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

da Silva, Carlos B.; Malico, Isabel; Coelho, Pedro J.; Pereira, José Carlos F.
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.

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

Allison, Colin Bidden
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...

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

Ferrari, Raffaele; Flierl, Glenn R.; Legg, Sonya
Fonte: MIT - Massachusetts Institute of Technology Publicador: MIT - Massachusetts Institute of Technology
EN-US
Relevância na Pesquisa
36.56%
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.

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

Rodríguez Imazio, Paola Carolina
Tipo: info:eu-repo/semantics/doctoralThesis; tesis doctoral; info:eu-repo/semantics/publishedVersion Formato: application/pdf
Relevância na Pesquisa
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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...

## Nonlocality in Homogeneous Superfluid Turbulence

Dix, O. M.; Zieve, R. J.
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.

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

Benzi, Roberto; Ching, Emily S. C.; Procaccia, Itamar
Tipo: Artigo de Revista Científica
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

## A Statistical Theory of Homogeneous Isotropic Turbulence

de Divitiis, Nicola
Tipo: Artigo de Revista Científica
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...

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

Benzi, Roberto; De Angelis, Elisabetta; Govindarajan, Rama; Procaccia, Itamar
Tipo: Artigo de Revista Científica
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.

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

Frisch, Uriel; Bec, Jeremie; Aurell, Erik
Tipo: Artigo de Revista Científica
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

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

de Divitiis, Nicola
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...

## Relating statistics to dynamics in axisymmetric homogeneous turbulence

Godeferd, Fabien S.
Tipo: Artigo de Revista Científica
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

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

Wang, Shaojie
Tipo: Artigo de Revista Científica
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.

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

Kleeorin, N.; Rogachevskii, I.
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

## Lyapunov Analysis of Homogeneous Isotropic Turbulence

de Divitiis, Nicola
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

## 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

Guadayol, Òscar; Peters, Francesc; Stiansen, Jan Erik; Marrasé, Cèlia; Lohrmann, Atle
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...

## Velocity autocorrelations of decaying isotropic homogeneous turbulence

Huang, Mei-Jiau; Leonard, Anthony
Fonte: Instituto de Tecnologia da Califórnia Publicador: Instituto de Tecnologia da Califórnia
Tipo: Article; PeerReviewed Formato: application/pdf
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.

## Note on decay of homogeneous turbulence

Saffman, P. G.
Fonte: Instituto de Tecnologia da Califórnia Publicador: Instituto de Tecnologia da Califórnia
Tipo: Article; PeerReviewed Formato: application/pdf
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.

## The large-scale structure of homogeneous turbulence

Saffman, P. G.
Fonte: Instituto de Tecnologia da Califórnia Publicador: Instituto de Tecnologia da Califórnia
Tipo: Article; PeerReviewed Formato: application/pdf
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.

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

Mohseni, Kamran; Shkoller, Steve; Kosović, Branko; Marsden, Jerrold E.; Carati, Daniele; Wray, Alan; Rogallo, Robert
Fonte: Center for Turbulence Research Publicador: Center for Turbulence Research
Tipo: Book Section; PeerReviewed Formato: application/pdf; application/pdf