Nos séminaires avec cadence mensuelle ont lieu les jeudis en début d'après-midi, ou exceptionnellement dans d'autres jours/horaire pour les professeurs et chercheurs invités. 

Les séminaires sont organisés par Stefano Berti stefano.berti[chez]polytech-lille[point].fr , responsable pour la mécanique des fluides,  et par Toufik Kanit toufik.kanit[chez]univ-lille1[point].fr , responsable pour la mécanique des solides.

2021

• 16 decembre à 12h30

Ksenia Guseva, Centre for Microbiology and Environmental Systems Science, University of Vienna, Austria

Interplay between physical and biological processes in fluid flows: from chemotaxis of marine bacteria to algal bloom formation

Biological processes taking place in the ocean are affected by physical processes that involve multiple spatial and temporal scales. In this talk I will highlight two theoretical and computational models that couple specific physical and biological processes in marine systems, corresponding, respectively, to the mesoscale currents (~10-100 km) affecting the occurrence of algal blooms, and to the smallest scale of the turbulent flow (< 1cm) affecting the motility of bacteria. The larger scale the ocean's hydrodynamic flow is characterized by the presence of large eddies (vortices), which play a very important role in phytoplankton ecosystems. Another important large scale phenomenon is upwelling, which brings a load of nutrients to the surface, sometimes triggering algal blooms. We investigate the effect of intermittent upwelling on the biological community in the presence of large eddies formed in the wake of an island. We observe plankton blooms when nutrients are trapped by vortices, and analyse how this dynamics depends on the timing and intensity of upwelling events. At a smaller scale the marine environment is characterized by a pronounced spatial heterogeneity. In these highly dynamic conditions, small microorganisms such as bacteria depend on efficient strategies to track sparse nutrient sources. I describe a simple theoretical model of the interplay of chemotaxis and different swimming patterns of bacteria in a fluid flow. With this model we investigate in particular the advantage of the run-reverse motility pattern, typical of marine bacteria, to track marine aggregates (food particles) advected by a flow field.

• 9 decembre à 13h

Volfango Bertola, University of Liverpool, School of Engineering, UK

Drop impact phenomena on heated surfaces: Newtonian vs. non-Newtonian fluids [video of the seminar]

The impact of a liquid drop on a solid surface is a fascinating phenomenon, that we observe frequently in our everyday life. Despite its apparent simplicity, drop impact conceals some nontrivial and challenging physics. The interest in understanding drop impact phenomena is also practical, because they play a major role in the optimization of several applications, such as spray applications, which are ubiquitous in industrial as well as in domestic processes, from painting or cleaning surfaces to injecting fuel into internal combustion engines. In this seminar some recent results about the impact of Newtonian and non-Newtonian drops on heated solid surfaces are reviewed, with focus on viscoelastic fluids (dilute polymer solutions) and viscoplastic (or yield-stress) fluids. 

• 18 novembre à 13h

Elena Alekseenko, Laboratoire d’Océanologie et de Géosciences, ULCO, Wimereux , France

Numerical modeling of coupled physico-biological processes and particle dynamics [video of the seminar]
Modeling is a powerful tool for coupling processes under different disciplines and scales. Models integrating hydrodynamical, biogeochemical and particulate transport allow (i) to study spatio-temporal variability and flow of suspended particles of inorganic and organic origin and (ii) to understand the contribution of material flows related to processes, such as: forcing and hydrodynamic regimes, resuspension and sedimentation process, terrestrial and atmospheric deposition, biogeochemical processes (growth, predation, excretion, mortality, mineralization, etc) according to the choices of the marine food-web functional groups. During this seminar I will talk about my experience in coupled modelling and about my previous projects related to this topic, and also about my research objectives in the LOG for long- and short-terms. 

• 14 octobre à 13h

Joran Rolland (Laboratoire de Mécanique des Fluides de Lille Kampé de Fériet, Ecole Centrale, Lille)

Experimental study and modeling of turbulent wake bistability [video of the seminar]
Many out of equilibrium turbulent flow organise themselves into a non-trivial large-scale flow, like a recirculation, a jet etc. There are more and more reports of turbulent flows where several distinct large scale flow configurations are possible for given values of the control parameters. These turbulent flows are multistable: observation indicates that they spend a lot of time in one configuration suddenly transit to another.
I will present the properties of such a bistable turbulent flow in the wake of two parallel bars, realised in the LMFL wind tunnel. This is ongoing work started this spring which is already showing promising results. In that case, the jet that comes out of the bars can either point left or right for a long time instead of fluttering around the central plane. The position of the jet can be followed either with a simple weighted average or using the amplitude of the first two POD modes of the flow. From this, the pdf of the position, as well as the pdf of the waiting times before a switch of position can be sampled. They clearly indicate bistability. In order to understand the bistable flow in more details, a stochastic model is constructed from the data, that can be used to regenerate new time evolutions of the large- scale flow. Eventually, I will present new, time revolved data that has been sampled to study in more details the switching events.
This work has been done using the data sampled experimentally by Jiangang Chen, Pierre Braganca et Christophe Cuvier, and has been processed with the help of Antoine Barlet (now starting a PhD in CEA Saclay) and Indra Kanshana (starting a PhD in LMFL). 

• 22 septembre à 13h

Badr Abou El Majd, Centre de Recherche INRIA Lille  et  Université Mohammed V, Rabat Maroc  

Ingénierie concourante en optimisation de forme multi-disciplinaire [Abstract]

En bureau d’études, les problèmes d’optimisation que soulèvent les concepteurs de systèmes complexes sont par nature multicritères. Par exemple, en optimisation de forme aérodynamique pour la conception d’avions commerciaux, on s’intéresse simultanément au critère de portance dans les phases critiques du décollage ou de l’atterrissage, au critère de traînée en régime de croisière, qui affecte la consommation et le rayon d’action, ainsi qu’à d’autres critères liés à la stabilité ou la manoeuvrabilité de l’engin, et à des critères de fabrication, etc. Par conséquent, l’évaluation de tels critères exige, dans les modèles de haut niveau, la simulation numérique efficace de plusieurs écoulements par les techniques d’approximation des équations de la mécanique des fluides, typiquement par volumes finis. Enfin, l’aérodynamique est généralement couplée à d’autres disciplines, le calcul des structures, l’acoustique, la thermique, etc, ce qui pose des problèmes difficiles de couplage en optimisation multidisciplinaire. Mais le couplage multidisciplinaire en optimisation peut également revêtir une autre forme : dans le cas de deux, ou plusieurs disciplines soumises à un jeu commun de paramètres de conception, comment optimiser ces paramètres pour prendre en compte concouramment de critères antagonistes issus de ces disciplines? On se focalise plus particulièrement sur ce type de problématique, quelquefois référée sous le terme d’ingénierie concourante.

