Página 1 dos resultados de 562 itens digitais encontrados em 0.004 segundos

- Universidade de Évora
- Universidade de Adelaide
- MIT - Massachusetts Institute of Technology
- Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires
- Universidade Cornell
- American Society of Limnology and Oceanography
- Instituto de Tecnologia da Califórnia
- Center for Turbulence Research
- Mais Publicadores...

## An exploratory investigation of radiation statistics in homogeneous isotropic turbulence

Fonte: Universidade de Évora
Publicador: Universidade de Évora

Tipo: Artigo de Revista Científica

ENG

Relevância na Pesquisa

36.4%

A fundamental study of radiation statistics in homogeneous isotropic turbulence is presented. A pseudo-spectral code is used to simulate isotropic turbulence by means of DNS of the full Navier-Stokes equations. The instantaneous scalar data is used to calculate the radiation intensity along a line of sight using the statistical narrow band model. The mean, variance, skewness and flatness of radiation intensity were obtained for conditions observed downstream of the flame tip of a piloted turbulent jet flame, where the statistics of the flow field are close to the ones found in isotropic turbulence. The joint probability density function between the temperature and the radiation intensity is presented, as well as the spectra for the radiation intensity. The present one way coupling philosophy used to connect isotropic turbulence data with radiation computations shows the correct trends and allows one to study the detailed effects of the turbulent characteristics upon the structure of the radiation intensity field.

Link permanente para citações:

## The effects of turbulence structures on the air-side performance of compact tube-fin heat exchangers.

Fonte: Universidade de Adelaide
Publicador: Universidade de Adelaide

Tipo: Tese de Doutorado

Publicado em //2006

Relevância na Pesquisa

46.42%

Energy is an essential and critical commodity and our reliance on it has fuelled much of the debate and interest in society and academia alike. Environmental concerns, depleted energy resources and higher energy prices are the main factors that drive this interest. Energy efficiency is one of the main avenues to preserve and better utilize this valuable commodity. The energy exchange by employment of heat exchangers is extensive and tube-fin heat exchangers are widely used in industrial and commercial applications. Smarter designs could not only improve energy efficiency but may also save on material costs. Although mass production and improved manufacturing techniques have reduced manufacturing costs, tube fin heat exchangers have not evolved greatly to take advantage of these improvements. There has been a large range of fin surface enhancements proposed, such as waffled fins or louvres and while limited improvements in capacity have been achieved, this is generally accomplished at a much larger pressure drop penalty. Numerous studies have been performed in order to examine the potential of various surface enhancement geometries on an ad hoc basis. These presumably operate on the basis of enhanced convection due to increased turbulence levels. However very few of these studies examine the actual nature of turbulence that is responsible for convection enhancement. A series of experiments and numerical studies have been conducted to quantify the effect of the turbulence vortex characteristics on the air side heat convection of a tube-fin heat exchanger. Homogeneous...

Link permanente para citações:

## 12.820 / 12.822 Turbulence in Geophysical Systems, Spring 2003; Turbulence in Geophysical Systems

Fonte: MIT - Massachusetts Institute of Technology
Publicador: MIT - Massachusetts Institute of Technology

EN-US

Relevância na Pesquisa

36.56%

#oceans#atmosphere#fine structure#planetary scale motions#homogeneous flows#geostrophic motions#shear flows#convection#boundary layers#stably stratified flows#internal waves

This course presents the phenomena, theory, and modeling of turbulence in the Earth's oceans and atmosphere. The scope ranges from the fine structure to planetary scale motions. The regimes of turbulence include homogeneous flows in two and three dimensions, geostrophic motions, shear flows, convection, boundary layers, stably stratified flows, and internal waves.

Link permanente para citações:

## Transporte anisótropo de escalares pasivos en turbulencia rotante; Anisotropic transport of passive scalars in rotating turbulence

Fonte: Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires
Publicador: Facultad de Ciencias Exactas y Naturales. Universidad de Buenos Aires

Tipo: info:eu-repo/semantics/doctoralThesis; tesis doctoral; info:eu-repo/semantics/publishedVersion
Formato: application/pdf

