8 results
Lagrangian diffusion properties of a free shear turbulent jet
- Bianca Viggiano, Thomas Basset, Stephen Solovitz, Thomas Barois, Mathieu Gibert, Nicolas Mordant, Laurent Chevillard, Romain Volk, Mickaël Bourgoin, Raúl Bayoán Cal
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- Journal:
- Journal of Fluid Mechanics / Volume 918 / 10 July 2021
- Published online by Cambridge University Press:
- 11 May 2021, A25
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A Lagrangian experimental study of an axisymmetric turbulent water jet is performed to investigate the highly anisotropic and inhomogeneous flow field. Measurements are conducted within a Lagrangian exploration module, an icosahedron apparatus, to facilitate optical access of three cameras. Stereoscopic particle tracking velocimetry results in three-component tracks of position, velocity and acceleration of the tracer particles within the vertically oriented jet with a Taylor-based Reynolds number ${\textit {Re}}_\lambda \simeq 230$. Analysis is performed at seven locations from 15 diameters up to 45 diameters downstream. Eulerian analysis is first carried out to obtain critical parameters of the jet and relevant scales, namely the Kolmogorov and large (integral) scales as well as the energy dissipation rate. Lagrangian statistical analysis is then performed on velocity components stationarised following methods inspired by Batchelor (J. Fluid Mech., vol. 3, 1957, pp. 67–80), which aim to extend stationary Lagrangian theory of turbulent diffusion by Taylor to the case of self-similar flows. The evolution of typical Lagrangian scaling parameters as a function of the developing jet is explored and results show validation of the proposed stationarisation. The universal scaling constant $C_0$ (for the Lagrangian second-order structure function), as well as Eulerian and Lagrangian integral time scales, are discussed in this context. Constant $C_0$ is found to converge to a constant value (of the order of $C_0 = 3$) within 30 diameters downstream of the nozzle. Finally, the occurrence of finite particle size effects is investigated through consideration of acceleration-dependent quantities.
Lagrangian turbulence in the woods
- Laurent Chevillard
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- Journal:
- Journal of Fluid Mechanics / Volume 916 / 10 June 2021
- Published online by Cambridge University Press:
- 06 April 2021, F1
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A recent statistical analysis, proposed by Shnapp (J. Fluid Mech., vol. 913, 2021, R2), of Lagrangian velocity measurements in a wind tunnel in the presence of a canopy (a forest or urban morphology), using three-dimensional particle tracking velocimetry (Shnapp et al., Sci. Rep., vol. 9, issue 1, 2019, pp. 1–13), is a great read. In this strongly anisotropic situation, despite the additional roughness induced by the canopy, it is shown that fluctuations of Lagrangian velocity increments over small time scales display very similar behaviour as those observed in homogeneous and isotropic turbulent flows. This is all the more true when focussing on the non-Gaussian and intermittent nature of these fluctuations. At much larger time scales, of the order and greater than the characteristic turnover time scale of the flow, anisotropies implied by the presence of the canopy are quantified using averages of the fluctuating kinetic energy conditioned upon the direction of Lagrangian velocity with respect to the mean Eulerian flow. Shnapp (2021) evidences that, indeed, the canopy modifies the velocity along the trajectories at large scales, in particular its variance, but leaves unchanged its local regularity, as it is pinpointed by the power-law exponents of the structure functions.
Modelling Lagrangian velocity and acceleration in turbulent flows as infinitely differentiable stochastic processes
- Bianca Viggiano, Jan Friedrich, Romain Volk, Mickael Bourgoin, Raúl Bayoán Cal, Laurent Chevillard
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- Journal:
- Journal of Fluid Mechanics / Volume 900 / 10 October 2020
- Published online by Cambridge University Press:
- 11 August 2020, A27
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We develop a stochastic model for Lagrangian velocity as it is observed in experimental and numerical fully developed turbulent flows. We define it as the unique statistically stationary solution of a causal dynamics, given by a stochastic differential equation. In comparison with previously proposed stochastic models, the obtained process is infinitely differentiable at a given finite Reynolds number, and its second-order statistical properties converge to those of an Ornstein–Uhlenbeck process in the infinite Reynolds number limit. In this limit, it exhibits furthermore intermittent scaling properties, as they can be quantified using higher-order statistics. To achieve this, we begin with generalizing the two-layered embedded stochastic process of Sawford (Phys. Fluids A, vol. 3 (6), 1991, pp. 1577–1586) by considering an infinite number of layers. We then study, both theoretically and numerically, the convergence towards a smooth (i.e. infinitely differentiable) Gaussian process. To include intermittent corrections, we follow similar considerations as for the multifractal random walk of Bacry et al. (Phys. Rev. E, vol. 64, 2001, 026103). We derive in an exact manner the statistical properties of this process, and compare them with those estimated from Lagrangian trajectories extracted from numerically simulated turbulent flows. Key predictions of the multifractal formalism regarding the acceleration correlation function and high-order structure functions are also derived. Through these predictions, we understand phenomenologically peculiar behaviours of the fluctuations in the dissipative range, that are not reproduced by our stochastic process. The proposed theoretical method regarding the modelling of infinitely differentiability opens the route to the full stochastic modelling of velocity, including the peculiar action of viscosity on the very fine scales.
