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Due to their high cost of acquisition and operation, there are still a limited number of high-yield, high-flux neutron source facilities worldwide. In this context, laser-driven neutron sources offer a promising, cheaper alternative to those based on large-scale accelerators, with, in addition, the potential of generating compact neutron beams of high brightness and ultra-short duration. In particular, the predicted capability of next-generation petawatt (PW)-class lasers to accelerate protons beyond the 100 MeV range should unlock efficient neutron generation through spallation reactions. In this paper, this scenario is investigated numerically through particle-in-cell and Monte Carlo simulations, modeling, respectively, the laser acceleration of protons from thin-foil targets and their subsequent conversion into neutrons in secondary heavy-ion targets. Laser parameters relevant to the 1 PW LMJ-PETAL and 1-10 PW Apollon systems are considered. Under such conditions, neutron fluxes exceeding $10^{23}\,\rm n\,cm^{-2}\,s^{-1}$ are predicted, opening up attractive fundamental and applicative prospects., Comment: 16 pages, 13 figures

Several biological and social contagion phenomena, such as superspreading events or social reinforcement, are the results of multi-body interactions, for which hypergraphs offer a natural mathematical description. In this paper, we develop a novel mathematical framework based on approximate master equations to study contagions on random hypergraphs with a heterogeneous structure, both in terms of group size (hyperedge cardinality) and of membership of nodes to groups (hyperdegree). The characterization of the inner dynamics of groups provides an accurate description of the contagion process, without losing the analytical tractability. Using a contagion model where multi-body interactions are mapped onto a nonlinear infection rate, our two main results show how large groups are influential, in the sense that they drive both the early spread of a contagion and its endemic state (i.e., its stationary state). First, we provide a detailed characterization of the phase transition, which can be continuous or discontinuous with a bistable regime, and derive analytical expressions for the critical and tricritical points. We find that large values of the third moment of the membership distribution suppress the emergence of a discontinuous phase transition. Furthermore, the combination of heterogeneous group sizes and nonlinear contagion facilitates the onset of a mesoscopic localization phase, where contagion is sustained only by the largest groups, thereby inhibiting bistability as well. Second, we formulate the problem of optimal seeding for hypergraph contagion, and we compare two strategies: tuning the allocation of seeds according to either individual node or group properties. We find that, when the contagion is sufficiently nonlinear, groups are more effective seeds of contagion than individual nodes., 23 pages, 12 figures (main text); 6 pages, 3 figures (supplementary information)

This chapter shows how evaporation and condensation can modify the wetting conditions. First, we review the microscopic-scale phenomena acting near the contact line: Kelvin effect, hydrodynamic slip, vapor recoil, surface forces, and interfacial resistance. Then we address the theory of liquid flow in the wedge under evaporation at partial wetting conditions, more common in practice than complete wetting. The importance of the correct formulation of the boundary conditions at the contact line is shown. Two main evaporation regimes are addressed next. First, the evaporation into a pure vapor atmosphere is considered (like in bubble growth in boiling). It is controlled by the flow in the liquid. In the presence of contact line receding, this problem is solved by asymptotic matching of the three liquid regions: (i) the microregion near the contact line controlled by the phenomena described above, (ii) the intermediate region where the surface tension competes with the viscous effects, and (iii) the macroregion controlled by the liquid bulk effects. The asymptotic matching results in an expression for the apparent contact angle that depends both on the evaporation rate and on the contact line velocity. From such an analysis, the contact line receding dynamics caused by evaporation can be found. The theory is then compared to the available experimental data. Finally, we consider another regime of wedge evaporation, that in the atmosphere of the neutral gas, controlled by the vapor diffusion in the diffusion boundary layer. As the evaporation is weaker in this case, its effect on the apparent contact angle is smaller. However, we show how it depends on the key parameters of evaporation, e.g., the boundary layer thickness.

International audience; Successful design of aerospace missions requires accurate modelling of the physical phenomena in a hypersonic boundary layer. The behaviour of hypersonic boundary layers is strongly influenced by finite-rate thermochemical effects. These effects can be captured by including finite-rate thermochemistry models in computational tools. Such models are typically highly parametrised, introducing non-equilibrium features into the flow that generate significant uncertainty, which reflects on the output quantities of interest. The way such phenomena interact with solid boundaries and roughness is fundamentally unknown and the additional uncertainty is unquantifiable. In the present work, we propose the investigation of the linear response of n-periodic roughness arrays using an efficient mathematical framework. We extend an existing computational tool to investigate the effect of roughness, including real-gas and finite-rate chemistry effects by coupling with the Mutation++ library. The proposed framework allows to study efficiently and in parallel the linear flow response and wake synchronisation without the restricting idealised periodicity constraint. The results are extracted from reduced-order geometries using automatic linearisation tools. This framework can be potentially combined with sensitivity analysis tools to identify critical roughness configurations. In this paper, we provide the necessary background and present preliminary results for canonical flat plate hypersonic boundary layers.

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International audience; The shear instability occurring at the interface between a slow water layer and a fast air stream is a complex phenomenon driven by momentum and viscosity differences across the interface, velocity gradients as well as by injector geometries. Simulating such an instability under experimental conditions is numerically challenging and few studies exist in the literature. This work aims at filling a part of this gap by presenting a study of the convergence between two-dimensional simulations, linear theory and experiments, in regimes where the instability is triggered by the confinement, i.e. finite thicknesses of gas and liquid streams. It is found that very good agreement between the three approaches is obtained. Moreover, using simulations and linear theory, we explore in detail the effects of confinement on the stability of the flow and on the transition between absolute and convective instability regimes, which is shown to depend on the length scale of the confinement as well as on the dynamic pressure ratio. In the absolute regime under study, the interfacial wave frequency is found to be inversely proportional to the smallest injector size (liquid or gas).

The present work is devoted to the phenomenon of induced side branching stemming from the disruption of free dendrite growth. We postulate that the secondary branching instability can be triggered by the departure of the morphology of the dendrite from its steady state shape. Thence, the instability results from the thermodynamic trade-off between non monotonic variations of interface temperature, surface energy, kinetic anisotropy and interface velocity within the Gibbs–Thomson equation. For the purposes of illustration, the toy model of capillary anisotropy modulation is prospected both analytically and numerically by means of phase-field simulations. It is evidenced that side branching can befall both smooth and faceted dendrites, at a normal angle from the front tip which is specific to the nature of the capillary anisotropy shift applied. This article is part of the theme issue ‘Transport phenomena in complex systems (part 2)’.