Publicado em //2014
SPA

Relevância na Pesquisa

46.18%

#TURBULENCE#PASIVE SCALARS#ANISOTROPIC TRANSPORT#TURBULENCIA#ESCALARES PASIVOS#TRANSPORTE ANISOTROPO

Un escalar pasivo es un contaminante difusivo presente en un fluido con un grado de concentración tan pequeño que no produce un efecto dinámico en el campo de velocidad. Aerosoles, contaminantes en pequeñas concentraciones en la atmósfera y los océanos, elementos químicos en el interior de las estrellas, o bajo ciertas aproximaciones pequeñas fluctuaciones en la temperatura de un fluido son ejemplos de escalares pasivos. La correcta comprensión de la dinámica de estas magnitudes es relevante para muchas aplicaciones, pero también tiene implicancias importantes en el entendimiento general de la turbulencia. Por este motivo, muchos estudios de escalares pasivos se concentraron en el caso de flujos turbulentos isótropos y homogéneos, aunque en flujos geofísicos, astrofísicos e industriales muchas veces la presencia de fuerzas externas introduce anisotropías en el campo de velocidad que no pueden despreciarse. En esta tesis se presenta un estudio detallado de la advección, el mezclado y la difusión de escalares pasivos en flujos turbulentos rotantes, con y sin helicidad neta. La rotación está presente en muchos flujos en la atmósfera, mientras que se cree que la helicidad (una magnitud conservada por un fluido ideal...

Link permanente para citações:

## Nonlocality in Homogeneous Superfluid Turbulence

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

36.45%

Simulating superfluid turbulence using the localized induction approximation
in periodic bound- aries produces open-orbit vortices, which make superfluid
turbulence unsustainable. Calculating with the fully nonlocal Biot-Savart law
prevents the open-orbit state from forming, but also in- creases computation
time. We use a truncated Biot-Savart integral to investigate the effects of
nonlocality on homogeneous turbulence. We find that including the nonlocal
interaction up to the average intervortex spacing prevents this open-orbit
state from forming, yielding an accurate model of homogeneous superfluid
turbulence with less computation time.

Link permanente para citações:

## Drag Reduction in Homogeneous Turbulence by Scale-Dependent Effective Viscosity

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 19/02/2004

Relevância na Pesquisa

46.3%

The phenomenon of drag reduction by polymer additives had been studied in
simulations on the basis of non-Newtonian fluid mechanical models that take
into account the field of polymer extension (conformation tensor) and its
interaction with the velocity field. Drag reduction was found in both
homogeneous and wall bounded turbulent flows. In the latter case it was shown
recently that the notion of scale-dependent effective viscosity allows
quantitative predictions of the characteristics of drag reduction in close
correspondence with experiments. In this paper we demonstrate that also drag
reduction in homogeneous turbulence is usefully discussed in terms of a
scale-dependent viscosity. In other words, the essence of the phenomena under
study can be recaptured by an "equivalent" equation of motion for the velocity
field alone, with a judiciously chosen scale-dependent effective viscosity that
succinctly summarizes the important aspects of the interaction between the
polymer conformation tensor field and the velocity field. We will also clarify
here the differences between drag reduction in homogeneous and wall bounded
flows.; Comment: tex file+ 11 figures

Link permanente para citações:

## A Statistical Theory of Homogeneous Isotropic Turbulence

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 11/02/2009

Relevância na Pesquisa

36.4%

The present work proposes a theory of isotropic and homogeneous turbulence
for incompressible fluids, which assumes that the turbulence is due to the
bifurcations associated to the velocity field. The theory is formulated using a
representation of the fluid motion which is more general than the classical
Navier-Stokes equations, where the fluid state variables are expressed in terms
of the referential coordinates.
The theory is developed according to the following four items: 1) Study of
the route toward the turbulence through the bifurcations analysis of the
kinematic equations. 2) Referential description of the motion and calculation
of the velocity fluctuation using the Lyapunov analysis of the local
deformation. 3) Study of the mechanism of the energy cascade from large to
small scales through the Lyapunov analysis of the relative kinematics equations
of motion. 4) Determination of the statistics of the velocity difference with
the Fourier analysis. Each item contributes to the formulation of the theory.
The theory gives the connection between number of bifurcations, scales and
Reynolds number at the onset of the turbulence and supplies an explanation for
the mechanism of the energy cascade which leads to the closure of the von
K\'arm\'an-Howarth equation. The theory also gives the statistics of the
velocity difference fluctuation and permits the calculation of its PDF.
The presented results show that the proposed theory describes quite well the
properties of the isotropic turbulence.; Comment: 34 pages...

