86 research outputs found
Study of the origin of three dimensional structures and chaos in an externally forced free shear layer
Experiments on an externally forced free shear layer are performed which study the origin of three-dimensional structures
and chaos in shear flows. Transition routes between the laminar two-dimensional stages of shear flows and their final
complex three-dimensional stages are examined. Two avenues of investigation are pursued. First, the general idea of a
multi-frequency route to chaos is examined which treats the shear flow as an open dynamical system. An attempt is
made to apply concepts from nonlinear dynamics to these systems. Secondly, a new approach to generating three-dimensional
structures in shear flows which involves the creation of a spatial shear in the frequency of external
perturbations is presented. In these experiments, a variety of vortex reconnection behaviors is observed at the
discontinuity
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
Extended self-similarity of the small-scale cosmic microwave background anisotropy
The Extended Self-Similarity (ESS) of cosmic microwave background (CMB)
radiation has been studied using recent data obtained by the space-craft based
Wilkinson Microwave Anisotropy Probe. Using the ESS and the high angular scale
resolution (arcminutes) of the data it is shown that the CMB temperature space
{\it increments} exhibit considerable and systematic declination from
Gaussianity for high order moments at the small angular scales. Moreover, the
CMB space increment ESS exponents have remarkably close values to the ESS
exponents observed in turbulence (in magnetohydrodynamic turbulence)
Yakhot's model of strong turbulence: A generalization of scaling models of turbulence
We report on some implications of the theory of turbulence developed by V.
Yakhot [V. Yakhot, Phys. Rev. E {\bf 57}(2) (1998)]. In particular we focus on
the expression for the scaling exponents . We show that Yakhot's
result contains three well known scaling models as special cases, namely K41,
K62 and the theory by V. L'vov and I. Procaccia [V. L'vov & I. Procaccia, Phys.
Rev. E {\bf 62}(6) (2000)]. The model furthermore yields a theoretical
justification for the method of extended self--similarity (ESS).Comment: 8 page
Probability density function of turbulent velocity fluctuations in rough-wall boundary layer
The probability density function of single-point velocity fluctuations in
turbulence is studied systematically using Fourier coefficients in the
energy-containing range. In ideal turbulence where energy-containing motions
are random and independent, the Fourier coefficients tend to Gaussian and
independent of each other. Velocity fluctuations accordingly tend to Gaussian.
However, if energy-containing motions are intermittent or contaminated with
bounded-amplitude motions such as wavy wakes, the Fourier coefficients tend to
non-Gaussian and dependent of each other. Velocity fluctuations accordingly
tend to non-Gaussian. These situations are found in our experiment of a
rough-wall boundary layer.Comment: 6 pages, to appear in Physical Review
Synthesis of Na+/Ca2+ ions modified TiO2 xerogels through co precipitation method
747-751We report a facile method for the synthesis of Na+/Ca2+ ions modified TiO2 xerogels by coprecipitation followed by calcination process. The resultant materials are well characterized by powder X-ray diffraction, Raman spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy and high resolution transmission electron microscopy. Presence of sodium and calcium ions can influence the crystallinity of rutile TiO2. The photocatalytic performances of bare rutile TiO2 along with modified materials such as Na-TiO2, Ca-TiO2, Na-Ca-TiO2 materials are evaluated by calculating the amount of hydrogen evolved during the photocatalytic decomposition of water under light irradiation.This study will be effective informulating the effect of alkali/alkaline earth metal ions on the photocatalytic activity of rutile TiO2
Local properties of extended self-similarity in 3D turbulence
Using a generalization of extended self-similarity we have studied local
scaling properties of 3D turbulence in a direct numerical simulation. We have
found that these properties are consistent with lognormal-like behavior of
energy dissipation fluctuations with moderate amplitudes for space scales
beginning from Kolmogorov length up to the largest scales, and in the
whole range of the Reynolds numbers: . The
locally determined intermittency exponent varies with ; it has a
maximum at scale , independent of .Comment: 4 pages, 5 figure
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
Experiments on the Forced Wake of an Airfoil
Presented here is an experimental effort which attempts to
understand the nature of the wake of an airfoil in a controlled
environment. The frequency of oscillation in the wake (the vortex
shedding frequency) is controlled through the introduction
of an external perturbation. Strip heaters are used to introduce
waves into the top and bottom boundary layers of a thin symmetric
airfoil which are amplified and introduced to the wake.
The linear and nonlinear interactions of these waves in the wake
are studied in detail. Three modes of interaction have been
observed through flow visualization and velocity measurements:
frequency locking in which the vortex shedding frequency is the
same as the forcing frequency, quasiperiodic vortex interaction
in which periodic clusters of vortices are observed in the wake,
and chaotic vortex interaction in which the vortices in the wake
have a three dimensional random structure
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