Fluid Mechanics and Acoustics Laboratory - UMR 5509

TBA

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

TBA

Bos Wouter — 2026-05-23T13:38:26Z

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.

Unexpected Crossover from Weak to Strong Magnetohydrodynamic Turbulence

Miquel Benjamin — 2026-04-20T14:27:04Z

The quest for a consistent theory of magnetohydrodynamic (MHD) turbulence has remained one of the most enduring challenges in physics. In this talk, I will present a universal crossover between weak and strong turbulence regimes. We demonstrate that spectral scaling is fundamentally governed by a competition between the external and local magnetic fields. We show that large mean magnetic field yields Iroshnikov–Kraichnan -3/2 scaling due to weak modulation of Alfvén waves, but moderate and weak magnetic fields produce Kolmogorov's -5/3 scaling driven by strong modulation from the local magnetic field. I connect the above result to the long-sought physical explanation for the evolution of the solar wind spectral index and corona heating in the quiet and active zones.

Turbulence in vegetated environments : numerical modelling and field measurement

Miquel Benjamin — 2026-04-13T06:35:38Z

Aquatic vegetation plays a crucial role in flow hydrodynamics, natural habitats, and morphodynamic evolution, all of which are strongly influenced by the turbulence generated through plant–flow interactions. Understanding the processes controlling turbulence in such environments, and developing predictive models, is therefore of major importance. In this talk, I will focus on rigid vegetation typical of mangrove environments. After reviewing the existing semi-empirical laboratory-based formulations to predict turbulence intensity and bed shear stress in vegetated flows, I will present both numerical and field-measurement results.

First, 3D Large Eddy Simulations (LES) are used to investigate the flow structure and the mechanisms governing the production and evolution of vegetation-induced turbulence, for different Reynolds numbers and a wide range of vegetation densities. Based on these high-resolution simulations, a reduced vegetation-averaged model is developed, together with a new turbulence closure accounting explicitly for vegetation-generated turbulence.

In a second part, I will present results from a field campaign conducted in a mangrove coastal zone in New-Zealand, where turbulence measurements were collected. Acoustic instruments provided high-resolution velocity measurements in a heterogeneous field environment over six tidal cycles. Although the measurements are generally consistent with laboratory-derived formulations, they also highlight the need to better account for and parameterize vegetation heterogeneity in natural environments.

Turbulence géostrophique : une version laboratoire des atmosphères planétaires

Scolan Hélène — 2026-04-03T12:27:21Z

Les travaux de Shanshan Ding et al dans le groupe de Peter Read au laboratoire Atmospheric, Oceanic and Planetary Physics à l'Université d'Oxford en collaboration avec Hélène Scolan (LMFA, anciennement en postdoctorat dans le groupe ) et récemment publiés dans le journal Physical Review Letters ont été sélectionnés et mis en avant sous la forme d'un Focus A Lab Version of Planetary Atmospheres et par Physics Today.

Extrait de l'article Ding et al: Schéma de l'expérience de l'anneau différentiellement chauffé avec forcage thermique localisé. Mesures de la vorticité verticale relative instantanée et spectres d'énergie cinétique.

L'article intitulé Stratification-Dependent Enstrophy-Controlled Regime in Geostrophic Turbulence s'intéresse en effet à reproduire les principales caractéristiques de la turbulence atmosphérique à l'aide d'une nouvelle expérience composée d'une cuve tournante d'un mètre de diamètre et d'un forcage thermique localisé (A rotating annulus driven by localized convective forcing: a new atmosphere-like experiment ). Le fluide (mélange eau-glycérol) contenu dans la cuve est en effet chauffé au fond, près de la paroi extérieure, reproduisant ainsi le réchauffement de l'atmosphère par la lumière du soleil frappant le sol près de l'équateur. Dans le même temps, le fluide est refroidi en haut du réservoir, près de la paroi intérieure, reproduisant ainsi le refroidissement radiatif de l'atmosphère aux pôles. Le processus de convection qui en résulte au sein du fluide constitue la principale source d'énergie à l'origine de la turbulence.