International audience; In this work, we consider the momentum transport of a incompressible fluid in a like Beavers and Joseph (1967) system. For this purpose, in the context of the volume averaging method, we use a one-domain approach (ODA). Thus, the momentum generalized transport equations (GTE), which are written in terms of position-dependent effective medium coefficients, are valid everywhere in the system and contains two Brinkman corrections in addition to a Darcy’s term. The ODA predictions are tested against the results obtained from averaging the local profiles resulting from pore-scale simulations. One of the key points for solving the ODA remains on the prediction of the permeability, which in this work is obtained either by solving the associated local closure problem or from pore-scale profiles. Our analysis shows that the GTE for momentum transport accurately predicts the average velocity profiles everywhere in the system. To this end, the first and the second Brinkman’s corrections, as well as a position-dependent intrinsic permeability tensor in Darcy’s term must be included.

International audience; An analytical formula for the ionizing dose of low energy electrons is proposed. The expressions have been validated for 11 monatomic elements (C, Be, Al, Si, Ti, Ni, Cu, Ge, Ag, Fe and W) and for energies ranging from ∼ 10 eV up to 14.5 keV by the use of low energy Monte Carlo simulations. These expressions can be used to evaluate the energy loss of low energy electrons as an input for other simulation codes.; Une formule analytique pour la dose ionisante des électrons de basse énergie est proposée. Les expressions ont été validées pour 11 éléments monatomiques (C, Be, Al, Si, Ti, Ni, Cu, Ge, Ag, Fe et W) et pour des énergies allant de ∼ 10 eV jusqu'à 14,5 keV par l'utilisation de simulations Monte Carlo de basse énergie. Ces expressions peuvent être utilisées pour évaluer la perte d'énergie des électrons de basse énergie comme entrée pour d'autres codes de simulation.

Natural convection in a melting material (hexadecane) is studied experimentally using Magnetic Resonance Imaging. This imaging technique allows to discriminate solid and liquid phases and to measure velocity vectors in the volume sample, even through/in opaque phases. Within our experimental conditions, the convection occurs above R a ≈ 1430 . The thermo-convective flow affects the melting interface leading to higher liquid height in uprising flow and lower liquid height in downward flow regions. Our experiments show that (i) the transient evolution of the mean liquid height h ¯ is enhanced with convection since h ¯ ∝ t 0.8 ( h ¯ ∝ t 1 2 − 3 β with β = 1 / 4 ), while h ¯ ∝ t 0.5 in the conductive regime similarly to the Stefan’s problem; (ii) the steady averaged liquid height h ¯ increases 4 times larger in the convective regime than in the conductive regime. In our experiments the range of A, ratio between the solid and liquid heights, is such as A > 1.5 . Within this range of values we obtain convective pattern under the form of hexagons / polygons. As the melting boundary grows, we observe a decrease in the polygon number. The polygonal pattern is characterized by a constant dimensionless wavelength, i.e. λ / h ¯ ≈ 2 , and thus a constant wavenumber around 3.1 similarly to the primary bifurcation in the classical Rayleigh–Benard Convection (RBC). Heat transfer is evaluated via the Nusselt number N u . Close to the onset of convection and above, our results highlight a scaling law N u ∝ R a β with β = 1 / 4 similarly to the classical RBC. The kinetic energy E c is also evaluated via velocity measurements leading to E c / E 0 ∝ N u 4 and therefore to R e ∝ R a 2 β ∝ R a 1 / 2 . Within the frame of our experiments, all scaling laws are converging to β = 1 / 4 . They reflect clearly an increase in the convective intensity from both thermal and dynamical points of view.

International audience; This study proposed to develop a model coupling hydrodynamics and mass transfer in order to gain an understanding of measurements taken on air-water bubble columns with low gas hold-up. Three experimental datasets with various operating conditions (water quality, liquid height, air flow range) were chosen. The model analyzes and interprets the significant impact of the local hydrostatic pressure and the effects of contamination on hydrodynamic and mass transfer parameters. The oxygen concentration in gas significantly depletes with the distance from diffusers, which explains the difference between the calculated mean of the local < k(L)a > and global K(L)a coefficients. This difference is highly significant for a high bubble column and/or systems with a low mean bubble size. The impact of water quality on mass transfer can be characterized by the contamination angle using comprehensive 1-D modeling and highlights a differentiated impact on the hydrodynamic or mass transfer parameters.

9 pages, 6 color figures, submitted to Phys. Rev. A; International audience; Dispersion relations are fundamental characteristics of the dynamics of quantum and wave systems. In this work we introduce a simple technique to generate arbitrary dispersion relations in a modulated tilted lattice. The technique is illustrated by important examples: the Dirac, Bogoliubov and Landau dispersion relations (the latter exhibiting the roton and the maxon). We show that adding a slow chirp to the lattice modulation allows one to reconstruct the dispersion relation from dynamical quantities. Finally, we generalize the technique to higher dimensions, and generate graphene-like Dirac points and flat bands in two dimensions.

Network dynamics with point-process-based interactions are of paramount modeling interest. Unfortunately, most relevant dynamics involve complex graphs of interactions for which an exact computational treatment is impossible. To circumvent this difficulty, the replica-mean-field approach focuses on randomly interacting replicas of the networks of interest. In the limit of an infinite number of replicas, these networks become analytically tractable under the so-called ‘Poisson hypothesis’. However, in most applications this hypothesis is only conjectured. In this paper we establish the Poisson hypothesis for a general class of discrete-time, point-process-based dynamics that we propose to call fragmentation-interaction-aggregation processes, and which are introduced here. These processes feature a network of nodes, each endowed with a state governing their random activation. Each activation triggers the fragmentation of the activated node state and the transmission of interaction signals to downstream nodes. In turn, the signals received by nodes are aggregated to their state. Our main contribution is a proof of the Poisson hypothesis for the replica-mean-field version of any network in this class. The proof is obtained by establishing the propagation of asymptotic independence for state variables in the limit of an infinite number of replicas. Discrete-time Galves–Löcherbach neural networks are used as a basic instance and illustration of our analysis.