Link permanente para citações:

## Shell Model for Drag Reduction with Polymer Additive in Homogeneous Turbulence

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 24/04/2003

Relevância na Pesquisa

46.26%

Recent direct numerical simulations of the FENE-P model of non-Newtonian
hydrodynamics revealed that the phenomenon of drag reduction by polymer
additives exists (albeit in reduced form) also in homogeneous turbulence. We
introduce here a simple shell model for homogeneous viscoelastic flows that
recaptures the essential observations of the full simulations. The simplicity
of the shell model allows us to offer a transparent explanation of the main
observations. It is shown that the mechanism for drag reduction operates mainly
on the large scales. Understanding the mechanism allows us to predict how the
amount of drag reduction depends of the various parameters in the model. The
main conclusion is that drag reduction is not a universal phenomenon, it peaks
in a window of parameters like Reynolds number and the relaxation rate of the
polymer.

Link permanente para citações:

## "Locally homogeneous turbulence" Is it an inconsistent framework?

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 21/02/2005

Relevância na Pesquisa

46.26%

In his first 1941 paper Kolmogorov assumed that the velocity has increments
which are homogeneous and independent of the velocity at a suitable reference
point. This assumption of local homogeneity is consistent with the nonlinear
dynamics only in an asymptotic sense when the reference point is far away. The
inconsistency is illustrated numerically using the Burgers equation.
Kolmogorov's derivation of the four-fifths law for the third-order structure
function and its anisotropic generalization are actually valid only for
homogeneous turbulence, but a local version due to Duchon and Robert still
holds. A Kolomogorov--Landau approach is proposed to handle the effect of
fluctuations in the large-scale velocity on small-scale statistical properties;
it is is only a mild extension of the 1941 theory and does not incorporate
intermittency effects.; Comment: 4 pages, 2 figures

Link permanente para citações:

## Steady Homogeneous Turbulence in the Presence of an Average Velocity Gradient

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

46.41%

We study the homogeneous turbulence in the presence of a constant average
velocity gradient in an infinite fluid domain, with a novel finite-scale
Lyapunov analysis, presented in a previous work dealing with the homogeneous
isotropic turbulence.
Here, the energy spectrum is studied introducing the spherical averaged pair
correlation function, whereas the anisotropy caused by the velocity gradient is
analyzed using the equation of the two points velocity distribution function
which is determined through the Liouville theorem. As a result, we obtain the
evolution equation of this velocity correlation function which is shown to be
valid also when the fluid motion is referred with respect to a rotating
reference frame. This equation tends to the classical von K\'arm\'an-Howarth
equation when the average velocity gradient vanishes.
We show that, the steady energy spectrum, instead of following the Kolmogorov
law $\kappa^{-5/3}$, varies as $\kappa^{-2}$. Accordingly, the structure
function of the longitudinal velocity difference $<\Delta u_r^n> \approx
r^{\zeta_n}$ exhibits the anomalous scaling $\zeta_n \approx n/2$, and the
integral scales of the correlation function are much smaller than those of the
isotropic turbulence.; Comment: 33 pages...

Link permanente para citações:

## Relating statistics to dynamics in axisymmetric homogeneous turbulence

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 09/02/2012

Relevância na Pesquisa

46.47%

The structure and the dynamics of homogeneous turbulence are modified by the
presence of body forces such that the Coriolis or the buoyancy forces, which
may render a wide range of turbulence scales anisotropic. The corresponding
statistical characterization of such effects is done in physical space using
structure functions, as well as in spectral space with spectra of two-point
correlations, providing two complementary viewpoints. In this framework,
second-order and third-order structure functions are put in parallel with
spectra of two-point second- and third-order velocity correlation functions,
using passage relations. Such relations apply in the isotropic case, or for
isotropically averaged statistics, which, however, do not reflect the actual
more complex structure of anisotropic turbulence submitted to rotation or
stratification. This complexity is demonstrated in this paper by
orientation-dependent energy and energy transfer spectra produced in both cases
by means of a two-point statistical model for axisymmetric turbulence. We show
that, to date, the anisotropic formalism used in the spectral transfer
statistics is especially well-suited to analyze the refined dynamics of
anisotropic homogeneous turbulence, and that it can help in the analysis of
isotropically computed third-order structure function statistics often used to
characterize anisotropic contexts.; Comment: Physica D

Link permanente para citações:

## Transport equation for plasmas in a stationary-homogeneous turbulence

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Publicado em 04/12/2015

Relevância na Pesquisa

46.27%

For a plasma in a stationary homogeneous turbulence, the Fokker-Planck
equation is derived from the nonlinear Vlasov equation by introducing the
entropy principle. The ensemble average in evaluating the kinetic diffusion
tensor, whose symmetry has been proved, can be computed in a straightforward
way when the fluctuating particle trajectories are provided. As an application,
it has been shown that a mean electric filed can drive a particle flux through
the Stokes-Einstein relation, independent of the details of turbulence.