Mineralogy, geochemistry and emplacement of the Conakry Igneous Complex, Guinea: implications for the Ni–Cu–PGE mineralization
- Thierry Augé, Éric Gloaguen, Matthieu Chevillard, Laurent Bailly
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- Journal:
- Mineralogical Magazine / Volume 82 / Issue 3 / June 2018
- Published online by Cambridge University Press:
- 15 April 2018, pp. 593-624
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The Conakry Igneous Complex is a mafic-ultramafic intrusion emplaced contemporaneously with the opening of the Atlantic, forming a complex, 55 km x 5 km dyke-like body within which three main episodes of injection have been recognized, characterized by a lack of mineral layering. Unit 1 consists of dunite and related facies, Unit 2 of wehrlite and pyroxene peridotite and Unit 3 corresponds to various gabbro facies. Units 1 and 2 constitute the Kaloum Peninsula; Unit 3 is its NW extension, forming the 1010 m high Mount Kakoulima. Unit 3 intrudes the two previous units and corresponds to a tholeiitic liquid that crystallized in an almost closed system, and thus exhibits a strong differentiation trend, in contrast to Units 1 and 2. Mineral compositions suggest the existence of a deeper magma chamber where a first stage of differentiation occurred.
Disseminated base-metal sulfides (BMS) are present in all units of the complex and earlier descriptions have mentioned a “massive sulfide layer” with 2 to 4 g/t PGE. Platinum-group minerals (PGM) are almost everywhere included in or attached to composite Ni–Fe–Cu sulfides. Most PGM grains form complex associations resulting either from exsolution or alteration. It is characteristic of the Conakry Igneous Complex PGM, described here for the first time, to be dominated by (Pd,Pt)(Te,Bi) minerals with rare Pd,Sn and Pd,Pb compositions and an absence of Pt,Pd sulfides and Pt,Pd antimonides.
The constant association of the PGM with the BMS shows that the magmatic sulfide liquid acts as an efficient collector of PGE. In such a dynamic environment, the process leading to the formation of massive sulfides must be sought in the accumulation of sulfides in the conduit following host-rock assimilation. Accordingly, considering the multiple injection processes that characterize the whole intrusion, the potential for discovering additional Ni–Cu–PGE mineralization in the Conakry Igneous Complex remains high.