In the present work we investigate for the first time the 2D fluid transport of the plasma in WEST during an entire discharge from the start-up to the ramp-down (shot #54487). The evolution of density profile, electron and ion temperatures together with the experimental magnetic equilibrium, total current and gas-puff rate is investigated. Comparisons with the interferometry diagnostic show a remarkable overall qualitative agreement during the discharge that can be quantitative at some locations in the plasma core. If at the onset of the X-points during the ramp-up the electron heat flux is dominant at the target, present results show that the ion heat flux becomes dominant during the stationary phase of the discharge. Using a simple model for erosion, present results assess the tungsten sputtering due to deuterium ions during the start-up and ramp-up phases of the discharge and confirm the need to consider full discharge simulation to accurately treat the W source of contamination. This work also demonstrates the interest of developing magnetic equilibrium free solver including efficient time integration to step toward predictive capabilities in the future for fusion operation.

A low-temperature plasmas jet is generated by a dielectric barrier discharge powered by a pulsed high voltage in helium flow (3 L min−1) at atmospheric pressure in the presence of different targets (a glass slide, ultra-pure water and a grounded metal plate) positioned perpendicular to the plasma propagation axis. Experimental electrical characterizations such as discharge current, voltage and power, and optical characterizations such as intensified camera ICCD, Schlieren imaging and emission spectroscopy to follow specific excited species have been achieved. The transition from laminar to turbulent regime was observed during the discharge ignition with a larger spreading of the plasma on the surface target with lower dielectric permittivity. The generation of two discharges during each voltage pulse is highlighted during the propagation of the ionization wave which has a variable speed along the plasma axis not depending on the target kind. The evolution of some active species (such as OH, O and excited nitrogen and helium) are investigated using time resolved mapping of the emissions of radiative excited species propagating in ambient air between the plasma jet output and the target. For a low relative permittivity target (glass), the volume ionization wave at its arrival on the target spreads on its surface thus behaving as a surface ionization wave. For the highest relative permittivity (metal), a conductive channel appears between the target surface and the plasma jet during the first discharge, followed by a diffuse plasma plume from the target surface towards the plasma jet after the impact of the ionization wave on the target. A hybrid behavior is highlighted for the ultra-pure water which leads to a short spreading of the ionization wave on the target surface, the formation of a conductive channel in ambient air between tube output and target and the formation of a plasma plume on the target surface.

International audience; Generating meshes with the right resolution is crucial for hybrid RANS/LES simulations of high Reynolds number flow with complex physical phenomena and geometries. This makes automatic mesh generation through adaptive refinement an interesting option. However, since the behavior of these turbulence models depends on the local grid size, mesh changes in time as a result of adaptive refinement may affect the production and destruction of turbulence. Therefore, grid adaptation with refinement criteria based on time-averaged quantities is proposed here to produce meshes that do not evolve rapidly in time and that are suitable for capturing the unsteady flow at each instant. Two averaging approaches, averaging over the instantaneous flow field, and averaging over the refinement criterion, are tested to simulate a turbulent flow behind a backward-facing step with a Detached Eddy Simulation type turbulence model. Compared to grid adaption based on instantaneous solutions, the computational cost is reduced, the accuracy of the solutions improves and an adapted mesh which has a generally static topology based on the main flow features is obtained. The investigation of this refinement process for two realistic test cases, a ship and a hybrid delta wing, confirms the reliability of the average-based adaptation and shows that automatic meshing for hybrid RANS/LES simulations of complex flows is possible.

International audience; Bubbles bursting at the ocean surface are an important source of sea-spray aerosol. Indeed, a bubble bursting at the surface of a liquid produces a jet that then breaks up leading to several droplets. Here we simulate the bursting of a single bubble by numerical simulation of the axisymmetric two phases air-water Navier-Stokes equations, in the presence of small initial perturbations in the liquid phase. We describe the impact of this random noise on the size and velocity of the drops for a wide range of control parameters. We then demonstrate how the process of jet drop production is robust to perturbations in the initial conditions, which allows an accurate prediction of the size and speed of jet droplets over the entire parameter space. We propose a mechanism to explain why the existence of some droplets, called "primary droplets" are robust to initial conditions; and discuss why the generation of secondary droplets, much smaller than the primary ones, are sensitive to the same initial conditions. The production of these secondary droplets is sensitive to perturbations in the initial conditions, through a mechanism of pinch-off escape. This can explain previously published experimental observations of bi-modal distributions of jet drops.

International audience; This paper presents a method for calculating Region of Attraction of a target set (not necessarilyan equilibrium) for controlled polynomial dynamical systems, using a hierarchy of semidefinite pro-gramming problems (SDPs). Our approach builds on previous work and addresses its main issue, thefast-growing memory demands for solving large-scale SDPs.The main idea in this work is in dissecting the original resource-demanding problem into multiplesmaller, interconnected, and easier to solve problems. This is achieved by spatio-temporal splitting akinto methods based on partial differential equations. We show that the splitting procedure retains theconvergence and outer-approximation guarantees of the previous work, while achieving higher precisionin less time and with smaller memory footprint.

International audience; This paper investigates the use of data-driven methods for the reconstruction of unsteady fluid flow fields. The proposed framework is based on the combination of machine learning tools: dimensionality reduction to extract dominant spatial directions from data, reconstruction algorithm to recover encoded data by limited measurements and cross-validation for hyperparameter optimization. For the encoding part, linear and nonlinear extraction of patterns are considered: proper orthogonal decomposition (POD), linear autoencoder (LAE) and variational autoencoder (VAE). For the reconstruction part, regressive reconstruction (neural network, linear, support vector, gradient boosting) and library-based reconstruction are compared, each method being cross-validated to ensure good generalization on testing data. The position of sensors is optimized using an enhanced clustering algorithm. The robustness of regressive reconstructions to noise measurements is also addressed, showing the benefits of variational approaches in the reduction phase. The strategy is tested for three increasing complexity flows: 2D vortex shedding (Re = 200), 2D spatial mixing layer and 3D vortex shedding (Re = 20000). The results suggest that proper machine learning approaches to fluid flow data can lead to effective reconstruction models that can be used for the rapid estimation of complex flows.; Ce papier s'intéresse à l'utilisation de méthodes orientée données pour la reconstruction de champs de vitesse instationnaires. La stratégie proposée est basée sur la combinaison d'outils du machine learning : réduction de dimension pour extraire les directions dominantes de l'écoulement, algorithme de reconstruction pour estimer les données encodées à partir de mesures et validation croisée pour l’optimisation des hyperparamètres. Pour la partie «encodage», des transformations linéaires et non linéaires sont considérées : décomposition en modes propres orthogonaux, auto-encodeur linéaire et auto encodeur variationnel. Pour la partie «reconstruction», une méthode régressive (réseau de neurones, linéaire, machine à vecteurs support, boosting du gradient) et une méthode basée sur une bibliothèque de référence sont comparées. La position des capteurs est optimisée par clustering amélioré. La robustesse au bruit de mesures est étudiée, montrant l'intérêt des approches variationnelles lors de l'étape de réduction. La stratégie est testée sur trois cas de mécanique des fluides : écoulement derrière un cyclindre (Re 200), couche de mélange spatiale (Re 500) et écoulement derrière un cylindre 3D (Re 20000). Les résultats montrent qu'une bonne approche machine learning permet d'apprendre des modèles robustes et efficients pour l'estimation rapide d'écoulements complexes.

