CiteULike: l-alex's lagrangian

An experimental investigation on Lagrangian correlations of small-scale turbulence at low Reynolds number

Michele Guala — 2008-07-17T20:39:52-00:00

Journal of Fluid Mechanics, Vol. 574, No. -1. (2007), pp. 405-427.

Lagrangian auto- and cross-correlation functions of the rate of strain s2, enstrophy ω2, their respective production terms −sijsjkski and ωiωjsij, and material derivatives, Ds2/Dt and Dω2/Dt are estimated using experimental results obtained through three-dimensional particle tracking velocimetry (three-dimensional-PTV) in homogeneous turbulence at Reλ=50. The autocorrelation functions are used to estimate the Lagrangian time scales of different quantities, while the cross-correlation functions are used to clarify some aspects of the interaction mechanisms between vorticity ω and the rate of strain tensor sij, that are responsible for the statistically stationary, in the Eulerian sense, levels of enstrophy and rate of strain in homogeneous turbulent flow. Results show that at the Reynolds number of the experiment these quantities exhibit different time scales, varying from the relatively long time scale of ω2 to the relatively shorter time scales of s2, ωiωjsij and −sijsjkski. Cross-correlation functions suggest that the dynamics of enstrophy and strain, in this flow, is driven by a set of different-time-scale processes that depend on the local magnitudes of s2 and ω2. In particular, there are indications that, in a statistical sense, (i) strain production anticipates enstrophy production in low-strain–low-enstrophy regions (ii) strain production and enstrophy production display high correlation in high-strain–high-enstrophy regions, (iii) vorticity dampening in high-enstrophy regions is associated with weak correlations between −sijsjkski and s2 and between −sijsjkski and Ds2/Dt, in addition to a marked anti-correlation between ωiωjsij and Ds2/Dt. Vorticity dampening in high-enstrophy regions is thus related to the decay of s2 and its production term, −sijsjkski.

On the distribution of Lagrangian accelerations in turbulent flows

AM Reynolds — 2008-07-13T07:38:40-00:00

New J Physics (2005)

Reynolds number dependence of relative dispersion statistics in isotropic turbulence

Brian Sawford — 2008-07-01T13:49:26-00:00

Physics of Fluids, Vol. 20, No. 6. (2008)

Chaotic advection and heat transfer enhancement in Stokes flows

A Lefèvre — 2008-06-16T08:49:21-00:00

International Journal of Heat and Fluid Flow, Vol. 24, No. 3. (June 2003), pp. 310-321.

The heat transfer rate from a solid boundary to a highly viscous fluid can be enhanced significantly by a phenomenon which is called chaotic advection or Lagrangian turbulence. Although the flow is laminar and dominated by viscous forces, some fluid particle trajectories are chaotic due either to a suitable boundary displacement protocol or to a change in geometry. As in turbulent flow, the heat transfer rate enhancement between the boundary and the fluid is intimately linked to the mixing of fluid in the system. Chaotic advection in real Stokes flows, i.e. flows governed by viscous forces and that can be constructed experimentally, is reviewed in this paper. An emphasis is made on recent new results on 3-D time-periodic open flows which are particularly important in industry.

Chaotic mixing and mixing efficiency in a short time

Mitsuaki Funakoshi — 2008-06-16T08:44:03-00:00

Fluid Dynamics Research, Vol. 40, No. 1. (January 2008), pp. 1-33.

Several studies of the chaotic motion of fluid particles by two-dimensional time-periodic flows or three-dimensional steady flows, called Lagrangian chaos, are first introduced. Secondly, some of the studies on efficient mixing caused by Lagrangian chaos, called chaotic mixing, are reviewed with discussion of several indices for the estimation of mixing efficiency. Finally, several indices to estimate the efficiency of mixing in a short time, such as those related to transport matrices, stable and unstable manifolds of hyperbolic periodic points of Poincaré maps, and lines of separation, are explained by showing examples of mixing by two-dimensional time-periodic flows between eccentric rotating cylinders and mixing by three-dimensional steady flows in a model of static mixers.

Statistics of passive tracers in three-dimensional magnetohydrodynamic turbulence

Angela Busse — 2008-01-08T21:56:56-00:00

Physics of Plasmas, Vol. 14, No. 12. (2007)

Eulerian diagnostics for Lagrangian chaos in three-dimensional Navier-Stokes flows

AN Yannacopoulos — 2007-12-28T22:43:11-00:00

Physical Review E, Vol. 57, No. 1. (January 1998), 482.

