594 research outputs found
Topological Shocks in Burgers Turbulence
The dynamics of the multi-dimensional randomly forced Burgers equation is
studied in the limit of vanishing viscosity. It is shown both theoretically and
numerically that the shocks have a universal global structure which is
determined by the topology of the configuration space. This structure is shown
to be particularly rigid for the case of periodic boundary conditions.Comment: 4 pages, 4 figures, RevTex4, published versio
Turbulence attenuation by large neutrally buoyant particles
Turbulence modulation by inertial-range-size, neutrally-buoyant particles is
investigated experimentally in a von K\'arm\'an flow. Increasing the particle
volume fraction , maintaining constant impellers Reynolds
number attenuates the fluid turbulence. The inertial-range energy transfer rate
decreases as , suggesting that only particles
located on a surface affect the flow. Small-scale turbulent properties, such as
structure functions or acceleration distribution, are unchanged. Finally,
measurements hint at the existence of a transition between two different
regimes occurring when the average distance between large particles is of the
order of the thickness of their boundary layers.Comment: 7 pages, 4 figure
Universality of Velocity Gradients in Forced Burgers Turbulence
It is demonstrated that Burgers turbulence subject to large-scale
white-noise-in-time random forcing has a universal power-law tail with exponent
-7/2 in the probability density function of negative velocity gradients, as
predicted by E, Khanin, Mazel and Sinai (1997, Phys. Rev. Lett. 78, 1904). A
particle and shock tracking numerical method gives about five decades of
scaling. Using a Lagrangian approach, the -7/2 law is related to the shape of
the unstable manifold associated to the global minimizer.Comment: 4 pages, 2 figures, RevTex4, published versio
Dynamics and statistics of heavy particles in turbulent flows
We present the results of Direct Numerical Simulations (DNS) of turbulent
flows seeded with millions of passive inertial particles. The maximum Taylor's
Reynolds number is around 200. We consider particles much heavier than the
carrier flow in the limit when the Stokes drag force dominates their dynamical
evolution. We discuss both the transient and the stationary regimes. In the
transient regime, we study the growt of inhomogeneities in the particle spatial
distribution driven by the preferential concentration out of intense vortex
filaments. In the stationary regime, we study the acceleration fluctuations as
a function of the Stokes number in the range [0.16:3.3]. We also compare our
results with those of pure fluid tracers (St=0) and we find a critical behavior
of inertia for small Stokes values. Starting from the pure monodisperse
statistics we also characterize polydisperse suspensions with a given mean
Stokes.Comment: 13 pages, 10 figures, 2 table
Clustering of passive impurities in MHD turbulence
The transport of heavy, neutral or charged, point-like particles by
incompressible, resistive magnetohydrodynamic (MHD) turbulence is investigated
by means of high-resolution numerical simulations. The spatial distribution of
such impurities is observed to display strong deviations from homogeneity, both
at dissipative and inertial range scales. Neutral particles tend to cluster in
the vicinity of coherent vortex sheets due to their viscous drag with the flow,
leading to the simultaneous presence of very concentrated and almost empty
regions. The signature of clustering is different for charged particles. These
exhibit in addition to the drag the Lorentz-force. The regions of spatial
inhomogeneities change due to attractive and repulsive vortex sheets. While
small charges increase clustering, larger charges have a reverse effect.Comment: 9 pages, 13 figure
Population dynamics at high Reynolds number
We study the statistical properties of population dynamics evolving in a
realistic two-dimensional compressible turbulent velocity field. We show that
the interplay between turbulent dynamics and population growth and saturation
leads to quasi-localization and a remarkable reduction in the carrying
capacity. The statistical properties of the population density are investigated
and quantified via multifractal scaling analysis. We also investigate
numerically the singular limit of negligibly small growth rates and
delocalization of population ridges triggered by uniform advection.Comment: 5 pages, 5 figure
Geometry and violent events in turbulent pair dispersion
The statistics of Lagrangian pair dispersion in a homogeneous isotropic flow
is investigated by means of direct numerical simulations. The focus is on
deviations from Richardson eddy-diffusivity model and in particular on the
strong fluctuations experienced by tracers. Evidence is obtained that the
distribution of distances attains an almost self-similar regime characterized
by a very weak intermittency. The timescale of convergence to this behavior is
found to be given by the kinetic energy dissipation time measured at the scale
of the initial separation. Conversely the velocity differences between tracers
are displaying a strongly anomalous behavior whose scaling properties are very
close to that of Lagrangian structure functions. These violent fluctuations are
interpreted geometrically and are shown to be responsible for a long-term
memory of the initial separation. Despite this strong intermittency, it is
found that the mixed moment defined by the ratio between the cube of the
longitudinal velocity difference and the distance attains a statistically
stationary regime on very short timescales. These results are brought together
to address the question of violent events in the distribution of distances. It
is found that distances much larger than the average are reached by pairs that
have always separated faster since the initial time. They contribute a
stretched exponential behavior in the tail of the inter-tracer distance
probability distribution. The tail approaches a pure exponential at large
times, contradicting Richardson diffusive approach. At the same time, the
distance distribution displays a time-dependent power-law behavior at very
small values, which is interpreted in terms of fractal geometry. It is argued
and demonstrated numerically that the exponent converges to one at large time,
again in conflict with Richardson's distribution.Comment: 21 page
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