220 research outputs found
Statistics of defect motion in spatiotemporal chaos in inclined layer convection
We report experiments on defect-tracking in the state of undulation chaos
observed in thermal convection of an inclined fluid layer. We characterize the
ensemble of defect trajectories according to their velocities, relative
positions, diffusion, and gain and loss rates. In particular, the defects
exhibit incidents of rapid transverse motion which result in power law
distributions for a number of quantitative measures. We examine connections
between this behavior and L\'evy flights and anomalous diffusion. In addition,
we describe time-reversal and system size invariance for defect creation and
annihilation rates.Comment: (21 pages, 17 figures
Azimuthal diffusion of the large-scale-circulation plane, and absence of significant non-Boussinesq effects, in turbulent convection near the ultimate-state transition
We present measurements of the orientation and temperature
amplitude of the large-scale circulation in a cylindrical sample of
turbulent Rayleigh-Benard convection (RBC) with aspect ratio ( and are the diameter and height respectively) and for the
Prandtl number . Results for revealed a preferred
orientation with upflow in the West, consistent with a broken azimuthal
invariance due to Earth's Coriolis force [see \cite{BA06b}]. They yielded the
azimuthal diffusivity and a corresponding Reynolds number
for Rayleigh numbers over the range . In the classical state () the results
were consistent with the measurements by \cite{BA06a} for and
which gave , and with the
Prandtl-number dependence as found previously
also for the velocity-fluctuation Reynolds number \cite[]{HGBA15b}. At
larger the data for revealed a transition to a new
state, known as the "ultimate" state, which was first seen in the Nusselt
number and in at and
. In the ultimate state we found .
Recently \cite{SU15} claimed that non-Oberbeck-Boussinesq effects on the
Nusselt and Reynolds numbers of turbulent RBC may have been interpreted
erroneously as a transition to a new state. We demonstrate that their reasoning
is incorrect and that the transition observed in the G\"ottingen experiments
and discussed in the present paper is indeed to a new state of RBC referred to
as "ultimate".Comment: 12 pages, 4 figures, to be pub. in JFM
Logarithmic temperature profiles of turbulent Rayleigh-B\'enard convection in the classical and ultimate state for a Prandtl number of 0.8
We report on experimental determinations of the temperature field in the
interior (bulk) of turbulent Rayleigh-Benard convection for a cylindrical
sample with aspect ratio (diameter over height) of 0.50, both in the classical
and in the ultimate state. The Prandtl number was close to 0.8. We find a
"logarithmic layer" in which the temperature varies as A*ln(z/L) + B with the
distance z from the bottom plate of the sample. The amplitude A varies with
radial position r. In the classical state these results are in good agreement
with direct numerical simulations (DNS); in the ultimate state there are as yet
no DNS. A close analogy between the temperature field in the classical state
and the "Law of the Wall" for the time-averaged down-stream velocity in shear
flow is discussed.Comment: 27 pages, 15 figure
Lagrangian view of time irreversibility of fluid turbulence
A turbulent flow is maintained by an external supply of kinetic energy, which
is eventually dissipated into heat at steep velocity gradients. The scale at
which energy is supplied greatly differs from the scale at which energy is
dissipated, the more so as the turbulent intensity (the Reynolds number) is
larger. The resulting energy flux over the range of scales, intermediate
between energy injection and dissipation, acts as a source of time
irreversibility. As it is now possible to follow accurately fluid particles in
a turbulent flow field, both from laboratory experiments and from numerical
simulations, a natural question arises: how do we detect time irreversibility
from these Lagrangian data? Here we discuss recent results concerning this
problem. For Lagrangian statistics involving more than one fluid particle, the
distance between fluid particles introduces an intrinsic length scale into the
problem. The evolution of quantities dependent on the relative motion between
these fluid particles, including the kinetic energy in the relative motion, or
the configuration of an initially isotropic structure can be related to the
equal-time correlation functions of the velocity field, and is therefore
sensitive to the energy flux through scales, hence to the irreversibility of
the flow. In contrast, for single-particle Lagrangian statistics, the most
often studied velocity structure functions cannot distinguish the "arrow of
time." Recent observations from experimental and numerical simulation data,
however, show that the change of kinetic energy following the particle motion,
is sensitive to time-reversal. We end the survey with a brief discussion of the
implication of this line of work.Comment: accepted for publication in Science China - Physics, Mechanics &
Astronom
Dissipative Effects on Inertial-Range Statistics at High Reynolds numbers
Using the unique capabilities of the Variable Density Turbulence Tunnel at
the Max Planck Institute for Dynamics and Self-Organization, G\"{o}ttingen, we
report experimental result on classical grid turbulence that uncover fine, yet
important details of the structure functions in the inertial range. This was
made possible by measuring extremely long time series of up to
samples of the turbulent fluctuating velocity, which corresponds to
large eddy turnover times. These classical grid
measurements were conducted in a well-controlled environment at a wide range of
high Reynolds numbers from up to , using both
traditional hot-wire probes as well as NSTAP probes developed at Princeton
University. We found that deviations from ideal scaling are anchored to the
small scales and that dissipation influences the inertial-range statistics at
scales larger than the near-dissipation range.Comment: 6 pages, 5 figure
Evolution of geometric structures in intense turbulence
We report measurements of the evolution of lines, planes, and volumes in an
intensely turbulent laboratory flow using high-speed particle tracking. We find
that the classical characteristic time scale of an eddy at the initial scale of
the object considered is the natural time scale for the subsequent evolution.
The initial separation may only be neglected if this time scale is much smaller
than the largest turbulence time scale, implying extremely high turbulence
levels.Comment: 10 pages, 6 figures, added more detail
Defect turbulence and generalized statistical mechanics
We present experimental evidence that the motion of point defects in thermal
convection patterns in an inclined fluid layer is well-described by Tsallis
statistics with an entropic index . The dynamical properties of
the defects (anomalous diffusion, shape of velocity distributions, power law
decay of correlations) are in good agreement with typical predictions of
nonextensive models, over a range of driving parameters
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