Based on symmetry considerations; Eulerian quantities are defined which can serve as diagnostics for the regions of the flow where Lagrangian chaos is possible in a three-dimensional Navier-Stokes flow. The applicability of the diagnostics is tested in two model flows which are perturbative solutions of the three-dimensional Navier-Stokes equation: the eccentric Taylor vortex and the (concentric) wavy Taylor vortex.

Evaluation of some proposed forms of Lagrangian velocity correlation coefficient

K Manomaiphiboon — 2007-11-24T08:17:19-00:00

International Journal of Heat and Fluid Flow, Vol. 24, No. 5. (October 2003), pp. 709-712.

This work evaluates four different forms of Lagrangian velocity correlation coefficient for stationary homogeneous turbulence at very large Reynolds numbers through consideration of simple mathematical and physical requirements. It is shown that some of them do not comply well with the requirements and may not be appropriate for use.

Turbulence modulation in dispersed two-phase flow laden with solids from a Lagrangian perspective

S Lain — 2007-06-15T09:16:34-00:00

International Journal of Heat and Fluid Flow, Vol. 24, No. 4. (August 2003), pp. 616-625.

In this paper numerical calculations of dispersed particulate two-phase flows, based on the Euler/Lagrange approach, are presented with special emphasis on two-way coupling and turbulence modification. The performance of the Lagrangian consistent terms, including the so-called wake-induced turbulence, that influence the continuous phase turbulent variables in a two-phase flow is compared with the standard terms resulting from the Reynolds average procedure. For this purpose two different gas-solid flow configurations laden with small and large particles are considered, namely an axisymmetric particle-laden jet flow and an upward pipe flow. As a result, the consistent terms are able to reproduce both situations, suppression and enhancement of fluid turbulence for small and large particles, whereas the standard terms can only deal with situations implying turbulence attenuation.

Lagrangian dispersion in turbulent channel flow and its relationship to Eulerian statistics

Jianping Luo — 2007-09-19T21:30:49-00:00

International Journal of Heat and Fluid Flow, Vol. 28, No. 5. (October 2007), pp. 871-881.

Lagrangian and Eulerian statistics were obtained from the direct numerical simulation of a turbulent channel flow. The Eulerian integral time scales and time microscales were compared to their Lagrangian equivalents. It is found that the Lagrangian time scales equal the Eulerian time scales at the wall. Otherwise, these proved to be consistently larger than the Eulerian time scales, and the latter are also found to be scaled by the propagation velocity rather than the mean velocity. The near-wall behavior of the ratio of the Lagrangian to Eulerian integral time scales is explained by the difference between the mean velocity and the propagation velocity of the turbulent structure. The ratio is proportional to the inverse of the turbulence intensity (TL/TE = [beta]U/[sigma]). [beta] for the streamwise component is nearly a constant value of 0.6 (20 < y+ < 70), in agreement with atmospheric data and analyses. The ratio of the Lagrangian to the Eulerian time microscales is fairly constant away from the wall (y+ > 40). After period over the integral time scale elapsed, the effect of shear on turbulent diffusion appears and the mean-square dispersion is almost proportional to t3 in agreement with the predictions of Riley and Corrsin [The relation of turbulent diffusivities to Lagrangian velocity statistics for the simplest shear flow. J. Geophys. Res. 79 (1974) 1768-1771] for homogeneous shear flow. After particles are distributed fairly uniformly between two walls, the mean-square dispersion becomes proportional to t and in agreement with theory of Taylor's longitudinal dispersion.

Walls Inhibit Chaotic Mixing

E Gouillart — 2007-09-14T19:42:32-00:00

Physical Review Letters, Vol. 99, No. 11. (2007)

We report on experiments of chaotic mixing in a closed vessel, in which a highly viscous fluid is stirred by a moving rod. We analyze quantitatively how the concentration field of a low-diffusivity dye relaxes towards homogeneity, and we observe a slow algebraic decay of the inhomogeneity, at odds with the exponential decay predicted by most previous studies. Visual observations reveal the dominant role of the vessel wall, which strongly influences the concentration field in the entire domain and causes the anomalous scaling. A simplified 1D model supports our experimental results. Quantitative analysis of the concentration pattern leads to scalings for the distributions and the variance of the concentration field consistent with experimental and numerical results.

