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This paper presents a methodology to analyse debris flows focusing at the surge scale rather than the full scale of the debris-flow event, as well as its application to a French site. Providing bulk surge features like volume, peak discharge, front height, front velocity and Froude numbers allows for numerical and experimental debris-flow investigations to be designed with narrower physical ranges and thus for deeper scientific questions to be explored. We suggest a method to access such features at the surge scale that can be applied to a wide variety of monitoring stations. Requirements for monitoring stations for the methodology to be applicable include (i) flow height measurements, (ii) a cross-section assumption and (iii) a velocity estimation. Raw data from three monitoring stations on the Réal torrent (drainage area: 2 km2, southeastern France) are used to illustrate an application to 34 surges measured from 2011 to 2020 at three monitoring stations. Volumes of debris-flow surges on the Réal torrent are typically sized at a few thousand cubic metres. The peak flow height of surges ranges from 1 to 2 m. The peak discharge range is around a few dozen cubic metres per second. Finally, we show that Froude numbers of such surges are near critical.

International audience; The purpose of this paper is to propose a time-step-robust cell-to-cell integration of particle trajectories in 3-D unstructured meshes in particle/mesh Lagrangian stochastic methods. The main idea is to dynamically update the mean fields used in the time integration by splitting, for each particle, the time step into sub-steps such that each of these sub-steps corresponds to particle cell residence times. This reduces the spatial discretization error. Given the stochastic nature of the models, a key aspect is to derive estimations of the residence times that do not anticipate the future of the Wiener process. To that effect, the new algorithm relies on a virtual particle, attached to each stochastic one, whose mean conditional behavior provides free-of-statistical-bias predictions of residence times. After consistency checks, this new algorithm is validated on two representative test cases: particle dispersion in a statistically uniform flow and particle dynamics in a non-uniform flow.; L’objectif de ce papier est de proposer un algorithme robuste en pas de temps permettant l’intégration face à face de la trajectoire de particules dans des maillage 3-D non-structurés pour des méthodes lagrangiennes stochastiques. L’idée principale est de mettre à jour de façon dynamique les champs moyens utilisés dans l’intégration temporelle. Pour ce faire, pour chaque particule, on subdivise le pas de temps en sous pas de temps, de façon à ce que chacun de ceux-ci correspondent au temps de résidence de la particule dans une cellule. Ce faisant on réduit l’erreur de discrétisation spatiale. Étant donnée la nature stochastique du modèle, un aspect prépondérant est de pouvoir dériver des estimations du temps de résidence des particules dans les cellules qui n’anticipent pas le futur des processus de Wiener. Dans ce but, le nouvel algorithme est basé sur une particule virtuelle attachée à chaque particule stochastique, dont le comportement conditionné moyen permet d’obtenir des temps de résidence qui ne sont entachés d’aucun biais statistique. Après une vérification de consistance, ce nouvel algorithme est validé sur deux cas de tests représentatifs : une dispersion de polluants dans un écoulement statistiquement uniforme et un écoulement non-uniforme dans lequel on traque la dynamique des particules.

International audience; This numerical study aims at getting further insight into singular solutions of the 2D Euler equations for non-smooth initial data, in particular for vortex sheets, similar to Caflisch et al 1. For this, high resolution simulations of vortex layers in 2D incompressible Euler flows are performed using the characteristic mapping method 2. An example of such a flow can be found in figue 1a. The method is a semi-Lagrangian method that evolves the flow map using the gradient-augmented level set method. Due to the semi-group structure of the flow map, it can be decomposed into sub-maps (each over a finite time interval), and thus the error can be controlled by choosing appropriate remapping times. Composing the flow map then yields exponential resolution in linear time and thus fine scale flow structures can be resolved in unprecedented detail. Here the roll-up process of vortex layers is studied varying the thickness of the layer showing its impact on the growth of palinstrophy and curvature of the center-line. The self-similar structure of the vortex core is investigated in the vanishing thickness limit, as can be seen in figure 1b. Conclusions on the non-uniqueness of weak solutions of 2D Euler for non-smooth initial data are drawn and the presence of flow singularities is revealed.

International audience; The objective of the paper is to better understand and quantify the flow structure in fractured rocks from flow logs, and to propose relevant indicators for validating, calibrating or even rejecting hydrogeological models. We first studied what the inflow distribution tells us about the permeability structure from a series of analyses: distribution of transmissivities as a function of depth, proportion of flowing sections as a function of section scale, and scaling of the arithmetically-averaged and geometrically-averaged permeability. We then define three indicators that describe few fundamental characteristics of the flow/permeability, whatever the scale: a percolation scale , the way permeability increases with scale above , and the variability of permeability. A 4th indicator on the representative elemental volume could in principle be defined but the data show that this volume/scale is beyond the 300 m investigated. We tested a series of numerical models built in three steps: the geo-DFN based on the observed fracture network, the open-DFN which is the part of the geo-DFN where fractures are open, and a transmissivity model applying on each fracture of the open-DFN (Discrete Fracture Network). The analysis of the models showed that the percolation scale is controlled by the open-DFN structure and that the percolation scale can be predicted from a scale analysis of the percolation parameter (basically, the third moment of the fracture size distribution that provides a measure of the network connectivity). The way permeability increases with scale above the percolation threshold is controlled by the transmissivity model and in particular by the dependence of the fracture transmissivity on either the orientation of the fractures via a stress-controlled transmissivity or their size or both. The comparison with data on the first two indicators shows that a model that matches the characteristics of the geo-DFN with an open fraction of 15% as measured adequately fits the data provided that the large fractures remain open and that the fracture transmissivity model is well selected. Most of the other models show unacceptable differences with data but other models or model combinations has still to be explored before rejecting them. The third indicator on model variability is still problematic since the natural data show a higher variability than the models but the open fraction is also much more variable in the data than in the models.

International audience; On-chip study of blood flow has emerged as a powerful tool to assess the contribution of each component of blood to its overall function. Blood has indeed many functions, from gas and nutrient transport to immune response and thermal regulation. Red blood cells play a central role therein, in particular through their specific mechanical properties, that directly influence pressure regulation, oxygen perfusion, or platelet and white cells segregation towards endothelial walls. As the bloom of in-vitro studies has led to the apparition of various storage and sample preparation protocols, we address the question of the robustness of the results involving cell mechanical behavior against this diversity. The effects of three conservation media (EDTA, citrate and glucose-albumin-sodium-phosphate) and storage time on the red blood cell mechanical behavior are assessed under different flow conditions: cell deformability by ektacytometry, shape recovery of cells flowing out of a microfluidic constriction, and cell flipping dynamics under shear flow. The impact of buffer solutions (phosphate-buffered saline and density-matched suspension using iodixanol/Optiprep) are also studied by investigating individual cell flipping dynamics, relative viscosity of cell suspensions and cell structuration under Poiseuille flow. Our results reveal that storing blood samples up to seven days after withdrawal and suspending them in adequate density-matched buffer solutions has in most experiments a moderate effect on the overall mechanical response, with a possible rapid evolution in the first three days after sample collection. SIGNIFICANCE Blood is in intimate contact with all organs in the body, supplying oxygen, nutrients and drugs while removing waste. It carries cells involved in immune response, wound repair and tumor dissemination. Blood is easily collected, revealing the presence of disease through biomarker analysis. It is storable and transfusable. Thus, blood is the subject of many in-vitro studies for research and medical purposes. Guidelines associated to sample preparation or storage conditions have been established, but these may affect its mechanical behavior. In this collaborative study, we provide new guidelines to minimize the impact of specific experimental requirements (e.g. density matching, blood freshness) by focusing on the single or collective motion of red blood cells in a large range of flow conditions.

