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International audience; The aim of this study is to investigate experimentally the transition from a rigid regime to a deformed regime for flexible discs freely advected in turbulent flows. For a given disc, the amplitude of the deformation is expected to increase when its bending modulus decreases or when the turbulent kinetic energy increases. To quantify this qualitative argument, experiments are performed where the deformation of flexible discs is measured using three cameras. The amplitude of the deformation has been characterised by the eigenvalues of the moment of inertia tensor. Experimental results exhibit a transition from a rigid regime to a deformed regime that depends on the size, the density and the flexibility of the disc and the turbulent kinetic energy. The modelling of this transition is a generalisation and an extension of the previous models used to characterise the deformation of flexible fibres in turbulent flows.

We report quantitative imaging experiments describing the three-dimensional (3D) bursting cascade of droplets from a liquid melt reacting with the oxygen of air which explode sequentially to produce ever smaller fragments. The 3D space-time resolved trajectories of the fragmenting drops reveal an arborescent structure of branchings defining the cascade steps, each random in direction and shortening along the cascade, in a way we determine. The phenomenon is a unique and prototypical illustration of the so-called Richardson regime, namely, an accelerated cascade towards smaller scales. The phenomenon, which coincides with the early time dispersion period of a Brownian motion, featuring here ever shrinking steps, is well captured by a Langevin dynamics.

International audience; The aim of this study is to investigate experimentally the transition from a rigid regime to a deformed regime for flexible discs freely advected in turbulent flows. For a given disc, the amplitude of the deformation is expected to increase when its bending modulus decreases or when the turbulent kinetic energy increases. To quantify this qualitative argument, experiments are performed where the deformation of flexible discs is measured using three cameras. The amplitude of the deformation has been characterised by the eigenvalues of the moment of inertia tensor. Experimental results exhibit a transition from a rigid regime to a deformed regime that depends on the size, the density and the flexibility of the disc and the turbulent kinetic energy. The modelling of this transition is a generalisation and an extension of the previous models used to characterise the deformation of flexible fibres in turbulent flows.

We simultaneously measure both spinning and tumbling components of rotation for long near-neutrally buoyant fibers in homogeneous and isotropic turbulence. The lengths and diameters of the measured fibers extend to several orders of the Kolmogorov length of the surrounding turbulent flow. Our measurements show that the variance of the spinning rate follows a -4/3 power law scaling with the fiber diameter ($d$) and is always larger than the variance of the tumbling rate. This behavior surprisingly resembles that observed previously for sub-Kolmogorov fibers. The general picture that emerges from this study is that long fibers preferentially align with vortex filaments that can be as long as the integral length of turbulence. We compute the Lagrangian time scale and the distribution of both tumbling and spinning that supports this outlook. Our measurements also allow us to quantify the importance of the Coriolis term on the rotational dynamics of fibers in turbulent flows.

We study the fragmentation of deformable and brittle fibers in the inertial range of turbulence using laboratory experiments and numerical simulations. The fragmentation process is shown to be limited at small scales by a physical cut-off length due to fluid-structure interactions of the object with turbulence, and thus independent of the fiber brittleness. This scenario, comprehensively modeled by an evolution equation, leads to the accumulation of fragments slightly longer than the cut-off scale, as smaller fragments are too short to be deformed and broken by the turbulence. This result may improve our understanding of microplastic formation in the ocean.

Turbulence induces rotation in the living and the non-living materials in the ocean. The time scale of rotation for a living organism is important in understanding an organism's feeding efficiency, mating, prey capture rate etc. This time scale is also crucial for understanding the migration of non-living materials such as microplastics. Herein, we investigate the tumbling motion of mesoscale particles that resemble organisms of intermediate size, such as zooplankton that appear in the ocean. Using time-resolved measurements of the orientation of rigid inertial fibers in a turbulence-tank, we analyze the autocorrelation of their tumbling rate. The correlation time ($\tau_d$) is well predicted by Kolmogorov inertial-range scaling based on the fiber length ($L$) when the fiber inertia can be neglected. For inertial fibers, we propose a simple model considering fiber inertia (measured by a tumbling Stokes number) and a viscous torque which accurately predicts both the correlation time and the variance of the tumbling rate. Our measurements and the theoretical model provide a basic understanding of the rotational response of an intermediately sized organism to the surrounding turbulence in its non-active state.