Modification of the turbulent energy cascade by polymer additives

Nicholas Ouellette — 2007-09-04T21:50:13-00:00

(29 Aug 2007)

By tracking small particles in the bulk of an intensely turbulent flow, we show that even a very small concentration of long-chain polymers disrupts the usual turbulent energy cascade. The polymers affect scales much larger than their physical size. We also show that the small-scale dynamics depend strongly on the polymer concentration. We find that the inertial-range statistics return to their Newtonian levels for small concentrations (less than 5 parts per million by weight for the polymer we used), while for larger concentrations we find new, concentration-dependent results.

Evolution of geometric structures in intense turbulence

Haitao Xu — 2007-09-04T21:30:28-00:00

(30 Aug 2007)

We report measurements of the evolution of lines, planes, and volumes in an intensely turbulent laboratory flow using high-speed particle tracking. We find that a time scale based on the initial size of the object considered is the natural time scale for its further evolution. The initial separation may only be neglected if this time scale is much smaller than the largest turbulence time scale, implying extremely high turbulence levels. We also show that turbulence tends to flatten material volumes to nearly planar shapes.

Experimental measurement of acceleration correlations and pressure structure functions in high Reynolds number turbulence

Haitao Xu — 2007-09-04T21:15:08-00:00

(29 Aug 2007)

We present measurements of fluid particle accelerations in turbulent water flows between counter-rotating disks using three-dimensional Lagrangian particle tracking. By simultaneously following multiple particles with sub-Kolmogorov-time-scale temporal resolution, we measured the spatial correlation of fluid particle acceleration at Taylor microscale Reynolds numbers between 200 and 690. We also obtained indirect, non-intrusive measurements of the Eulerian pressure structure functions by integrating the acceleration correlations. Our experimental data provide strong support to the theoretical predictions of the acceleration correlations and the pressure structure function in isotropic high Reynolds number turbulence by Obukhov and Yaglom in 1951. The measured pressure structure functions display K41 scaling in the inertial range.

Lagrangian Particle Statistics in Turbulent Flows from a Simple Vortex Model

M Wilczek — 2007-07-05T12:01:44-00:00

(3 Jul 2007)

The statistics of Lagrangian particles in turbulent flows is considered in the framework of a simple vortex model. Here, the turbulent velocity field is represented by a temporal sequence of Burgers vortices of different circulation, strain, and orientation. Based on suitable assumptions about the vortices' statistical properties, the statistics of the velocity increments is derived. In particular, the origin and nature of small-scale intermittency in this model is investigated both numerically and analytically.

Eulerian and Lagrangian studies in surface flow turbulence

Ohn Cressman — 2007-04-27T13:07:25-00:00

New Journal of Physics, Vol. 6 (2004), 53.

10.1088/1367-2630/6/1/053

Determination of the coefficients of Langevin models for inhomogeneous turbulent flows by three-dimensional particle tracking velocimetry and direct numerical simulation

RJE Walpot — 2007-04-15T18:38:06-00:00

Physics of Fluids, Vol. 19, No. 4. (2007)

A promising and, in terms of computer power, low-cost way of describing flow properties such as turbulent diffusion is by Langevin models. The development of such models requires knowledge of Lagrangian statistics of turbulent flows. Our aim is to determine Lagrangian statistics of inhomogeneous flows, as most turbulent flows found in practical applications are inhomogeneous. The present paper describes how a Lagrangian measurement technique, three-dimensional particle tracking velocimetry, has been developed and applied to the most common example of inhomogeneous flows: turbulent pipe flow. A new direct numerical simulation (DNS) code has been developed and experimental results have been compared with results of this DNS code. The results concern Eulerian and Lagrangian velocity statistics at two Reynolds numbers. Based on these, coefficients of the Langevin model have been determined and physical consequences for Langevin modeling and turbulent dispersion have been explained. ©2007 American Institute of Physics

Uncovering the Lagrangian Skeleton of Turbulence

Manikandan Mathur — 2007-04-06T22:49:35-00:00

Physical Review Letters, Vol. 98, No. 14. (2007)

We present a technique that uncovers the Lagrangian building blocks of turbulence, and apply this technique to a quasi-two-dimensional turbulent flow experiment. Our analysis identifies an intricate network of attracting and repelling material lines. This chaotic tangle, the Lagrangian skeleton of turbulence, shows a level of complexity found previously only in theoretical and numerical examples of strange attractors. We quantify the strength (hyperbolicity) of each material line in the skeleton and demonstrate dramatically different mixing properties in different parts of the tangle.