In this paper, we assess the capabilities of the Arbitrary Lagrangian–Eulerian (ALE) method implemented in the open-source code TrioCFD to tackle down two fluid–structure interaction problems involving moving boundaries. To test the code, we first consider the bi-dimensional case of two coaxial cylinders moving in a viscous fluid. We show that the two fluid forces acting on the cylinders are in phase opposition, with amplitude and phase that only depend on the Stokes number, the dimensionless separation distance and the Keulegan–Carpenter number. Throughout a detailed parametric study, we show that the self (resp. cross) added mass and damping coefficients decrease (resp. increase) with the Stokes number and the separation distance. Our numerical results are in perfect agreement with the theoretical predictions of the literature, thereby validating the robustness of the ALE method implemented in TrioCFD. Then, we challenge the code by considering the case of a vibrating cylinder located in the central position of a square tube bundle. In parallel to the numerical investigations, we also present a new experimental setup for the measurement of the added coefficient, using the direct method introduced by Tanaka. The numerical predictions for the self-added coefficients are shown to be in very good agreement with a theoretical estimation used as a reference by engineers. A good agreement with the experimental results is also obtained for moderate and large Stokes numbers, whereas an important deviation due to parasitic frequencies in the experimental setup appears for low Stokes number. Still, this study clearly confirms that the ALE method implemented in TrioCFD is particularly efficient in solving fluid–structure interaction problems. As an open-source code, and given its ease of use and its flexibility, we believe that TrioCFD is thus perfectly adapted to engineers who need simple numerical tools to tackle down complex industrial problems.

To simulate the interaction of ocean waves with marine structures, coupling approaches between a potential flow model and a viscous model are investigated. The first model is a fully nonlinear potential flow (FNPF) model based on the Harmonic Polynomial Cell (HPC) method, which is highly accurate and best suited for representing long distance wave propagation. The second model is a CFD code, solving the Reynolds-Averaged Navier-Stokes (RANS) equations within the \openfoam toolkit, more suited to represent viscous and turbulent effects at local scale in the body vicinity. Two one-way coupling strategies are developed and compared in two dimensions, considering fully submerged and fixed structures. A domain decomposition (DD) strategy is first considered, introducing a refined mesh in the body vicinity on which the RANS equations are solved. Boundary conditions and interpolation operators from the FNPF results are developed in order to enforce values at its outer boundary. The second coupling strategy considers a decomposition of variables (functional decomposition, FD) on the local grid. As the FNPF simulation provides fields of variables satisfying the irrotational Euler equations, complementary velocity and pressure components are introduced as the difference between the total flow variables and the potential ones. Those complementary variables are solutions of modified RANS equations. Comparisons are presented for nonlinear waves interacting with a horizontal cylinder of rectangular cross-section. The loads exerted on the body computed from the four simulation methods (standalone FNPF, standalone CFD, DD and FD coupling schemes) are compared with experimental data. It is shown that both coupling approaches produce an accurate representation of the loads and associated hydrodynamic coefficients over a large range of incident wave steepness and Keulegan-Carpenter number., Comment: 39 Pages, 12 figures, 2 tables

A rotating/stratified fluid supports waves that propagate in directions that depend on their frequency. If the fluid is uniformly rotating around an axis Oz with a rotation rate Ω and/or uniformly stratified along the same axis with a Br¨unt-V¨aiss¨al¨a frequency N, harmonic waves of frequency ω propagate along characteristic lines that make an angle θ with respect to the horizontal plane Oxy with θ satisfying the relation ω^2 = 4Ω^2 cos^2 θ + N^2sin^2 θ. Any infinitesimal oscillation of the boundary is a source of waves but when this boundary is tangent to a direction of propagation of the waves, a singularity generically appears. This critical slope singularity propagates along the criticalray tangent to the boundary. When weak viscous effects are considered, this singular ray forms aconcentrated self-similar wave beam with an amplitude that depends on the nature of the singularity and a width of order (νxk/ω) 1/3, where ν is the kinematic viscosity and xk the distance to the critical point.The goal of the present work is to provide information on the type of singularities that can be generated in an infinite domain, and therefore on the amplitude and nature of the concentrated wave beams that can be created from critical points. We analyse in a 2D framework two generic configurations corresponding to oscillations normal and tangent to the boundary, respectively. In the first case (oscillations normal to the boundary), we obtain an amplitude scaling in (ωτ^3c /(νxll))^1/6 corresponding to an inviscid singularity in x⊥^−1/2 , where x⊥ is the distance to the critical ray and rc the curvature radius at the critical point. In the second case (oscillations tangent to the boundary), a weaker beam in (νr^3c /(ωx^5ll))^1/12 is obtained corresponding to a stronger singularity in x⊥^−5/4. In that case, the beam is generated by the peculiar viscous boundary layer flow obtained close to the critical point and the problem can be completely solved by a local analysis. A general expression forthe beam amplitude is derived that depends on the fluid characteristics (Ω, N), the wave frequency ω, the velocity amplitude of imposed tangential oscillations and the local curvature radius at the critical point. Finally, the first order viscous correction to the critical slope beam induced by the no-slip boundary condition on the surface is also calculated.

International audience; The understanding and analyzing of solid particle behavior in a liquid is a challenge in numerouse fields and engineering industry as the petroleum or the cosmetic one. It is indeed essential to know the behavior of soft matter process to avoid problems and ensures the product quality. This study presents the viscosimeter development working on a resonant optical signal principle by measuring the Free Spectral Range (FSR) parameter of a resonant optical mode during nanoparticles (NPs) sedimentation in a liquid which consists of a water/glycerol mixture. The photonic structure is composed of racetracks micro-resonators made of a UV210 polymer fabricated by deep-UV photolitography developped on an oxydated silicon layer to get a Si/SiO 2 bi-layer. The chip is then integrated in an optical bench to track the evolution of the FSR during the complet sedimentation process. The resonant signal analyse established by an adapted signal processing of silica nanoparticles sedimentation in different water/glycerol concentrations allows us to determine stages and velocity rate of the sedimentation process to finaly access to their viscosity. At the same time, measures are performed on a commercial mechanical rheometer so as to compare the dynamic evolution of their viscosity and their associated FSR. The plot of those data versus the glycerol concentration in water obviously shows a possible mathematical transformation between viscosity and FSR slope. There is therefore a good agreement between mechanical and resonant optical measures if we consider the dynamic evolution of both curves ; so this work proves the feasability of an optical viscosimeter based on resonant signal.

International audience; A stable scheme is proposed in this paper in order to obtain approximate solutions of second-moment turbulent models for incompressible flows with or without thermal transport equation. The analysis of the convective terms, which includes the solution of the associated Riemann problem, enables to propose a standard projection scheme, and to get rid of spurious oscillations.

