Unité de Mécanique de Lille - Joseph Boussinesq

Convective flows coupled with solidification or melting in water bodies play a major role in shaping geophysical landscapes. Particularly in relation to the global climate warming scenario, it is essential to be able to accurately quantify how water-body environments dynamically interplay with ice formation or melting process. Previous studies have revealed the complex nature of the icing process, but have often ignored one of the most remarkable particularities of water, its density anomaly, and the induced stratification layers interacting and coupling in a complex way in the presence of turbulence. By combining experiments, numerical simulations, and theoretical modeling, we investigate solidification of freshwater, properly considering phase transition, water density anomaly, and real physical properties of ice and water phases, which we show to be essential for correctly predicting the different qualitative and quantitative behaviors. We identify, with increasing thermal driving, four distinct flow-dynamics regimes, where different levels of coupling among ice front and stably and unstably stratified water layers occur. Despite the complex interaction between the ice front and fluid motions, remarkably, the average ice thickness and growth rate can be well captured with the theoretical model. It is revealed that the thermal driving has major effects on the temporal evolution of the global icing process, which can vary from a few days to a few hours in the current parameter regime. Our model can be applied to general situations where the icing dynamics occur under different thermal and geometrical conditions.

The evolution of heat flux through an initially solid pure substance that is heated from below and that undergoes both phase-change and natural convection is studied numerically and contrasted with the dynamics of the Rayleigh-B\'enard system in laminar and turbulent regimes.

International audience

Settling of inertial particles in basally heated fluids is a topic of great significance in nature and, in particular, for the study of how magma cools and solidifies. The residence time of particles in Rayleigh-B\'enard convection is computed for a broad range of flow and particle parameters, and a general analytic formula is designed that captures the results. It is found that particles tend to settle rapidly compared with the characteristic solidification timescale of magmatic systems. In addition, the horizontal distribution of settling events shows a surprising pattern: Heavy particles settle preferentially below clusters of upwelling plumes.

Sea ice is crucial in many natural processes and human activities. Understanding the dynamical couplings between the inception, growth and equilibrium of sea ice and the rich fluid mechanical processes occurring at its interface and interior is of relevance in many domains, ranging from geophysics to marine engineering. Here we investigate experimentally the complete freezing process of water with dissolved salt in a standard natural convection system, i.e. the prototypical Rayleigh–Bénard cell. Due to the presence of a mushy phase, the studied system is considerably more complex than the freezing of fresh water in the same conditions (Wang et al. , Proc. Natl Acad. Sci. USA , vol. 118, issue 10, 2021, e2012870118). We measure the ice thickness and porosity at the dynamical equilibrium state for different initial salinities of the solution and temperature gaps across the cell. These observables are non-trivially related to the controlling parameters of the system as they depend on the heat transport mode across the cell. We identify in the experiments five out of the six possible modes of heat transport. We highlight the occurrence of brine convection through the mushy ice and of penetrative convection in stably stratified liquid underlying the ice. A one-dimensional multi-layer heat flux model built on the known scaling relations of global heat transport in natural convection systems in liquids and porous media is proposed. Given the measured porosity of the ice, it allows us to predict the corresponding ice thickness, in a unified framework.

The coupling between turbulent convecting water and the freezing/melting process leads to intriguingly complex phenomena which are of pressing importance for applications in environmental and climatological sciences. Here we study the extent and the morphology of ice forming in a differentially heated cavity filled with water by means of laboratory-scale experiments and numerical simulations. We demonstrate that the characteristic ice shape formed in our system is the result of the competition of two counterrotating convective rolls whose strength depends on the externally prescribed thermal gap.

