591 research outputs found
Vortex tubes in velocity fields of laboratory isotropic turbulence: dependence on the Reynolds number
The streamwise and transverse velocities are measured simultaneously in
isotropic grid turbulence at relatively high Reynolds numbers, Re(lambda) =
110-330. Using a conditional averaging technique, we extract typical
intermittency patterns, which are consistent with velocity profiles of a model
for a vortex tube, i.e., Burgers vortex. The radii of the vortex tubes are
several of the Kolmogorov length regardless of the Reynolds number. Using the
distribution of an interval between successive enhancements of a small-scale
velocity increment, we study the spatial distribution of vortex tubes. The
vortex tubes tend to cluster together. This tendency is increasingly
significant with the Reynolds number. Using statistics of velocity increments,
we also study the energetical importance of vortex tubes as a function of the
scale. The vortex tubes are important over the background flow at small scales
especially below the Taylor microscale. At a fixed scale, the importance is
increasingly significant with the Reynolds number.Comment: 8 pages, 3 PS files for 8 figures, to appear in Physical Review
Dual-camera system for high-speed imaging in particle image velocimetry
Particle image velocimetry is an important technique in experimental fluid
mechanics, for which it has been essential to use a specialized high-speed
camera. However, the high speed is at the expense of other performances of the
camera, i.e., sensitivity and image resolution. Here, we demonstrate that the
high-speed imaging is also possible with a pair of still cameras.Comment: 4 pages, accepted by Journal of Visualization (see
http://www.springerlink.com
Quantifying Suppression of the Cosmological 21-cm Signal due to Direction Dependent Gain Calibration in Radio Interferometers
The 21-cm signal of neutral hydrogen - emitted during the Epoch of
Reionization - promises to be an important source of information for the study
of the infant universe. However, its detection is impossible without sufficient
mitigation of other strong signals in the data, which requires an accurate
knowledge of the instrument. Using the result of instrument calibration, a
large part of the contaminating signals are removed and the resulting residual
data is further analyzed in order to detect the 21-cm signal. Direction
dependent calibration (DDC) can strongly affect the 21-cm signal, however, its
effect has not been precisely quantified.
In the analysis presented here we show how to exactly calculate what part of
the 21-cm signal is removed as a result of the DDC. We also show how a-priori
information about the frequency behavior of the instrument can be used to
reduce signal suppression. The theoretical results are tested using a realistic
simulation based on the LOFAR setup. Our results show that low-order smooth
gain functions (e.g. polynomials) over a bandwidth of ~10\,MHz - over which the
signal is expected to be stationary - is sufficient to allow for calibration
with limited, quantifiable, signal suppression in its power spectrum. We also
show mathematically and in simulations that more incomplete sky models lead to
larger 21-cm signal suppression, even if the gain models are enforced to be
fully smooth. This result has immediate consequences for current and future
radio telescopes with non-identical station beams, where DDC might be necessary
(e.g. SKA-low).Comment: Submitted to MNRAS on 10-Aug-201
Statistical mechanics and large-scale velocity fluctuations of turbulence
Turbulence exhibits significant velocity fluctuations even if the scale is
much larger than the scale of the energy supply. Since any spatial correlation
is negligible, these large-scale fluctuations have many degrees of freedom and
are thereby analogous to thermal fluctuations studied in the statistical
mechanics. By using this analogy, we describe the large-scale fluctuations of
turbulence in a formalism that has the same mathematical structure as used for
canonical ensembles in the statistical mechanics. The formalism yields a
universal law for the energy distribution of the fluctuations, which is
confirmed with experiments of a variety of turbulent flows. Thus, through the
large-scale fluctuations, turbulence is related to the statistical mechanics.Comment: 7 pages, accepted by Physics of Fluids (see http://pof.aip.org/
Two-point velocity average of turbulence: statistics and their implications
For turbulence, although the two-point velocity difference u(x+r)-u(x) at
each scale r has been studied in detail, the velocity average [u(x+r)+u(x)]/2
has not thus far. Theoretically or experimentally, we find interesting features
of the velocity average. It satisfies an exact scale-by-scale energy budget
equation. The flatness factor varies with the scale r in a universal manner.
These features are not consistent with the existing assumption that the
velocity average is independent of r and represents energy-containing
large-scale motions alone. We accordingly propose that it represents motions
over scales >= r as long as the velocity difference represents motions at the
scale r.Comment: 8 pages, accepted by Physics of Fluids (see http://pof.aip.org/
Probability density function of turbulent velocity fluctuation
The probability density function (PDF) of velocity fluctuations is studied
experimentally for grid turbulence in a systematical manner. At small distances
from the grid, where the turbulence is still developing, the PDF is
sub-Gaussian. At intermediate distances, where the turbulence is fully
developed, the PDF is Gaussian. At large distances, where the turbulence has
decayed, the PDF is hyper-Gaussian. The Fourier transforms of the velocity
fluctuations always have Gaussian PDFs. At intermediate distances from the
grid, the Fourier transforms are statistically independent of each other. This
is the necessary and sufficient condition for Gaussianity of the velocity
fluctuations. At small and large distances, the Fourier transforms are
dependent.Comment: 7 pages, 8 figures in a PS file, to appear in Physical Review
Fluctuations of statistics among subregions of a turbulence velocity field
To study subregions of a turbulence velocity field, a long record of velocity
data of grid turbulence is divided into smaller segments. For each segment, we
calculate statistics such as the mean rate of energy dissipation and the mean
energy at each scale. Their values significantly fluctuate, in lognormal
distributions at least as a good approximation. Each segment is not under
equilibrium between the mean rate of energy dissipation and the mean rate of
energy transfer that determines the mean energy. These two rates still
correlate among segments when their length exceeds the correlation length. Also
between the mean rate of energy dissipation and the mean total energy, there is
a correlation characterized by the Reynolds number for the whole record,
implying that the large-scale flow affects each of the segments.Comment: 7 pages, accepted by Physics of Fluids (see http://pof.aip.org/
- …