National audience; La modélisation de la turbulence en convection naturelle est un problème à la fois difficile et important au regard des enjeux industriels et environnementaux. La flottabilité couple fortement la dynamique et la thermique et a une influence sur la plupart des termes à modéliser qui est souvent mal connue ou comprise expérimentalement. Si la modélisation au second ordre permet de prendre en compte en partie la subtilité des interactions complexes en jeu, de nombreuses questions restent ouvertes sur les échelles de temps ou la dissipation, par exemple. De nombreuses applications industrielles se basent sur des modèles à viscosité turbulente et corrigent les modèles artificiellement en diminuant le nombre de Prandtl turbulent. Une manière plus physique de procéder consiste à prendre en compte la production par la flottabilité dans la loi de comportement. Ce type de modifications, applicables aux modèles RANS et hybride RANS/LES, permettent d'améliorer la représentation de la physique dans certaines configurations non-stratifiées, mais leur influence reste marginale dans d'autre configurations. Un des problèmes non-résolus est la cohabitation de régions laminaires et turbulentes, la transition dans les couches limites en convection naturelle restant globalement un sujet complètement ouvert en modélisation.

The emergence of new nanoporous materials, based e.g. on 2D materials, offers new avenues for water filtration and energy. There is accordingly a need to investigate the molecular mechanisms at the root of the advanced performances of these systems in terms of nanofluidic and ionic transport. In this work, we introduce a novel unified methodology for Non-Equilibrium classical Molecular Dynamic simulations (NEMD), allowing to apply likewise pressure, chemical potential and voltage drops across nanoporous membranes and quantifying the resulting observables characterizing confined liquid transport under such external stimuli. We apply the NEMD methodology to study a new type of synthetic Carbon NanoMembranes (CNM), which have recently shown outstanding performances for desalination, keeping high water permeability while maintaining full salt rejection. The high water permeance of CNM, as measured experimentally, is shown to originate in prominent entrance effects associated with negligible friction inside the nanopore. Beyond, our methodology allows to fully calculate the symmetric transport matrix and the cross-phenomena such as electro-osmosis, diffusio-osmosis, streaming currents, etc. In particular, we predict a large diffusio-osmotic current across the CNM pore under concentration gradient, despite the absence of surface charges. This suggests that CNMs are outstanding candidates as alternative, scalable membranes for osmotic energy harvesting., Comment: 13 pages, 16 figures, submitted to J. Chem Phys

Advanced courses; International audience

International audience; Superhydrophobic surfaces (SHSs) can reduce the friction drag in turbulent flows. In the laminar regime, it has been shown that trace amounts of surfactant can negate this drag reduction, at times rendering these surfaces no better than solid walls (Peaudecerf et al., Proc. Natl. Acad. Sci. USA 114(28), 7254-9, 2017). However, surfactant effects on the drag-reducing properties of SHSs have not yet been studied under turbulent flow conditions, where predicting the effects of surfactant in direct numerical simulations remains expensive by today’s standards. We present a model for turbulent flow inclusive of surfactant, in either a channel or boundary-layer configuration, over long but finite-length streamwise ridges that are periodic in the spanwise direction, with period and gas fraction . We adopt a technique based on a shifted log law to acquire an expression for the drag reduction. The average streamwise and spanwise slip lengths are derived by introducing a local laminar model within the viscous sublayer, whereby the effect of surfactant is modelled by modifying the average streamwise and spanwise slip lengths. Our model agrees with available laboratory experimental data from the literature when conditions are clean (surfactant-free), or when there are low surfactant levels. However, we find an appreciable drag increase for larger background surfactant concentrations that are characteristic of turbulent flows over SHSs for marine applications.

In this paper, we build and analyze the stability of a collocated scheme, involving a specific numerical diffusion operator, for the isentropic Euler equations. This scheme is based on the numerical diffusion operator of a family of staggered finite volume schemes introduced in [1]. The properties of this operator allowed to understand the L 2-stability of staggered finite volume methods. Staggered schemes are popular in the thermal hydraulics community for their reported robustness and lack of spurious oscillations. The contributions of this paper are twofold: we firstly build a colocated scheme with a staggered-based numerical diffusion operator, hence the proposed terminology of "pseudo-staggered" scheme, and we secondly present a rigorous TVD analysis, in order to explain the non-oscillatory behaviour of staggered discretisations.

International audience; Chemical and biological reactions at fluid-solid interfaces are central to a broad range of porous material applications and research. Pore-scale solute transport limitations can reduce reaction rates, with marked consequences for a wide spectrum of natural and engineered processes. Recent advances show that chaotic mixing occurs spontaneously in porous media, but its impact on surface reactions is unknown. We show that pore-scale chaotic mixing significantly increases reaction efficiency compared to nonchaotic flows. We find that reaction rates are well described in terms of diffusive first-passage times of reactants to the solid interface subjected to a stochastic restart process resulting from Lagrangian chaos. Under chaotic mixing, the shear layer at no-slip interfaces sets the restart rate and leads to a characteristic scaling of reaction efficiency with Péclet number, in excellent agreement with numerical simulations. Reaction rates are insensitive to the flow topology as long as flow is chaotic, suggesting the relevance of this process to a broad range of porous materials.

International audience; The separation efficiency and pressure drop of the dynamic separator of cement particles can be affected by many factors, like structural type, geometric parameters, and operating characteristics. In this paper, CFD modeling is applied to investigate the fluid flow behavior and the efficiency of the industrial dynamic separator with different heights of the inner cone called the vortex breaker. Simulations are based on the RSM and the DPM models. A CFD comparison of the original design and new designs has been performed. The simulation results showed that the fluid flow inside the industrial air separator is greatly dependent on the height of the vortex breaker. Interesting phenomena were observed by the numerical simulations and the results revealed that an increase in the height of the vortex breaker up to three-quarters of the magnitude of the fine powder outlet duct can improve the performances of particle separation not only by reducing 29% the cut size, and by 40% the bypassing of fine particles but also by increasing 30% the separation sharpness while keeping the pressure drop substantially unchanged.

International audience; This study aims to characterize some safety aspects by examining the geometries of the infrastructure, currently used by societies, against the accumulation of hazardous hydrogen clouds during an accidental leak in areas with limited ventilation. Using ANSYS FLUENT as a modeling tool, the influence of garage roof shape; pyramidal and domed roof compared with the basic model (flat roof), for different leak times, on dispersion and stratification of hydrogen layers, is analyzed. As a result, the domed roof promotes to have a lower hydrogen concentration and presents two remarkable peaks of the Richardson number (Ri) with the highest value more than 2 × 105, which is three times higher than the flat roof. Besides, the influence of the leak time on the dynamic of the flow, concentration, and stratification process are observed: the mole fraction of hydrogen is more than 0.25 after 1 h of leak, whereas it is lower than 0.05 after 100 s. The volume flow and therefore the flammable volume increase. This study highlights the importance of geometrical and sizing parameters on the characteristics of hydrogen leaks and subsequently gives insights to establish performance standards for the availability and reliability of safety critical systems.

The drying of human whole blood pools has been studied experimentally to improve the understanding of drops and pools of human whole blood. Here blood is assimilated to a complex fluid made of several bio-colloids. The blood pools were created inside a glove box under controlled conditions. We performed experiments at temperatures of 21 °C, 29 °C, and 37 °C, and relative humidity ranging from 20 % to 70 %. The air inside the glove box was still, and the evaporation process was mainly purely diffusive. A human blood diffusion coefficient of (1.08 ± 0.02) × 10 −9 m 2 /s was obtained. This value is in agreement with literature values for polymers and colloidal films. Moreover, we elaborated an updated model using Page's model to describe the drying kinetics of human blood pools. The effect of the surface area of pools was considered using a diffusive characteristics time. These findings are of interest for biomedical and forensic applications.