Link permanente para citações:

## Mean-field dynamo in a turbulence with shear and kinetic helicity fluctuations

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

36.45%

We study effects of kinetic helicity fluctuations in a turbulence with
large-scale shear using two different approaches: the spectral
tau-approximation and the second order correlation approximation (or
first-order smoothing approximation). These two approaches demonstrate that
homogeneous kinetic helicity fluctuations alone with zero mean value in a
sheared homogeneous turbulence cannot cause large-scale dynamo. Mean-field
dynamo can be possible when kinetic helicity fluctuations are inhomogeneous
which cause a nonzero mean alpha effect in a sheared turbulence. On the other
hand, shear-current effect can generate large-scale magnetic field even in a
homogeneous nonhelical turbulence with large-scale shear. This effect was
investigated previously for large hydrodynamic and magnetic Reynolds numbers.
In this study we examine the threshold required for the shear-current dynamo
versus Reynolds number. We demonstrate that there is no need for a developed
inertial range in order to maintain the shear-current dynamo (e.g., the
threshold in the Reynolds number is of the order of 1).; Comment: 12 pages, 3 Figures, small corrections to match the final published
version, Physical Review E, in press

Link permanente para citações:

## Lyapunov Analysis of Homogeneous Isotropic Turbulence

Fonte: Universidade Cornell
Publicador: Universidade Cornell

Tipo: Artigo de Revista Científica

Relevância na Pesquisa

36.41%

The present work studies the isotropic and homogeneous turbulence for
incompressible fluids through a specific Lyapunov analysis, assuming that the
turbulence is due to the bifurcations associated to the velocity field.
The analysis consists in the study of the mechanism of the energy cascade
from large to small scales through the Lyapunov analysis of the relative motion
between two particles and in the calculation of the velocity fluctuation
through the Lyapunov analysis of the local deformation and the Navier-Stokes
equations.
The analysis provides an explanation for the mechanism of the energy cascade,
leads to the closure of the von K\'arm\'an-Howarth equation, and describes the
statistics of the velocity difference.
Several tests and numerical results are presented.; Comment: 31 pages, 14 figures, Revision of "Lyapunov Analysis of Homogeneous
Isotropic Turbulence", Submitted on Phys. Rev. E

Link permanente para citações:

## Evaluation of oscillating grids and orbital shakers as means to generate isotropic and homogeneous small-scale turbulence in laboratory enclosures commonly used in plankton studies

Fonte: American Society of Limnology and Oceanography
Publicador: American Society of Limnology and Oceanography

Tipo: Artículo
Formato: 180710 bytes; application/pdf

ENG

Relevância na Pesquisa

36.45%

17 pages, 7 figures, 6 tables.-- Full-text version available Open Access at: http://www.icm.csic.es/bio/projects/icmicrobis/pdf/guadayol_2009_L&O_mt.pdf; The effects of turbulent motion on planktonic organisms have mainly been studied in the laboratory with devices capable of generating controlled turbulent conditions. Owing to technical and logistical difficulties, thorough assessments of hydrodynamics in such experiments are not routinely made. In this study, we examined the suitability of two widely used systems to generate isotropic, homogeneous, and stationary turbulence in laboratory containers: oscillating grid devices with large stroke length and relatively low frequencies of oscillation and orbital shaker tables. Turbulent kinetic energy dissipation rates were estimated from velocity measurements made with acoustic Doppler velocimeters. Both systems were shown to generate isotropic conditions in a relatively broad range of dissipation rates. Grid-stirred tanks produce homogeneous turbulence in both the horizontal and vertical dimensions, as long as stroke length is comparable to the height of the container. Turbulence in orbital shakers is not completely homogeneous, as it depends on the distance to the wall and to the surface. Empirical models are derived as a tool for the calculation of dissipation rates in the two systems within the ranges and conditions examined in this study.; O.G. received a Spanish CSIC-I3P fellowship sponsored by INNOVA Oceanografía Litoral...