Establishing accurate structure-property relationships for intervertebral disc annulus fibrosus tissue is a fundamental task for a reliable computer simulation of the human spine but needs excessive theoretical-numerical-experimental works. The difficulty emanates from multiaxiality and anisotropy of the tissue response along with regional dependency of a complex hierarchic structure interacting with the surrounding environment. We present a new and simple hybrid microstructure-based experimental/modeling strategy allowing adaptation of animal disc model to human one. The trans-species strategy requires solely the basic knowledge of the uniaxial circumferential response of two different animal disc regions to predict the multiaxial response of any human disc region. This work demonstrates for the first time the determining role of the interlamellar matrix connecting the fibers-reinforced lamellae in the disc multiaxial response. Our approach shows encouraging multiaxial predictive capabilities making it a promising tool for human spine long-term prediction.

Air purifiers are limited to small polluting airborne particles and poor air circulation (fan) for bringing airborne particles inside the device. Thus, the optimal utility of domestic air purifiers (DAPs) for eliminating airborne viruses is still ambiguous. This paper addresses the above limitations using computational fluid dynamics modeling and simulations to investigate the optimal local design of a DAP in an indoor space. We also investigate the integrated fan system and the local transport of airborne viruses. Three different scenarios of using standard DAP equipment (144 m3/h) are explored in an indoor space comprising a furnished living room 6×6×2.5 m3. We show that the local positioning of a purifier indoors and the fan system embedded inside it can significantly alter the indoor airborne virus transmission risk. Finally, we propose a new indoor air circulation system that better ensures indoor airborne viruses' local orientation more efficiently than a fan embedded in a standard DAP.

Date palm fibers have been studied as potential reinforcement in composite materials. Their density is equal to 1.22 g.cm−3; making them suitable for use in lightweight applications. In order to efficiently use these fibers and understand their mechanical properties, quasi-static tensile tests and cyclic fiber fatigue tests were performed to determine the mechanical properties. Their internal structure, elementary fibers and porosity were analyzed by scanning electron microscopy. The results show that the mechanical properties are clearly improved by taking into account the porosity by calculating the real section of the fibers by using the imageJ software a cross-sectional pore fraction of the fibers has been determined and the ultimate properties of the pore-free bulky fibers has been calculated with stress and Young’s modulus increased by approx. 54% as compared to the highly porous natural fibers.

We study the conductive and convective states of phase-change of pure water in a rectangular container where two opposite walls are kept respectively at temperatures below and above the freezing point and all the other boundaries are thermally insulating.The global ice content at the equilibrium and the corresponding shape of the ice-water interface are examined, extending the available experimental measurements and numerical simulations.We first address the effect of the initial condition, either fully liquid or fully frozen, on the system evolution.Secondly, we explore the influence of the aspect ratio of the cell, both in the configurations where the background thermal-gradient is antiparallel to the gravity, namely the Rayleigh-Bénard (RB) setting, and when they are perpendicular, i.e., vertical convection (VC).We find that for a set of well-identified conditions the system in the RB configuration displays multiple equilibrium states, either conductive rather than convective, or convective but with different ice front patterns.The shape of the ice front appears to be always determined by the large scale circulation in the system.In RB, the precise shape depends on the degree of lateral confinement.In the VC case the ice front morphology is more robust, due to the presence of two vertically stacked counter-rotating convective rolls for all the studied cell aspect-ratios.

