Laboratoire de Mécanique des Fluides et d'Acoustique - UMR 5509

Informations pratiques

Gostiaux Louis — 2100-10-17T10:08:00Z

Les séminaires du LMFA ont lieu le vendredi à 11h, et s'adressent à l'ensemble du laboratoire.
contact : louis.gostiaux@ec-lyon.fr, wouter.bos@ec-lyon.fr, benjamin.miquel@ec-lyon.fr
Pour accéder au laboratoire voir ici

Documents utiles :

Smiron Varghese

Bos Wouter — 2026-07-07T14:53:15Z

Linear Analysis of Rotating Double-Diffusive Convection

Jury :

Sébastien AUMAÎTRE, Directeur de recherche, CEA (SPEC) Rapporteur
Le FANG, Professeur, (Ecole Centrale Pekin) Rapporteur
Florence RAYNAL, Directrice de recherche, CNRS (LMFA) Examinatrice
Jérémie VIDAL, Chargé de recherche, CNRS (LGL-TPE) Examinateur
Wouter BOS, Directeur de recherche, CNRS (LMFA) Directeur de thèse
Benjamin MIQUEL, Chargé de recherche, CNRS (LMFA) Co-directeur de thèse

Résumé :

This thesis examines the influence of rotation on the salt-fingering branch of double-diffusive instability in stably stratified fluid layers of geophysical and astrophysical rel-evance. In many planetary cores and stellar interiors, density depends simultaneously on temperature and composition. The marked disparity between their diffusivities can destabilize a layer that would otherwise remain stable to ordinary overturning convection. The manner in which rapid rotation modifies this instability remains incompletely understood. The present work addresses this question in a plane-layer geometry.

A central part of the work is devoted to a detailed linear stability analysis of rotating double-diffusive convection. The instability is studied through normal-mode analysis in a wall-bounded layer with impenetrable horizontal boundaries. In the rapidly rotating limit, the study shows how rotation alters both the threshold and the spatial structure of the instability, leading to a progressive shift toward smaller horizontal scales. Reduced descriptions are then derived in the limits of large Lewis number and large Schmidt number, which are especially pertinent to astrophysical and liquid-metal contexts. Numerical linear stability shows that these results are roughly insensitive to the exact type of boundary conditions at the wall. Preliminary results using direct numerical simulations are used to illustrate the dynamics of the nonlinear saturated regime.

The manuscript also explores the question of helicity amplification in the rotating fingering system. A linear analysis is performed in a 2D3C limit to assess possible mechanisms of helicity growth in the absence of solid walls.

Journée des doctorant.e.s du LMFA 2026

Scolan Hélène — 2026-07-07T12:50:12Z

Jeudi 9 juillet sur le campus de Centrale Lyon a lieu la journée des doctorant.e.s du LMFA.

Cette journée est l'occasion :

de découvrir ou redécouvrir quelques activités de recherche faites par les doctorants au sein du laboratoire
d'échanger sur les difficultés et les astuces que l'on peut rencontrer pendant une thèse
de partager un moment convivial

Venez découvrir le travail des doctorants en amphi 3 !

Programme complet :

Programme JDD 2026

Ecole d'été - ED MEGA et Matériaux 2026

Scolan Hélène — 2026-06-29T11:49:27Z

Approches multidisciplinaires en mécanique ED TRAMME

Comme chaque année, cette école doctorale thématique en partenariat avec les ED MEGA et Matériaux se déroulera du 7 au 8 juillet 2026 sur le campus de la Doua.

L'inscription est obligatoire mais gratuite, inclut les déjeuners et donne droit à la validation de 14 heures de formation scientifique pour les doctorants.

Inscription par mail à Nadira Matar (voir bulletin à remplir en fin d'article).
Informations complémentaires : Sophie Miralles

Programme :

Mardi 7 juillet :

8h30 - 12h	Sébastien Manneville (École Normale Supérieure de Lyon)	« Mécanique des fluides à seuil et des solides mous : des ultrasons aux rayons X couplés à la rhéométrie »
13h30 - 17h	Axelle Amon (Université de Rennes)	« Plasticité des milieux granulaires : des événements localisés aux bandes de cisaillement »

Mercredi 8 juillet :

8h30 - 12h	Gergely Molnar (INSA de Lyon)	« Modélisation de la ruine des matériaux à différentes échelles »
13h30 - 17h	Guillaume Kermouche, (Mines Saint-Etienne)	« La micromécanique expérimentale ou l'art de mesurer la plasticité et la rupture des matériaux à l'échelle de quelques microns »

Les interventions sont prévues en français mais les supports sont souvent en anglais.

