559 research outputs found
Tangling clustering of inertial particles in stably stratified turbulence
We have predicted theoretically and detected in laboratory experiments a new
type of particle clustering (tangling clustering of inertial particles) in a
stably stratified turbulence with imposed mean vertical temperature gradient.
In this stratified turbulence a spatial distribution of the mean particle
number density is nonuniform due to the phenomenon of turbulent thermal
diffusion, that results in formation of a gradient of the mean particle number
density, \nabla N, and generation of fluctuations of the particle number
density by tangling of the gradient, \nabla N, by velocity fluctuations. The
mean temperature gradient, \nabla T, produces the temperature fluctuations by
tangling of the gradient, \nabla T, by velocity fluctuations. These
fluctuations increase the rate of formation of the particle clusters in small
scales. In the laboratory stratified turbulence this tangling clustering is
much more effective than a pure inertial clustering that has been observed in
isothermal turbulence. In particular, in our experiments in oscillating grid
isothermal turbulence in air without imposed mean temperature gradient, the
inertial clustering is very weak for solid particles with the diameter 10
microns and Reynolds numbers Re =250. Our theoretical predictions are in a good
agreement with the obtained experimental results.Comment: 16 pages, 4 figures, REVTEX4, revised versio
Transition phenomena in unstably stratified turbulent flows
We study experimentally and theoretically transition phenomena caused by the
external forcing from Rayleigh-Benard convection with the large-scale
circulation (LSC) to the limiting regime of unstably stratified turbulent flow
without LSC whereby the temperature field behaves like a passive scalar. In the
experiments we use the Rayleigh-B\'enard apparatus with an additional source of
turbulence produced by two oscillating grids located nearby the side walls of
the chamber. When the frequency of the grid oscillations is larger than 2 Hz,
the large-scale circulation (LSC) in turbulent convection is destroyed, and the
destruction of the LSC is accompanied by a strong change of the mean
temperature distribution. However, in all regimes of the unstably stratified
turbulent flow the ratio varies slightly (even in the range
of parameters whereby the behaviour of the temperature field is different from
that of the passive scalar). Here are the integral scales of
turbulence along x, y, z directions, T and \theta are the mean and fluctuating
parts of the fluid temperature. At all frequencies of the grid oscillations we
have detected the long-term nonlinear oscillations of the mean temperature. The
theoretical predictions based on the budget equations for turbulent kinetic
energy, turbulent temperature fluctuations and turbulent heat flux, are in
agreement with the experimental results.Comment: 14 pages, 14 figures, REVTEX4-1, revised versio
An assessment on the unsteady flow distortion generated by an S-duct intake
Closer integration between the fuselage and the propulsion system is expected for futureaircraft toreducefuel consumption, emissions, weight and drag. The use of embedded or partially embedded propulsion systems may require the use of complex intakes. However, thiscanresult in unsteady flow distortion which can adversely affect the propulsion system efficiency and stability. This works assesses the characteristics of the unsteady flow with a view to the potential flow distortion presented to the compression system.Particle image velocimetry is used to measure the flow distortion generated by an S-shaped intake.The time-resolved tracking of the idealized relative incidence angle revealed that most frequent distortion events exhibited90°exposure sector and upto±5°meanrelativeincidence. The imposition of a thicker boundary at the S-duct inlet increased the probability of distortion events that are characterized by a longer exposure sector and higher relative incidence angles.Because of these characteristics, thedistortion caused by the S-duct intake could induce instabilities that are detrimental for the propulsion system performances and stability. Overall, this work proposes a new method to assess thepossible relativeincidence angle on the compressor rotor taking into account the intake flow unsteadiness
Mixing at the external boundary of a submerged turbulent jet
We study experimentally and theoretically mixing at the external boundary of
a submerged turbulent jet. In the experimental study we use Particle Image
Velocimetry and an Image Processing Technique based on the analysis of the
intensity of the Mie scattering to determine the spatial distribution of tracer
particles. An air jet is seeded with the incense smoke particles which are