National audience; The understanding and analyzing of solid particle behavior in a liquid is a challenge in numerouse fields and engineering industry as the petroleum or the cosmetic one. It is indeed essential to know the behavior of soft matter process to avoid problems and ensures the product quality. This study presents the viscosimeter development working on a resonant optical signal principle by measuring the Free Spectral Range (FSR) parameter of a resonant optical mode during nanoparticles (NPs) sedimentation in a liquid which consists of a water/glycerol mixture. The photonic structure is composed of racetracks micro-resonators made of a UV210 polymer fabricated by deep-UV photolitography developped on an oxydated silicon layer to get a Si/SiO2 bi-layer. The chip is then integrated in an optical bench to track the evolution of the FSR during the complet sedimentation process. The resonant signal analyse established by an adapted signal processing of silica nanoparticles sedimentation in different water/glycerol concentrations allows us to determine stages and velocity rate of the sedimentation process to finaly access to their viscosity. At the same time, measures are performed on a commercial mechanical rheometer so as to compare the dynamic evolution of their viscosity and their associated FSR. The plot of those data versus the glycerol concentration in water obviously shows a possible mathematical transformation between viscosity and FSR slope. There is therefore a good agreement between mechanical and resonant optical measures ; so this work proves the feasability of an optical viscosimeter based on resonant signal.

International audience

Recent palaeomagnetic and seismological studies have strengthened the suggestion that the Moon once possessed a core dynamo. Despite its importance as a constraint on lunar evolution, there is currently no consensus on how such a dynamo was driven. Two groups working independently have arrived at the idea that the lunar dynamo was powered by mechanical stirring of the liquid core. Dwyer et al. investigate the mechanism of a dynamo driven by continuous mechanical stirring arising from the differential motion between the solid silicate mantle and the liquid core beneath. They show that the fluid motions and the power required to drive a dynamo operating continuously for more than 1 billion years are readily obtained by such mechanical stirring. Le Bars et al. propose a model whereby the dynamo action comes from impact-induced changes in the Moon's rotation rate. They show that basin-forming impact events are energetic enough to have unlocked the Moon from synchronous rotation, and that the subsequent large-scale fluid flows in the core, excited by the tidal distortion of the core–mantle boundary, could have powered a lunar dynamo. The origin of lunar magnetic anomalies1,2,3,4,5 remains unresolved after their discovery more than four decades ago. A commonly invoked hypothesis is that the Moon might once have possessed a thermally driven core dynamo3, but this theory is problematical given the small size of the core and the required surface magnetic field strengths6. An alternative hypothesis is that impact events might have amplified ambient fields near the antipodes of the largest basins7, but many magnetic anomalies exist that are not associated with basin antipodes. Here we propose a new model for magnetic field generation, in which dynamo action comes from impact-induced changes in the Moon’s rotation rate. Basin-forming impact events are energetic enough to have unlocked the Moon from synchronous rotation8, and we demonstrate that the subsequent large-scale fluid flows in the core, excited by the tidal distortion of the core–mantle boundary9, could have powered a lunar dynamo. Predicted surface magnetic field strengths are on the order of several microteslas, consistent with palaeomagnetic measurements5, and the duration of these fields is sufficient to explain the central magnetic anomalies associated with several large impact basins.

International audience; Due to the steep relief of the mountain massifs, the valleys are strategic locations for the establishment of human activities. Over last decades, the floodplain of alpine rivers has been highly anthropized. However, mountain rivers have a strong erosive power that can threaten human infrastructures. To protect them, riverbanks are regularly protected with civil engineering works. Yet, this environment is home to a high level of biodiversity. Thus, soil water bioengineering techniques appear sustainable by allowing both to protect the anthropogenic assets and to welcome the rich riparian biodiversity and the associated ecological services. To implement these Nature-Based Solutions, it is necessary to draw inspiration from the biotic and abiotic components of natural riverbanks, which are still poorly understood at high altitude due to several pressures (climatic, hydrologic, hydraulic or morphologic). In order to understand the structure and functioning of natural models of alpine riverbanks, a protocol has been implemented on 21 mountain banks with mature vegetation and no signs of erosion. The sites were located in the Vanoise massif (France) on an altitudinal gradient (1331 – 2131 m) ranging from montane to subalpine belts with a wide range of channel slope (0,8 – 28,4 %). By focusing on the bank toe, biotic (vegetation cover and species biological features) and abiotic (altitude and hydrogeomorphology) components have been measured. First observations showed that the grain size distribution of the sediments forming the bank was coarser and more homogeneous than those forming the channel bed. For the step-pool channels, bank grain sizes were coarser than for plane-bed channels. Shields parameters for the 21 banks were well below the thresholds for sediment motion, and were therefore relatively stable. Furthermore, on all the sites the shear stresses were relatively high but according to the literature values, classic bioengineering structures could be implemented on 16 of the sites and would withstand a 100-year return period flood, which is consistent with our observation of mature, stable and healthy vegetation patches. The mineral component of the bank toe is closely linked to the woody plant structure. The vegetation cover was dominated by 12 species of willow and the green alder. Green alder cover increased with the altitude and the bed slope. Willow species cover varied with altitude and was less important for steep river. Thus, the natural models of banks were: (i) for the rivers with steep slope, models mixing mineral and plant units which both took part jointly in the stability of the bank; and (ii) for the rivers with lower slope the stability of the bank was mainly ensured by the vegetation. This work provided additional knowledge on the structure and functioning of the banks of stable and mature high-altitude rivers. Accordingly, these results will allow to design soil water bioengineering techniques more adapted to mountain environments (plant composition and structure, mineral unit size).

International audience; Willow fascine is the soil bioengineering technique for riverbank stabilization the most frequently used in France. Made up of bundles of living branches fixed between stakes, the fascine presents various possible configurations, adaptable to each site. The structure, with high theoretical resistance, is implemented at the bottom of the riverbank joining the riverbed and the riverbank, which makes it subject to strong constraints. Terrain return of experience showed that 22 % of the fascines did not start or did not stand after few years. Punctual observations in the field do not give the possibility to understand the whole process behind fascines structural failure nor to test various fascine configurations over a short period of time. The study based on physical modelling (scale 1:25) describes how riverbanks with fascines are destabilized and develops which fascine design could better stand. The techniques are replicated on the extrados of three meanders created in the flume. Each bank protection technique undergoes the same flood hydrogram. Three various bank toe protection techniques are tested with and without geotextile to protect the rest of the bank. Bank toe without protection is confronted to designs of (i) fascines with one bundle, (ii) fascines with two bundles and (iii) ripraps with the thickness of the two bundles fascine. Bank pressure is measured continuously, the topography is established by photogrammetry before and after each experiment and direct observations are conducted. As first main result, the process of destabilization of riverbanks with fascines could be descripted. Scour holes, naturally occurring at meanders extrados, develop bellow the fascine level of implementation. The bank material initially stocked behind the fascine is than free to fall in the scour hole. The fascine ends up isolated in the river. Without contact to the substrate, the living material cannot start anymore. Moreover, consecutive to the scour holes development, the fascines stakes fall into the river leading to the fascine structural failure. As second result, between the three bank toe protection techniques tested, the fascine with two bundles and geotextile stabilized the best the bank. However, this strong configuration resulted with deeper scour holes at the bottom of the fascine. Finally as third result, with the sediment material used, the water pressure differential between the riverbank and the running water did not enhance any bank erosion. The destabilization of riverbank with fascines is mainly driven by natural morphologic river adjustment. Consequently, the fascine design could be adjusted as follow. (i) The bank slope must be reduced to its maximum to minimize the scour hole. (ii) The fascine must be deeply anchored by the stakes. (iii) The fascine toe, below the level of vegetation start, is preferentially reinforced by extra bundles, wood pieces or riprap, in order to confined the bank material.