A multifractal model for the velocity gradient dynamics in turbulent flows
- Rodrigo M. Pereira, Luca Moriconi, Laurent Chevillard
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- Journal:
- Journal of Fluid Mechanics / Volume 839 / 25 March 2018
- Published online by Cambridge University Press:
- 01 February 2018, pp. 430-467
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We develop a stochastic model for the velocity gradient dynamics along a Lagrangian trajectory in isotropic and homogeneous turbulent flows. Comparing with different attempts proposed in the literature, the present model, at the cost of introducing a free parameter known in turbulence phenomenology as the intermittency coefficient, gives a realistic picture of velocity gradient statistics at any Reynolds number. To achieve this level of accuracy, we use as a first modelling step a regularized self-stretching term in the framework of the recent fluid deformation (RFD) approximation that was shown to give a realistic picture of small-scale statistics of turbulence only up to moderate Reynolds numbers. As a second step, we constrain the dynamics, in the spirit of Girimaji & Pope (Phys. Fluids A, vol. 2, 1990, p. 242), in order to impose a peculiar statistical structure to the dissipation seen by the Lagrangian particle. This probabilistic closure uses as a building block a random field that fulfils the statistical description of the intermittency, i.e. multifractal, phenomenon. To do so, we define and generalize to a statistically stationary framework a proposition made by Schmitt (Eur. Phys. J. B, vol. 34, 2003, p. 85). These considerations lead us to propose a nonlinear and non-Markovian closed dynamics for the elements of the velocity gradient tensor. We numerically integrate this dynamics and observe that a stationary regime is indeed reached, in which (i) the gradient variance is proportional to the Reynolds number, (ii) gradients are typically correlated over the (small) Kolmogorov time scale and gradient norms over the (large) integral time scale, (iii) the joint probability distribution function of the two non-vanishing invariants $Q$ and $R$ reproduces the characteristic teardrop shape, (iv) vorticity becomes preferentially aligned with the intermediate eigendirection of the deformation tensor and (v) gradients are strongly non-Gaussian and intermittent, a behaviour that we quantify by appropriate high-order moments. Additionally, we examine the problem of rotation rate statistics of (axisymmetric) anisotropic particles as observed in direct numerical simulations. Although our realistic picture of velocity gradient fluctuations leads to better results when compared to the former RFD approximation, it is still unable to provide an accurate description for the rotation rate variance of oblate spheroids.
A dissipative random velocity field for fully developed fluid turbulence
- Rodrigo M. Pereira, Christophe Garban, Laurent Chevillard
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- Journal:
- Journal of Fluid Mechanics / Volume 794 / 10 May 2016
- Published online by Cambridge University Press:
- 04 April 2016, pp. 369-408
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We investigate the statistical properties, based on numerical simulations and analytical calculations, of a recently proposed stochastic model for the velocity field (Chevillard et al., Europhys. Lett., vol. 89, 2010, 54002) of an incompressible, homogeneous, isotropic and fully developed turbulent flow. A key step in the construction of this model is the introduction of some aspects of the vorticity stretching mechanism that governs the dynamics of fluid particles along their trajectories. An additional further phenomenological step aimed at including the long range correlated nature of turbulence makes this model dependent on a single free parameter, ${\it\gamma}$, that can be estimated from experimental measurements. We confirm the realism of the model regarding the geometry of the velocity gradient tensor, the power-law behaviour of the moments of velocity increments (i.e. the structure functions) including the intermittent corrections and the existence of energy transfer across scales. We quantify the dependence of these basic properties of turbulent flows on the free parameter ${\it\gamma}$ and derive analytically the spectrum of exponents of the structure functions in a simplified non-dissipative case. A perturbative expansion in power of ${\it\gamma}$ shows that energy transfer, at leading order, indeed take place, justifying the dissipative nature of this random field.
Orientation dynamics of small, triaxial–ellipsoidal particles in isotropic turbulence
- Laurent Chevillard, Charles Meneveau
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- Journal:
- Journal of Fluid Mechanics / Volume 737 / 25 December 2013
- Published online by Cambridge University Press:
- 27 November 2013, pp. 571-596
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The orientation dynamics of small anisotropic tracer particles in turbulent flows is studied using direct numerical simulation (DNS) and results are compared with Lagrangian stochastic models. Generalizing earlier analysis for axisymmetric ellipsoidal particles (Parsa et al., Phys. Rev. Lett., vol. 109, 2012, 134501), we measure the orientation statistics and rotation rates of general, triaxial–ellipsoidal tracer particles using Lagrangian tracking in DNS of isotropic turbulence. Triaxial ellipsoids that are very long in one direction, very thin in another and of intermediate size in the third direction exhibit reduced rotation rates that are similar to those of rods in the ellipsoid’s longest direction, while exhibiting increased rotation rates that are similar to those of axisymmetric discs in the thinnest direction. DNS results differ significantly from the case when the particle orientations are assumed to be statistically independent from the velocity gradient tensor. They are also different from predictions of a Gaussian process for the velocity gradient tensor, which does not provide realistic preferred vorticity–strain-rate tensor alignments. DNS results are also compared with a stochastic model for the velocity gradient tensor based on the recent fluid deformation approximation (RFDA). Unlike the Gaussian model, the stochastic model accurately predicts the reduction in rotation rate in the longest direction of triaxial ellipsoids since this direction aligns with the flow’s vorticity, with its rotation perpendicular to the vorticity being reduced. For disc-like particles, or in directions perpendicular to the longest direction in triaxial particles, the model predicts noticeably smaller rotation rates than those observed in DNS, a behaviour that can be understood based on the probability of vorticity orientation with the most contracting strain-rate eigendirection in the model.