Comité d'organisation :
Sophie Miralles et Guilhem Mollon, Nadira Matar, Valéry Botton

L'inscription est obligatoire pour prendre les repas de midi et valider les heures.
Merci de renvoyer le bulletin d'inscription auprès de Nadira Matar par email.

Détails et bulletin d'inscription :

2-3 Juillet 2026 : Journées de rencontre de la Graduate Initiative EIF

Scolan Hélène — 2026-06-29T11:28:38Z

Les journées de la Graduate Initiative Énergie et Industrie du Futur (GI-EIF) ont lieu le 2 et 3 Juillet à la Médiathèque Paul ZECH sur le Campus Rockefeller Lyon 1.

La GI EIF finance des actions à destination des étudiants pour les former par la recherche (Stages de Master, Bourses d'Excellence, Prof Invités, Missions pédagogiques, journées d'études). Toutes les actions réalisées en 2026 sont présentées lors de ces journées. Les actions de type stages sont exposés en tandem (Stagiaire + encadrant).

Journées gratuites mais sur inscription.

Tous les membres permanents, post doctorants, doctorants et stagiaires des laboratoires sont cordialement invités à venir à ces journées en s'inscrivant sur ce lien avant mardi 30 juin soir : Lien inscription

Prix
Les prix (bons Fnac) seront décernés le vendredi en fin d'après midi par un vote sur l'ensemble des présentations.

Venez voir les présentations et voter !

Comité d'organisation :
Laurent Martin Witkowski, Marc Buffat

Fabien Depoilly

Scolan Hélène — 2026-06-18T11:34:48Z

Stabilité d'écoulement de films de fluides non newtoniens sur un plan incliné

Jury :
Silvia Hirata, Maîtresse de conférences, UML, Université de Lille, Rapporteure
Yoël Forterre, Directeur de recherche CNRS, IUSTI, Aix-Marseille Université, Rapporteur
Ivana Vinkovic, Professeure des universités, LMFA, Lyon 1 Université, Examinatrice
Gianluca Lavalle, Maître assistant , LGF, École des Mines de Saint-Étienne, Examinateur
Guillaume Chambon, Directeur de recherche , IGE, INRAÉ Lyon-Grenoble Auvergne-Rhône-Alpes, Examinateur
Hamda Ben Hadid, Professeur des universités, LMFA, Lyon 1 Université, Directeur
Séverine Millet, Maîtresse de conférences, LMFA, Lyon 1 Université, Co-encadrante
Simon Dagois—Bohy, Maître de conférences, LMFA, Lyon 1 Université Co-encadrant
François Rousset, Maître de conférences, CETHIL, INSA Lyon, Invité

Résumé :

Ce travail de thèse porte sur la stabilité des écoulements en film mince de fluides non newtoniens sur un plan incliné.
Après un panorama des comportements rhéologiques non newtoniens, un cadre mathématique général a été développé pour l'analyse de la stabilité linéaire de films de fluides newtoniens généralisés, en traitant la fonction de viscosité de manière générique.
La résolution du champ de base et l'analyse asymptotique aux ondes longues ont conduit à une expression analytique générale de la vitesse de phase des instabilités et du nombre de Reynolds critique, validée sur une large gamme de rhéologies.
Sur le plan numérique, deux méthodes complémentaires — collocation pseudo-spectrale de Tchebychev et méthode de tir — ont été mises en œuvre pour résoudre l'équation d'Orr–Sommerfeld pour les fluides newtoniens généralisés, permettant d'accéder aussi bien au spectre complet des valeurs propres qu'au mode le plus instable en stabilité temporelle et spatiale.
Une nouvelle loi de régularisation des fluides de Herschel–Bulkley, plus reliée à la physique que les lois empiriques communément rencontrées a par ailleurs été proposée.
Enfin, une campagne expérimentale dédiée aux fluides viscoplastiques a permis de confronter les prédictions numériques aux observations, confirmant la pertinence de la loi rhéologique non alignée retenue.
Ces travaux établissent le formalisme du fluide newtonien généralisé comme un cadre unificateur, ouvrant des perspectives vers des rhéologies plus complexes, des configurations géométriques variées et des régimes non linéaires.