International audience; A focus on a novel conjugate heat transfer method used to couple the arc-plasma andworkpiece (weld pool included) domains in a unified Tungsten Inert Gas welding modelis presented in this paper. The method uses a non-associated relation to implement anasymmetric thermal constraint when coupling the arc and workpiece domains. Themethod is shown to be conservative and robust. The algorithm is used in a unified 3DTIG welding model with displacement effects. The promising simulation results arecompared to experimental work from the literature.

Being the world’s largest freshwater resource, groundwater is at a continuous risk of overabstraction for human water use. Beside substantial drops in groundwater levels that are the consequence of unsustainable groundwater abstraction, which modify the recharge/discharge relationships between large-scale hydrogeological units, this anthropogenic hydraulic forcing is also responsible for changes in thermal regimes within the critical zone. While the impact of global groundwater pumping on the hydrogeological cycle has long been demonstrated, we still have insufficient knowledge on the influence of human activities on groundwater temperatures and, as a consequence, on stream thermal regimes and groundwater quality.In this contribution we discuss temperature anomalies that develop in the shallow subsurface as a result of localized groundwater extraction. We study different hydrogeological settings, i.e., porous and fractured aquifers, that we explore via numerical modelling and comparison with field observations. In the field, we use repeated temperature-depth borehole profiles separated by decades, the advantage of which is that differencing the temperature logs for individual boreholes yields real temperature change and eliminates steady-state sources of curvature. Thus, it enables us to detect changes in subsurface thermal regimes, resulting from transient conditions, i.e., climate change and changes in groundwater hydrodynamics.

The study of heat transport in porous media has recently attracted a lot of attention due to the wide range of industrial and geological applications, yet the impact of the structural heterogeneity of naturally occurring aquifers on their hydraulic and thermal properties is often disregarded. In that regard, a novel application of phosphor thermometry to porous media is proposed with the aim of examining under which conditions the validity of existing thermal transfer models in complex natural saturated porous media can be questioned. This experimental technique relies on monitoring the temperature-dependent luminescence properties of solid phosphor particles seeded into the fluid as tracers, using light sources and cameras. It offers the possibility of characterizing quantitatively the interaction between flow and heat transport processes at the pore scale in transparent analog porous media, with minimal interference and from spatially-resolved measurements, hereby overcoming the technical limitations of current experimental techniques, which are constrained to point temperature measurements.Here, as proof of concept, we present a demonstration experiment performed on a slow-moving flow in a synthetic porous medium with a heterogenous size distribution, and using YAG:Cr3+, a thermographic phosphor with a temperature sensitivity exceeding 0.3%/K [1]. The measurements are performed using a modulated light source and are recorded at a sampling rate of 1 kHz during continuous injection of an aqueous solution which is initially at a constant temperature, different from that of the resident solution. The results show the dynamics of the spatial temperature distribution in the porous medium with a precision of ±0.3°C.[1] J. L. Bonilla and B. Fond, "Phosphor thermometry using the phase-shift method: optimization and comparison with decay time method," 2022.

Repeated alpine glaciations during the Quaternary partly reached far into the foreland and caused profound landscape changes beyond glacial margins in northern Switzerland. Climate-driven glacier growth and decay and commensurate variations in water and sediment delivery caused river systems to aggrade and to incise, leading to the widespread occurrence of glacio-fluvial deposits (Deckenschotter) and associated terraces. Mapping and numerical dating of these depositional complexes increasingly offer insights into the spatial patterns and timing of Quaternary glaciations, and associated changes in (glacio-)fluvial dynamics. An improved understanding of how fluvial systems respond to glacial-interglacial cycles will help to assess the erosion potential around repository sites of nuclear waste over the next one million years. In this study, we contribute to close this research gap using numerical landscape evolution modelling (LEM).We use EROS, a numerical landscape evolution model, which implements a particle-based approach to simulate water and sediment fluxes that interact with topography through erosive and depositional actions. Unlike LEMs based on the stream-power incision law, the method solves the 2D shallow water equations with both basal and lateral erosion and deposition, which allows for variations in width and lateral mobility of rivers; these variations induce changes in the transport capacity of the sediments that cause specific patterns of deposition and erosion to emerge. We adjusted the model so that it is capable to run over 1 Myrs, and imposed boundary conditions that - informed by estimates on longterm erosion rates – reflect rock uplift and plausible variations in water and sediment fluxes following a 100-kyrs glacial cycle. Our model relies on digital elevation models and sediment thickness data with 60 m spatial resolution and is applied to the Hochrhein river between Stein am Rhein and Basel and the Aare river downstream of the area, where Limmat and Reuss enter.Our simulations show that the model reproduces widespread aggradation, reworking of the sediments by highly laterally mobile, braided river systems and incision during periods of increased runoff and low sediment availability.Our model setup and parametrization features several uncertainties. For example, the capacity of rivers to laterally erode strongly determines the thickness and extent of depositional complexes lining the Hochrhein and Aare system. Also, our model is sensitive to temporally varying boundary conditions of water and sediment input about which precise estimates are lacking. Regardless, the more detailed and realistic representation of hydraulic and sediment transport processes by EROS compared to conventionally used landscape evolution models at this spatial and temporal scale provide the opportunity to test different hypotheses using numerical experiments and link the results to field evidence. Further sensitivity analyses and uncertainty quantification will enable us to use our model as simulation tool to hindcast and investigate the behaviour of fluvial system in response to different tectonic and climatic scenarios, thus helping to better understand potential spatial patterns of and sediment assemblages within widespread glacio-fluvial deposits.

The Brague River basin (68 km2) is located on the French Mediterranean coast. It experiences a high risk of flash floods. The potential efficacy and efficiency of flood protection strategies based on green (Nature Based Solutions: NBS) or grey (civil-engineering) measures, as well as their co-benefits, are studied in this chapter. Two NBS flood alleviation strategies combine both small natural water retention areas, along with a widening of the river corridor. Two more classical grey scenarios were based on large wood-trapping racks, and another based on retention dams. This chapter synthesizes (i) the flood risk assessment; (ii) the estimation of total costs; (iii) how benefits related to each strategy were estimated. Benefits were evaluated with both a top-down method (avoided damages and transfer of valuations made elsewhere for co-benefits) and with a bottom-up approach surveying citizen willingness to pay. The cost-benefit analysis demonstrates that costs are higher than the avoided damage, and that co-benefits are much higher than avoided damage for most strategies and for both approaches. Depending on the number of household considered in the co-benefits valuation, the balance may reach higher benefits than costs for NBS strategies, though not for the grey solution based on large dams.

Recently a lumped-parameter model of the cardiovascular system was proposed to simulate the hemodynamics response to partial hepatectomy and evaluate the risk of portal hypertension (PHT) due to this surgery. Model parameters are tuned based on each patient data. This work focuses on a global sensitivity analysis (SA) study of such model to better understand the main drivers of the clinical outputs of interest. The analysis suggests which parameters should be considered patient-specific and which can be assumed constant without losing in accuracy in the predictions. While performing the SA, model outputs need to be constrained to physiological ranges. An innovative approach exploits the features of the polynomial chaos expansion method to reduce the overall computational cost. The computed results give new insights on how to improve the calibration of some model parameters. Moreover the final parameter distributions enable the creation of a virtual population available for future works. Although this work is focused on partial hepatectomy, the pipeline can be applied to other cardiovascular hemodynamics models to gain insights for patient-specific parameterization and to define a physiologically relevant virtual population.