Inertial particles driven by a telegraph noise

G Falkovich — 2007-04-02T05:13:51-00:00

(28 Mar 2007)

We present a model for the Lagrangian dynamics of inertial particles in a compressible flow, where fluid velocity gradients are modelled by a telegraph noise. The model allows for an analytic investigation of the role of time correlation of the flow in the aggregation-disorder transition of inertial particle. The dependence on Stokes and Kubo numbers of the Lyapunov exponent of particle trajectories reveals the presence of a region in parameter space (St, Ku) where the leading Lyapunov exponent changes sign, thus signaling the transition. The asymptotics of short and long-correlated flows are discussed, as well as the fluid-tracer limit.

Curvature of Lagrangian Trajectories in Turbulence

Haitao Xu — 2007-02-06T00:41:26-00:00

Physical Review Letters, Vol. 98, No. 5. (2007)

We report measurements of the curvature of Lagrangian trajectories in an intensely turbulent laboratory water flow measured with a high-speed particle-tracking system. The probability density function (PDF) of the instantaneous curvature is shown to have robust power-law tails. We propose a model for the instantaneous curvature PDF, assuming that the acceleration and velocity are uncorrelated Gaussian random variables, and show that our model reproduces the tails of our measured PDFs. We also predict the scaling of the most probable vorticity magnitude in turbulence, assuming Heisenberg-Yaglom scaling. Finally, we average the curvature along trajectories and show that, by removing the effects of large-scale flow reversals, the filtered curvature reveals the turbulent features.

A stochastic Lagrangian representation of the 3-dimensional incompressible Navier-Stokes equations

Peter Constantin — 2005-11-05T09:22:32-00:00

(3 Nov 2005)

In this paper we derive a representation of the deterministic 3-dimensional Navier-Stokes equations based on stochastic Lagrangian paths. The particle trajectories obey SDEs driven by a uniform Wiener process; the inviscid Weber formula for the Euler equations of ideal fluids is used to recover the velocity field. This method admits a self-contained proof of local existence for the nonlinear stochastic system, and can be extended to formulate stochastic representations of related hydrodynamic-type equations, including viscous Burgers equations and LANS-alpha models.

Application of the finite-size Lyapunov exponent to particle tracking velocimetry in fluid mechanics experiments

Natalie Kleinfelter — 2007-01-15T20:31:05-00:00

Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 72, No. 5. (2005)

A finite-size (or scale) Lyapunov exponent (FSLE), a(x), is presented in a statistical mechanical framework and employed to characterize mixing in a variety of laboratory and computational fluid mechanics experiments. The FSLE is the exponential rate at which two particles separate from a distance x to ax. Laboratory particle tracking experiments are used to study penetrative convection and flow in porous media while computational experiments are used to study Lévy processes and deterministic diffusion. The apparent scaling relation a(x)~Cax–(a) of the FSLE holds over intermediate initial separations where the laboratory experiment data is most accurate and asymptotically for the computational experiments. The dependence of the exponent on a decreases with increasing a. In the matched index porous system, Ca is also a function of mean fluid velocity. The exponent is when the Lévy process is -stable and in this case is independent of a.

Statistics of 3-dimensional Lagrangian turbulence

Christian Beck — 2007-01-09T07:04:57-00:00

(8 Jan 2007)

We consider a superstatistical dynamical model for the 3-d movement of a Lagrangian tracer particle embedded in a high-Reynolds number turbulent flow. The analytical model predictions are in excellent agreement with recent experimental data for flow between counter-rotating disks. In particular, we calculate the Lagrangian scaling exponents zeta_j for our system, and show that they agree well with the measured exponents reported in [X. Hu et al., PRL 96, 114503 (2006)]. Moreover, the model correctly predicts the shape of velocity difference and acceleration probability densities, the fast decay of component correlation functions and the slow decay of the modulus, as well as the statistical dependence between acceleration components. Finally, the model explains the numerically [P.K. Yeung and S.B. Pope, J. Fluid Mech. 207, 531 (1989)] and experimentally observed fact [B.W. Zeff et al., Nature 421, 146 (2003)] that enstrophy lags behind dissipation.