Summary :

This doctoral work focuses on the stability of thin-film flows of non-Newtonian fluids on an inclined plane.
Following an overview of non-Newtonian rheological behaviours, a general mathematical framework was developed for the linear stability analysis of generalised Newtonian fluid films, treating the viscosity function in a generic manner.
Solving the base flow and performing a long-wave asymptotic analysis led to general analytical expressions for the phase velocity of instabilities and the critical Reynolds number, validated over a wide range of rheologies.
On the numerical side, two complementary methods — Chebyshev pseudo-spectral collocation and the shooting method — were implemented to solve the Orr–Sommerfeld equation for generalised Newtonian fluids, providing access to both the full eigenvalue spectrum and the most unstable mode in temporal and spatial stability.
A new regularisation law for Herschel–Bulkley fluids, more physically grounded than the empirical laws commonly encountered in the literature, was also proposed.
Finally, an experimental campaign dedicated to viscoplastic fluids allowed numerical predictions to be compared with observations, confirming the relevance of the chosen non-aligned rheological law.
This work establishes the generalised Newtonian fluid formalism as a unifying framework, opening perspectives towards more complex rheologies, varied geometric configurations, and nonlinear regimes.

A unifying variational framework for data-driven linear modeling

Miquel Benjamin — 2026-06-01T10:57:08Z

In contemporary engineering and scientific computing, the explosion of high-fidelity experimental and numerical data has driven a paradigm shift toward data-driven modeling.
In fluid mechanics, capturing the low-dimensional skeleton of complex, high-dimensional transport and coherent structures is paramount for physical understanding, design optimization, and flow control.
This seminar addresses a fundamental question at the intersection of data science and physics : Can we establish a singular mathematical framework that encapsulates, connects, and generalizes the diverse family of data-driven linear modeling techniques ?
We answer this in the affirmative by demonstrating that a broad spectrum of ubiquitous architectures — including Proper Orthogonal Decomposition (POD / PCA), Canonical Component Analysis (CCA), Dynamic Mode Decomposition (DMD), and classical Linear System Identification (such as N4SID, MOESP, and CVA) — can be elegantly derived as specific, parameterized instances of a single global variational principle.

The underlying problem takes the core matrix form of minimizing a weighted Frobenius norm :

$$\text{minimize} \quad \| M^{1/2} ( Y - FX ) W^{1/2} \|_F^2$$

$$\text{subject~to} \quad \textrm{rank}~F = r, \quad r \leq \min(m, n)$$

where $\mathbf{X}$ and $\mathbf{Y}$ represent the input and output data structures, while $M$ and $W$ act as custom state and sample weight (symmetric positive definite) matrices.
Crucially, though rank-constrained optimizations are inherently non-convex and typically deemed computationally intractable for large systems, we show that this specific variational formulation resolves analytically into a tractable generalized eigenvalue decomposition.
By introducing appropriate choices for $M$ and $W$, we bypass the limitations of simple truncated SVD heuristics or heavy convex relaxations (such as nuclear norm regularization) that incur prohibitive $O(mn)$ storage costs and lack explicit low-rank design guarantees.
For example, setting the mass matrix $M = I$ and equating input and output variables natively yields classical POD, whereas selecting $M = C_{yy}^{-1}$ recovers Hotelling's Canonical Component Analysis, maximizing mutual information in latent spaces rather than simple mean-squared prediction error.
We showcase this framework direct impact on high-dimensional fluid systems, including the data-driven modeling of pressure and vorticity fields in cylinder wakes, where CCA reveals highly synchronized latent space coordinates that outperform traditional multi-dimensional linear stochastic estimations.
Furthermore, we bridge this formulation to modern operator-theoretic methodologies by demonstrating how the statistical approximation of the infinite-dimensional Koopman operator from data can be framed entirely within the same Reduced-Rank Regression variational shell.
Using the chaotic Lorenz attractor as a benchmark, we highlight how the resulting generalized eigenvalues provide an optimal $R_E$-score metric that is perfectly tailored for rigorous cross-validation, functional basis selection, and predictive lobe-switching control.

Dynamics of the structures of intense vorticity in isotropic turbulence

Bos Wouter — 2026-05-23T13:38:26Z

One of the most distinctive characteristics of turbulent flows is the presence of a large range of eddy structures, defined loosely as regions of concentrated vorticity and low pressure, with a life time which is large compared with the characteristic timescale of the flow (for the biggest of these structures). The smallest existing eddies, where the most intense vorticity of the flow is concentrated, are usually called ‘worms' or intense vorticity structures (IVS). Their relevance in turbulence is connected with their relation with the viscous dissipation mechanism, internal intermittency, and with small scale mixing, notably to the so-called 'nibbling' mechanism which ultimately causes the turbulent entrainment. Recent direct numerical simulations (DNS) and an new innovative time tracking algorithm allows the time tracking of a very large number of IVS, and a detailed characterisation of their kinematic and dynamic features from ‘birth' to ‘death'. The presentation will focus on the dynamics of the ‘death' of these structures and on its possible relation with some of the characteristics of these structures. Possible implications for the ability to control the entrainment and mixing rates in turbulent flows are discussed.