We investigate the dependency of the magnitude of heat transfer in a convection cell as a function of its inclination by means of experiments and simulations. The study is performed with a working fluid of large Prandtl number, $\mathrm{Pr}\ensuremath{\simeq}480$, and at Rayleigh numbers $\mathrm{Ra}\ensuremath{\simeq}{10}^{8}$ and $\mathrm{Ra}\ensuremath{\simeq}5\ifmmode\times\else\texttimes\fi{}{10}^{8}$ in a quasi-two-dimensional rectangular cell with unit aspect ratio. By changing the inclination angle $(\ensuremath{\beta})$ of the convection cell, the character of the flow can be changed from moderately turbulent, for $\ensuremath{\beta}={0}^{\ensuremath{\circ}}$, to laminar and steady at $\ensuremath{\beta}={90}^{\ensuremath{\circ}}$. The global heat transfer is found to be insensitive to the drastic reduction of turbulent intensity, with maximal relative variations of the order of $20%$ at $\mathrm{Ra}\ensuremath{\simeq}{10}^{8}$ and $10%$ at $\mathrm{Ra}\ensuremath{\simeq}5\ifmmode\times\else\texttimes\fi{}{10}^{8}$, while the Reynolds number, based on the global root-mean-square velocity, is strongly affected with a decay of more than $85%$ occurring in the laminar regime. We show that the intensity of the heat flux in the turbulent regime can be only weakly enhanced by establishing a large-scale circulation flow by means of small inclinations. However, in the laminar regime the heat is transported solely by a slow large-scale circulation flow which exhibits large correlations between the velocity and temperature fields. For inclination angles close to the transition regime in-between the turbulentlike and laminar state, a quasiperiodic heat-flow bursting phenomenon is observed.

Purpose A comprehensive investigation on the outlet air position effects on the thermal comfort and air quality has been achieved. In addition, airflow and temperature distributions in ventilated cavities filled with an air-CO 2 mixture with mixed convection are predicted. The airflow enters from the cavity through an opening in the lower side of the left vertical wall and exits through the opening in one wall of the cavity. This paper aims to investigate the outlet location effect, four different placement configurations of output ports are considered. Three of them are placed on the upper side and the fourth on top of the opposite side of the inlet opening. A uniform heat and CO 2 contaminant source are applied on the left vertical wall, while the remaining walls are impermeable and adiabatic to heat and solute. The cooling efficiency inside the enclosure and the average fluid temperature are computed for different Reynolds and Rayleigh numbers to find the most suitable fluid outlet position that ensures indoor comfortable conditions while effectively removing heat and the contaminant. This is demonstrated by three relevant indices, namely, the effectiveness for heat removal, the contaminant removal and the index of indoor air quality. Design/methodology/approach The simulations were performed via the finite-volume scSTREAM CFD solver V11. Three different values of CO 2 amount are considered, namely, 10 3 , 2 × 10 3 and 3 × 10 3 ppm, the Reynolds number being in the range 100 ≤ Re ≤ 800. Findings Based on the findings obtained, it is the configuration whose air outlet is placed near the heat source and the contaminant, which provides a better air distribution and a ventilation efficiency compared to the others ventilation strategies. Originality/value The studies on heat and mass transfers by natural and forced convection in ventilated cavities remain a fruitful research topic. Thereby, such a study deals with different ventilation strategies through cavities containing an air-CO 2 mixture subjected to a mixed regime. In particular, the air inlet velocity and contaminant sources’ effects on thermal comfort and air quality have been investigated.

In the present paper, we investigate the polymer–turbulence interaction by discriminating between the mechanical responses of this system to three different subdomains: elliptical, parabolic, and hyperbolic, corresponding to regions where the magnitude of vorticity is greater than, equal to, or less than the magnitude of the rate of strain, respectively, in accordance with the Q-criterion. Recently, it was recognized that hyperbolic structures play a crucial role in the drag reduction phenomenon of viscoelastic turbulent flows, thanks to the observation that hyperbolic structures, as well as vortical ones, are weakened by the action of polymers in turbulent flows in a process that can be referred to as flow parabolization. We employ direct numerical simulations of a viscoelastic finite extensible nonlinear elastic model with the Peterlin approximation to examine the transient evolution and statistically steady regimes of a plane Couette flow that has been perturbed from a laminar flow at an initial time and developed a turbulent regime as a result of this perturbation. We have found that even more activity is located within the confines of the hyperbolic structures than in the elliptical ones, which highlights the importance of considering the role of hyperbolic structures in the drag reduction mechanism.