We present the first observations of a large-scale coexistence between rod-like and lamellar eutectic growth patterns during directional solidification of a eutectic alloy. In situ experiments with real-time optical monitoring were carried out under microgravity onboard the International Space Station (ISS). We used the transparent succinonitrile-d,camphor eutectic alloy that ordinarily forms rod-like patterns. At low growth velocity, short lamellae stabilized at the contact line with a sample glass wall. In the presence of a controlled transverse temperature gradient, the coupled-growth pattern experienced a global drift along an inclined isotherm, and a stable lamellar domain spread over the solidification front. The propagative nature of the lamellar-to-rod transition was evidenced. The advance of the lamellar/rod domain boundary was determined by a slowly amplifying varicose instability of the lamellae. On a large scale, the domain boundary underwent a dynamic faceting.

International audience; This paper investigates the primary atomization of airblasted liquid sheet using detailed numerical simulations. The atomization of liquid sheet under airblasting conditions involve complex mechanisms and a thorough understanding is necessary. A planar pre-filming airblast atomization configuration have been chosen to study the breakup of liquid sheet/film injected on a solid flat plate. We have investigated an operating point directly relevant for high altitude relight condition of the aircraft. This configuration has been chosen based on the experimental investigation of Gepperth et al [S. Gepperth, A. Müller, R. Koch, H.-J. Bauer, Ligament and droplet characteristics in pre-filming airblast atomization, Proceedings of the ICLASS, 12th Triennial International Conference on Liquid Atomization and Spray Systems, September 2-6, Heidelberg, Germany, 2012] for the airblast atomization. The numerical simulations have been performed using in-house Navier-Stokes solver that uses consistent mass and momentum flux computation technique. The purpose of this work is to provide a comprehensive database and analyses of the airblast atomization of liquid sheet. This include studies on the effect of velocity profile on the atomization characteristics, occurrence of secondary atomization and drop coalescence, and extraction of near-field atomization characteristics. The qualitative analyses of the results from the simulations showed that there are two major atomization mechanisms of liquid film breakup -- sheet/bag breakup and ligament breakup. The drop diameter and velocity distributions computed from the simulations was found to be of the same order of magnitude although under-predicting the experimental data. Based on the atomized drop data, both the secondary atomization and drop coalescence have been observed to occur in the simulations. The quantitative analyses of the near-field liquid ligaments results revealed the lengths of these ligaments are of the same order of magnitude as the experimental data while an under-prediction in the ligament velocity has been observed. Finally, an excellent agreement between simulations and experimental data has been found for the Sauter Mean Diameter (SMD) of the atomized droplets.

International audience; Soft-elastic reactor (SER) has been demonstrated as an alternative approach to effective mixing of viscous liquid. Here, the mixing performance of Newtonian fluid-glycerin water solutions (1200±50 mPa⋅s) was experimentally investigated for the first time inside a simple cylindrical SER with soft-elastic baffle (SEB) aligned in parallel to the SER axis. Visualization based on an acid-base reaction was used to reveal the presence and the disappearance of isolated mixing regions. The mixing performance was characterized using the mixing curve and the mixing time. The effect of SEB positioning on mixing intensification was explored. Results show that a single SEB could improve mixing efficiency with the optimal positioning angle of 90 • With the increase of the number of the SEBs, the mixing efficiency was found to be improved regardless of positioning angles. Compared with the SER without SEB, adding a single SEB could reduce the mixing time by about 34% at the probe penetration depth of 2.5 cm and the penetration frequency of 1.0 Hz. Efficient mixing for SERs of large length to diameter ratios was also achieved with the presence of SEB.

International audience; In this work, we focus on the numerical approximation of a hybrid fluid-kinetic plasma model for electrons, in which energetic electrons are described by a Vlasov kinetic model whereas a fluid model is used for the cold population of electrons. First, we study the validity of this hybrid modelling in a two dimensional context (one dimension in space and one dimension in velocity) against the full (stiff) Vlasov kinetic model and second, a four dimensional configuration is considered(one dimension in space and three dimensions in velocity) following [1]. To do so, we consider two numerical Eulerian methods. The first one is based on the Hamiltonian structure of the hybrid system and the second approach, which is based on exponential integrators, enables to derive high order integrator and remove the CFL condition induced by the linear part. The efficiency of these methods, which are combined with an adaptive time stepping strategy, are discussed in the different configurations and in the linear and nonlinear regimes.

International audience; Understanding the dynamics and generation of coherent structures in wind-turbine wakes is crucial for efficiency improvement of wind farms, which will most probably represent one of the main renewable power generation sources in 2050. In this paper, we investigate the origin of such coherent structures by performing modal and non-modal stability analysis of the mean flow downstream of a wind-turbine rotor. The database consists of large-eddysimulation results. Bi-local linear-stability and optimal-forcing analyses are performed at several wake's cross-sections. Most unstable perturbations are compared with the most energetic coherent structures recovered by the proper orthogonal decomposition (POD) analysis, showing a good agreement close to the rotor. Further downstream, these modes are overtaken by others with wavenumbers departing from those of the main POD modes. However, optimal-forcing analysis shows that asymptotically stable modes can be amplified by more than one order of magnitude via quasi-resonance mechanisms, bypassing the growth of the most unstable modes in the far wake. This suggests that the most energetic structures are originated by modal instabilities, which trigger quasiresonance mechanisms in the far wake, determining the emergence of specific frequencies in the turbulent flow. These findings are crucial for designing efficient control systems to optimize wind farm performance.

