Lagrangian acceleration statistics in a turbulent channel flow

Lagrangian acceleration statistics in a fully developed turbulent channel flow at $Re_\tau = 1440$ are investigated, based on tracer particle tracking in experiments and direct numerical simulations. The evolution with wall distance of the Lagrangian velocity and acceleration time scales is analyzed. Dependency between acceleration components in the near-wall region is described using cross-correlations and joint probability density functions. The strong streamwise coherent vortices typical of wall-bounded turbulent flows are shown to have a significant impact on the dynamics. This results in a strong anisotropy at small scales in the near-wall region that remains present in most of the channel. Such statistical properties may be used as constraints in building advanced Lagrangian stochastic models to predict the dispersion and mixing of chemical components for combustion or environmental studies.Comment: accepted for publication in Physical Review Fluid

Retailing farmed fish In southern Europe: supermarkets lead other seafood retailers.

Seafood retail channels in Southern Europe include four main classes: traditional fish markets and fishmongers, supermarkets and hypermarkets. Location, primary products and typical level of interaction with customers vary among the formats. In a survey in Spain, price was much more important for customers of supermarkets and hypermarkets than for those of the traditional channels, who rated quality more important. The empathy and reliability of vendors were scored higher for the traditional stores

Utilización de materiales compuestos en actividades de reconstrucción

El presente trabajo se enmarca dentro del conjunto de acciones y pautas previstas para la realización de un estudio previo de viabilidad técnica para la redacción del proyecto de reconstrucción de la Isla Horadada de la bahía de Santander, España. La Isla de la Horadada es una formación rocosa, de aproximadamente 70 m2, en el centro de la bahía de Santander y por ello expuesta a las inclemencias del tiempo, de las mareas y de los temporales. La formación presentaba hasta el año 2005 una curiosa estructura geométrica natural en forma de arco. La parte superior de la estructura colapsó con motivo de un temporal invernal. Como consecuencia de que la historia de la ciudad de Santander se encuentra estrechamente arraigada a la estructura objeto de estudio, se impulsó, por parte del Ayuntamiento de la ciudad, una serie de actuaciones cuyo objetivo fue estudiar la viabilidad técnica de la reconstrucción del arco de la isla. El carácter multidisciplinar de este tipo de actuaciones hace necesaria una distribución racional de las tareas a realizar. Dentro de este marco general, y en base a la contrastada experiencia de los integrantes de los equipos de investigación participantes, se encargaron labores experimentales encaminadas a lograr la caracterización física, mecánica y de durabilidad de los materiales originales y de los susceptibles de ser utilizados en la reconstrucción, así como del estudio de la compatibilidad entre ellos

PencilArrays.jl: Distributed Julia arrays using the MPI protocol

PencilArrays v0.19.1 Diff since v0.19.0 Merged pull requests: Allow HDF5.jl 0.17 (#85) (@jipolanco) Closed issues: Type Issue with Transforms.NoTransform!() (#84)If you use this software, please cite it as below

Propriétés lagrangiennes d’un écoulement de canal turbulent : une étude numérique

The Lagrangian perspective, describing a flow from the trajectories of fluid tracers, isa natural framework for studying dispersion phenomena in turbulent flows. In wall-boundedturbulence, the motion of fluid tracers is affected by mean shear and by strong inhomogeneityand anisotropy near walls. We investigate the Lagrangian properties of a turbulent channel flowusing direct numerical simulations at a moderate Reynolds number. Lagrangian accelerationstatistics are compared to particle tracking experiments performed in parallel to this work. Asin homogeneous isotropic turbulence (HIT), the acceleration components along Lagrangianpaths decorrelate over time scales representative of the smallest scales of the flow, while theacceleration norm stays correlated for much longer. The persistence of small-scale anisotropy farfrom the wall is demonstrated in the form of a non-zero cross-correlation between accelerationcomponents. As a result of the average fluxes of kinetic energy in wall turbulence, tracers initiallylocated close to the wall travel and spread over longer distances when tracked backwardsin time than forwards. The relative dispersion of tracer pairs is finally investigated. At shorttimes, pair separation is ballistic for all wall distances. As in HIT, relative dispersion is timeasymmetric, with tracers separating faster when tracked backwards in time. At longer times,mean shear dominates leading to rapid separation in the mean flow direction. A ballisticcascade model previously proposed for HIT is adapted to inhomogeneous flowsLa perspective lagrangienne, décrivant un écoulement selon les trajectoires de traceurs fluides, est une approche naturelle pour étudier les phénomènes de dispersion dans les écoulements turbulents. En turbulence de paroi, le mouvement des traceurs est influencé par le cisaillement moyen et par une forte inhomogénéité et anisotropie en proche paroi. On étudie les propriétés lagrangiennes d’un écoulement de canal turbulent par simulation numérique directe à un nombre de Reynolds modéré. Les statistiques d’accélération lagrangienne sont comparées aux expériences de suivi de particules réalisées en parallèle à ce travail. Comme en turbulence homogène isotrope (THI), les composantes d’accélération le long des trajectoires lagrangiennes se décorrèlent sur des temps comparables aux plus petites échelles de l’écoulement, tandis que la norme de l’accélération reste corrélée plus longtemps. La persistance d’anisotropie à petite échelle loin de la paroi est constatée par l’existence d’une corrélation croisée non nulle entredeux composantes de l’accélération. On montre que, en conséquence des flux moyens d’énergie cinétique en turbulence de paroi, près des parois les traceurs se déplacent et s’étalent sur des plus grandes distances quand ils sont suivis en arrière dans le temps qu’en avant. La dispersion relative de paires de traceurs est aussi étudiée. Aux temps courts, la séparation des paires est balistique pour toutes les distances à la paroi. Comme en THI, les traceurs se séparent plus rapidement lorsqu’ils sont suivis en arrière dans le temps. Aux temps plus longs, le cisaillement moyen accélère la séparation dans la direction de l’écoulement moyen. Un modèle de cascade balistique initialement proposé pour la THI est adapté aux écoulements inhomogène