International audience; We numerically investigated the transport, deformation and buckling events of an isolated elastic fiber in Taylor-Green vortices and studied the dynamics of long filaments in homogeneous isotropic turbulence. The fiber is modelled by an assembly of spherical beads. The contact between beads enforces the inextensibility of the filament while bending is accounted for by the Gears Bead Model (GBM) proposed by Delmotte et al. (2015). In the cellular Taylor-Green flow, the buckling probability is a function of a dimensionless number, called Sperm number, which is a balance between the compression rate of the flow and the elastic response of the filament. The shapes of the filament and its ability to buckle have been successfully validated through comparisons with experiments from the work by Quennouz et al. (2015). The deformation statistics of long flexible fibers in sustained homogeneous isotropic turbulence were analyzed for various flow and fiber material conditions. Two regimes have been identified depending on the ratio of fiber length to persistence length which is a measure of turbulent forcing to flexibility. The numerical results are in good agreement with existing experimental data (C. Brouzet et al., Phys. Rev. Lett. 112, 074501 (2014)) validating the assumptions of our model for the configurations we investigated.

Anisotropic particles play a major role in many environmental and industrial turbulent flows. The modeling of their rotation dynamics is a fundamental challenge which has some important consequences in industrial processes, such as in the paper making industry. This study investigates the rotation rate of neutrally buoyant fibers longer than the Kolmogorov length η_{K}. We show that the fiber inertia is at the origin of a decrease of the rotation rate. We propose a model which describes this phenomenon. We introduce also a new Stokes number which defines the limit of validity of the classical slender body approximation.

Particle aggregation is an ubiquitous phenomenon encountered in many natural and industrial environments. We describe here the equilibrium state of the ag-gregation/fragmentation process of inertial scale particles clustering in turbulence i.e. when the particles and the aggregates are larger than the Kolmogorov dissipa-tive scale η. For this purpose, we seeded a high Reynolds number turbulent von Kármán flow with millimetric nearly neutrally buoyant magnetic particles. Each magnetic dipole imposes a torque and a force on the other magnets at the origin of the cohesion of the aggregates. On the contrary, turbulent fluctuations impose external stresses which tend to fragment the aggregates. Using video image analyses , we have performed the three-dimensional reconstruction of the aggregates and measured their characteristic sizes. The average number of particles inside each aggregate as a function of the intensity of turbulence can then be deduced. Assuming a Kolmogorov inertial scaling law for the turbulent velocity increments, we predict theoretically the aggregate mean size which is in agreement with our experimental results. 2 LED camera 1 camera 2 FIG. 1. Sketch of the turbulent von Kármán flow setup.

International audience; Sea balls also called aegagropila, can be found on Mediterranean beaches. They are made of Posidonia fibers, which aggregate on the seabed due to the sea motions. To understand the mechanism of aggregation and compaction of these structures , we have performed a laboratory investigation on the dynamics of aggregation of fibers by surface gravity waves generated in a water tank. Amazingly, depending on the flexibility of the fibers, two different sites of aggregation are observed. Following our experiments, we propose an analytical derivation of the clustering of particles by the Stokes drift. This theoretical calculation is quite general and emphasizes the respective roles of the Stokes number and the density of the particles; it also underlines the importance of the fiber flexibility in their drift and explains our experimental observations.

International audience; Fiber networks encompass a wide range of natural and man-made materials. The threads or filaments from which they are formed span a wide range of length scales: from nanometers, as in biological tissues and bundles of carbon nanotubes, to millimeters , as in paper and insulation materials. The mechanical and thermal behavior of these complex structures depends on both the individual response of the constituent fibers and the density and degree of entanglement of the network. A question of paramount importance is how to control the formation of a given fiber network to optimize a desired function. The study of fiber clustering of natural flocs could be useful for improving fabrication processes, such as in the paper and textile industries. Here, we use the example of aegagropilae that are the remains of a seagrass (Posidonia oceanica) found on Mediterranean beaches. First, we characterize different aspects of their structure and mechanical response, and second, we draw conclusions on their formation process. We show that these natural aggregates are formed in open sea by random aggregation and compaction of fibers held together by friction forces. Although formed in a natural environment, thus under relatively unconstrained conditions, the geometrical and mechanical properties of the resulting fiber aggregates are quite robust. This study opens perspectives for manufacturing complex fiber network materials. fiber network | fiber aggregation | Posidonia oceanica | fibrous material A egagropilae are a natural aggregate of fibers produced by the decomposition of leaves and roots of Posidonia ocean-ica. The fibers are entangled by sea motion until the clusters reach the shore (1) (Fig. 1). P. oceanica is an endemic plant of the Mediterranean Sea with very long and thin leaves (about 1-m long, 1-cm wide, and less than 1-mm thick). The name aega-gropilae originates from the Greek [α´ιγαγρoςα´ιγαγρoς (wild goat) and π˜ιλoςπ˜ιλoς (fur)] and refers to the resemblance between the shape of these balls and those regurgitated by goats. Natural fiber clustering occurs for different species of aquatic plants, such as the so-called seaballs that can be found on the Atlantic Ocean and lake shores (2, 3). P. oceanica meadows play an important ecological role in the preservation of Mediterranean coasts. They constitute plant barriers that promote sediment trapping and oxygen production in seabed, and the accumulated remains protect the beaches from erosion. Moreover, aegagropilae fibers are suitable materials for insulation in construction and automotive industries. After submitting them to various tests, they recently landed in the marketplace under the name of Neptutherm. Beyond the curiosity that these natural objects can provoke, aegagropilae samples found on beaches raise several fundamental questions. The first set of questions is on their formation process. How can fibers be entangled and packed by a flow without any confinement? How long does it take to form a cluster? Can one explain the size distribution of these aggregates? The second set of questions is on the cohesion of these structures. How can we relate the apparent stiffness of these balls to the interaction of the fibers and the topological properties of the network? Natural and manmade fiber networks are abundant structures and arise on a wide range of length scales. Examples may be found in both biological systems, such as the cytoskeleton of a cell (4), blood clots (5), and biological tissues (6), and technology , such as nanotube bundles (7), paper (8), textiles, and felts (9–11). The mechanical response of fiber assemblies depends on both the properties of the elementary thread and the density , connectivity, and ordering of the network. Therefore, the functional properties of these materials can be tuned by controlling their formation processes. The manufacturing of some athermal networks, such as paper, involves transport of fibers in a fluid flow (8). During this process, fibers are advected and deformed elastically, and they interact through interfiber friction (12, 13). From this perspective, aegagropilae are an archetype of these fiber networks, and understanding the clustering mechanism can shed light on the fundamental aspects of fiber aggre-gation dynamics. Here, we perform various measurements on aegagropilae to characterize their structural and mechanical properties. Our observations provide a qualitative understanding of the formation process of this fiber network. Structural Properties of Aegagropilae Aegagropilae were collected at two different locations of the Mediterranean shore: at Six Fours, France (43 • 06 03 N, 5 • 49 20 E) and on Porquerolles Island, France (43 • 00 02 N, 6 • 13 38 E) with the authorization of Port-Cros National Park. On the first beach, nearly 2,000 samples were collected to determine their size and mass distributions. To avoid any bias in the sampling caused by damage of aegagropilae by human activity, all samples were collected on a morning in winter, the day after a storm. Moreover, only balls found at a distance less than 10 m from the sea were collected, assuming that the ones located farther away from the sea might have been released by a precedent storm. At the second spot, few samples were picked up in seabed to investigate the geometrical and mechanical properties of fiber Significance Aegagropilae are centimeter-sized, nearly spherical fiber aggregate found on Mediterranean beaches. They result from aggregation and compaction of the remains of the seagrass Posidonia oceanica. We find that they possess remarkable mechanical properties, which are unexpected for a structure formed by random agitation caused by sea motion and held together solely by friction between the fibers. The study of such material offers perspectives for complex materials, such as felts and insulation materials.

International audience; A growing number of studies is devoted to anisotropic particles in turbulent flows. In most cases the particles are assumed to be rigid and their deformations are neglected. We present an adaptation of classical computer vision tools to reconstruct from two different images the 3D conformation of a fiber distorted by the turbulent fluctuations in a von Kármán flow. This technique allows us notably to characterize the fiber deformation by computing the correlation function of the orientation of the tangent vector. This function allows us to tackle the analogy between polymers and flexible fibers proposed by Brouzet et al. (Phys. Rev. Lett. 112(7) 074501 (2014)). We show that this function depends on an elastic length e which characterizes the particle flexibility, as is the case for polymers, but also on the fiber length L, contrary to polymers.

International audience; This paper reports an experimental investigation of a turbulent flow confined within a rotor/stator cavity of aspect ratio close to unity at high Reynolds number. The experiments have been driven by changing both the rotation rate of the disk and the thermodynamical properties of the working fluid. This fluid is sulfur hexafluoride (SF 6) whose physical properties are adjusted by imposing the operating temperature and the absolute pressure in the pressurized vessel, especially near the critical point of SF 6 reached for T c = 45.58 • C, P c = 37.55 bar. This original setup allows to obtain Reynolds numbers as high as 2 10 7 together with compressibility effects as the Mach number can reach 0.5. Pressure measurements reveal that the resulting fully turbulent flow shows both a direct and an inverse cascade as observed in rotating turbulence and in accordance with Kraichnan conjecture for 2D-turbulence. The spectra are however dominated by low-frequency peaks, which are subharmonics of the rotating disk frequency, involving large scale structures at small azimuthal wavenumbers. These modes appear for a Reynolds number around 10 5 and experience a transition at a critical Reynolds number Re c ≈ 10 6. Moreover they show an unexpected non linear behavior that we understand with the help of a low dimensional amplitude equations.

The influence of an externally applied magnetic field on flow turbulence is investigated in liquid-gallium von-Kármán (VK) swirling flows. Time-resolved measurements of global variables (such as the flow power consumption) and local recordings of the induced magnetic field are made. From these measurements, an effective Reynolds number is introduced as ${\mathit{Rm}}_{\mathit{eff}} = \mathit{Rm}(1\ensuremath{-} \ensuremath{\alpha} \sqrt{N} )$, so as to take into account the influence of the interaction parameter $N$. This effective magnetic Reynolds number leads to unified scalings for both global variables and the locally induced magnetic field. In addition, when the flow rotation axis is perpendicular to the direction of the applied magnetic field, significant flow and induced magnetic field fluctuations are observed at low interaction parameter values, but corresponding to an Alfvèn speed ${v}_{A} $ of the order of the fluid velocity fluctuations ${u}_{\mathit{rms}} $. This strong increase in the flow fluctuations is attributed to chaotic changes between hydrodynamic and magnetohydrodynamic velocity profiles.

We describe, for the first time, an experiment devoted to the study of the spatial conformation of a flexible fiber in a turbulent flow. We propose a model for the transition from rigid to flexible regimes as the intensity of turbulence is increased or the elastic energy of the fiber is decreased. This transition occurs for a fiber typical length which is observed experimentally and recovered by our analysis. We also demonstrate that the conformations of flexible fibers in a turbulent flow are analog to conformations of flexible polymers in a good solvent. This last result opens some new and creative ways to model flexible fiber distortions in turbulent flows while addressing fundamental problems in polymer dynamics.

We analyze time series stemming from experiments and direct numerical simulations of hydrodynamic and magnetohydrodynamic turbulence. Simulations are done in periodic boxes, but with a volumetric forcing chosen to mimic the geometry of the flow in the experiments, the von K\'arm\'an swirling flow between two counter-rotating impellers. Parameters in the simulations are chosen to (within computational limitations) allow comparisons between the experiments and the numerical results. Conducting fluids are considered in all cases. Two different configurations are considered: a case with a weak externally imposed magnetic field, and a case with self-sustained magnetic fields. Evidence of long-term memory and $1/f$ noise is observed in experiments and simulations, in the case with weak magnetic field associated with the hydrodynamic behavior of the shear layer in the von K\'arm\'an flow, and in the dynamo case associated with slow magnetohydrodynamic behavior of the large scale magnetic field., Comment: 10 pages, 9 figures

Hydrodynamic and magnetic behaviors in a modified experimental setup of the von Kármán sodium flow-where one disk has been replaced by a propeller-are investigated. When the rotation frequencies of the disk and the propeller are different, we show that the fully turbulent hydrodynamic flow undergoes a global bifurcation between two configurations. The bistability of these flow configurations is associated with the dynamics of the central shear layer. The bistable flows are shown to have different dynamo efficiencies; thus for a given rotation rate of the soft-iron disk, two distinct magnetic behaviors are observed depending on the flow configuration. The hydrodynamic transition controls the magnetic field behavior, and bifurcations between high and low magnetic field branches are investigated.

International audience; Predicting dynamo self-generation in liquidmetal experiments has been an ongoing question for many years. In contrast to simple dynamical systems for which reliable techniques have been developed, the ability to predict the dynamo capacity of a flow and the estimate of the corresponding critical value of the magnetic Reynolds number (the control parameter of the instability) has been elusive, partly due to the high level of turbulent fluctuations of flows in such experiments (with kinetic Reynolds numbers in excess of 10^6).We address these issues here, using the von Karman sodium experiment and studying its response to an externally applied magnetic field. We first show that a dynamo threshold can be estimated from analysis related to critical slowing down and susceptibility divergence, in configurations for which dynamo action is indeed observed. These approaches are then applied to flow configurations that have failed to self-generate magnetic fields within operational limits, and we quantify the dynamo capacity of these configurations.

International audience; We study different types of stationary dynamos observed in the Von Karman sodium (VKS) experiment when varying the electromagneticboundary conditions on (and in) the impellers. The flow is driven with two impellers made of soft iron (Monchaux et al 2007 Phys Rev. Lett. 98 044502)or using one soft-iron impeller and one stainless steel impeller. The magnetic field is mapped using 40 three-dimensional probes distributed within the flow and its surroundings. Symmetry and coupling properties are then retrieved from direct probe measurements and/or from the field structure as reconstructed using the inversion procedure described by Boisson and Dubrulle (2011 New J. Phys. 13 023037). Several salient results are obtained: (i) dynamo action is not achieved unless at least one iron impeller is rotating, at a frequency larger than 15 Hz; (ii) the resulting dynamo is a dipolar, mostly axisymmetric structure; and (iii) the self-sustained magnetic field properties depend on the sodium flow structure between the two impellers. We propose to interpret the stationary dynamos generation as the (constructive or destructive) superposition of two one-impeller fluid dynamos generated close to the soft-iron impellers, nonlinearly coupled through the turbulent flow, as suggested by Verhille et al (2010 New J. Phys. 12 033006). The normal form equation describing this coupling is similar to the one obtained in a theoretical model (Petrelis et al 2009 Phys. Rev. Lett. 102 144503).

International audience; We report the first experimental observation of a spatially localized dynamo magnetic field, a common feature of astrophysical dynamos and convective dynamo simulations. When the two propellers of the von Ka ́rma ́n sodium experiment are driven at frequencies that differ by 15%, the mean magnetic field's energy measured close to the slower disk is nearly 10 times larger than the one close to the faster one. This strong localization of the magnetic field when a symmetry of the forcing is broken is in good agreement with a prediction based on the interaction between a dipolar and a quadrupolar magnetic mode.

International audience; A new type of velocimeter, capable of local velocity measurements in conducting fluids, is introduced. The principle of the " magnetic-distortion probe " is based on the measurement of the induced magnetic field by the flow of a conducting fluid in the vicinity of a localized magnetic field. The new velocimeter has no moving parts, and can be enclosed in a sealed cap, easing the implementation in harsh environments, such as liquid metals. The proposed method allows one to probe both the continuous part and fluctuations of the velocity, the temporal and spatial resolution being linked to the actual geometric configuration of the probe. A prototype probe has been tested in a gallinstan pipe flow and in a fully turbulent flow of liquid gallium generated by the counter rotation of two coaxial impellers in a cylinder. The signals have been compared to a reference potential probe and show very good agreement both for time-averaged velocities and turbulent fluctuations. The prototype is shown to detect motion from a few cm s −1 to a few m s −1. Moreover, the use of the magnetic-distortion probe with large-scale applied magnetic field is discussed.

We study magnetohydrodynamics in a von K\'arm\'an flow driven by the rotation of impellers made of material with varying electrical conductivity and magnetic permeability. Gallium is the working fluid and magnetic Reynolds numbers of order unity are achieved. We find that specific induction effects arise when the impeller's electric and magnetic characteristics differ from that of the fluid. Implications in regards to the VKS dynamo are discussed., Comment: 14 pages, 7 figures

International audience; Since the turn of the century, experiments have produced laboratory fluid dynamos that enable a study of the effect in controlled conditions. We review here magnetic induction processes that are believed to underlie dynamo action, and we present results of these dynamo experiments. In particular, we detail progress that have been made through the study of von Kármán flows, using gallium or sodium as working fluids.

International audience; A synthetic fluid dynamo built in the spirit of the Bullard device (Bullard, The stability of a homopolar dynamo, Proc. Camb. Phil. Soc. 1955, 51, 744) is investigated. It is a two-step dynamo in which one process stems from the fluid turbulence, while the other part is achieved by a linear amplification of currents in external coils (Bourgoin et al., A bullard-von Karman dynamo, New J. Phys., 2006, 8, 329). Modifications in the forcing are introduced in order to change the dynamics of the flow, and hence the dynamo behavior. Some features, such as on-off intermittency at onset of dynamo action, are very robust. Large-scale fluctuations have a significant impact on the resulting dynamo, in particular in the observation of magnetic field reversals.

We compare experimental data and numerical simulations for the dynamics of inertial particles with finite density in turbulence. In the experiment, bubbles and solid particles are optically tracked in a turbulent flow of water using an Extended Laser Doppler Velocimetry technique. The probability density functions (PDF) of particle accelerations and their auto-correlation in time are computed. Numerical results are obtained from a direct numerical simulation in which a suspension of passive pointwise particles is tracked, with the same finite density and the same response time as in the experiment. We observe a good agreement for both the variance of acceleration and the autocorrelation timescale of the dynamics; small discrepancies on the shape of the acceleration PDF are observed. We discuss the effects induced by the finite size of the particles, not taken into account in the present numerical simulations., Comment: 7 pages, 4 figures

We use an extended laser Doppler technique to track optically the velocity of individual particles in a high Reynolds number turbulent flow. The particle sizes are of the order of the Kolmogorov scale and the time resolution, 30 microseconds, resolves the fastest scales of the fluid motion. Particles are tracked for mean durations of the order of 10 Kolmogorov time scales and their accelerations are measured. For neutrally buoyant particles (fluid tracers), this technique matches the performance of the silicon strip detector technique introduced at Cornell University (Voth G. A. et al., J. Fluid Mech., 469 (2002) 121). This reference dynamics is then compared to that of slightly heavier solid particles (density 1.4) and to air bubbles. We observe that the dynamics of the particles strongly depends on their density. Bubbles have a much faster dynamics and experience much higher accelerations than fluid tracers. Although the particles dynamics are different, we find that the probability distribution functions of accelerations normalized to the variance always remain very close to the one for the fluid tracers.