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

The objective of this experimental investigation is to characterize the gas–surface interaction under different flow conditions. Therefore, the mass flow rates driven by a pressure gradient under isothermal conditions and by only a temperature gradient under constant pressure conditions are measured in the same microchannel for five different gases: helium, neon, nitrogen, argon and krypton. The pressure driven experiments are carried out in the hydrodynamic and slip flow regimes, 0.0016

The capability of an Eulerian two-phase model, SedFoam, in simulating the onset of scour underneath a pipeline and the backfill process is investigated. When a pipeline is slightly buried in the sediment bed, the scour onset can be caused by the piping process, which is due to seepage flow moving underneath the pipeline driven by the upstream–downstream pressure difference. To directly resolve piping as part of the scour simulation has been a challenge in the single-phase models. Alternatively, the two-phase models may be capable of simulating piping and backfill as it can resolve the interactions between the flow, structure, sediment transport, and seepage flow using a single set of governing equations and closures. To prove this point, SedFoam is validated by two laboratory experiments for the onset of scour underneath a pipeline. For piping driven by a prescribed upstream–downstream pressure difference, the model captures the temporal evolution of the pore-pressure gradient and the resulting fine-scale bathymetric change around the pipe consistent with the measured data. The model further provides insight into the seepage flow and the creeping movement of sediments during the onset of scour. When simulating piping driven by a unidirectional current, SedFoam successfully predicts piping driven by the upstream–downstream pressure gradient due to the incoming flow deceleration by the presence of pipeline and flow separation. As a proof-of-concept application, SedFoam is applied to simulate pipeline scour driven by an oscillatory flow. Although the boundary layer streaming effect is neglected, the model result shows realistic scour onset and the development of a scour hole. To demonstrate the model’s capability to simulate the backfill process, the pipeline is then lowered artificially into the scour hole as an idealized treatment of the complex pipeline sinking process during scour. The resulting burial depth due to backfill is similar to that predicted by the empirical formula.

International audience; This work is an experimental investigation of the effect of the nature of surfactants on oxygen mass transfer. The study focuses on three cationic surfactants with different hydrophobic chain lengths, and four nonionic surfactants with different hydrophilic chain lengths. Equilibrium adsorption isotherms are calculated for each surfactant from experimental values of surface tension in static conditions. Surfactant solutions at concentrations between 2.5×10-8 and 5×10-3 mol/L were prepared and oxygen transfer from millimetric air bubbles (between 0.82 and 1.08 mm) was measured by Planar Laser Induced Fluorescence with Inhibition (PLIF-I). When the bulk concentration of surfactant was increased, results showed a sharp decrease of bubble velocity, in the range of 283-75 mm/s, and of liquid-side mass transfer coefficient, in the range of 5.6 ×10-4-0.4 ×10-4 m/s. This effect was observed for all surfactants studied. However, the length of the hydrophilic chain did not appear to affect the hydrodynamics of the rising bubble or the oxygen transfer at the same bulk concentration. Furthermore, for the same bulk concentration, increasing the hydrophobic chain length had an impact on the velocity and the mass transfer coefficient of oxygen. Finally, the Sherwood number was calculated in each medium and compared with classical correlations for gas-liquid mass transfer prediction. Those correlations seemed to reach a limit for a very concentrated medium.

International audience; A high-fidelity simulation of the massively separated shock/ transitional boundary layer interaction caused by a 15-degrees axisymmetrical compression ramp is performed at a free stream Mach number of 6 and a transitional Reynolds number. The chosen configuration yields a strongly multiscale dynamics of the flow as the separated region oscillates at low-frequency, and high-frequency transitional instabilities are triggered by the injection of a generic noise at the inlet of the simulation. The simulation is post-processed using Proper Orthogonal Decomposition to extract the large scale low-frequency dynamics of the recirculation region. The bubble dynamics from the simulation is then compared to the results of a global linear stability analysis about the mean flow. A critical interpretation of the eigenspectrum of the linearized Navier-Stokes operator is presented. The recirculation region dynamics is found to be dominated by two coexisting modes, a quasi-steady one that expresses itself mainly in the reattachment region and that is caused by the interaction of two self-sustained instabilities, and an unsteady one linked with the separation shock-wave and the mixing layer. The unsteady mode is driven by a feedback loop in the recirculation region, which may also be relevant for other unsteady shock-motion already documented for shock-wave/turbulent boundary layer interaction. The impact of the large-scale dynamics on the transitional one is then assessed through the numerical filtering of those low wavenumber modes; they are found to have no impact on the transitional dynamics.

Articles in Advance; International audience; This paper investigates the effects of a porous wall on the development of Tollmien–Schlichting (TS) waves propagating inside a boundary layer. An experimental study performed in a low-speed wind tunnel showed that microperforated panels over hollow chambers could promote laminar–turbulent transition. First, measurements of the streamwise evolution of the TS instabilities reveal that greater amplification occurs over perforated panels compared to a solid wall. This effect was attributed to a wall impedance condition imposed by the perforated panels over the chambers. This setup is somewhat analogous to an acoustic liner for which the wall response is represented by a complex impedance. The impedance of the experimental setup was thus measured in situ. These experimental impedance values were then used as wall boundary conditions of a linear local stability calculation to quantify the influence of the wall impedance condition on TS wave growth. The eN approach was applied to predict the corresponding transition positions. Comparison between experimental and numerical results shows that the use of impedance boundary conditions is a promising approach to predict transition location, particularly in the case of the panel with the lowest resistance.