Two-particle description of turbulence, Markov property, and intermittency

EA Novikov — 2006-12-28T20:44:52-00:00

Physics of Fluids A: Fluid Dynamics, Vol. 1, No. 2. (1989), pp. 326-330.

It is shown that the hypothesis of independent increments for velocity, which is widely used by many authors [e.g., A. M. Obukhov, Adv. Geophys. 6, 113 (1959)] in the Lagrangian description of turbulence, is inconsistent with the Navier–Stokes equations in a fundamental way. A more general Lagrangian description of turbulent velocity such as the Markov process with dependent increments, which recognizes the condition of incompressibility and the important phenomenon of intermittency, is proposed. A model of intermittent relative motion of fluid particles in turbulent flow is presented. The high-order Lagrangian moments and the probability distribution are obtained. The distribution for the intermittent vorticity is also proposed. Physics of Fluids A: Fluid Dynamics is copyrighted by The American Institute of Physics.

Trajectory Curvature As A Selection Criterion For valid Lagrangian Stochastic Dispersion Models

John Wilson — 2006-12-28T20:20:48-00:00

Boundary-Layer Meteorology, Vol. V84, No. 3. (1997), pp. 411-425.

Lagrangian Statistics from Numerically Integrated Turbulent Shear Flow

JW Deardorff — 2006-09-22T16:40:09-00:00

Physics of Fluids, Vol. 13, No. 3. (1970), pp. 584-595.

Lagrangian turbulence statistics were obtained from a three-dimensional numerical model of plane Poiseuille flow at large Reynolds number. Single-particle Lagrangian correlations were computed and compared with Eulerian space correlations. Although the curves were not self-similar, a dimensionless scale ratio defined at the e-folding time was comparable to Eulerian-Lagrangian scale ratios found in the literature. The numerically obtained mean-square particle displacements exhibited correct short- and long-time behavior. Mean-square particle separations were analyzed, and two-particle Lagrangian velocity correlations taken at the same time were more persistent than Lagrangian autocorrelations. A semiempirical functional form was constructed for the two-particle velocity correlations which yielded two-particle distance correlations in good agreement with those of the numerical model. The effect of mean shear on downstream separation was examined. Results indicate a t3 or steeper dependence for downstream mean-square separation (xa – xb)2, with strong shear and Reynolds stress. Batchelor's similarity law, namely, that (xaj – xbj)2 t3 in directions not controlled by shear, is postulated for the direction of shear when shear generates . This postulate was tested numerically and found to be consistent. ©1970 American Institute of Physics

Measurement of Lagrangian Velocity in Fully Developed Turbulence

N Mordant — 2006-09-21T13:48:33-00:00

Physical Review Letters, Vol. 87, No. 21. (2 November 2001), 214501.

We have developed a new experimental technique to measure the Lagrangian velocity of tracer particles in a turbulent flow; based on ultrasonic Doppler tracking. This method yields a direct access to the velocity of a single particle at a turbulent Reynolds number R Î» Â =Â 740; with two decades of time resolution; below the Lagrangian correlation time. We observe that the Lagrangian velocity spectrum has a Lorentzian form E L (Ï)Â =Â u rms 2 T L /[1+( T L Ï) 2 ]; in agreement with a Kolmogorov-like scaling in the inertial range. The probability density functions of the velocity time increments display an intermittency which is more pronounced than that of the corresponding Eulerian spatial increments.

Lagrangian statistics from direct numerical simulations of isotropic turbulence

PK Yeung — 2006-09-20T12:31:54-00:00

J. Fluid Mech, Vol. 207 (1989), pp. 531-586.

Lagrangian characteristics of turbulence and scalar transport in direct numerical simulations

PK Yeung — 2006-09-20T12:27:53-00:00

J. Fluid Mech, Vol. 427 (2001), pp. 241-274.

One- and two-particle Lagrangian acceleration correlations in numerically simulated homogeneous turbulence

PK Yeung — 2006-09-20T12:25:59-00:00

Phys. Fluids, Vol. 9, No. 10. (1997), pp. 2981-2990.