Flow regimes and bifurcations in spherical Couette flows

Miquel Benjamin — 2026-05-20T15:54:17Z

Spherical Couette flow, generated by the differential rotation between concentric spheres, exhibits a wide range of complex flow instabilities and transition mechanisms relevant to geophysical and engineering applications. In this talk, numerical simulations of incompressible spherical Couette flow are presented for a narrow-gap configuration with only the inner sphere rotating. The governing Navier–Stokes equations in spherical coordinates are solved using a pseudo-spectral method. Particular emphasis is placed on the sensitivity of the flow to initial conditions and the resulting multiplicity of solution branches. Using different initial conditions, multiple branches of the bifurcation diagram are identified, each dominated by distinct instability mechanisms. Three primary branches
are observed : an axisymmetric branch, a travelling-wave instability branch, and an equatorial instability branch. The axisymmetric branch becomes unsteady at sufficiently large Reynolds numbers. The travelling-wave branch is characterized by spiral instabilities concentrated near the polar regions and exhibits an interesting reversal in the propagation direction of the spiral waves as the Reynolds number increases. At higher Reynolds numbers, this branch further develops multi-mode equatorial instabilities. The equatorial instability branch is associated with the formation of twin jet-like structures on either side of the equator, which themselves become unstable as the Reynolds number is increased. The flow topology and dynamical behaviour of these branches are further analysed in phase space. The travelling-wave branch, in particular, demonstrates
signatures of chaotic dynamics at large Reynolds numbers. The results provide insights into the nonlinear dynamics, multiplicity, and transition characteristics of spherical Couette flow in narrow-gap geometries.

Giovanni Coco

Scolan Hélène — 2026-05-07T10:19:48Z

Propagation of Shock Waves Generated by un Unducted Single Fan : Acoustic Signature on the Fuselage

Jury :
M. Benjamin Cotté, Maître de conférences HDR, ENSTA Paris, Rapporteur
M. Francesco Avallone, Professeur, Politecnico di Torino, Rapporteur
M. Régis Marchiano, Professeur, Sorbonne Université, Examinateur
M. Christophe Bailly, Professeur, École Centrale de Lyon, Directeur de thèse
M. Didier Dragna, Maître de conférences HDR, École Centrale de Lyon, Co-encadrant
Mme Hélène de Laborderie, Ingénieure experte, Safran Aircraft Engines, Co-encadrante

Summary :

The transition toward sustainable aviation has intensified interest in unducted aero-engines as a means of achieving substantial reductions in fuel consumption. The absence of a conventional engine nacelle in such configurations, however, removes a primary location for acoustic treatment and allows non-linear acoustic fluctuations generated at blade tips to propagate directly toward the aircraft fuselage. Existing aeroacoustic prediction methods often rely on linear acoustic analogies that neglect non-linear propagation effects or require computationally expensive body-fitted meshes to account for installation phenomena.
The objective of this dissertation is to develop and validate a high-fidelity numerical framework for predicting the acoustic signature induced by an unducted single fan engine on the fuselage, with particular emphasis on non-linear propagation and installation effects. The work addresses key methodological challenges related to robust CFD/CAA coupling and the efficient representation of rigid boundaries within Cartesian computational domains.
The proposed framework is based on a three-dimensional non-linear Euler solver employing high-order, low-dispersion numerical schemes combined with a dedicated shock-capturing strategy. A major contribution of this thesis is the development of a volumetric source method that enables the injection of prescribed acoustic fields from analytical solutions or independent flow simulations without generating spurious reflections. In addition, a novel reflection approach is introduced to model the fuselage as a perfectly rigid obstacle through forcing terms in the momentum equations, enabling accurate boundary treatment on uniform Cartesian grids.
Application of the developed tools provides new physical insight into the mechanisms governing fuselage noise. The results indicate that non-linear propagation effects lead to a limited reduction in sound pressure levels for upstream-propagating waves, whereas refraction by the fuselage boundary layer represents the dominant factor shaping the acoustic signature. Parametric analyses further show that shielding effectiveness increases monotonically with boundary layer thickness and that laminar velocity profiles yield substantially higher attenuation than turbulent profiles.
Overall, the findings suggest that conventional linear approaches may overestimate forward-fuselage noise levels and demonstrate the relevance of boundary layer characterization in installed noise prediction. The numerical framework established in this work offers an efficient and reliable tool for the aeroacoustic analysis and preliminary design of future unducted propulsion systems.