phenomena in a turbulent flow
I present empirical evidence that turbulent flows are closely analogous to
critical phenomena, from a reanalysis of friction factor measurements in rough
pipes. The data collapse found here corresponds to Widom scaling near critical
points, and implies that a full understanding of turbulence requires explicit
accounting for boundary roughness
Dimension reduction for systems with slow relaxation
We develop reduced, stochastic models for high dimensional, dissipative
dynamical systems that relax very slowly to equilibrium and can encode long
term memory. We present a variety of empirical and first principles approaches
for model reduction, and build a mathematical framework for analyzing the
reduced models. We introduce the notions of universal and asymptotic filters to
characterize `optimal' model reductions for sloppy linear models. We illustrate
our methods by applying them to the practically important problem of modeling
evaporation in oil spills.Comment: 48 Pages, 13 figures. Paper dedicated to the memory of Leo Kadanof
Continuous, Semi-discrete, and Fully Discretized Navier-Stokes Equations
The Navier--Stokes equations are commonly used to model and to simulate flow
phenomena. We introduce the basic equations and discuss the standard methods
for the spatial and temporal discretization. We analyse the semi-discrete
equations -- a semi-explicit nonlinear DAE -- in terms of the strangeness index
and quantify the numerical difficulties in the fully discrete schemes, that are
induced by the strangeness of the system. By analyzing the Kronecker index of
the difference-algebraic equations, that represent commonly and successfully
used time stepping schemes for the Navier--Stokes equations, we show that those
time-integration schemes factually remove the strangeness. The theoretical
considerations are backed and illustrated by numerical examples.Comment: 28 pages, 2 figure, code available under DOI: 10.5281/zenodo.998909,
https://doi.org/10.5281/zenodo.99890
A new doubly discrete analogue of smoke ring flow and the real time simulation of fluid flow
Modelling incompressible ideal fluids as a finite collection of vortex
filaments is important in physics (super-fluidity, models for the onset of
turbulence) as well as for numerical algorithms used in computer graphics for
the real time simulation of smoke. Here we introduce a time-discrete evolution
equation for arbitrary closed polygons in 3-space that is a discretisation of
the localised induction approximation of filament motion. This discretisation
shares with its continuum limit the property that it is a completely integrable
system. We apply this polygon evolution to a significant improvement of the
numerical algorithms used in Computer Graphics.Comment: 15 pages, 3 figure
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