International audience; The present work focuses on the development of a performing numerical methodology to solve the steady state Population Balance Equation (PBE) including nucleation, independent size growth and loose agglomeration as crystallization mechanisms. The methodology is based on the solution of two PBEs: one for the isolated crystallites and one describing the loose agglomerates formation. Both are solved by a discretization method and only the last one is reformulated as a fixed point problem. The algorithm solving PBE for agglomeration includes the crossed-secant algorithm as a fixed point acceleration method. The numerical PBE solution method is first validated by comparison to analytical solutions and then applied to the neodymium oxalate precipitation in order to compare to experimental results in a wide range of operating conditions. The methodology is tested under highly restrictive numerical conditions: narrow tolerances, a large amount of points in the discretization scheme and a zero vector as initial condition. The crossed-secant method demonstrates to improve the robustness of the standard fixed point iterations by ensuring the convergence of the agglomerates PBE when penalizing conditions are applied and by reducing the number of iterations otherwise. In all cases, the developed methodology predicts accurately the crystal size distribution under the experimental uncertainty in a reasonable computation time and number of iterations.

International audience; We present some sharper finite extinction time results for solutions of a class of damped nonlinear Schrödinger equations when the nonlinear damping term corresponds to the limit cases of some ``saturating non-Kerr law'' $F(|u|^2)u=\frac{a}{\varepsilon+(|u|^2)^\alpha}u,$ with $a\in\mathbb{C},$ $\varepsilon\geqslant0,$ $2\alpha=(1-m)$ and $m\in[0,1).$ To carry out the improvement of previous results in the literature we present in this paper a careful revision of the existence and regularity of weak solutions under very general assumptions on the data. We prove that the problem can be solved in the very general framework of the maximal monotone operators theory, even under a lack of regularity of the damping term. This allows us to consider, among other things, the singular case $m=0.$ We replace the above approximation of the damping term by a different one which keeps the monotonicity for any $\varepsilon\geqslant0$. We prove that, when $m=0,$ the finite extinction time of the solution arises for merely bounded right hand side data $f(t,x).$ This is specially useful in the applications in which the Schrödinger equation is coupled with some other functions satisfying some additional equations.

This paper is focused on SIS (Susceptible-Infected-Susceptible) epidemic dynamics (also known as the contact process) on populations modelled by homogeneous Poisson point processes of the Euclidean plane, where the infection rate of a susceptible individual is proportional to the number of infected individuals in a disc around it. The main focus of the paper is a model where points are also subject to some random motion. Conservation equations for moment measures are leveraged to analyze the stationary regime of the point processes of infected and susceptible individuals. A heuristic factorization of the third moment measure is then proposed to obtain simple polynomial equations allowing one to derive closed form approximations for the fraction of infected individuals in the steady state. These polynomial equations also lead to a phase diagram which tentatively delineates the regions of the space of parameters (population density, infection radius, infection and recovery rate, and motion rate) where the epidemic survives and those where there is extinction. A key take-away from this phase diagram is that the extinction of the epidemic is not always aided by a decrease in the motion rate. These results are substantiated by simulations on large two dimensional tori. These simulations show that the polynomial equations accurately predict the fraction of infected individuals when the epidemic survives. The simulations also show that the proposed phase diagram accurately predicts the parameter regions where the mean survival time of the epidemic increases (resp. decreases) with motion rate., post-revision, JMB

An experimental investigation on the flow topology over a rotor operating atlow Reynolds numbers is presented. The feasibility of laminar to turbulenttransition experiments over small rotors is demonstrated. Phase-locked infraredthermography coupled with simultaneous force and torque measurements wereused to study a three bladed rotor with a NACA0012 aerofoil section set at auniform angle of incidence of 10◦. Boundary layer transition was forced usingtwo-dimensional (2D) and three-dimensional (3D) roughness elements, placedat approximately 5% and 28% of the rotor blades chord. In the 3D roughnessconfigurations once the critical roughness Reynolds was attained the streaksdownstream developed into fully turbulent wedges. For the current rotor con-figuration, it was found that the state of the boundary layer can significantlyaffect its performance, with the non-optimal forcing of laminar to turbulenttransition generally resulting in a loss of performance when compared to thesmooth reference rotor case.Keywords:Infrared Thermography; Boundary layer transition; Low Reynoldsnumber; rotor

This study presents a method for the experimental determination of the local volumetric mass transfer coefficient k L a L in a high-pressure two-phase flow of water ( H 2 O ) and carbon dioxide ( CO 2 ) in a micro-capillary using a colorimetric method. H 2 O and CO 2 are fed continuously and co-axially injected at high-pressure (10 MPa) and moderate temperature (303 K) into a microcapillary. Under the flow conditions studied, a segmented flow of CO2 in H 2 O is formed. The CO2 dissolves into the H2O-rich phase, thereby reducing the pH to about 3.3, depending on the pressure and temperature. The pH of the H2O-rich phase is determined over the entire length of the capillary using a pH sensitive indicator coupled with high-speed imaging and analysis. The concentration of CO 2 in the water-rich phase is deduced from this pH value using literature experimental data. The CO 2 concentration data and the unit-cell model, which has been modified to account for high pressure conditions, have then been used to determine the volumetric mass transfer coefficient, k L a L , of CO2 into the liquid phase along the entire length of the microreactor. The experimentally derived k L a L ​​ranges between 1 and 13 s−1.

International audience

International audience; View Video Presentation: https://doi.org/10.2514/6.2022-1073.vid Predicting the laminar to turbulent transition is an important aspect of computational fluid dynamics because of its impact on skin friction. The traditional methods of transition prediction do not make it possible to consider configurations where the boundary layer develops in the presence of surface defects (bumps, steps, gaps, etc.). A neural network approach is used in this paper, based on an extensive database of boundary layer stability studies in the presence of gap-like surface defects. These studies consists on Linearized Navier-Stokes (LNS) calculations and provide information on the effect of surface irregularity geometry and aerodynamic conditions on the transition to turbulence. The physical and geometrical parameters characteristic of the defect and the flow are then provided to a neural network whose outputs inform about the effect of a given gap on the transition through the ΔN method.

We propose a new experimental protocol to investigate the atmospheric dispersion of a dense gas in wind-tunnel experiments. Simultaneous measurements of concentration and velocity are performed by coupling a flame ionisation detector (FID) and a hot-wire anemometer (HWA), whose probes are located sufficiently close to each other so that the respective measurements correspond to a same sampling volume. The heavy gas emission consists in a mixture of air, carbon dioxide, and ethane. Carbon dioxide is used as a buoyant agent, and ethane is used as a tracer, its concentration being detected by the FID. In this context, the main metrological issue of the setup concerns the response of the FID and the HWA to different proportions of the two gases in the mixture. Notably, the presence of carbon dioxide affects the linear response of the FID, which can attain a saturation level, as well as the response of the HWA, due to the varying density of the gas. This can be compensated by determining a non-linear calibration curve of the FID, which is used to determine the instantaneous fluid density, and to apply a time-dependent correction to the HWA response. To analyse the performances of the coupled HWA–FID system we realised wind-tunnel experiments of tracer dispersion in a turbulent boundary layer. The estimate of mass fluxes through different sections downwind the source enlightens the reliability of the experimental results, and show that the measurements within a negatively buoyant gas plume are as accurate as those within a passive scalar plume.