Link permanente para citações:

## Velocity autocorrelations of decaying isotropic homogeneous turbulence

Fonte: Instituto de Tecnologia da Califórnia
Publicador: Instituto de Tecnologia da Califórnia

Tipo: Article; PeerReviewed
Formato: application/pdf

Publicado em /10/1995

Relevância na Pesquisa

46.18%

Velocity autocorrelations and the mean-square displacements of fluid particles are obtained for decaying, isotropic homogeneous turbulence by numerical simulation of the flow field, using 1283 and 2563 grids, and tracking several tens of thousands of fluid particles, using a third-order interpolation scheme. A self-preserving Lagrangian velocity autocorrelation coefficient is found in terms of a dimensionless time variable s, defined by ds=dt/[script T]s(t), under the observation of a power-law energy decay and the assumption that [script T]s(t) is proportional to the Lagrangian integral timescale [script T]

_{[script L]}. This timescale is in turn assumed to be proportional to the length scale of the energy-containing eddies [script L]e~K3/2/epsilon divided by the turbulent velocity u[prime], where K=3/2u[prime]2 is turbulent energy and epsilon is the energy dissipation rate.Link permanente para citações:

## Note on decay of homogeneous turbulence

Fonte: Instituto de Tecnologia da Califórnia
Publicador: Instituto de Tecnologia da Califórnia

Tipo: Article; PeerReviewed
Formato: application/pdf

Publicado em /06/1967

Relevância na Pesquisa

46.18%

The assumption of self-similarity and the existence of an exact invariant are combined to predict the decay rate of homogeneous turbulence.

Link permanente para citações:

## The large-scale structure of homogeneous turbulence

Fonte: Instituto de Tecnologia da Califórnia
Publicador: Instituto de Tecnologia da Califórnia

Tipo: Article; PeerReviewed
Formato: application/pdf

Publicado em /02/1967

Relevância na Pesquisa

46.27%

A field of homogeneous turbulence generated at an initial instant by a distribution of random impulsive forces is considered. The statistical properties of the forces are assumed to be such that the integral moments of the cumulants of the force system all exist. The motion generated has the property that at the initial instant E(kappa) = Ckappa^2 + o(kappa^2) where E(k) is the energy spectrum function, k is the wave-number magnitude, and C is a positive number which is not in general zero. The corresponding forms of the velocity covariance spectral tensor and correlation tensor are determined. It is found that the terms in the velocity covariance Rij(r) are O(r^−3) for large values of the separation magnitude r.
An argument based on the conservation of momentum is used to show that C is a dynamical invariant and that the forms of the velocity covariance at large separation and the spectral tensor at small wave number are likewise invariant. For isotropic turbulence, the Loitsianski integral diverges but the integral \[ \int_0^{\infty} r^2R(r)dr = \frac{1}{2}\pi C \] exists and is invariant.

Link permanente para citações:

## Numerical Simulations of Homogeneous Turbulence using Lagrangian-Averaged Navier-Stokes Equations

Fonte: Center for Turbulence Research
Publicador: Center for Turbulence Research

Tipo: Book Section; PeerReviewed
Formato: application/pdf; application/pdf

Publicado em /11/2000

Relevância na Pesquisa

56.33%

The Lagrangian-averaged Navier-Stokes equations (LANS) are numerically evaluated as
a turbulence closure. They are derived from a novel Lagrangian averaging procedure on
the space of all volume-preserving maps and can be viewed as a numerical algorithm
which removes the energy content from the small scales (smaller than some a priori fixed
spatial scale α) using a dispersive rather than dissipative mechanism, thus maintaining
the crucial features of the large scale flow. We examine the modeling capabilities of the
LANS equations for decaying homogeneous turbulence, ascertain their ability to track
the energy spectrum of fully resolved direct numerical simulations (DNS), compare the
relative energy decay rates, and compare LANS with well-accepted LES models.

Link permanente para citações:

## Pressure spectra for vortex models of fine-scale homogeneous turbulence

Fonte: Instituto de Tecnologia da Califórnia
Publicador: Instituto de Tecnologia da Califórnia

Tipo: Article; PeerReviewed
Formato: application/pdf

Publicado em /04/1995

Relevância na Pesquisa

46.18%

Pressure spectra at large wave numbers are calculated for Lundgren–Townsend vortex models of the fine scales of homogeneous turbulence. Specific results are given for the Burgers vortex and also for the Lundgren-strained spiral vortex. For the latter case, it is found that the contribution to the shell-summed spectrum produced by the interaction between the axisymmetric and nonaxisymmetric components of the velocity field is proportional to k^–7/3 (k=||k|| is the modulus of the wave number) in agreement with Kolmogorov-type dimensional arguments. Numerical estimates of the dimensionless prefactors for this component are obtained in Kolmogorov scaling variables and comparisons are made with results from the Batchelor–Kolmogorov theory, and with experimental measurement.

Link permanente para citações: