Papers by Matthieu Mercier
Most gravitational currents occur on sloping topographies, often in the presence of particles tha... more Most gravitational currents occur on sloping topographies, often in the presence of particles that can settle during the current propagation. Yet, an exhaustive exploration of associated parameters in experimental devices is still lacking. Here, we present an extensive experimental investigation on the slumping regime of turbidity (particle-laden) currents in two lock-release (dam-break) systems with inclined bottoms. We identify 3 regimes controlled by the ratio between settling and current inertia. (i) For negligible settling, the turbidity current morphodynamics correspond to those of saline homogeneous gravity currents, in terms of velocity, slumping (constant-velocity) regime duration and current morphology. (ii) For intermediate settling, the slumping regime duration decreases to become fully controlled by a particle settling characteristic time. (iii) When settling overcomes the current initial inertia, the slumping (constant-velocity) regime is not detected anymore. In the f...
Physical Review Fluids
Oscillating grid turbulence (OGT) is a specific situation allowing to investigate the equilibrium... more Oscillating grid turbulence (OGT) is a specific situation allowing to investigate the equilibrium between the turbulent kinetic energy, k, and the dissipation rate, , without any mean flow. One of its main features is that, in homogeneous fluids, the turbulence intensity decreases with the distance to the vibrating grid used to generate the forcing. OGT is thus usually described in terms of depth profiles of turbulent quantities. In this paper, we discuss experiments realized in two different setups and compare our results for all turbulent quantities with past studies. The results on the turbulent kinetic energy and the integral length scale lead us to propose a parametric model for the eddy viscosity, νt, taking into account the spatial decay of the turbulence being slightly different than previous descriptions. Indeed, we find that νt is best described as a constant value over a certain depth, Hs, before decreasing quickly as a power law in z −3/2. The specific depth Hs is defined at the depth where a discontinuity of the integral length scale is observed. Physically, it corresponds to the depth at which the presence of the grid no longer influences the flow. We also describe OGT in the case of a two-layer fluid. The description of the turbulence at depths z far from the position of the interface h (0 < z/h < 0.6) is very similar to the case of an homogeneous fluid, whereas the region near the interface (0.6 < z/h < 1) is strongly altered by the presence of the fluid boundary. Surprisingly, νt (at the difference of the others turbulent quantities) no longer depends on z in such configuration. Finally, we investigate mixing processes at the interface in this context. Thanks to large field measurement (about the width of the tank), we were able to identify large scale coherent structures as well as the turbulent flow. We discuss the entrainment law of the interface, and its relationship with the Richardson number based on different definitions used in previous studies, as well as by using our observations of the turbulent flow properties at the interface. Our observations suggest that mixing at the interface in OGT is controlled by the turbulent flow properties at the interface but could also be influenced by weak mean flow features, this could explain the difficulty to discriminate a simple dependency of the entrainment rate with the Richardson number.
24e Congrès Français de Mécanique, Aug 26, 2019
Nous étudions ici la turbulence de surface générée par un dispositif expérimental de grille oscil... more Nous étudions ici la turbulence de surface générée par un dispositif expérimental de grille oscillante et son influence sur la distribution verticale de particules millimétriques de plastiques légèrement flottantes. Dans un premier temps, la description détaillée de cette turbulence est faite grâce à des mesures PIV (Particle Image Velocimetry) pour remonter aux profils verticaux des grandeurs caractéristiques de l'écoulement (échelles intégrale et de Kolmogorov, énergie cinétique turbulente, taux de dissipation). Dans un second temps, le lien avec la distribution de particules légèrement flottantes dont la taille est comprise entre l'échelle de Kolmogorov et celle intégrale est fait. Nous comparons nos résultats avec des modèles existants, dans le cadre de la pollution plastique océanique.
We present preliminary results from a realistic experiment that took place at the Coriolis turnta... more We present preliminary results from a realistic experiment that took place at the Coriolis turntable (Grenoble, France) investigating the baroclinic tide generated at the Luzon Strait. Careful scalings of the spatial and temporal coordinates have been done to preserve dynamic similarity with the ocean’s problem. The study of the barotropic and baroclinic diurnal tides reveals good agreement with observations and models. 1
We revisit experimental studies performed by Ekman on dead-water (Ekman, 1904) using modern techn... more We revisit experimental studies performed by Ekman on dead-water (Ekman, 1904) using modern techniques in order to present new insights on this peculiar phenomenon. We extend its description to more general situations such as a three-layer fluid or a linearly stratified fluid in presence of a pycnocline, showing the robustness of dead-water phenomenon. We observe large amplitude nonlinear internal waves which are coupled to the boat dynamics, and we emphasize that the modeling of the wave-induced drag requires more analysis, taking into account nonlinear effects.
We model internal tides generated by the interaction of a barotropic tide with variable topograph... more We model internal tides generated by the interaction of a barotropic tide with variable topography. For the barotropic part, an asymptotic solution valid over the variable topography is considered. The resulting non-uniform ambient flow is used as a prescribed barotropic forcing for the baroclinic equations (linearized, non-hydrostatic, Euler equations within the Boussinesq approximation).
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific re... more HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Nousétudions la dynamiqueà bas nombre de Reynolds d'une suspension de particules non-Browniennes ... more Nousétudions la dynamiqueà bas nombre de Reynolds d'une suspension de particules non-Browniennes initialement réparties de façon uniforme dans un fluide visqueux linéairement stratifié en sel. Une caméra permet de visualiser dans un plan vertical, grâceà deux sources lumineuses (laser et lumière blanche) synchronisées, l'évolution temporelle de la répartition spatiale et du champ de vitesses des particules. Nous quantifions les valeurs moyennes et fluctuantes de ces quantités pour différentes stratifications, et les comparonsà des mesures similaires réalisées en fluide homogène.
Bulletin of the American Physical Society, 2014
Submitted for the DFD14 Meeting of The American Physical Society Sedimentation dynamics of disks ... more Submitted for the DFD14 Meeting of The American Physical Society Sedimentation dynamics of disks in a linearly stratified fluid MATTHIEU MERCIER, JUSTIN PEMEJA, PATRICIA ERN, IMFT (Institut de Mecanique des Fluides de Toulouse)-The settling dynamics of small objects in a stratified fluid is important to understand the fate of the biomass in lakes or oceanic environments, for industrial applications such as waste-water disposal for instance. Recent numerical and theoretical studies dedicated to freely falling (or rising) bodies in stratified environments have shown some important differences compared to the same problem in a homogeneous fluid. Experimental results are still needed for validation, especially at low and moderate values of the Reynolds number, Re = U d/ν ≤ 100, with U the instantaneous vertical velocity of the object, d its characteristic length, and ν the kinematic velocity of the fluid. We present original experimental results of freely falling disks of finite thickness in a linearly stratified fluid. Three-dimensional trajectories and the wake of the object are obtained using a pair of cameras visualizing two perpendicular planes, revealing a strong influence of the stratification on the dynamics of the object. In particular, the stratification enhances the steady drag experienced by the disks when falling broadside; and generates a change of stability for the disk orientation (from horizontal to vertical) when the Re number decreases below a threshold value.
Turbulent mixing is a vital component of vertical particulate transport, but ocean global circula... more Turbulent mixing is a vital component of vertical particulate transport, but ocean global circulation models (OGCMs) generally have low resolution representations of near-surface mixing. Furthermore, turbulence data is often not provided in reanalysis products. We present 1D parametrizations of wind-driven turbulent mixing in the ocean surface mixed layer, which are designed to be easily included in 3D Lagrangian model experiments. Stochastic transport is computed by Markov-0 or Markov-1 models, and we discuss the advantages/disadvantages of two vertical profiles for the vertical diffusion coefficient K z. All vertical diffusion profiles and stochastic transport models lead to stable concentration profiles for buoyant particles, which for particles with rise velocities of 0.03 and 0.003 m s −1 agree relatively well with concentration profiles from field measurements of microplastics. Markov-0 models provide good model performance for integration timesteps of ∆t ≈ 30 seconds, and can be readily applied in studying the behaviour of buoyant particulates in the ocean. Markov-1 models do not consistently improve model performance relative to Markov-0 models, and require an additional parameter that is poorly constrained.
We model linear, inviscid non-hydrostatic internal tides generated by the interaction of a barotr... more We model linear, inviscid non-hydrostatic internal tides generated by the interaction of a barotropic tide with variable topography in two dimensions. We first derive an asymptotic solution for the nonuniform barotropic flow over the topography that serves as forcing for the baroclinic equations. The resulting internal-tide generation problem is reformulated as a Coupled-Mode System (CMS) by means of a series decomposition of the baroclinic stream function in terms of vertical basis functions. We solve this CMS numerically and also provide a method for estimating the sea-surface signature of internal tides. We consider several seamounts and shelf profiles and perform calculations for a wide range of (topographic) heights and slopes. For subcritical topographies, the energy flux as a function of height exhibits local maxima, separated by cases of weaklyor even non-radiating topographies. For supercritical topographies, the energy flux generally increases with height and criticality. ...
Bulletin of the American Physical Society, 2014
Stratified flows over obstacles are important features in meteorology and oceanography. The chara... more Stratified flows over obstacles are important features in meteorology and oceanography. The characterization of these flows is crucial in order to propose models of geophysical processes such as mixing and ocean circulation or orographic drag in the atmosphere. For some specific stratification profiles, the energy of internal waves generated by the obstacle can be trapped at a given depth, at the base of the oceanic mixing layer or at the top of the atmospheric boundary layer for instance. This scenario can be modelled by a two-layer stratified fluid for which gravity waves spread at the interface between the two layers. The work presented here focuses on a two-layer flow over a 3D obstacle, or equivalently, an obstacle towed in a fluid at rest. Experiments performed both in the large-scale flume of CNRM-GAME Toulouse (METEO-FRANCE & CNRS) and in a smaller tank apparatus, are presented with a specific attention on the measurement of the 3D wave patterns. A non-hydrostatic linear ana...
L'assujettissement volontaire au statut du bail rural autorisé par la Cour de Cassation
Revue juridique de l'Ouest, 2010
National audienc
Journal of Fluid Mechanics, 2019
We consider the unbounded settling dynamics of a circular disk of diameter d and finite thickness... more We consider the unbounded settling dynamics of a circular disk of diameter d and finite thickness h evolving with a vertical speed U in a linearly stratified fluid of kinematic viscosity ν and diffusivity κ of the stratifying agent, at moderate Reynolds numbers (Re = U d/ν). The influence of the disk geometry (diameter d and aspect ratio χ = d/h) and of the stratified environment (buoyancy frequency N , viscosity and diffusivity) are experimentally and numerically investigated. Three regimes for the settling dynamics have been identified for a disk reaching its gravitational equilibrium level. The disk first falls broadside-on, experiencing an enhanced drag force that can be linked to the stratification. A second regime corresponds to a change of stability for the disk orientation, from broadside-on to edgewise settling. This occurs when the non-dimensional velocity U/ √ νN becomes smaller than some threshold value. Uncertainties in identifying the threshold value is discussed in terms of disk quality. It differs from the same problem in a homogeneous fluid which is associated with a fixed orientation (at its initial value) in the Stokes regime and a broadside-on settling orientation at low, but finite Reynolds numbers. Finally, the third regime corresponds to the disk returning to its broadside orientation after stopping at its neutrally buoyant level.
Journal of Fluid Mechanics, 2019
Stratification due to salt or heat gradients greatly affects the distribution of inert particles ... more Stratification due to salt or heat gradients greatly affects the distribution of inert particles and living organisms in the ocean and the lower atmosphere. Laboratory studies considering the settling of a sphere in a linearly stratified fluid confirmed that stratification may dramatically enhance the drag on the body, but failed to identify the generic physical mechanism responsible for this increase. We present a rigorous splitting scheme of the various contributions to the drag on a settling body, which allows them to be properly disentangled whatever the relative magnitude of inertial, viscous, diffusive and buoyancy effects. We apply this splitting procedure to data obtained via direct numerical simulation of the flow past a settling sphere over a range of parameters covering a variety of situations of laboratory and geophysical interest. Contrary to widespread belief, we show that, in the parameter range covered by the simulations, the drag enhancement is generally not primari...
Annual Review of Fluid Mechanics, 2019
Rigid or deformable bodies moving through continuously stratified layers or across sharp interfac... more Rigid or deformable bodies moving through continuously stratified layers or across sharp interfaces are involved in a wide variety of geophysical and engineering applications, with both miscible and immiscible fluids. In most cases, the body moves while pulling a column of fluid, in which density and possibly viscosity differ from those of the neighboring fluid. The presence of this column usually increases the fluid resistance to the relative body motion, frequently slowing down its settling or rise in a dramatic manner. This column also exhibits specific dynamics that depend on the nature of the fluids and on the various physical parameters of the system, especially the strength of the density/viscosity stratification and the relative magnitude of inertia and viscous effects. In the miscible case, as stratification increases, the wake becomes dominated by the presence of a downstream jet, which may undergo a specific instability. In immiscible fluids, the viscosity contrast combin...
Environmental Science & Technology, 2019
Estimates of cumulative plastic inputs into the oceans are e:xpressed in hundred million tons, wh... more Estimates of cumulative plastic inputs into the oceans are e:xpressed in hundred million tons, whereas the total R = ,,fITF mass of microplastics afl oat at sea is 3 orders of magnitude twt In couni. l.ottrge pla.<_; tki, (1-f> mm) below this. This l arg e gap is evidence of our ignorance about the fate of plastics, as well as transformations and sinks in the oceans. One of the current challenges consists of identifying and quantifying plastic particles at the microscale, the small Il"' �'• Sphcrc Microplastic Ellip,oid �! n ��"pl�!�, � (� 6 ln mass Smnll plastics Large 1>laslks (251un-l mm) (1-5 mm) microplastics (SMP, 25-1000 µm). The aim of the present study is to investigate SMP concentration in count and in mass at the sea surface in the North Atlantic subtropical gyre during the sea campaign Expedition i h Continent After isolation, From plastic dcbris SMP were characterized by micro Fourier transform infrared to models Rise velocities 1\, ficroplo.stics corroetcd concentrations spectroscopy. Microplastic distribution was modeled by a wind driven vertical mixing correction mode! taking into account individual particle properties (dimension, shape and density). We demonstrate that SMP buoyancy is significantly decreased compared to the l arg e microplastics (LMP, 1-5 mm) and consequently more susceptible to vertical transport. The uncorrected LMP concentration in count was between 13 000 and 174000 pieces 1an-2 , and was between 5 and 170 times more abundant for SMP. With a wind driven vertical mixing correction, we estimated that SMP were 300 to 70 000 times more abundant than LMP. When discussing this in terms of weight after correction, LMP concentrations were between 50 and 1000 g 1an-2 , and SMP concentrations were between 5 and 14000 g 1an-2 _
A sampling system using sample vials for high-temperature isothermal capillary gas chromatography
Journal of High Resolution Chromatography, 1979
A novel sampling system for isothermal capillary gas chromatography of medium- or low-volatility ... more A novel sampling system for isothermal capillary gas chromatography of medium- or low-volatility mixtures is described and test results are reported using flame ionization detection. The system features a loading and positioning chamber for glass vials which can carry a dilute sample in up to 0.2 ml of a volatile solvent. The output from the glass vaporization chamber is trapped onto a packed precolumn operated in the backflush mode. The analytes are further transferred over a wall-coated capillary trap prior to isothermal capillary GLC. Repeat-cycle operation of the sampling system requires 10 channels of a processor for time control over 5 solenoid valves and 3 static heaters. Preliminary results with long-chain n-alkanes in the submicrogram range showed no unwanted effects. The principal claim of the system is an improved rejection of non-volatiles.
La rétroactivité : (essai d'une théorie générale)
Http Www Theses Fr, 2003
Qu'elle procede d'un acte juridique de portee individuelle ou generale, la norme juridiqu... more Qu'elle procede d'un acte juridique de portee individuelle ou generale, la norme juridique doit etre rappelee a un modele de mesure du reel. Loin d'etre une fiction, la retroactivite constitue alors une modalite ordinaire d'application de la regle de droit dans le temps : une regle retroactive qualifie les faits passes et leur associe des effets juridiques au passe. Par suite, la regle retroactive se materialise dans la realite des sa creation. Exigence politique fondamentale, la securite juridique restreint en pratique l'usage de la retroactivite par le legislateur ou le droit prive. Les hypotheses de retroactivite peuvent pourtant correspondre a une necessite de securite juridique a vocation a retroagir.
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Papers by Matthieu Mercier