International audience; Kinematical and geometrical parameters of helical vortices winding in the same (right-handed) or opposite direction (left-handed) to the base Taylor-Couette-Poiseuile flow were studied at low Ta = R-1 d/v < 319 and Re = U-M d/v < 10.5 numbers. The inner cylinder (R-1 = 24.85 mm) rotated and the outer one (R-2 = 30.95 mm) was stationary. The parameters of helical vortices were obtained from flow visualizations and the wall shear rates were deduced from the electrodiffusion currents of three mass transfer probes flush mounted on the wall of the outer cylinder Kristiawan et al. [11] . The vector field of wall shear rate on the outer cylinder was reconstructed using a three-segment mass transfer probe. It is found that the axial component of the wall shear rate of the left-handed helices is greater than that of the right-handed helices. The motion of the flow structures near the outer wall was described by the velocity V resulting from the axial drifting velocity (U-D) and the azimuthal velocity W . It was shown that the axial velocity perceived by an observer or sensor fixed in the laboratory system U-OBS is composed of the drifting velocity U-D and the velocity due to the helix rotation U-Omega, known as Barber pole effect. New insight on the helical structures are revealed from studying the variations of different parameters such as the helix pitch p, the rotation rate and the observed angular velocity of the helices. (c) 2021 Elsevier Ltd. All rights reserved.

We describe a new field campaign over a steep, snowy $$30^{\circ }$$ alpine slope, designed to investigate three recurrent issues in experimental studies of steep-slope katabatic winds. (1) Entrainment is known to be present in katabatic jets and has been estimated at the interface between the jet and the boundary layer above it. However, to our knowledge, the slope-normal velocity component has never been measured in the katabatic jet. (2) It is hard to accurately measure turbulence in the first tens of centimetres above the surface using standard sonic anemometry due to the filtering effect of the long instrument path. The present field experiment used a three-dimensional multi-hole pitot-type probe with a high sampling frequency (1250 Hz) that was positioned as close to the surface as 3 cm. It provides three-dimensional mean velocity and Reynolds stress tensor from which dissipation can be estimated, as well as spectra for the turbulent quantities. Energy spectra reveal a well-developed inertial range and capture the inertial scales and some of the dissipative scales. (3) Measuring turbulence on a mast usually provides information about mean and turbulent quantities at certain discrete heights because the sensors are sparsely located inside the jet. We present the first measurements of well-developed katabatic flows where the full wind-speed and temperature profiles acquired, from tethered balloon are available at the location of the measurement mast, which comprises three-dimensional anemometry and thermometry.

Collisionless shocks are ubiquitous in the Universe and are held responsible for the production of nonthermal particles and high-energy radiation. In the absence of particle collisions in the system, theory shows that the interaction of an expanding plasma with a pre-existing electromagnetic structure (as in our case) is able to induce energy dissipation and allow shock formation. Shock formation can alternatively take place when two plasmas interact, through microscopic instabilities inducing electromagnetic fields that are able in turn to mediate energy dissipation and shock formation. Using our platform in which we couple a rapidly expanding plasma induced by high-power lasers (JLF/Titan at LLNL and LULI2000) with high-strength magnetic fields, we have investigated the generation of a magnetized collisionless shock and the associated particle energization. We have characterized the shock as being collisionless and supercritical. We report here on measurements of the plasma density and temperature, the electromagnetic field structures, and the particle energization in the experiments, under various conditions of ambient plasma and magnetic field. We have also modeled the formation of the shocks using macroscopic hydrodynamic simulations and the associated particle acceleration using kinetic particle-in-cell simulations. As a companion paper to Yao et al. [Nat. Phys. 17, 1177–1182 (2021)], here we show additional results of the experiments and simulations, providing more information to allow their reproduction and to demonstrate the robustness of our interpretation of the proton energization mechanism as being shock surfing acceleration.

Lattice Boltzmann simulations of corner separation flow in a compressor cascade are presented. The lattice Boltzmann approach is rather new in the context of turbomachinery, and the configuration is known to be particularly challenging for turbulence modeling. The present methodology is characterized by a quasi-autonomous meshing strategy and a limited computational cost (a net ratio of 5 compared to a previous finite-volume compressible Navier–Stokes simulation). The simulation of the reference case (4 deg incidence) shows a good agreement with the experimental data concerning the wall pressure distribution or the distribution of losses. A good description is also obtained when incidence angle is increased to 7 deg, with a span-wise development of the separation. Subsequently, the methodology is used to investigate the sensitivity of the flow to the end-wall boundary-layer thickness. A thinner boundary-layer results in a smaller corner separation, but not a complete elimination. Finally, the ingredients of the wall modeling are analyzed in details. On the one hand, the curvature correction term promotes transition to turbulence on the blade suction side and avoids a spurious separation. On the other hand, the addition of the pressure-gradient correction term allows a wider and more realistic corner separation.

International audience; We report on magnetic-torque and resistivity measurements of the heavy-fermion compound CeCoIn5 in static magnetic fields up to 36 T and temperatures down to 50 mK. While quantum oscillations of the de Haas-van Alphen (dHvA) as well as the Shubnikov-de Haas (SdH) effect confirm the previously reported Fermi surfaces, an analysis of the field dependence reveals two anomalous features. The first is seen at about 22 T as a sharp anomaly in the resistivity for current applied along the a direction. The second appears as nonmonotonic field-dependent oscillation frequencies and amplitudes in both dHvA and SdH signals. This second feature emerges at about 28 T. This field is close to that of the nematic transition reported for CeRhIn5 and the proposed Lifshitz transition in CeIrIn5. We discuss possible common grounds of these latter features that might originate from the very similar band structures of these materials.