• 9 juillet à 14h salle Agorà au bâtiment Esprit 

Silvia HIRATA - Soutenance d'HDR  [extended abstract]

Thermal convection and Instabilities

I will present a synthesis of the research work I have conducted since my arrival in the Université de Lille in September 2009. The investigations can be organized essentially in 3 axes: (i) thermoconvective instabilities in viscoelastic fluids; (ii) effects of viscous dissipation on the stability of mixed convection problems; (iii) convection and phase change in melt ponds; the common thread being the study of systems where convection is generated by an external temperature gradient or by an internal heat source. The research presented is part of the work developed by the Complex Fluid Mechanics team of the UML laboratory (Unité de Mécanique de Lille – ULR 7512).  

• 17 juin à 13h

Ramon Martins, Department of Mechanical Engineering, Universidade Vila Velha, Brazil [ video of the seminar ]

Sensitivity analysis for numerical simulations of disturbed flows aiming ultrasonic flow measurement

Transit-time ultrasonic flow meters are widely used for flare gas flow measurements. These flow meters use the so called profile (k) factor to relate the intrinsic measured velocity with the cross-section averaged velocity to calculate the flow rate. Under fully developed flow conditions, the profile factor is known to depend on the Reynolds number only. However, under disturbed flow conditions, the profile factor varies with the distance from the source of the disturbance and with the type of disturbance as well. Historically, experiments have been used to analyze ultrasonic flow measurements under disturbed conditions. Nevertheless, in the past few years, some regulatory agencies started to accept numerical analysis (by means of CFD) to evaluate the profile factor in nonconforming installations and diminish the uncertainty due to the velocity profile. Simulation parameters may cause a significant amount of uncertainty. The goal here is to perform a sensitivity analysis of various simulation parameters to evaluate how they can affect the results for the velocity profile and, consequently, for the profile factor of ultrasonic flow meters. Results show that several combinations of mesh refinement, turbulence model, wall model, interpolation schemes and boundary conditions (for instance) can lead to satisfactory results or not. Therefore, it is left as a good practice to always provide a detailed sensitivity analysis when performing numerical simulation of disturbed flows aiming ultrasonic flow measurement. 
 

• 10 juin à 13h

Anselmo Pereira (MINES ParisTech, Centre for material forming (CEMEF), Sophia-Antipolis)  [video of the seminar]

Multiphase flows with viscoplastic materials

Multiphase flows with viscoplastic materials are of paramount importance in many fields of science, being directly related to extremely important biomedical, environmental, and industrial situations. In this talk, two major multiphase flow scenarios involving viscoplastic materials will be discussed: the (1) viscoplastic dam-break; and (2) the vibration-induced bubble rising in a viscoplastic medium. These problems are analysed through a mixed approach combining both two- and three-dimensional numerical simulations, as well as experiments. Numerical simulations are performed based on an adaptive variational multi-scale method for two materials (viscoplastic medium/gas) and compared with experiments carried out with a variety of yield-stress materials, such as Carbopol gels, Hand Sanitisers, peanuts butter, mayonnaise, ketchup, and dense suspensions, among others. The results are explored in the light of energy budget analyses and scaling laws, thanks to which the physical mechanisms driven the investigated problems are stressed. Lastly, the results are summarised in two-dimensional diagrams linking different flow regimes with their driving dimensionless parameters. 

• 12 mai à 13h

Agnese Seminara, Institut de Physique de Nice, CNRS, Université Côte d’Azur   [video of the seminar]

Turbulent navigation in animals and fungi

Organisms navigate efficiently to reach food or mates, and to escape predators. Here I’ll first discuss an olfactory navigation task in mice, where animals navigate using alternatively their senses or information they memorize. The switch in behavior suggests olfactory navigation using turbulent cues is cognitively demanding. I will then discuss the challenges of olfactory navigation due to turbulence and propose a framework to think about predictions using turbulent cues, inspired by experiments with octopuses. I will show that a short time history of odor detections enables prediction of target location, if care is taken about correctly quantifying intermittent odor cues. I will conclude discussing how fungi navigate the atmosphere by releasing spores and how their strategies for spore release mirror the memory vs sensation divide.

• 15 avril à 13h

Himani Garg, LEGI, Univ. Grenoble 

Development of a numerical method for prediction of cavitation inception in the YALES2 code

In a liquid, cavitation is the appearance of vapor bubbles caused by the fluid passing below the vapor pressure, most often due to high-pressure variations around obstacles. The collapse of cavitation bubbles near walls is one of the primary reasons for the failure of technical devices involving liquids' processing at significant pressure differences. The bubble dynamics during the collapse shows two fundamental phenomena during the non-spherical cavitation bubble collapse process: first, the development of high-speed jets, and second the release of shock waves upon final bubble collapse. As a consequence of collapsing vapor cavities in cavitating flow near solid boundaries, high-pressure impact loads are generated. These pressure loads are believed to be responsible for the erosive damages on solid surfaces observed in most applications.
To better understand and possibly alleviate cavitation damage, we study the dynamics of cavitating flows around obstacles numerically. We investigate the dynamics of collapsing bubble, initially in a simple geometry, a single vapor bubble in an unconfined liquid collapsing due to the pressure difference between the bubble and the surrounding liquid. The numerical approach employs a simplified homogeneous mixture or ‘single fluid’ model with the barotropic assumption in a fully compressible finite volume solver. The numerical method results are validated against the well- known Rayleigh collapse of a 2D and 3D vapor bubble. By studying the academic situation of a flow around a cylinder, we have successfully improved the code's stability. Further, we extended the code to the simulation of cavitation inception, i.e., the appearance of vapor structures from a single-phase liquid flow, and exploited it at a macroscopic scale by studying flow around NACA0015 hydrofoil. 