12 - Genetics of human susceptibility to infection and hepatic disease caused by schistosomes
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- By Alain J. Dessein, Immunologie et Génétique des Maladies Parasitaires, Institut National de la Santé et de la Recherche Médicale, Laboratoire de Parasitologie-Mycologie, Faculté de Médecine, Marseille, France, Sandrine Marquet, Immunologie et Génétique des Maladies Parasitaires, Institut National de la Santé et de la Recherche Médicale, Laboratoire de Parasitologie-Mycologie, Faculté de Médecine, Marseille, France, Carole Eboumbou Moukoko, Immunologie et Génétique des Maladies Parasitaires, Institut National de la Santé et de la Recherche Médicale, Laboratoire de Parasitologie-Mycologie, Faculté de Médecine, Marseille, France, Hèlia Dessein, Immunologie et Génétique des Maladies Parasitaires, Institut National de la Santé et de la Recherche Médicale, Laboratoire de Parasitologie-Mycologie, Faculté de Médecine, Marseille, France, Laurent Argiro, Immunologie et Génétique des Maladies Parasitaires, Institut National de la Santé et de la Recherche Médicale, Laboratoire de Parasitologie-Mycologie, Faculté de Médecine, Marseille, France, Sandrine Henri, Immunologie et Génétique des Maladies Parasitaires, Institut National de la Santé et de la Recherche Médicale, Laboratoire de Parasitologie-Mycologie, Faculté de Médecine, Marseille, France, Dominique Hillaire, Immunologie et Génétique des Maladies Parasitaires, Institut National de la Santé et de la Recherche Médicale, Laboratoire de Parasitologie-Mycologie, Faculté de Médecine, Marseille, France, Christophe Chevillard, Immunologie et Génétique des Maladies Parasitaires, Institut National de la Santé et de la Recherche Médicale, Laboratoire de Parasitologie-Mycologie, Faculté de Médecine, Marseille, France, Nasureldin El Wali, Institute of Nuclear Medicine and Molecular Biology, University of Gezira, Wad Medani, Sudan, Mubarak Magzoub, Institute of Nuclear Medicine and Molecular Biology, University of Gezira, Wad Medani, Sudan, Laurent Abel, Génétique Humaine des Maladies Infectieuses, Institut National de la Santé et de la Recherche Médicale, Faculté de Médecine Necker, Paris, Virmondes Rodrigues, Jr, Faculty of Medicine do Triangulo Mineiro, Ubéraba, Brazil, Aluizio Prata, Faculty of Medicine do Triangulo Mineiro, Ubéraba, Brazil, Gachuhi Kimani, Kenya Medical Research Institute, Biomedical Sciences Research Centre, Nairobi, Kenya
- Edited by Richard Bellamy, Kintampo Health Research Centre, Ghana
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- Book:
- Susceptibility to Infectious Diseases
- Published online:
- 14 August 2009
- Print publication:
- 22 December 2003, pp 337-360
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Summary
Schistosome infections cause much suffering in millions of people living in tropical regions of Africa, Asia, and South America (Prata, 1987; Chitsulo et al., 2000). The most severe clinical symptoms affect the kidneys and urinary tract. However, schistosomes also cause various other disorders such as heart failure and neurological diseases. Three species of schistosome are responsible for most human infections (Schistosoma mansoni, Schistosoma japonicum, and Schistosoma haematobium). These species are found in different geographical locations, have different vectors, and cause different symptoms. Schistosomes are multicellular parasites that are disseminated as free swimming larvae (cercariae) in ponds, lakes, and rivers by snails. Humans become infected when they stay in contaminated water for a few minutes. The cercariae penetrate the human skin and develop into male or female adult schistosomes within 5 or 6 weeks. These small worms (Fig. 12.1A) can live in the vascular system of their vertebrate host for 2 to 5 years. Schistosomes do not multiply within their vertebrate host. The female worms, however, lay hundreds of eggs per day in the mesenteric or vesical veins of their host. Most of the symptoms associated with these infections are caused by the inflammation that is induced by the immunogenic and toxic substances produced by the eggs. The chronic cellular reaction that develops around the eggs is organised in a granuloma (Von Lichtenberg, 1962; Warren et al., 1967).