Counterflow-Induced Inverse Energy Cascade in Three-Dimensional Superfluid Turbulence

International audienceFinite-temperature quantum turbulence is often described in terms of two immiscible fluids that can flow with a nonzero-mean relative velocity. Such out-of-equilibrium state is known as counterflow superfluid turbulence. We report here the emergence of a counterflow-induced inverse energy cascade in three-dimensional superfluid flows by performing extensive numerical simulations of the Hall-Vinen-Bekarevich-Khalatnikov model. As the intensity of the mean counterflow is increased, an abrupt transition, from a fully three-dimensional turbulent flow to a quasi-two-dimensional system exhibiting a split cascade, is observed. The findings of this work could motivate new experimental settings to study quasi-two-dimensional superfluid turbulence in the bulk of three-dimensional experiments. They might also find applications beyond superfluids in systems described by more than one fluid component

Inhomogeneous distribution of particles in coflow and counterflow quantum turbulence

International audienc

Multi-particle Lagrangian statistics in homogeneous rotating turbulence

Geophysical flows are often turbulent and subject to rotation. This rotation modifies the structure of turbulence and is thereby expected to sensibly affect its Lagrangian properties. Here, we investigate the relative dispersion and geometry of pairs, triads and tetrads in homogeneous rotating turbulence, by using direct numerical simulations at different rotation rates. Pair dispersion is shown to be faster in the vertical direction (along the rotation axis) than in the horizontal one. At long times, in Taylor's regime, this is due to the slower decorrelation of the vertical velocity component as compared to the horizontal one. At short times, in the ballistic regime, this result can be interpreted by considering pairs of different orientations at the release time, and is a signature of the anisotropy of Eulerian second-order functions. Rotation also enhances the distortion of triads and tetrads also present in homogeneous and isotropic turbulence. In particular, at long times, the flattening of tetrads increases with the rotation rate. The maximal dimension of triads and tetrads is shown to be preferentially aligned with the rotation axis, in agreement with our observations for pairs.Comment: 18 pages, 14 figure

Vortex clustering, polarisation and circulation intermittency in classical and quantum turbulence

International audienceThe understanding of turbulent flows is one of the biggest current challenges in physics, as no first-principles theory exists to explain their observed spatio-temporal intermittency. Turbulent flows may be regarded as an intricate collection of mutually-interacting vortices. This picture becomes accurate in quantum turbulence, which is built on tangles of discrete vortex filaments. Here, we study the statistics of velocity circulation in quantum and classical turbulence. We show that, in quantum flows, Kolmogorov turbulence emerges from the correlation of vortex orientations, while deviations-associated with intermittency-originate from their non-trivial spatial arrangement. We then link the spatial distribution of vortices in quantum turbulence to the coarse-grained energy dissipation in classical turbulence, enabling the application of existent models of classical turbulence intermittency to the quantum case. Our results provide a connection between the intermittency of quantum and classical turbulence and initiate a promising path to a better understanding of the latter

Intermittency of Velocity Circulation in Quantum Turbulence

International audienceThe velocity circulation, a measure of the rotation of a fluid within a closed path, is a fundamental observable in classical and quantum flows. It is indeed a Lagrangian invariant in inviscid classical fluids. In quantum flows, circulation is quantized, taking discrete values that are directly related to the number and the orientation of thin vortex filaments enclosed by the path. By varying the size of such closed loops, the circulation provides a measure of the dependence of the flow structure on the considered scale. Here, we consider the scale dependence of circulation statistics in quantum turbulence, using high-resolution direct numerical simulations of a generalized Gross-Pitaevskii model. Results are compared to the circulation statistics obtained from simulations of the incompressible Navier-Stokes equations. When the integration path is smaller than the mean intervortex distance, the statistics of circulation in quantum turbulence displays extreme intermittent behavior due to the quantization of circulation, in stark contrast with the viscous scales of classical flows. In contrast, at larger scales, circulation moments display striking similarities with the statistics probed in the inertial range of classical turbulence. In particular, we observe the emergence of the power-law scalings predicted by Kolmogorov's 1941 theory, as well as intermittency deviations that closely follow the recently proposed bifractal model for circulation moments in classical flows. To date, these findings are the most convincing evidence of intermittency in the large scales of quantum turbulence. Moreover, our results strongly reinforce the resemblance between classical and quantum turbulence, highlighting the universality of inertial-range dynamics, including intermittency, across these two a priori very different systems. This work paves the way for an interpretation of inertialrange dynamics in terms of the polarization and spatial arrangement of vortex filaments