characterized by large Schmidt number and small Stokes number. We determine the
spatial distributions of the jet fluid characterized by a high concentration of
the particles and of the ambient fluid characterized by a low concentration of
the tracer particles. In the data analysis we use two approaches, whereby one
approach is based on the measured phase function for the study of the mixed
state of two fluids. The other approach is based on the analysis of the
two-point second-order correlation function of the particle number density
fluctuations generated by tangling of the gradient of the mean particle number
density by the turbulent velocity field. This gradient is formed at the
external boundary of a submerged turbulent jet. We demonstrate that PDF of the
phase function of a jet fluid penetrating into an external flow and the
two-point second-order correlation function of the particle number density do
not have universal scaling and cannot be described by a power-law function. The
theoretical predictions made in this study are in a qualitative agreement with
the obtained experimental results.Comment: 13 pages, 13 figures, REVTEX
Novel Relapsing Fever Spirochete in Bat Tick
Novel Relapsing Fever Spirochete in Bat Tic
Hysteresis phenomenon in turbulent convection
Coherent large-scale circulations of turbulent thermal convection in air have
been studied experimentally in a rectangular box heated from below and cooled
from above using Particle Image Velocimetry. The hysteresis phenomenon in
turbulent convection was found by varying the temperature difference between
the bottom and the top walls of the chamber (the Rayleigh number was changed
within the range of ). The hysteresis loop comprises the one-cell
and two-cells flow patterns while the aspect ratio is kept constant (). We found that the change of the sign of the degree of the anisotropy of
turbulence was accompanied by the change of the flow pattern. The developed
theory of coherent structures in turbulent convection (Elperin et al. 2002;
2005) is in agreement with the experimental observations. The observed coherent
structures are superimposed on a small-scale turbulent convection. The
redistribution of the turbulent heat flux plays a crucial role in the formation
of coherent large-scale circulations in turbulent convection.Comment: 10 pages, 9 figures, REVTEX4, Experiments in Fluids, 2006, in pres
Effect of large-scale coherent structures on turbulent convection
We study an effect of large-scale coherent structures on global properties of
turbulent convection in laboratory experiments in air flow in a rectangular
chamber with aspect ratios and (with the Rayleigh
numbers varying in the range from to ). The large-scale
coherent structures comprise the one-cell and two-cell flow patterns. We found
that a main contribution to the turbulence kinetic energy production in
turbulent convection with large-scale coherent structures is due to the
non-uniform large-scale motions. Turbulence in large Rayleigh number convection
with coherent structures is produced by shear, rather than by buoyancy. We
determined the scalings of global parameters (e.g., the production and
dissipation of turbulent kinetic energy, the turbulent velocity and integral
turbulent scale, the large-scale shear, etc.) of turbulent convection versus
the temperature difference between the bottom and the top walls of the chamber.
These scalings are in an agreement with our theoretical predictions. We
demonstrated that the degree of inhomogeneity of the turbulent convection with
large-scale coherent structures is small.Comment: 10 pages, 12 figures, REVTEX
Quantitative imaging of concentrated suspensions under flow
We review recent advances in imaging the flow of concentrated suspensions,
focussing on the use of confocal microscopy to obtain time-resolved information
on the single-particle level in these systems. After motivating the need for
quantitative (confocal) imaging in suspension rheology, we briefly describe the
particles, sample environments, microscopy tools and analysis algorithms needed
to perform this kind of experiments. The second part of the review focusses on
microscopic aspects of the flow of concentrated model hard-sphere-like
suspensions, and the relation to non-linear rheological phenomena such as
yielding, shear localization, wall slip and shear-induced ordering. Both
Brownian and non-Brownian systems will be described. We show how quantitative
imaging can improve our understanding of the connection between microscopic
dynamics and bulk flow.Comment: Review on imaging hard-sphere suspensions, incl summary of
methodology. Submitted for special volume 'High Solid Dispersions' ed. M.
Cloitre, Vol. xx of 'Advances and Polymer Science' (Springer, Berlin, 2009);
22 pages, 16 fig
- …