Particle suspensions in non-Newtonian liquid matrices are frequently encountered in nature and industrial applications. We here study the Taylor–Couette flow (TCF) of semidilute spherical particle suspensions (volume fraction $\leq 0.1$ ) in viscoelastic, constant-viscosity liquids (Boger fluids). We describe the influence of particle load on various flow transitions encountered in TCF of such fluids, and on the nature of these transitions. Particle addition is found to delay the onset of first- and second-order transitions, thus stabilising laminar flows. It also renders them hysteretic, suggesting an effect on the nature of bifurcations. The transition to elasto-inertial turbulence (EIT) is shown to be delayed by the presence of particles, and the features of EIT altered, with preserved spatio-temporal large scales. These results imply that particle loading and viscoelasticity, which are known to destabilise the flow when considered separately, can on the other hand compete with one another and ultimately stabilise the flow when considered together.

A complete chain from a non-linear MHD plasma model simulation through full-wave code simulations implementing synthetic conventional reflectometry is established. For this purpose, the two-dimensional full-wave code REFMUL is employed together with MHD descriptions obtained from the JOREK code. First results of the integrated modeling are presented here where a type-I ELM crash, leading to a fast collapse of the H-mode pedestal, was taken as case-study. The REFMUL simulations were customized to implement synthetic reflectometry in conventional set-up using fixed frequency probing with O-mode waves. Posing challenging conditions for reflectometry, the type-I ELM crash reveals some of the merits and caveats of the diagnostic technique. This work also opens up the possibility to extend modeling to other MHD or ELM studies and provide support to experimental observations with reflectometry.

International audience; This work deals with the inference of catalytic recombination parameters from plasma wind tunnel experiments for reusable thermal protection materials. One of the critical factors affecting the performance of such materials is the contribution to the heat flux of the exothermic recombination reactions at the vehicle surface. The main objective of this work is to develop a dedicated Bayesian framework that allows us to compare uncertain measurements with model predictions which depend on the catalytic parameter values. Our framework accounts for uncertainties involved in the model definition and incorporates all measured variables with their respective uncertainties. The physical model used for the estimation consists of a 1D boundary layer solver along the stagnation line. The chemical production term included in the surface mass balance depends on the catalytic recombination eciency. As not all the different quantities needed to simulate a reacting boundary layer can be measured or known (such as the flow enthalpy at the inlet boundary), we propose an optimization procedure built on the construction of the likelihood function to determine their most likely values based on the available experimental data. This procedure avoids the need to introduce any a priori estimates on the nuisance quantities, namely, the boundary layer edge enthalpy, wall temperatures, static and dynamic pressures, which would entail the use of very wide priors. Furthermore, we substitute the optimal likelihood of the experimental measurements with a surrogate model to make the inference procedure both faster and more robust. We show that the resulting Bayesian formulation yields meaningful and accurate posterior probability distributions of the catalytic parameters with a reduction of more than 20% of the standard deviation with respect to previous works. We also study the implications of an extension of the experimental procedure on the improvement of the quality of the inference.

An intercomparison between 10 single-column (SCM) and 5 large-eddy simulation (LES) models is presented for a radiation fog case study inspired by the LANFEX field campaign. 7 of the SCMs represent single-column equivalents of operational numerical weather prediction (NWP) models, whilst 3 are research-grade SCMs designed for fog simulation, and the LES are designed to reproduce in the best manner currently possible the underlying physical processes governing fog formation. The LES model results are of variable quality, and do not provide a consistent baseline against which to compare the NWP models, particularly under high aerosol or cloud droplet number (CDNC) conditions. The main SCM bias appears to be toward over-development of fog, i.e. fog which is too thick, although the inter-model variability is large. In reality there is a subtle balance between water lost to the surface and water condensed into fog, and the ability of a model to accurately simulate this process strongly determines the quality of its forecast. Some NWP-SCMs do not represent fundamental components of this process (e.g. cloud droplet sedimentation) and therefore are naturally hampered in their ability to deliver accurate simulations. Finally, we show that modelled fog development is as sensitive to the shape of the cloud droplet size distribution, a rarely studied or modified part of the microphysical parametrization, as it is to the underlying aerosol or CDNC.

International audience; We report on the development and implementation of methodologies dedicated to soft X-ray imaging by coherent scattering in reflection mode. Two complementary approaches are tested, based on Fourier transform holography and on ptychography. A new method for designing holographic masks has been developed. Our results represent a feasibility test and highlight the potential and limitations of imaging in reflection mode. Reflectivity is less efficient than transmission at soft X-ray wavelengths, hampering the acquisition of good quality images. Nonetheless, it has the potential to image a wider set of samples, notably those that are not transparent to soft X-rays. Although the images obtained so far are of modest quality, these results are extremely encouraging for continuing the development of coherent soft X-ray imaging in reflection mode.

International audience; The blade element momentum (BEM) theory introduced by Lock et al. and formulated in its modern form by Glauert provides a framework to model the aerodynamic interaction between a turbine and a fluid flow. This theory is used either to estimate turbine efficiency or as a design aid. However, a lack of mathematical interpretation limits the understanding of some of its issues. The aim of this paper is to propose an analysis of BEM equations. Our approach is based on a reformulation of Glauert's model which enables us to identify criteria to guarantee the existence of solutions, analyze the convergence of usual and new (and more efficient) solution algorithms, and study turbine design procedures. The mathematical analysis is completed by numerical experiments.

It is typically interpreted that more moisture in the atmosphere leads to more intense rains. This notion may be supported, for example, by taking a scatterplot between rain and column precipitable water. The present paper suggests, however, that the main consequence of intense rains with more moisture in the atmosphere is that there is a higher chance of occurrence rather than an increase in the expected magnitude. This tendency equally applies to any rains above 1 mm (6 h)−1 to a lesser extent. The result is derived from an analysis of 33 local rain gauge station data and a shared sounding over Friuli Venezia Giulia, northeast Italy. Significance Statement Moisture is the source of clouds. Clouds, in turn, are source of rain. So we may expect that more moisture in the atmosphere causes more intense rains. We may further speculate that with more moisture in the atmosphere as a consequence of the global warning, we must face more catastrophic rain events and floods. However, this paper, by analyzing data over Friuli Venezia Giulia, northeast Italy, suggests otherwise: more moisture indeed increases frequencies of intense rains, but not their magnitudes as much.