Ch4-project is a developing computational fluid dynamics code that is used for the investigation of a range of different turbulent flows – periodic or bounded, thermally driven, with phase change interfaces – and a variety of Lagrangian phenomena, such as particles’ transport, mixing and clustering. After an introduction on the genesis of this project and its position on the wide landscape of computational fluid dynamics, its current structure and features are briefly presented. An overview of its achievements on topics as varied as global scaling in non-ideal high-Rayleigh number convection, acceleration statistics of bubbles in developed turbulence, time-series analysis from propelled probes in a fluid environment along with ongoing studies are provided.

Lignocellulosic fibers from date palm are one of the most available biomasses without significant valorization in the world. This work makes use of an experimental Taguchi design with three factors to determine the optimal conditions of three parameters related to date palm leaflets (gauge length, loading speed and fiber position). The paper also shows an analysis of variance for identifying the parameters that provide a maximization of their tensile properties and obtaining a significant linear model for the manufacturing of natural fibers derived from those date palm components. The results show that the factor that most affects the tensile properties of those leaflets is the gauge length, followed by the fiber within the leaflets position. The best properties were obtained for the top part of the leaflets, 20 mm gauge length and 1 mm/min speed. Moisture tests for date palm leaflets have been also carried out; a drying period of 48 h led to a decrease in water content from 53% to 6%.

Identifying the convective/absolute instability nature of a local base flow requires an analysis of its linear impulse response. One must find the appropriate singularity in the eigenvalue problem with complex frequencies and wavenumbers and prove causality. One way to do so is to show that the appropriate integration contour of this response, a steepest decent path through the relevant singularity, exists. Due to the inherent difficulties of such a proof, one often verifies instead whether this singularity satisfies the collision criterion. In other words, one must show that the branches involved in the formation of this singularity come from distinct halves of the complex wavenumber plane. However, this graphical search is computationally intensive in a single plane and essentially prohibitive in two planes. A significant computational cost reduction can be achieved when root finding procedures are applied instead of graphical ones to search for singularities. They focus on locating these points, with causality being verified graphically a posteriori for a small parametric sample size. The use of root-finding procedures require auxiliary equations, often derived by applying the zero group velocity conditions to the dispersion relation. This relation, in turn, is derived by applying matrix forming to the differential eigenvalue problem and taking the determinant of the resulting system of algebraic equations. Taking the derivative of the dispersion relation with respect to the wavenumbers generates the auxiliary equations. If the algebraic system is decoupled, this derivation is straightforward. However, its computational cost is often prohibitive when the algebraic system is coupled. Other methods exist, but often they can also be too costly and/or not reliable for two wavenumber plane searches. This paper describes an alternative methodology based on sensitivity analysis and adjoints that allow the zero group velocity conditions to be applied directly to the differential eigenvalue problem. In doing so, the direct and auxiliary differential eigenvalue problems can be solved simultaneously using standard shooting methods to directly locate singularities. Auxiliary dispersion relations no longer have to be derived, although it is shown that they are the algebraic equivalent of the auxiliary differential eigenvalue problems obtained by this alternative methodology. Using the latter dramatically reduces computational costs. The search for arbitrary singularities is then not only accelerated in single wavenumber planes but it also becomes viable in two wavenumber planes. Finally, the new method also allows group velocity calculations, greatly facilitating the verification of causality. Several test cases are presented to illustrate the capabilities of this new method.

Abstract Lightweight and inexpensive natural fiber‐reinforced composites are gaining market share due to their environment‐friendly and sustainable nature. Among these, thermoplastic composites are preferred due to their recycling ability. However, their fabrication is a difficult task due to the higher viscosity of thermoplastic matrices. In the present study, we present the comparison of mechanical properties of jute/polypropylene composite samples fabricated using the conventional thermoforming technique with those fabricated using the commingled yarn technique. The effect of singeing of jute yarn on its mechanical properties as well on the weaving efficiency of jute fabric as reinforcement for composites was also studied for the first time. Mechanical properties, that is, tensile, flexural, and instrumented Charpy impact properties, were tested using standard testing methods. It was found that singeing improves mechanical properties of jute yarn as well as its performance during weaving. Furthermore, the yarn commingling not only simplifies/shorten the composite fabrication process but also improves the mechanical properties of the developed composites significantly in comparison to conventional thermoforming technique.