• mardi 9 mars  à 13h

Harshit Bhatia, CEA Saclay, France

Surfacing and clustering of gyrotactic micro-swimmers in free-surface turbulence

In this work, behavior of gyrotactic (spherical and elongated shape) micro-swimmers in a free- surface turbulence has been investigated. This setup mimics the dynamics of phytoplankton in water bodies when surface waves and ripples are smooth or absent. Direct numerical simulation of open- channel turbulence at varying shear Reynolds number from 170 to 1020 based on the channel height) was performed and inertia-less gyrotactic swimmers with stability number covering 2 orders of magnitude were released in the flow in uniform distribution. Behavior and evolution of was tracked till statistical convergence. Both spherical and elongated swimmers were considered in the study, the latter being modeled as prolate ellipsoids with aspect ratio up to 10. Statistics show that vertical migration of the swimmers depends on both their reorientation ability and shape. This effect reflects in the velocity, orientation and concentration statistics. 
 

• jeudi 25 février 2021 à 13h

Karim Kandil, UML et LGCGE, Lille

Multi-physics and multi-scale modeling of the multi-axial response of human intervertebral disc

The intervertebral disc is probably the most extraordinary tissue that the nature produces, mainly for its unusual time- dependent properties strongly influenced by the biochemical environment and the applied mechanical loading. Establishing accurate structure-property relationships for intervertebral disc annulus fibrosus tissue is a fundamental task for a reliable computer simulation of the human spine. The difficulty emanates from the multi-axiality and the anisotropy of the tissue response along with regional dependency of a complex hierarchic structure interacting with the biochemical environment. A physically-based chemo-viscoelastic constitutive model that takes into account an accurate disc annulus structure in relation with the biochemical environment is proposed. Numerical models of annulus specimens and lumbar functional spinal units are designed while taking into consideration the interlamellar matrix connecting the fibers-reinforced lamellae. At the specimen scale, the model capabilities are verified by experimental comparisons under various conditions in terms of osmolarity, strain-rate and multi-axiality while considering the regional dependency. The different scenarios applied to lumbar units show encouraging multi-axial predictive capabilities of our approach making it a promising tool for human spine behavior long-term prediction including age-dependency.

• jeudi 25 février 2021 à 13h

Wenwei WU - Soutenance de thèse / PhD defence (online)

Réactions chimiques en milieu turbulent : étude numérique par simulation numérique directe

The present work focuses on the statistical properties of reactive scalars undergoing reversible chemical reactionsin incompressible turbulence. Theoretical analysis about the statistical properties of scalars at different order of moments were carried out based on appropriately proposed approximations and models. The theoretically derived results were then compared with numerical results obtained by direct numerical simulation (DNS).

• vendredi 19 février 2021 à 14h

Muzzamal Hussain - Soutenance de thèse / PhD defence 

Couplage procédé / propriétés mécaniques des matériaux sandwiches métal / composite hybride à base de tissus en jute

• jeudi 18 fevrier

Holger Stark, Technische Universität, Berlin (Germany)

Elastic turbulence in the two-dimensional Taylor-Couette flow and its control 

Viscoelastic fluids show elastic turbulence although Reynolds number is close to zero. It is generated by elastic stresses, which cause phenomena such as the Weissenberg effect. I first review our work on elastic turbulence in the two-dimensional Taylor-Couette geometry using the program OpenFoam to numerically solve the Oldroyd-B model [1]. We observe a supercritical transition from laminar to turbulent flow at a critical Weissenberg number and quantify it by an order parameter, the time average of the secondary-flow strength. Close to the transition it scales with an exponent close to 0.5. We present temporal and spatial power spectra of the velocity fluctuations and discuss the characteristic exponents of their power-law decay. Second, we apply active open-loop control to elastic turbulence by periodically modulating the angular velocity of the outer cylinder of the Taylor-Couette cell [2]. In the turbulent state we observe that the order parameter continuously approaches zero when increasing the oscillation frequency or Deborah number. Thus, beyond a critical value laminar flow is recovered since for fast oscillations the necessary elastic stresses to induce elastic turbulence cannot build up. We rationalize this view using the linear Oldroyd-B model and introduce an effective Weissenberg number to approximate the critical Deborah number. In the end, we also show first results on elastic turbulence in the van Karman swirling flow.
[1] R. van Buel, C. Schaaf, and H. Stark, Elastic turbulence in two-dimensional Taylor-Couette flows, EPL 124, 14001 (2018).
[2] R. van Buel and H. Stark, Active open-loop control of elastic turbulence, Sci. Rep. 10, 15704 (2020). 

• vendredi 29 janvier, Bet. ESPRIT

Quoc Hoan PHAM - Soutenance de thèse / PhD defence 

Analyse des réponses balistiques des fribres d'un matériau tissé à l'échelle microscopique basée sur l'homogenisation numérique

• 14 janvier

​Alessandro Sozza, ISC-CNR, Institute for Complex Systems, Roma (Zoom)  

Nutrient uptake by phytoplankton in turbulent flows

Aquatic microorganisms, like bacteria and phytoplankton, are known to influence large-scale processes in the sea and to play a fundamental role in Earth climate dynamics, by recycling dissolved organic matter and producing half of the oxygen of the planet. As they interact with the environment at the microscale, phytoplankton takes up dissolved nutrients from the surroundings via osmosis. Therefore, scalar absorption is controlled by diffusion and enhanced by turbulence. Mean field approaches offer good predictions of such increase but cannot describe the effect of turbulent fluctuations. In this talk, we present an efficient point-particle approach to simulate reaction- diffusion processes of spherical absorbing particles in the diffusion-limited regime. First, we demonstrate the potential of the method to resolve diffusive interactions by examining configurations of static absorbers with increasing complexity. Then, we investigate the effects of turbulent fluctuations on the Lagrangian statistics of absorption of a scalar field by tracer particles. By means of extensive direct numerical simulations, we quantify the increase of the scalar uptake induced by turbulence and its dependence on the particle properties. We study the statistics of the uptake rate in relation with the statistics of the local shear rate experienced by the particles along their trajectories. Finally, the implications of our results for aquatic microorganisms are discussed. 

