933 research outputs found
On the self-similarity of line segments in decaying homogeneous isotropic turbulence
The self-similarity of a passive scalar in homogeneous isotropic decaying
turbulence is investigated by the method of line segments (M. Gauding et al.,
Physics of Fluids 27.9 (2015): 095102). The analysis is based on a highly
resolved direct numerical simulation of decaying turbulence. The method of line
segments is used to perform a decomposition of the scalar field into smaller
sub-units based on the extremal points of the scalar along a straight line.
These sub-units (the so-called line segments) are parameterized by their length
and the difference of the scalar field between the ending
points. Line segments can be understood as thin local convective-diffusive
structures in which diffusive processes are enhanced by compressive strain.
From DNS, it is shown that the marginal distribution function of the
length~ assumes complete self-similarity when re-scaled by the mean
length . The joint statistics of and , from which
the local gradient can be defined, play an important role
in understanding the turbulence mixing and flow structure. Large values of
occur at a small but finite length scale. Statistics of are characterized
by rare but strong deviations that exceed the standard deviation by more than
one order of magnitude. It is shown that these events break complete
self-similarity of line segments, which confirms the standard paradigm of
turbulence that intense events (which are known as internal intermittency) are
not self-similar
Lagrangian Structure Functions in Turbulence: A Quantitative Comparison between Experiment and Direct Numerical Simulation
A detailed comparison between data from experimental measurements and
numerical simulations of Lagrangian velocity structure functions in turbulence
is presented. By integrating information from experiments and numerics, a
quantitative understanding of the velocity scaling properties over a wide range
of time scales and Reynolds numbers is achieved. The local scaling properties
of the Lagrangian velocity increments for the experimental and numerical data
are in good quantitative agreement for all time lags. The degree of
intermittency changes when measured close to the Kolmogorov time scales or at
larger time lags. This study resolves apparent disagreements between experiment
and numerics.Comment: 13 RevTeX pages (2 columns) + 8 figures include
On the Scales of Turbulent Motion at High Reynolds Numbers
Turbulence is a physical state of a fluid far from equilibrium. In turbulent flows, a huge number of degrees of freedom is excited and a wide range of interacting scales determines the flow characteristics. Turbulent flows are nonlinear and non-local. They exhibit chaotic spatial and temporal dynamics and extreme events are likely to occur. Up to today, there is no unified theory of turbulence, very few exact predictions from the governing equations are available and the precise predictability of the behavior of turbulent flows is limited. Additionally, it is not known exactly, how the flow quantities depend on the turbulent flow�s vigorousness that is given by the so-called Reynolds number. In this thesis, high-Reynolds number turbulence and its dependencies on the Reynolds number are investigated by the means of hot-wire measurements in the Variable Density Turbulence Tunnel at the Max-Planck-Institute for Dynamics and Self-Organization in G?ttingen. The Reynolds number dependence of the decay exponent of freely decaying turbulence is found to be consistent with Saffmans prediction. Furthermore, with extremely long datasets, the statistical properties of turbulence in the inertial range are investigated in great detail, finding deviations from the expected scaling behavior
Long Term Evolution of Magnetic Turbulence in Relativistic Collisionless Shocks: Electron-Positron Plasmas
We study the long term evolution of magnetic fields generated by a
collisionless relativistic shock which is initially unmagnetized. Our
2D particle-in-cell numerical simulations show that downstream of such a
Weibel-mediated shock, particle distributions are close to isotropic,
relativistic Maxwellians, and the magnetic turbulence is highly intermittent
spatially, with the non-propagating magnetic fields forming relatively isolated
regions with transverse dimension skin depths. These structures
decay in amplitude, with little sign of downstream merging. The fields start
with magnetic energy density of the upstream kinetic energy
within the shock transition, but rapid downstream decay drives the fields to
much smaller values, below of equipartition after skin depths.
In an attempt to construct a theory that follows field decay to these smaller
values, we explore the hypothesis that the observed damping is a variant of
Landau damping in an unmagnetized plasma. The model is based on the small value
of the downstream magnetic energy density, which suggests that particle orbits
are only weakly perturbed from straight line motion, if the turbulence is
homogeneous. Using linear kinetic theory applied to electromagnetic fields in
an isotropic, relativistic Maxwellian plasma, we find a simple analytic form
for the damping rates, , in two and three dimensions for small
amplitude, subluminous electromagnetic fields. We find that magnetic energy
does damp due to phase mixing of current carrying particles as with . (abridged)Comment: 10 pages, 6 figures, accepted to ApJ; Downsampled version for arXiv.
Full resolution figures available at
http://astro.berkeley.edu/~pchang/full_res_weibel.pd
Astrophysical turbulence modeling
The role of turbulence in various astrophysical settings is reviewed. Among
the differences to laboratory and atmospheric turbulence we highlight the
ubiquitous presence of magnetic fields that are generally produced and
maintained by dynamo action. The extreme temperature and density contrasts and
stratifications are emphasized in connection with turbulence in the
interstellar medium and in stars with outer convection zones, respectively. In
many cases turbulence plays an essential role in facilitating enhanced
transport of mass, momentum, energy, and magnetic fields in terms of the
corresponding coarse-grained mean fields. Those transport properties are
usually strongly modified by anisotropies and often completely new effects
emerge in such a description that have no correspondence in terms of the
original (non coarse-grained) fields.Comment: 88 pages, 26 figures, published in Reports on Progress in Physic
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