We reprove the fact, due to Backus, that the Poincar{\'e} operator in ellipsoids admits a pure point spectrum with polynomial eigenfunctions.We then show that the eigenvalues of the Poincar{\'e} operator restricted to polynomial vector fields of fixed degree admitsa limit repartition given by a probability measure that we construct explicitely. For that, we use Fourier integral operators and ideas comingfrom Alan Weinstein and the first author in the seventies. The starting observation is that the orthogonal polynomials in ellipsoids satisfy a PDE.

International audience

In crystalline bedrock, the open fraction of the fracture network constitutes the main pathways for fluids. Many observations point out that the state of stress influences the open fraction, likely indicating recent reactivation. But how this occurs is still unresolved. We analyse the conditions for fracture reactivation from fracture data collected in the uppermost 1 km of bedrock in Forsmark, Sweden. The open fraction is mainly correlated to the stress acting normally on the fracture but even away from critical failure, leading us to analyse the potential fluid pressure required for reactivation, $${P}_{c}$$ P c . We observe that 100% of the fractures are open when $${P}_{c}$$ P c is hydrostatic, and the ratio decreases exponentially to a plateau of ~ 17% when $${P}_{c}$$ P c is lithostatic and above. Exceptions are the oldest fractures, having a low open fraction independent of $${P}_{c}$$ P c . We suggest that these results reflect past pressure build-ups, potentially related to recent glaciations, and developing only if the preexisting open fraction is large enough.

International audience; Transport of solutes in aquifers is controlled by the heterogeneous spatial distribution of hydraulic properties, but the characterization of aquifer heterogeneity is quite challenging with conventional methods. Hydraulic tomography (HT) was developed to define the heterogeneous distribution of hydraulic conductivity (K) and specific storage (Ss). HT involves the emission of a series of hydraulic head perturbations in a stressed well and the recording of this signal at several levels in the stressed and observation wells. All recorded hydraulic head responses are simultaneously analyzed through numerical inversion, which provides the spatial distribution of hydraulic properties at a scale relevant for local site investigations.This communication reports on a tomographic experiment carried out in a heterogeneous and highly anisotropic granular aquifer at the Saint-Lambert research site near Quebec City, Canada. This site has already been the object of detailed characterizations with multiple hydraulic methods: pumping tests, packer slug tests, flowmeter profiles, vertical interference tests, and slug test tomography. A relatively new approach named oscillatory hydraulic tomography (OHT) was tested, in which multi-frequency oscillatory head perturbations are induced in an interval isolated by packers of the stressed well by a submerged rod that is electronically controlled by a winch system. Hydraulic responses are measured in the stressed intervals and in multiple intervals of an observation well.This study was primarily aimed at testing, first on an operational level, if the OHT signal could be generated in the stressed well and propagated to the observation well in a highly anisotropic granular aquifer. Second, the study developed a rigorous workflow for the treatment of the measured hydraulic heads. Third, in terms of characterization efficacy, the study aimed to determine if multiple controlled frequencies would allow the assessment of K spatial distribution.Results show that the field experiment provided clear measured hydraulic responses that could be used to obtain the 2D distribution of hydraulic properties from the inversion of OHT measurements. Comparison was made of inversion results using a single oscillatory frequency and multiple frequencies. Under conditions of realistic field measurement noise and uncertainty, it will be valuable in future work to compare the imaging capabilities of oscillatory hydraulic tomography against other tomographic methods. Further investigation is also needed to examine the information content of oscillatory hydraulic tomographic data for characterizing K and Ss heterogeneities through a sensitivity and resolution analysis. This study demonstrates the practical potential for the implementation of OHT experiments in relatively low permeability and highly anisotropic granular aquifers.

International audience; Flow and mixing processes in porous media control many natural and industrial systems, such as microbial clogging, oil extraction, and effluent disposal. In many systems, the porosity may evolve during mineral precipitation, such as in rocks, and control fluid mixing and fluid transport properties. Here, we use three-dimensional in situ dynamic neutron and X-ray micro-tomography imaging to explore fluid transport into Berea sandstone core samples during in-situ carbonate precipitation. Neutron imaging can track fluid flow inside the rock, whereas X-ray imaging illuminates the regions where mineral precipitation occurs. We control the precipitation of calcium carbonate in the rock through reactive-mixing between solutions containing CaCl2 and Na2CO3. By solving the advection-diffusion equation using the contrast in neutron attenuation from time-lapse images, we derive the 3D velocity field of the injected fluids and characterize the evolution of the permeability field into the rock during mineral precipitation. We also investigate the mixing between heavy water and a cadmium solution under the influence of mineral precipitation. Results show that, under the effect of mineral precipitation, a wide range of local flow velocities develop in the sample, under the same fluid injection rate, and we quantify the distribution of flow velocities in the sample. Moreover, we observe more efficient mixing between heavy water and a cadmium solution after mineral precipitation. The finding of this experimental study is useful in progressing the knowledge in the domain of reactive solute and contaminant transport in the subsurface.

A large fraction of greenhouse gases (about 60%) released into the atmosphere are due to CO2 emissions from industrial processes and the burning of fossil fuels [1]. One of the strategies employed to reduce the emissions is rapping them securely in the subsurface [2-4]. Dissolution trapping, in particular, involves injection of CO2 into the subsurface where the supercritical CO2 (sCO2) dissolves in the aquifer brine and forms a CO2 enriched layer within solution. The interface between the high density CO2 rich brine on the top and the ambient low density aquifer water below results in destabilization of the aforementioned layer [2-4]. This leads to a gravitational instability which then causes a natural convection of CO2 rich brine to lower layers, thereby accelerating further dissolution of the sCO2 into the fresh brine.The study of Brouzet et al. shows that traditional continuume scale, Darcy law-governed, models underestimate the timescales of the convective dissolution’s dynamics, owing to local heterogeneity in the pore-scale flow, and that it may thus be necessary to take pore-scale fluctuations into account [5]. We present here a 2D experimental study using miscible analog fluids with a contrast in densities to understand the convective transport of the dissolved sCO2. The fluids and the granular media are refractive index matched, which renders the medium transparent and helps in accurate quantification of experimental findings at various Rayleigh (Ra) and Darcy numbers (Da). Darcy scale simulations are used to complement the two-dimensional experimental measurements and it was found that Darcy scale simulations underpredict the experimental findings by several orders of magnitude, which is consistent with the findings by Brouzet et al. We investigate convective dynamics for various values of the number by changing the density of fluids, the properties of the granular medium (permeability, size of the granular medium) which determines the size of the instability with respect to pore size. When that number is much smaller than 1, obvious causes for the failure of the continuum scale description can be excluded, yet discrepancies remain between the experimental results and the simulations.References:[1] Contribution of Working Group III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, IPCC 2014.[2] Emami-Meybodi, H., Hassanzadeh, H., Green, C. P., & Ennis-King, J. (2015). Convective dissolution of CO2 in saline aquifers: Progress in modeling and experiments. International Journal of Greenhouse Gas Control, 40, 238-266.[3] Pau, G. S., Bell, J. B., Pruess, K., Almgren, A. S., Lijewski, M. J., & Zhang, K. (2010). High-resolution simulation and characterization of density-driven flow in CO2 storage in saline aquifers. Advances in Water Resources, 33(4), 443-455.[4] Meunier, P., & Nadal, F. (2018). From a steady plume to periodic puffs during confined carbon dioxide dissolution. Journal of Fluid Mechanics, 855, 1-27.[5] Brouzet, C., Méheust, Y., & Meunier, P. (2022). CO2 convective dissolution in a three-dimensional granular porous medium: An experimental study. Physical Review Fluids, 7(3), 033802.