2020

• 14 decembre

Clothilde Le Quiniou, LOG, ULCO, Wimeruex (Zoom)

​Copepods in turbulence: how to benefit from a chaotic environment?

Copepods are tiny crustaceans living for most of them as plankton during their entire life cycle. Their swim abilities were observed to depend on their physical environment, such as turbulence. In our work, the motions of copepods were filmed in a turbulent environment of different intensities. We expect turbulence to be an advantage at certain intensities only. We will present the experimental set-up Agiturb, that we build-up in order to obtain an isotropic and homogeneous turbulence representative of the natural environment. The flow was characterized at different turbulence intensities and with different tracer sizes. The derivatives (speed and acceleration) of the tracers were calculated despite the difficulties when the signal is acquired at very high frequency (high speed camera). Lastly, we will present our preliminary results obtained on copepods. 

• 12 novembre

Matteo Borgnino, Univ. Torino (Zoom) ​

To swim or not to swim: Phytoplankton dynamics in turbulent flows

Phytoplankton are among the most important life-forms on Earth: they produce an amount of oxygen comparable to all land plants and are crucial because of their functions in marine ecosystems. In particular phytoplankton patchiness as well as the ability to float within the water column have both an ecological relevance and an impact on the microorganisms population composition, modulating cells activities like the encounter rate, the predation and the reproduction. We use numerical simulations to investigate the dynamics and distribution of both motile and non-motile phytoplankton transported by turbulent flows. In particular we study how the presence of turbulence can have an impact on sedimentation statistics and lead to non-trivial spatial patterns. We first investigate, within a suspension of gyrotactic microorganisms, preferential sampling and small-scale clustering formation as a function of the swimming and shape parameters; in particular in the limits of spherical and rod-like particles. We then move our attention on non-motile microorganisms that, actively controlling their buoyancy, do not simply behave like passive tracers; here, using a minimal mechano-response model, we study microorganisms distribution and sedimentation statistics in presence of different turbulent intensities.
 

• jeudi 25 juin visioconference à 11h

Cuong Ha Minh (Laboratoire de Mécanique et Technologie, ENS Cachan)

Modélisation prédictive du comportement des matériaux tissus soumis à l’impact balistique

Ces travaux portent sur la modélisation prédictive du comportement des matériaux tissus soumis à l’impact balistique. Ce comportement est connu complexe due aux architectures spéciales de ces matériaux et à la vitesse de chargement ultrarapide. Le défi principal est une prédiction correcte des mécanismes complexes en même temps aux échelles différentes avec un cout de temps de calcul raisonnable. Ces travaux proposent en premier lieu des modèles numériques aux échelles différentes pour atteindre une bonne prédiction des mécanismes principaux de chaque échelle. Dans un deuxième temps, des modèles multi-échelles sont développés pour optimiser le cout de calcul en assurant ces mécanismes physiques. Les dispositifs expérimentaux ont également été proposés pour caractériser le comportement de ces matériaux aux échelles différentes et enfin valider la modélisation prédictive de manière quantitative et qualitative.

• jeudi 11 juin, seminaire en visioconference à 11h

Alice Jaccod (IJLRA, Sorbonne Univ., Paris)

Phytoplankton dynamics in turbulent fluid environments

In the marine environment, biological processes are strongly affected by oceanic currents, particularly by eddies formed in the wake of the Canary Islands. A direct numerical simulation of the fluid around the Gran Canary coupled to a predator–prey model of planktonic population dynamics is performed, in order to predict under which conditions an algal bloom is observed. Despite that excitability in the predator–prey dynamics is a transient phenomenon, we show that the presence of the wake behind the island entrains fluid and plankton, enhancing cross-wake transport. Different obstacle shapes are studied, as well as the impact of fluid and reaction parameters. The main outcome is that the response of the plankton depends crucially on two factors: the relation between the time scale of the vortex formation and the time related to biological growth, as well as the ratio of biomass consumed to biomass of new plankton produced.

• jeudi 14 mai, seminaire en visioconference à 11h

Francesco Romano’ (LMFL - Kampé de Fériet, Ensam, Lille) 

The effect of viscoelasticity and surfactant in an airway closure model [abstract]

The closure of a human lung airway is modeled as a pipe coated internally with a liquid. For a thick enough coating, the Plateau-Rayleigh instability creates a liquid plug which blocks the airway halting distal gas exchange. This airway closure flow induces high stress levels on the wall, which is the location of airway epithelial cells. The bi-frontal plug growth induces a high level of stress and stress gradients on the epithelial cells, which are large enough to damage them, causing sub-lethal or lethal responses. We simulate the effect of the viscoelastic properties of mucus by means of the Oldroyd-B model. Increasing the relaxation time speeds up the airway closure and, if the solvent concentration is low enough, the extra stresses induce a second lethal response of the epithelial cells because of an elastic instability. The effect of surfactant is also investigated and becomes relevant only if the surfactant concentration is high enough, causing a delay of the closure and a decrease of wall stresses.

• mardi 3 mars 2020, Bât. Esprit – Salle Atrium à 13h

Rossana Tazzioli (U. Lille, Lab. P. Painlevé)

Levi-Civita's contribution to hydrodynamics and its reception in France [abstract]

Tullio Levi-Civita (1871-1943) gave remarkable contributions to various mathematical fields, such as general relativity, the three-body problem, differential geometry, and hydrodynamics. Henri Villat and other French mathematicians assimilated Levi-Civita's works on hydrodynamics and took inspiration from them, especially from those concerning the wake hypothesis and the wave theory. This talk aimes at giving an image of Levi-Civita's work on hydrodynamics and of its reception in France, by using in particular the correspondence between Levi-Civita and Henri Villat contained in the Fondo Levi-Civita (Accademia dei Lincei, Rome) and in the Dossier Villat (Académie des Sciences, Paris).

• jeudi 13 février, Bât. Esprit – Salle Atrium à 13h

Frédéric Roger & Lahcen Khouchaf (UML, IMT Lille-Douai) 

Multiphysics and multiscale experimental analyses and simulations [abstract]

In the engineering sciences, analyse or transform the solid heterogeneous material often calls gas–solid interactions. Numerical simulation combined with imagery allows an exploration of heterogeneous materials and dynamically with observations of its behavior in its future environment. To understand these complex interactions and improve either the image quality or the process efficiency, numerical and experimental multiphysics and multiscale approach can be combined. 

• jeudi 30 janvier 2020, Bât. Esprit – Salle Atrium à 13h

Yongxiang Huang (State Key Lab of Marine Environmental Science, College of Ocean and Earth Sciences,  Xiamen University, China)

Intermittency and cascade in two-dimensional bacterial turbulence [abstract]

The active interaction between the bacteria and flow fluid (e.g., water) generates coherent flow pattern more than 10 times larger than their body size even at nearly zero Reynolds number. This fascinating phenomenon can be understood in the framework of turbulence, where the kinetic energy is injected into the system via a size of bacterial body length R, e.g., few μm, and is transferred to large scales. This dynamical procedure is recognized as an inverse energy cascade. In this talk, an experimental velocity field of the bacterial turbulence is quantitatively analyzed via two relative novel methodologies, which are the Hilbert-Huang transform and the streamline segment analysis. The former method is free with high-order harmonics in a joint physics-wavenumber domain, while the latter one is coordinate-free in the physical domain. For the Hilbert-based approach, a dual-power- law is observed respectively in the range 0.15 ≤ kR ≤ 0.5 and 0.03 ≤ kR ≤ 0.075, where k is the wavenumber. The corresponding intermittency parameter μ provided by the lognormal model, e.g., ζ(q) = qh – μ/2 (q2h2 − qh), is found to be μS= 0.26 and μL = 0.17. For the latter streamline-based method, the power-law behavior is observed in the range 2 ≤ r/R ≤ 10, which agrees with the observation of the Hilbert approach. The experimental intermittency parameter is found to be μ = 0.2, which is comparable with the one μS provided by the Hilbert method. This intermittency parameter μ is coincidentally in agreement with the three-dimensional hydrodynamic turbulence result, e.g., 0.2 ≤ μ ≤ 0.4, suggesting that the two-dimensional mesoscale bacterial turbulence and three-dimensional fluid turbulence may share universal intermittent feature. Moreover, a scale-to- scale energy flux is examined via the filter-space technique. An inverse energy cascade is evident. Finally, the lognormal model is justified by verifying the lognormal distribution assumption of the coarse-grained energy dissipation rate, suggesting that the intermittency correction is originated from the strong variation of the energy dissipation field. 

• jeudi 23 janvier 2020, Bât. Esprit – Salle Atrium à 13h

​J. John Soundar Jerome (LMFA, Lyon)

Non-modal stability analysis of wall-bounded parallel shear flows with/without heat addition [abstract]

In this talk, I shall present, some of the major results from my, almost a decade-old, investigations on the effect of an adverse temperature gradient on the transient growth phenomenon in plane Poiseuille flow and plane Couette flow. I wiill show that the largest transient growth is always attained for streamwise-uniform perturbations which produce large streamwise streaks and Rayleigh-Bénard convection rolls (RB). The short-time response is governed by the inviscid lift-up mechanism and that the influence of Rayleigh number on this mechanism is secondary and negligible. The optimal input for the largest long-time response is given by the adjoint of the dominant eigenmode with respect to the energy scalar product: the RB eigenmode without its streamwise velocity component.
In addition, I shall try to talk about some consequences of the well-known Squire transformation on the algebraic growth in linear stability analysis. Finally, I will also point out the possibility of a novel algebraic growth mechanism in such flows in the presence of internal viscous dissipation. 

2019

• vendredi 29 novembre 2019, Amphi Lebon - Polytech'Lille à 10h30 

Hamza Lamnii  - Soutenance de thèse / PhD defence Effets du vieillissement ultraviolet sur le comportement des polymères semi-cristallins en chargement cyclique et monotone : expérimentation et modélisation [abstract]

• vendredi 22 novembre 2019, Bât. Esprit – Salle Atrium à 14h 

Dario Oliveira Canossi   - Soutenance de thèse / PhD defence Numerical simulation of the transition to elastic turbulence in viscoelastic inertialess flows [abstract]

• jeudi 14 novembre, salle Agorà 1 au bâtiment Esprit à 13h 

Stefano Musacchio (Department of Physics, University of Torino, Italy)

The periodic Kolmogorov flow: a virtual channel [PDF]

The Kolmogorov flow provides an ideal instance of a virtual channel flow: It has no boundaries, but it possesses well-defined mean flow in each half wavelength. In this talk, I will discuss how this remarkable feature can be exploited for the purpose of investigating the interplay between the mean flow and the turbulent drag of the bulk flow. I will review the results of numerical simulations at increasing Reynolds number, which show the dependence of the bulk turbulent drag on the amplitude of the mean flow. Further, I will present a detailed analysis of the scale-by-scale energy balance, which describes how kinetic energy is redistributed among different regions of the flow while being transported toward small dissipative scales. I will also briefly review how the Kolmogorov flow has been used to investigate numerically the phenomena of drag reduction and elastic turbulence induced by the presence of polymer additives, and the turbophoresis of inertial particles 

• jeudi 10 octobre, salle D'arsonval à Polytech à 13h 

Fouad Erchiqui (Université du Québec en Abitibi-Témiscaminque (UQAT), Canada)

Hybrid enthalpy method for Finite Element Thermal Analysis of Anisotropic Multi-materials involving Multi-orientations of the thermal conductivity tensor [PDF]

The anisotropic problem of thermal conduction in a incompressible solid is generally treated in a reference coordinate system, which adequately describes its thermal conductivity tensor (Cartesian, cylindrical or spherical). For this problem, numerical treatment is delicate, especially if the thermophysical properties are non-linear or if the anisotropic medium undergoes a phase change. In this presentation, we propose an new approach using only one reference system (for example the Cartesian) to treat the anisotropic thermal conduction of problems for which the multi-material solid is characterized by a set of tensors of thermal conductivity of different natures (Cartesian and/or cylindrical and/or spherical), with or without phase change. The nonlinear heat conduction problem involving phase changes is solved using hybrid three-dimensional volumetric specific enthalpy based on finite-element analysis. The proposed approach is validated with recent analytical testing for two anisotropic media (cylinder and spherical with convective transfer with variable circumference) and with two experimental tests related to the heating of frozen woods. An example on the characterization of the phytosanitary treatment time of wood by microwaves, in accordance with FAO International Standard No.15, will be presented.

• lundi 7 octobre à 14h, Bat. M6

Yoann Réquilé  - Soutenance de thèse / PhD defence Effets de la dissipation visqueuse sur la convection des écoulements de Rayleigh-Bénard- Poiseuille/Couette de fluides Newtoniens ou viscoélastiques [résumé / abstract]

• jeudi 20 juin

Christian Kamdem  - Soutenance de thèse / PhD defence  Caractérisation et la modélisation du comportement mécanique des structures tissées aux températures cryogéniques

• ​jeudi  23 mai

Anke Lindner (PMMH-ESPCI, Paris)

Elastic flow instabilities in microfluidic serpentine channels [PDF]

We discuss the onset of a purely elastic flow instability in serpentine channels, using a combined experimental, numerical and theoretical investigation. Good qualitative agreement is obtained between experiments, using dilute solutions of flexible polymers in microfluidic devices, and three-dimensional numerical simulations using the upper-convected Maxwell model [1] The results are confirmed by a simple theoretical analysis, based on the dimensionless criterion proposed by Pakdel & McKinley (PRL, 1996). We then determine the influence of fluid shear thinning on the onset of such purely-elastic flow instabilities and observe that shear thinning has a stabilizing effect on the microfluidic flow [4]. Three-dimensional numerical simulations performed using the White–Metzner model predict similar trends, which are not captured by a simple scaling analysis using the Pakdel–McKinley criterion. The good understanding of the onset of elastic instabilities can also be used to determine relaxation times of unknown solutions and we describe a microfluidic rheometer using a serpentine flow channel [2]. In addition, we investigate the structure and magnitude of secondary flows, present in flows of visco-elastic fluids in curved geometries [3,5]. 

• jeudi 16 mai

Alexis Duchesne (IEMN, U. Lille)​

Playing with free surface flows : Leidenfrost drops on a liquid pool and The birth of the hydraulic jump  [PDF]

In this talk, I would like to address two different subjects related to free surface flow:
- Leidenfrost drops on a liquid pool
We will discuss the case of drops in levitation (so-called Leidenfrost effect) on a heated liquid pool. Particularly we will see that a volatile liquid drop placed on the surface of a non-volatile liquid pool reaches the Leidenfrost state as soon as the liquid of the pool is just hotter than the drop boiling point, with no apparent Leidenfrost threshold. We will also discuss the flow induced by the drop in the pool.
- Birth of a Hydraulic jump
The hydraulic jump, i.e., the sharp transition between a supercritical and a subcritical free-surface flow, has been extensively studied. However, an important question has been left unanswered: How does a hydraulic jump form? We present here an experimental and theoretical study of the formation of stationary hydraulic jumps in centimeter-sized channels. 
 

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• mardi 23 avril, au bat. M6 à 13h

Azma Putra (Centre for Advanced Research on Energy, Universiti Teknikal Malaysia Melaka)

Towards Eco-Friendly Acoustic Absorbers [PDF]

The acoustic absorber has been widely known for its applications for noise control in industries as well as for improving acoustic quality in buildings. Sound absorption materials were first substituted by synthetic fibers in the 1970s due to public health concerns on the original asbestos-based material, which is harmful to human health. Until recent years, when global warming has became an alarming issue, studies were conducted to investigate the utilisation of sound absorber materials. According to the study on the Life Cycle Assessment (LCA) on a range of synthetic and natural absorber materials, it is found that the production of synthetic materials requires more energy and thus provide greater global warming potential. In contrast, natural plant-based material contributed less carbon footprint.

The talk will present the research progress on the eco-friendly natural materials as acoustic materials. The materials include the natural wasted fibres and non-fibrous type materials such as hollow structures as well as micro-perforated panels. The general theoretical foundation will be presented and important findings on the absorption performance of the materials will be discussed. It will be shown that the performance of the ‘new’ eco-friendly absorber is comparable with that of the synthetic materials. 

• jeudi 4 avril, au bat. M6 à 13h 

 Karim Kandil (UML, U. Lille)

A chemo-viscoelastic finite element model for intervertebral disc [PDF]

Back pain is a common health problem affecting about 80% of humans. It is often related to the intervertebral discs (IVD) injuries and dysfunctions. The IVD tissue exhibits a complex anisotropic heterogeneous osmo-inelastic behavior described by rate dependency and hysteresis during cyclic loading. A healthy well-hydrated state of the IVD is ensured by the osmotic effect resulting from the bio-chemical interaction between the disc tissue and the surrounding environment. Towards a better understanding of the IVD behavior, a chemo-viscoelastic finite element model based on the microstructure of the soft tissue is developed in order to reproduce the intrinsic response of the IVD under different mechanical and chemical conditions. The model is applied to specimens extracted from different disc locations as well as entire functional units. The chemo-mechanical response of the disc soft tissue in terms of free swelling, stress-strain curves and transversal strains is replicated by the model and the results are compared with experimental tests.

• jeudi 21 mars, au bat. M6 à 13h 

Vojtech Patocka (DLR German Aerospace Centre, Berlin, Germany)

Viscoelastic mantle convection [PDF]

Present thermochemical convection models of planetary evolution often assume a purely viscous or viscoplastic rheology. Ignoring elasticity in the cold, outer boundary layer is, however, questionable since elastic effects may play an important role there and affect surface topography as well as the stress distribution within the stiff cold lithosphere. We present a modelling study focused on the combined effects of Maxwell viscoelastic rheology and a free surface in the stagnant lid planetary convection.  We perform thermal convection models of the cooling planetary mantles and demonstrate that while the global characteristics of the mantle flow do not change significantly when including viscoelasticity, the stress state of the cold lithosphere may be substantially different. Transient cooling of an initially thin upper thermal boundary layer results in a complex layered stress structure due to the memory effects of viscoelastic rheology. The stress state of the lid may thus contain a record of the planetary thermal evolution.

• vendredi 15 février , au bat. M6 à 14h 

​Himani Garg - Soutenance de thèse / PhD defence  Particle laden inhomogeneous elastic turbulence

2018

• jeudi 29 novembre, au bat. M6 à 13h 

Alexei Sentchev (Université du Littoral - Côte d'Opale,  Laboratoire d'Océanologie et de Géosciences, UMR 8187-LOG)

Assessing coastal ocean dynamics by using modeling and remote sensing: Application in La Manche

Recent research activities conducted in LOG in the field of coastal ocean circulation assessment will be presented. Two approaches are used for understanding the major features of coastal water dynamics and its variability at different space/time scales: modeling and remote sensing. Examples of high resolution modeling in La Manche will be used to illustrate some practical applications in the domain of coastal water resource management. More recent work is focused on the assessment of tidal energy resource in La Manche and off the western Brittany coast (Iroise Sea) by using the remote sensing of coastal currents and modeling. A method of improving the results of modeling by merging the model and the underway velocity measurements will be presented. It allows to considerably decrease the discrepancy between the modeled and observed velocities at a highly energetic site in Normandy – Raz Blanchard. Possibilities of joint research could be also discussed.

• jeudi 25 octobre, au bat. M6 à 13h 

Nader Ben-Cheikh (University of Tunis El-Menar, Tunisia)

A Multigrid Technic adapted to Large Eddy Simulation and Convection-Radiation of Incompressible Fluids

A finite-volume method is used to discretize the Navier–Stokes and energy equations. The advective terms are discretized by way of a QUICK third-order scheme in the momentum equation and a second order central differencing one in the energy equation. The discretized momentum and energy equations are solved using the red and black successive over relaxation method RBSOR, while the Poisson pressure correction equation is solved using a full multigrid method.The numerical method is first applied to the study of turbulent incompressible flows in lid driven cubical cavities using Large Eddy Simulation and two sub-grid scale models, i.e., the WALE (Wall Adapting Local Eddy-viscosity) model and the corresponding dynamic sub-grid model (DSGS).In a second step, a numerical tool for coupling natural convection in cavities with surface radiation and computations are presented for an air-filled square cavity whose four walls have the same emissivity. 

•  jeudi 18 octobre, au bat. M6 à 13h 

Leonardo Santos de Brito Alves (Fluminense Federal University, Rio de Janeiro, Brazil) 

Linear and nonlinear stability analyses to detect unsteady disturbance growth in Darcy-Bénard mixed convection under the influence of viscous dissipation [PDF]

The present work investigates the linear and nonlinear disturbance growth in time induced by viscous dissipation and external heating from below in a porous media flow. In order to do so, we employed linear and local stability analysis as well as nonlinear analysis, by means of a direct numerical simulation of the governing equations. Linear stability analysis consists in decomposing all variables of the problem in two parts: a steady state, around which the stability is analyzed, and infinitesimal disturbances. Furthermore, two different linear stability analyzes are employed here: modal and non-modal. Matrix forming using the Generalized Integral Transform Technique is used to generate the generalized eigenvalue problem whereas the fully nonlinear problem is solved using high-order finite differences. The non-modal analysis reveals that viscous dissipation makes the linear operator non-normal, but transient energy growth is small, which means non-monotonicity is weak. Hence, an asymptotic behavior in time is assumed to dominate and a modal analysis is performed to evaluate absolute stability. Viscous dissipation introduces new modes that compete to trigger transition from convective to absolute instability. A nonlinear analysis is then performed in order to validate and better understand the linear results. Since both modal and non-modal linear analyses are dominated by transverse modes, a 2D DNS is used. The transition from stable to convectively unstable is in agreement between the two approaches, but the transition from convective to absolute instability only agrees when viscous dissipation effects are weak, i.e. low velocities and low viscosity. When these effects become important, the results from linear analysis are no longer recovered. In this scenario, transition to absolute instability takes place in subcritical conditions, where a nonlinear group velocity is observed. In other words, the flow transitions from linearly convectively unstable to nonlinearly absolutely unstable.

•  jeudi 4 octobre, 13h-14h , bâtiment M6

Youssef Hammi (Mississippi State University, USA)

Fatigue Modeling of a Powder Metallurgy Main Bearing Cap

Developing the ability to predict density distribution, monotonic plasticity, damage and the cyclic damage progression is imperative for the design of Powder Metallurgy (PM) components that will experience overloads during in-service life due to impacts, rough ground, and crash environments. In this paper, mathematical-based models for PM manufacturing process are developed, validated and implemented in user material subroutines Vumat and Umat to model the compaction and sintering processes. The material history is initially captured and carried throughout the compaction and sintering processes in order to predict the density distribution. Knowing the density distribution, mechanical properties are mapped in the PM sintered part using tension, compression, and torsion mechanical tests performed on samples at different densities, temperatures and strain rates. A finite element analysis of an experimental fatigue fixture that reproduces similar loading conditions than those of in-service life conditions is performed. Using a Multi-Stage Fatigue (MSF) model implemented in Abaqus, the fatigue life is evaluated on a PM automotive component, the main bearing cap (MBC), and results are compared to experimental fatigue tests.

• jeudi 20 juin, 13h-14h, bâtiment M6.