Les mesures de vitesse au sein du fluide et les spectres d'énergie cinétique horizontale ont permis l'étude des cascades d'énergie et de vorticité. A faible nombre d'onde horizontal k , les spectres d'énergie cinétique présentent une loi d'échelle en k^-3 avec une amplitude spectrale qui dépend de N^2 où N est la fréquence de Brunt-Väisälä (caractéristique de la stratification en température). L'état turbulent observé présente une cascade directe d'enstrophie à toutes les échelles, ainsi qu'un transfert d'énergie bidirectionnel, mis en évidence par un renversement du signe du flux d'énergie spectral à une échelle proportionnelle au rayon de Rossby interne de déformation.

Ces résultats soulignent le rôle de l'instabilité barocline dans la formation de la distribution d'énergie à travers les échelles, avec des implications pour les écoulements turbulents à l'échelle synoptique près de la tropopause terrestre.

Lire en ligne :
Scolan, H., Read, P.L. A rotating annulus driven by localized convective forcing: a new atmosphere-like experiment. Exp Fluids 58, 75 (2017). doi:10.1007/s00348-017-2347-5
Focus: A Lab Version of Planetary Atmospheres, March 20, 2026 by M.Schirber - Physics 19, 40 doi:10.1103/Physics.19.40
Ding, S., Bobas,H, Scolan, H, Young,R.,and Read, P. - Stratification-Dependent Enstrophy-Controlled Regime in Geostrophic Turbulence, - Phys. Rev. Lett. 136, 114101 (2026) doi:10.1103/n2nj-dg5r
Article Physics today doi:10.1063/pt.8862a11b47

Interactions of ocean currents with the seafloor, energy dissipation and mixing

Miquel Benjamin — 2026-04-02T18:42:55Z

The oceanic circulation is primarily forced at the surface through momentum and heat fluxes with the atmosphere. In the ocean interior, small-scale, irreversible, turbulent mixing through density surfaces, called diapycnal mixing, is key to shape the large-scale circulation. Globally, the largest share of diapycnal mixing is fuelled by the frictional interactions between currents and the seafloor topography. Specifically, tidal currents interact with the seafloor topography and generate internal waves at tidal frequency called internal tides. Internal tides can propagate over long distances, interact with the background low-frequency circulation and seafloor slopes, and ultimately cascade down their energy to dissipative scales. Although the geography of internal tide generation is well known, the details of how and where the waves dissipate their energy remains poorly constrained, undersampled, and suffer from incomplete representation in climate models, with important consequences on the regulation of vertical fluxes of tracers relevant for climate and the biosphere, such as carbon and nutrients. In this presentation, I will first draw a general state of the art of the problem of turbulent energy dissipation in the ocean. Then I will present how turbulent kinetic energy dissipation can be measured in situ and how we try to link to larger-scale processes.

Lien visio

RHAPSODI: A Ring of HAPS for active noise control of turbofan noise

Miquel Benjamin — 2026-04-02T18:29:29Z

Active noise control (ANC) is well established through widely successful products such as active noise-cancelling headphones. However, multichannel active control remains an open research area, including the active control of turbofan noise within engine nacelles. The RHAPSODI project was initially motivated by the challenge of actively attenuating the intense tonal noise radiated by the fan. The main challenges are related to deploying a large number of secondary sources and microphones in a harsh environment. The originality of the proposed approach lies in the use of Harmonic Acoustic Pneumatic Sources (HAPS), capable of generating high acoustic levels.

The first part of the presentation reviews the historical development of compressed-air loudspeakers, from the auxetophone to HAPS. It covers both theoretical aspects and experimental results, spanning frequencies from low frequencies (below 100 Hz) to ultrasound (above 10 kHz), in both no-flow conditions and in flows up to Mach 0.5.
The second part addresses active control using multiple HAPS. Each HAPS must be controlled in both amplitude and phase, despite its relatively slow dynamic response. The signals from error microphones located near the sources must be compensated for near-field effects. Experimental results in tonal active control demonstrate noise reductions on the order of 20 dB SPL, achieved using a ring of six HAPS without flow (130 dB SWL), and with three HAPS in an infinite duct with flow (Mach 0.5, up to 144 dB).

The final part discusses future perspectives of this work. The control strategy developed within RHAPSODI can be extended to broadband noise, as demonstrated by its recent application to active double-glazing systems using electrodynamic loudspeakers. Ongoing work aims to accelerate the dynamic response of HAPS to enable subband control. Future directions include extending these concepts toward active control of sound directivity in free field.

Ce séminaire est labellisé par l'Ecole Doctorale MEGA.

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TBA

Bos Wouter — 2026-03-31T14:58:31Z

Lien vers la salle de visioconference Séminaire LMFA