International audience; Biot coefficient and Skempton coefficient are key descriptors of the coupled hydro-mechanical (HM) behavior of fluid-saturated porous materials. Biot coefficient defines a relationship between an applied load, fluid pressure and the stress that effectively acts on the solid skeleton. Skempton coefficient defines the temporary pore pressure variation caused by the application of a load in undrained conditions. The product of the two coefficients establishes the impact of an applied load on the solid skeleton, and thus the material deformation, under undrained conditions. The two coefficients are generally estimated through analytical expressions valid for isotropic homogeneous materials, or they are experimentally estimated at the laboratory sample-scale.In this work, we define a framework for the evaluation of equivalent Biot coefficient and Skempton coefficient at the scale of a fractured rock mass. We derive theoretical expressions that estimate the two equivalent coefficients from the properties of both the porous intact rock and the discrete fracture network (DFN), including fractures with different orientation, size, and mechanical properties. These formal expressions are validated against results from fully coupled hydro-mechanical simulations on systems with explicit representation of deformable fractures and rock blocks. We show that the coefficients largely vary with the fracture orientation and density, which implies that disregarding the presence of fractures may incur an incorrect evaluation of the HM response. We also discuss the variability of the coefficients under different settings of DFN properties, including realistic scaling conditions of size-dependent and stress-dependent fracture properties.

Multi grain-size transport models that simulate transport of various grain sizes along with the bed stratigraphy consider that only the sediment present in an active layer at the top of the substratum participates in sediment transport. The thickness of this well-mixed layer may be fixed but also calculated according to the coarsest grain size it contains or to the shear stress applied at the surface of the substratum. However, this approach puts the emphasis on the conservation of the active layer thickness and on the availability of the various sizes within this layer. This means there is little consideration i) for heterogeneity in grain size distribution when mixing together adjacent stratigraphic layers that differ significantly in composition and ii) for grain sizes that could prevent or slow down removal of the others. To cope with these limitations, we developed an algorithm with the ability to capture the transport of heterogeneous sediments and the related stratigraphic record of erosional and depositional events based on the behavior of the various sizes within the bed layers. We built a multi grain-size module based on the precipiton method: the time spent by a precipiton (volume of water that carries sediment) on a pixel determines the grain-size specific magnitude of deposition and erosion. The newness of our work is that the magnitudes of erosion may be corrected according to the sizes that slow down the erosion of the others (zero or slow erosion rate) and stratigraphic layers with similar composition only may be merged. A few tests were conducted to study the morphological evolution of a 1D-river reach under various conditions (water discharge, sediment source, etc.). A lake was added at the end of the reach to record the various sizes existing the reach over time. At low water discharge when only the threshold of fine grains is exceeded, an armoring layer made of coarse grains develop at the surface of the substrate. At a water discharge when all the grains are in motion, the finer the grains are, the further downstream they are transported. This downstream fining pattern may be associated with changes in the concavity of the river profile. This multi grain-size algorithm not restricted to the precipiton approach has the potential to unravel the role of heterogeneous sediments in the formation of sorting patterns and, therefore, it is to be implemented in the landscape evolution model RiverLab (former Eros).

Biogeochemical reactions at the interface between fluid and solid phases are of central importance to a broad range of natural and engineered processes in porous media. As dissolved reactants are transported through a porous medium, advection and diffusion act to homogenize their concentrations, in competition with reactive depletion at the solid interface. Transport limitations can limit reactant availability, leading to reduced reaction efficiency when compared to well-mixed conditions. Chaotic advection has been recently established to occur spontaneously in steady, three-dimensional flows through porous media. In this work, we explore and quantify its role in mitigating transport limitations and correspondingly increasing reaction efficiency. We employ the continuous time random walk framework to connect reaction delays due to transport limitations to the statistics of solute excursion times to the interface. Chaotic advection is associated with a rapid loss of memory of initial conditions and efficient exploration of the bulk of the pore space. We model the corresponding effect on excursion times through a stochastic restart process, such that reactant positions are randomly restarted homogeneously across the domain over a characteristic time scale that depends on flow and geometry. Processes that restart under some condition have received much attention in the context of search strategies, where it is known that they can increase the efficiency of the underlying process. Here, we find a corresponding effect on excursion times, and a consequent increase of reaction efficiency with Péclet number. As chaotic advection leads to efficient bulk exploration, low velocities near the interface due to no-slip boundary conditions become the limiting factor on mixing and thus control the restart rate. This has the surprising consequence that, while chaotic advection sets the stage for enhanced reaction efficiency, the increase is insensitive to the "strength" of chaos as quantified by the Lyapunov exponent, and is instead controlled by flow shear at the interface. The theoretical predictions are in excellent agreement with numerical simulations of reactive decay at solid surfaces in a crystalline porous medium, over a broad range of Péclet and Damköhler numbers.

International audience; We report the observation of nonlinear three-wave resonant interactions between two different branches of the dispersion relation of hydrodynamic waves, namely the gravity-capillary and sloshing modes. These atypical interactions are investigated within a torus of fluid for which the sloshing mode can be easily excited. A triadic resonance instability is then observed due to this three-wave two-branch interaction mechanism. An exponential growth of the instability and phase locking are evidenced. The efficiency of this interaction is found to be maximal when the gravity-capillary phase velocity matches the group velocity of the sloshing mode. For a stronger forcing, additional waves are generated by a cascade of three-wave interactions populating the wave spectrum. Such a three-wave two-branch interaction mechanism is probably not restricted to hydrodynamics and could be of interest in other systems involving several propagation modes.

International audience; Investigations of shock-induced cavitation is highly challenged by the multiphase nature of the mechanisms involved. Thermodynamically well-posed multiphase numerical models accouting for phase compression and expansion however allow to elucidate the underlying physics. A description of the bubble cloud growth and its effects on the early droplet dynamics is proposed, as well as a critical discussion on the analytical predictions of the cavitation event previously reported.

International audience; The relaxation dynamics of a weathercock free-to-rotate, in the presence of a uniform flow, as it aligns with the flow direction, is investigated experimentally in a wind-tunnel. The dynamics is observed to conveniently follow a damped harmonic oscillator behavior. At first order, the frequency is set by the aerodynamic coefficients. We show that a quasi static approach fails to precisely describe the relaxation dynamics and that non-stationary corrections are required to model the dynamics. A first strategy is to introduce added mass, added stiffness and added damping to the quasi-static approximation, following what is usually done in the context of vortex-induced vibrations. A second strategy is to introduce empirical corrections, whose scaling is obtained from the analysis of the experimental data. Finally, these two strategies are compared and we discuss the physical interpretations of the non-stationary corrections.