Thomas Gomez (LMFL, Univ. Lille) 

Analytical closure and helical turbulence

Since the first studies on turbulent flows, researchers’ attention has been drawn to helic- ity. Indeed, because the helicity is a non-viscous invariant of the 3D turbulent dynamics as well as energy or enstrophy in 2D, it is expected that its dynamics has a strong influence on inter-scales energy exchanges. Helical turbulence is also important because it corresponds to a first step towards more complex flows in which the homogeneous turbulence is no longer fully isotropic. This case denoted as skew-isotropic corresponds to a turbulence in which the mirror symmetry is broken. More recent results have shown that helicity is also involved in complex physical phenomena such as the dynamo effect in magnetohydrody- namics. After having defined the helicity and described its main properties, we will give an overview of the different models of analytical closure with special attention for the EDQNM model (Eddy Damped Quasi Normal Markonvian) introduced by Orszag in 1970 that we will use to illustrate the influence of helicity on the dynamics of free decaying turbulence. 

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• jeudi 12 avril, bâtiment M6 à 12h45 

Abdelghani Saouab  (Laboratoire Ondes et Milieux Complexes, Université Le Havre Normandie)

Numerical Modeling and Simulation of Liquid Composites Molding Processes 

For several years now, liquid composite molding (LCM) processes present a well‐established class of manufacturing technics for processing semi-structural and structural fiber-reinforced composite parts. These technics are widely used in many industrial domains and particularly in transportation (automotive, aeronautical, marine, etc.). Their principle is to inject or infuse a liquid within a fibrous preform, where the main objective is to reach a full impregnation as the liquid moves along between and inside the fiber bundles. The impregnation driving force is usually resulting from pressure gradient. During the LCM process, there is a strong coupling between the liquid flow, the preform compressibility, and the heat transfer between the liquid, the mold, and the fibrous medium. In this seminar, we will present our numerical modelling approach developed in this context, it concerns in particular the modeling of air bubble dynamics. In addition, a set of numerical and experimental results is presented to: validate the model, identify its parameters, and show some of its applications. 

• jeudi 15 mars, bâtiment M6 à 12h45 

Wouter Bos (Laboratoire de Mécanique des Fluides et d’Acoustique, Lyon)

Non-equilibrium turbulence

Recent experiments and simulations have shown that unsteady turbulent flows, before reaching a dynamic equilibrium state, display a universal behaviour. We show that the observed universal non-equilibrium scaling can be derived in a fairly simple manner. Given the universality of the experimental observations, the ideas presented here lay the foundation for the modeling of a wide class of unsteady turbulent flows and we show how engineering turbulence models (e.g. the k-epsilon model) can be modified to take this into account.

• jeudi 15 février, salle d'Arsonval à Polytech à 13h 

Moufid Mouwakeh (Department of Applied Mechanics, University of Aleppo, Syria)

Ductile Fracture of Cracked Pipes Using Limit Load Analysis

Using cracked pipes design codes and finite element (FE) analysis, limit load solutions of pipes containing surface cracks is determined. The study is performed on 5 pipes of different diameters with a constant crack length and depth. The crack geometry is semi-elliptical surface crack. The cracked pipes are subjected to internal pressures which are obtained from formulas of cracked pipes design codes. Due to ductile behavior of polyethylene pipes, failure occurs when the critical net stress reaches a value equal to ultimate tensile strength multiplied by constraint factor. Constraint factor is calculated and its evolution with pipe diameter is analyzed. Three different definitions of constraint factor based on global or local approach are also compared, so that a new failure criterion can be obtained. The new failure criterion enables us to predict the remaining life of the cracked pipes which allows programming the works of maintenance and replacement.

 2017

• vendredi 15 décembre, Amphi Migeon à Polytech à 10h 

Fouad Erchiqui (Université du Québec en Abitibi-Témiscaminque (UQAT), Canada)

Caractérisation, modélisation et optimisation en thermoformage

La simulation numérique des procédés de mise en forme des plastiques (thermoformage, moulage par soufflage, etc. ) nécessite une bonne connaissance, d’une part, du comportement des matériaux utilisés et, d’autre part, des intervalles en températures et en pressions (thermoformabilité) pour des applica! tions industrielles de ces matériaux en plasturgie. Ces matériaux sont souvent des polymères thermoplastiques (avec ou sans fibres) chauffés entre la température de transition vitreuse et de fusion pour être ensuite mis en forme. C’est dans ce contexte que la présentation est orientée et elle concerne trois volets : i) identification viscoélastique en grandes déformations des matériaux thermoplastiques (réseaux de neurones); ii) modélisation intégrée de thermoformage (étapes de chauffage infrarouge, de formage et de refroidissement) et iii) optimisation (métaheuristique).

• jeudi 30 novembre, bâtiment  M6 à 13h

Roney Thompson (Université Fédérale de Rio de Janeiro, Brésil)

Modeling thixotropic elasto-viscoplastic materials: ideas and challenges

There are examples of applied materials with high complexity that combine a diversity of aspects of material behavior. Recently, the scientific community started the endeavor to model these materials despite the fact that there are still unsolved problems in simpler materials. In this presentation we explore some ideas on modeling thixotropic elasto-viscoplastic materials and how a specific model in these lines perform in transient motions of free-surface problems. Challenges on developing even more generic models encompasses temperature dependency, better representation of microstructure, inclusion of non-viscometric data, transient experiments, transient microstructure evolution modeling, among others.

• jeudi 9 novembre, bâtiment M6 à 13h 

Louise Watremez (LOG, U. Lille)

Observation et modélisation géodynamique des structures extensives

Les plaques tectoniques sont considérées rigides et se déforment à leurs frontières. Il existe trois types de frontières de plaques tectoniques : les frontières convergentes (zones de subduction ou de collision continentale), les frontières coulissantes (failles décrochantes) et les frontières divergentes (rift continental ou océanique). L’étude de l’extension continentale est donc une des clés pour la compréhension des processus géodynamiques à grande échelle. Cette extension peut conduire à la formation (1) de grandes zones de lithosphère continentale amincie immergées dans lesquelles on retrouve notamment une grande partie des ressources pétrolières mondiales, voire (2) d'un nouvel océan. Cette étude, effectuée grâce à la combinaison de méthodes d’imagerie sismique à l’échelle de la croûte terrestre (~30 premiers kilomètres sous la surface) et de modélisation numérique thermomécanique à l’échelle de la lithosphère, permet de comprendre les processus entrant en jeu lors de la dislocation d’un continent ainsi que leur chronologie.

https://uml-jb.sciencesconf.org/