Lagrangian statistics in turbulent channel flow

Jung Choi — 2006-08-31T22:21:39-00:00

Physics of Fluids, Vol. 16, No. 3. (2004), pp. 779-793.

The Lagrangian dispersion of fluid particles in inhomogeneous turbulence is investigated by a direct numerical simulation of turbulent channel flow. Lagrangian velocity and acceleration along a particle trajectory are computed by employing several interpolation schemes. Among the schemes tested, the four-point Hermite interpolation in the homogeneous directions combined with Chebyshev polynomials in the wall-normal direction seems to produce most reliable Lagrangian statistics. Inhomogeneity of Lagrangian statistics in turbulent boundary layer is investigated by releasing many particles at several different wall-normal locations and tracking those particles. The fluid particle dispersion and Lagrangian structure function of velocity are investigated for the Kolmogorov similarity. The behavior of the Lagrangian integral time scales, Kolmogorov constants a0 and C0 of the velocity structure function near the wall are discussed. The intermittent behavior of the fluid particle acceleration is also examined by kurtosis of the Lagrangian structure function. Finally, the effect of the initial particle location on the dispersion is analyzed by the probability density function of particle position at several downstream locations. ©2004 American Institute of Physics.

Lagrangian multi-particle statistics

B Luethi — 2006-08-31T22:06:58-00:00

unpublished (2006)

Lagrangian microscale in turbulence

SB Pope — 2006-07-12T22:02:34-00:00

Phil. Trans. Phys. Sci. Eng., Vol. 333, No. 1631. (15 November 1990), pp. 309-319.

DNS, turbulent straining is not persistent, both acceleration and strain rate, directional information is lost within 2 Kolmogorov timescales whereas the amplitudes of those have longer timescales.

Lagrangian statistics for fluid particles and bubbles in turbulence

Irene Mazzitelli — 2006-07-02T20:10:49-00:00

New Journal of Physics, Vol. 6 (2004)

The dispersion of bubbles in homogeneous and isotropic turbulence is numerically examined. The fluid velocity field evolves according to the Navier–Stokes equations that are solved by direct numerical simulation. The bubble paths are followed by Lagrangian tracking. The aim of the work is to quantify dispersion properties of bubbles in a regime ranging from low- to high-turbulence velocity fluctuations as compared to the bubble velocity scale, and to compare them to those of fluid particles. Moreover, the forces that are relevant for the bubble dispersion are analysed and their probability density functions, as well as their intermittency characteristics, are compared to those of fluid particle accelerations.

Lagrangian dynamics and statistical geometric structure of turbulence

L Chevillard — 2006-06-19T15:34:31-00:00

(13 Jun 2006)

The local statistical and geometric structure of three-dimensional turbulent flow can be described by properties of the velocity gradient tensor. A stochastic model is developed for the Lagrangian time evolution of this tensor, in which the exact nonlinear self-stretching term accounts for the development of well-known non-Gaussian statistics and geometric alignment trends. The non-local pressure and viscous effects are accounted for by a closure that models the material deformation history of fluid elements. The system is forced with a simple, white in time, Gaussian noise. The resulting stochastic system reproduces many statistical and geometric trends observed in numerical and experimental 3D turbulent flows. Examples include the non-Gaussian statistics of velocity gradient components, the preferential aligment of vorticity, nearly log-normal statistics of the dissipation, and the tear-drop shape of the so-called R-Q joint probability density. A free parameter entering in the pressure Hessian is used to model finite Reynolds number effects and allows to reproduce anomalous scaling behaviours.

Dynamical Foundations of Nonextensive Statistical Mechanics

Christian Beck — 2006-06-15T19:01:51-00:00

Physical Review Letters, Vol. 87, No. 18. (10 October 2001), 180601.

We construct classes of stochastic differential equations with fluctuating friction forces that generate a dynamics correctly described by Tsallis statistics. These systems generalize the way in which ordinary Langevin equations underlie ordinary statistical mechanics to the more general nonextensive case. As a main example; we construct a dynamical model of velocity fluctuations in a turbulent flow; which generates probability densities that very well fit experimentally measured probability densities in Eulerian and Lagrangian turbulence. Our approach provides a dynamical reason why many physical systems with fluctuations in temperature or energy dissipation rate are correctly described by Tsallis statistics.