International audience; In this article we analyze several mathematical models with singularities where the classical cotangent model is replaced by a b-cotangent model. We provide physical interpretations of the singular symplectic geometry underlying in b-cotangent bundles featuring two models: the canonical (or nontwisted) model and the twisted one. The canonical one models systems on manifolds with boundary and the twisted one represents Hamiltonian systems with a singularity on the fiber. The twisted cotangent model includes (for linear potentials) the case of fluids with dissipation. We prove (non)existence of cotangent lift dynamics and show the existence of an infinite number of escape orbits in this model. We also discuss more general physical interpretations of the twisted and non-twisted b-symplectic models. Twisted b-symplectic models yield in a natural way escape orbits that go to the critical set. Under compactness assumptions those escape orbits are continued as singular periodic orbits in the sense of Miranda and Oms (2021) and Miranda (2020). These models offer a Hamiltonian formulation for systems which are dissipative, extending the horizons of Hamiltonian dynamics and opening a new approach to study non-conservative systems.

Air jets for active flow control have proved effective in postponing the onset of stall phenomenon in axial compressors. In this paper, we use a combination of machine learning and genetic algorithm to explore the optimal parameters of air jets to control rotating stall in the axial compressor CME2. Three control parameters are investigated: the absolute injection angle, the number of injector pairs and the injection velocity. Given an experimental dataset, the influence of the air jet parameters on the surge margin improvement and power balance is modeled using two shallow neural networks. Parameters of the air jets are then optimized using a genetic algorithm for three rotational velocities, i.e., Ω=3200RPM,4500RPMand6000RPM. First, surge margin improvement and power balance are being maximized independently. Then, a bi-objective optimization problem is posed to explore the trade-off between the two competing objectives. Based on the Pareto front, results suggest that a globally optimal set of parameters is obtained for a velocity ratio (defined as the ratio of the injection velocity to the rotor tip speed) ranging from 1.1 to 1.6 and an injection angle attack varying from 1∘to11∘. These outcomes point out a potential generalization of the control strategy applicable to other compressors.

We develop a numerical scheme for the flow of viscoelastic fluids, including the OldroydB and FENE-CR constitutive models. The space discretization is staggered, using either the Marker-And-Cell (MAC) scheme for structured nonuniform grids, or the Rannacher and Turek (RT) nonconforming low-order finite element approximation for general quandrangular or hexahedral meshes. The time discretization uses a fractional-step algorithm where the solution of the Navier-Stokes equations is first obtained by a projection method and then the transport-reaction equation for the conformation tensor is solved by a finite volume scheme. In order to obtain consistency, the space discretization of the divergence of the elastic part of the stress tensor in the momentum balance equation is derived using a weak form of the MAC scheme. For stability and accuracy purposes, the solution of the transport-reaction equation for the conformation tensor is split into pure convection steps, with a change of variable to the log-conformation tensor, and a reaction step, which consists in solving one ODE per cell via an Euler scheme with local sub-cycling. Numerical computations for the flow in the lid-driven cavity at Weissenberg numbers above one and the flow around a confined cylinder confirm the efficiency of the scheme.

International audience; Recent studies showed both experimental and numerical evidence that the occurrence probability of freak waves could be significantly enhanced as results of non-equilibrium dynamics induced by strong depth variations. The sea-state is characterized by strong non-Gaussian behavior in a short spatial extent after the depth transition, covering a few wavelengths. In this work, we investigate the complete equilibration process of an out-of-equilibrium sea-state via high-fidelity numerical simulations. In the simulations, the region after the depth transition is set as long as around one hundred wavelengths, such that the spectral adaptation develops and terminates eventually. The results are analyzed with spectral, cross-spectral and statistical approaches. It is shown that there are two stages with different spatial scales in the equilibration process. In the short scale, the sea-state is characterized by significant changes in wave statistics, freak wave occurrence probability is intensified. In the long scale, the wave spectrum undergoes strong modulation, the spectral peak disintegrate into a relative broad band, and low-frequency waves are enhanced as well. We show evidence that the spectral changes in the long scale are due to interactions of free components. The wave nonlinearity is shown to be positively correlated to the magnitude of the dynamical responses, but irrelevant to the length of the spatial scales in the equilibration process. In the established shallow-water equilibrium, the freak wave occurrence probability becomes less than Gaussian expectation and the waves are asymmetric in the vertical direction and symmetric in the horizontal.

This work explore the possibility to determine the spray characteristics at early stage of the atomization process. This approach has been firstly proposed in [7] propose to characterize the liquid-gas surface in term of surface curvature distribution allowing the determination of which part of the surface is already representative of the final spray. This innovative approach is based on numerical simulation but could be extended in principle on experimental imagery techniques for instance. In this work, the methodology is extended to include surface velocity measurements to provide the joint distribution of the spray in term of diameter and velocity. The configuration is representative of an aeronautic injector used in the European project CHAiRLIFT and focus on the prefilming airblast atomization. Such an atomization process involves a wide range of length scales. The diameter of the aeronautic atomizer could be three orders of magnitude higher than the diameter of the smallest droplet. Therefore, performing a CFD simulation is neither a straightforward nor a cheap process computationally speaking. Thus, a workflow, which divides the atomizer in less complex processes, is presented in this work to make the simulation affordable in an industrial perspective. The objective is to compute the properties that determine the spray behavior for the following combustion process. The OpenFOAM library is used to perform the simulation. In addition to the numerical strategy to capture first steps of the atomization process the focus of our presentation will be on the post processing of the simulation to extract the main features of the spray. The results presented concern the drop size distribution of the spray and the joint velocity distributions of the droplets. The post processing methodology is based on an analysis of the surface density joint distribution for curvatures and velocity. We believe this methodology can be used to enrich the spray injection models for further reactive spray flame simulations.

International audience; An investigation of the dispersion of pollutants ejected from a chim- ney around a three-dimensional cylindrical obstacle within a cross- flow air stream was conducted in this paper. The dynamic evolution of air seeded with glycerin particles ejected from an elevated jet around the isolated obstacle in a wind tunnel was experimentally studied using the particle image velocimetry technique and under different velocity ratios between the chimney ejection and the wind source. The dispersion of CO2 pollutant around the cylindrical build- ing was numerically predicted based on measured experimental data using the finite volume method and the Reynolds stress model. A good agreement between experimental and computational results was found. Velocity, temperature, and concentration results revealed that the velocity of the wind together with the presence of the isolated building obstacle influenced the flow structure.

Thermally driven flows of superfluid 4He display unique features, often related to the presence of quantized vortices—line singularities embedded in the liquid. Here, we focus on turbulent round jets, experimentally investigated using the flow visualization and second sound attenuation techniques, at Reynolds numbers exceeding 104. These turbulent flows are driven by releasing heat into a small volume of liquid, open to the surrounding bath through a cylindrical nozzle, 2 mm in diameter. Our measurements reveal in unprecedented detail how the tangle of quantized vortices associated with the jets arranges itself in space, for distances ranging from 9 to 34 nozzle diameters, at fluid temperatures between 1.64 and 2.10 K. We specifically find that the vortex tangle spreads in the radial direction, while it dilutes away from the nozzle. Additionally, the tangle density is found to systematically depend on the flow forcing. Two physical interpretations of the observed behavior are proposed, which could motivate further investigations of this peculiar flow. One leads us to conjecture a self-similar functional form of the vortex tangle density across counterflow jets. The other suggests that the position of the superfluid stagnation point—a characteristic feature of counterflow jets—could depend on the flow forcing as well.