The objective of this paper is to develop a global modelling approach, that simulates both the resin transfer molding (RTM) manufacturing and the prediction of the effective thermal conductivity (ETC) of a carbon–cpoxy (CE) laminated composite reinforced with particles. This numerical approach is based on two main stages. First, a numerical simulation of the suspension flow and the filtration of the charges during the RTM process. A method, for simulating the flow of a resin, loaded with particles in suspension through a fibrous medium, considering its double porosity scale, has been proposed. It is based on the description of the flow by Stokes–Darcy coupling, filtration phenomenon and particle dynamics. Secondly, the ETC of the composite thus produced is evaluated using a numerical homogenisation technique, considering the spherical particles inserted into the carbon–epoxy laminated composite. These obtained results have shown that, the incorporation of particles in the laminated composite leads to a significant increase in their effective thermal conductivity, which depends on their thermal conductivity. Finally, a simple linear thermal model has been proposed to predict the effective thermal conductivity of the composite carbon–epoxy–particles, as a function of that of the base composite carbon–epoxy and that of the particles.

The orientational dynamics of inertialess anisotropic particles transported by two-dimensional convective turbulent flows display a coexistence of regular and chaotic features. We numerically demonstrate that very elongated particles (rods) align preferentially with the direction of the fluid flow, i.e., horizontally close to the isothermal walls and dominantly vertically in the bulk. This behavior is due to the presence of a persistent large scale circulation flow structure, which induces strong shear at wall boundaries and in up/down-welling regions. The near-wall horizontal alignment of rods persists at increasing the Rayleigh number, while the vertical orientation in the bulk is progressively weakened by the corresponding increase in turbulence intensity. Furthermore, we show that very elongated particles are nearly orthogonal to the orientation of the temperature gradient, an alignment independent of the system dimensionality and which becomes exact only in the limit of infinite Prandtl numbers. Tumbling rates are extremely vigorous adjacent to the walls, where particles roughly perform Jeffery orbits. This implies that the root-mean-square near-wall tumbling rates for spheres are much stronger than for rods, up to O(10) times at Ra ≃ 109. In the turbulent bulk, the situation reverses and the rods tumble slightly faster than isotropic particles, in agreement with earlier observations in two-dimensional turbulence.

The transition from laminar to turbulent flows has challenged the scientific community since the seminal work of Reynolds ( Phil. Trans. R. Soc. Lond. A, vol. 174, 1883, pp. 935–982). Recently, experimental and numerical investigations on this matter have demonstrated that the spatio-temporal dynamics that are associated with transitional flows belong to the directed percolation class. In the present work, we explore the analysis of laminar–turbulent transition from the perspective of the recent theoretical development that concerns viscoelastic turbulence, i.e. the drag-reducing turbulent flow obtained from adding polymers to a Newtonian fluid. We found remarkable fingerprints of the variety of states that are present in both types of flows, as captured by a series of features that are known to be present in drag-reducing viscoelastic turbulence. In particular, when compared to a Newtonian fully turbulent flow, the universal nature of these flows includes: (i) the statistical dynamics of the alternation between active and hibernating turbulence; (ii) the weakening of elliptical and hyperbolic structures; (iii) the existence of high and low drag reduction regimes with the same boundary; (iv) the relative enhancement of the streamwise-normal stress; and (v) the slope of the energy spectrum decay with respect to the wavenumber. The maximum drag reduction profile was attained in a Newtonian flow with a Reynolds number near the boundary of the laminar regime and in a hibernating state. It is generally conjectured that, as the Reynolds number increases, the dynamics of the intermittency that characterises transitional flows migrate from a situation where heteroclinic connections between the upper and the lower branches of solutions are more frequent to another where homoclinic orbits around the upper solution become the general rule.