Experimental Lagrangian acceleration probability density function measurement

N Mordant — 2006-06-15T16:21:21-00:00

Physica D: Nonlinear Phenomena, Vol. 193, No. 1-4. (15 June 2004), pp. 245-251.

We report experimental results on the acceleration component probability distribution function at R[lambda]=690 to probabilities of less than 10-7. This is an improvement of more than an order of magnitude over past measurements and allows us to conclude that the fourth moment converges and the flatness is approximately 55. We compare our probability distribution to those predicted by several models inspired by non-extensive statistical mechanics. We also look at acceleration component probability distributions conditioned on a velocity component for conditioning velocities as high as three times the standard deviation and find them to be highly non-Gaussian.

Experimental Determination of Lagrangian Velocity Statistics in Turbulent Pipe Flow

R Walpot — 2006-06-07T12:16:34-00:00

Applied Scientific Research, Vol. 76, No. 2. (March 2006), pp. 163-175.

Statistics of particle pair separations in the elastic turbulent flow of a dilute polymer solution

T Burghelea — 2006-06-15T08:14:32-00:00

Europhys. Lett., Vol. 68, No. 4. (2004), pp. 529-535.

We investigate experimentally the statistics of a chaotic flow of a dilute polymer solution in a regime of elastic turbulence by using the Lagrangian coordinates approach. We show that due to flow smoothness at small scales the Finite Time Lyapunov Exponent (FTLE) technique can be successfully used to investigate the statistics of particle pair separations at different scales. We compare the measured FTLE with the characteristics of statistical description in the Eulerian coordinate presentation, namely the velocity correlation times and the average velocity gradients. We characterize the flow intermittency by measuring high-order moments of the statistics of the particle pair separations.

Transport effects associated with turbulence with particular attention to the influence of helicity

HK Moffatt — 2006-06-15T07:45:13-00:00

Reports on Progress in Physics, Vol. 46, No. 5. (1983), pp. 621-664.

The action of turbulence on a passive convected scalar field (e.g. temperature) or vector field (e.g. the magnetic field in an electrically conducting fluid) is reviewed, with particular attention paid to anomalous effects that can arise through the influence of Coriolis forces in a rotating system on the statistics of the turbulence. The simplest such effect (which corresponds to a breaking of the Onsager symmetry relations) is a 'skew-diffusion' effect, i.e. the appearance of a component of turbulent heat flux perpendicular to the local mean temperature gradient. The famous alpha effect of magnetohydrodynamic dynamo theory is also in this category, as is the more subtle Radler effect (the appearance of a mean electromotive force perpendicular to the mean current in a plasma). These effects are all associated with the helicity of a turbulent flow, i.e. the correlation between the velocity field u(x,t) and the vorticity field omega (x,t)=curl u.

Eulerian and Lagrangian time microscales in isotropic turbulence

H Tennekes — 2006-06-08T12:50:27-00:00

@JFM, Vol. 67, No. 3. (1975), pp. 561-567.

Lagrangian spectrum up to $(ε/ν)^1/2$, Eulerian extends to higher frequencies than its Lagrangian counterpart. As a consequence, $∂ u/∂ t$ is overestimated by a factor of $Re_λ^1/2$. The Eulerian spectrum has $ω^-5/3$ behaviour, which is not governed by Kolmogorov similarity.

The mathematical nature of the problem of relating Lagrangian and Eulerian statistical functions in turbulence

JL Lumley — 2006-05-08T13:16:01-00:00

(28 August 1961), pp. 17-26.

The mathematical nature of the problem of relating Lagrangian and Eulerian statistical functions in turbulence

JL Lumley — 2006-04-20T15:32:48-00:00

Mechanique de la Turbulence (28 August 1961), pp. 17-26.

Origin of Non-Gaussian Statistics in hydrodynamics turbulence

Yi Li — 2005-09-05T16:01:04-00:00

Physical Review Letters, Vol. 95, No. 164502. (2005)

We report the first experimental demonstration of coherent population transfer, induced by stimulated Raman adiabatic passage, via continuum states. Population is transferred from the metastable state 2s 1S0 to the excited state 4s 1S0 in helium atoms in a two-photon process mediated by coherent interaction with the ionization continuum. While incoherent techniques usually do not permit any population transfer in such a process, we show that stimulated Raman adiabatic passage allows significant population transfer to take place also via ultrafast decay channels.