**09/07/2012, Pino Martin**

Invité : Pino Martin,Associate Professor CRoCCo Laboratory Director Department of Aerospace Engineering, University of Maryland 01/06/2012 – 30/06/2012

Action 1.3 " Contrôle". Responsable : U. Ehrenstein

Title : Description of wall turbulence from DNS data

Abstract For over more than a decade, we have validated a numerical capability that allows for exact 3D, time-varying solutions of turbulence from first principles, using direct numerical simulations (DNS). The accuracy of the data sets has been validated against experiments at matching conditions. The increased computational resources and prolific algorithm development have revolutionized our research, and we are generating numerical data sets with unprecedented scope and accuracy. Our main efforts today are based on data-intensive science that is revolutionizing our understanding of the physics underlying the vast volumes of data, which is massive, complex stochastically and chaotic, and includes multi-scale dynamics, and long time behavior.

In this talk, I will present a sample problem, the interaction of a shockwave and a turbulent boundary layer, where spatially and temporally resolved data was indispensable to unveil the physical mechanisms at play. Then, I will show how we are using statistical descriptions of the data to validate (1) discrete definitions for the hierarchy of structures in wall bounded turbulence and (2) the resulting data interpretations, and how in turn we are using these to develop a catalog of feature-activity and feature‐evolutions to semantically characterize data across time and space.

M. Pino Martín has been an Associate Professor of Aerospace Engineering in the University of Maryland since 2009. She is also a Senior Research Scholar in the Mechanical and Aerospace Engineering Department in Princeton University and Affiliate Faculty in the University of Mryland Institute for Computer Studies. Her research interests include interdisciplinary approaches to the study and control of turbulent flow using joined theoretical, numerical and experimental studies. She is the recipient of an NSF CAREER award in 2003 and the 2007 Alfred Rheinstein’11 Princeton University Faculty Award for “excellence in her chosen research field”, and a Tewkesbury Merit Scholar award from the University of Melbourne in Australia. Prior to joining the University of Maryland, she was an assistant Professor in the Mechanical and Aerospace Engineering Department in Princeton University (2001‐2009) and a Postdoctoral Research Fellow at the Center for Turbulence Research in Stanford University (2001) and the University of Minnesota (2000), after receiving a B.A. from Boston University (Summa cum laude 1994) and Ph.D. in Aerospace Engineering (1999) from the University of Minnesota under the guidance of Graham Candler.

**23/05/2013 et 28/05/2013, Samy Missoum**

Invité : Samy Missoum, Aerospace and Mechanical Engineering Departement, University of Arizona 05/05/2013 – 14/06/2013

Action 1.2 " Dynamique non linéaire en acoustique et vibrations ". Volet "Vibrations non-linéaires et instruments de musique" Responsable : J. Kergomard

Titre : Introduction aux Techniques Modernes de Conception Optimale

Résumé : Les techniques de conception optimale permettent de trouver efficacement les meilleures solutions dans des espaces de conception parfois très larges et complexes. Elles sont maintenant d’utilisation courantes tant dans le monde académique que dans l’industrie. Ce cours présentera les notions fondamentales et méthodes classiques d'optimisation telles que la programmation non-linéaire. Les méthodes de calcul de sensibilité, souvent essentielles à l’optimisation, seront aussi présentées. Par ailleurs, les méthodes d'approximation basées sur les surfaces de réponse et les méta-modèles, importantes pour les problèmes gourmands en temps de calculs, seront décrites. Le cours discutera les différentes formes d'optimisation telles que l'optimisation de forme ou l'optimisation topologique ainsi que leur limitations actuelles. Enfin, une partie du cours sera dédiée à la prise en compte des incertitudes en conception optimale.

**25/06/2014 et 26/06/2014, Alexander Petrov et Vladimir Vanovsky**

Invité : Pr. Alexander Petrov, Moscow Institute of Physics and Technology 15/06/2014 – 05/07/2014

Invité : M.Vladimir Vanosky, Moscow Institute of Physics and Technology 15/06/2014 – 05/07/2014

A. Petrov et V. Vanovsky, Moscow Institute of Physics and Technology and Institute for Problems in Mechanics RAS

Action 2.3 "Hétérogénéité, Multi-échelles, changement d'échelles". Volet "Modélisation des systèmes hétérogènes" Responsable : R. Saurel

Seminaire 1 , 25 juin 2014, a 14h

Title : The Mystery of the Foucault Pendulum, the Spring Pendulum and other Hamiltonian Mechanics Problems

In famous Foucault experiment the Earth rotation was observed by the precession of the pendulum. However, the obtained value of the Earth rotation period was incorrect or even of the wrong sign for small pendulum lengths. We proved that the error was related to the pendulum nonlinearity and was unavoidable in the classical approach.

When a swinging spring oscillates, sometimes vertical spring oscillations are transformed to horizontal ones and back. The effect is observed only for specific pendulum lengths (when the condition of resonance is met). The plane of horizontal oscillations is changed by an arbitrary angle but in most cases the angle is near 90°.

In our talk we will solve these and other puzzles and propose a new approach for dealing with many challenging problems in Hamiltonian mechanics (for example covered by Mathieu equation) by using a very effective technique of Hamiltonian symmetrization.

Seminaire 2 , 26 juin 2014 a 11 h

Title : The Spring Analogy to the Resonant Mechanism of Gas Bubble Break-up

In this talk we will discuss in details the technique of Hamiltonian symmetrization. This new very efficient technique will be applied to the problem of swinging spring. The resonance condition and resonant energy transfer will be obtained.

Then the bubble break-up problem will be discussed. One of the possible mechanisms of bubble break-up is the one of the shape instability. The resonance 2:1 of radial and arbitrary deformational oscillation modes will be examined. The problem proved to be fully similar to that of a swinging spring. The full analytical solution will be found. The exact expression for the energy transfer period will be obtained. The resonant energy transfer and a huge growth of deformational mode amplitude determined by higher order Legendre polynomials could be an explanation of the bubble break-up.

**24/07/2014, 25/07/2014, Matthieu Wyart**

Invité : Matthieu Wyart, Assistant Professor, Department of Physics, New-York University 08/07/2014 – 07/08/2014

Action 3.3 "Milieux divisés". Volet "Milieux granulaires" Responsable : Y. Forterre

Dans le cadre du LABEX MEC, nous avons l’immense plaisir d’accueillir pendant un mois à marseille Matthieu Wyart du laboratoire de Physique de l’Université de New York. Matthieu Wyart est un éminent spécialiste des milieux amorphes proche du jamming. Il donnera deux cours ces jeudi 24 et vendredi 25 juillet à 11h à l’IUSTI en salle de séminaire (250). Les résumés sont joints ci-après:

These two talks propose two distinct approaches toward dense flows in driven amorphous solids, distinguishing the “granular” case where particles are hard and cannot load elastic energy (talk 1), from soft systems where elastic loading is possible and is interrupted by sudden plastic events (talk 2).

Talk 1: The memory of sand

(Jeudi 24 juillet, 11h, à l’IUSTI, salle 250)

Complex systems are characterized by an abundance ofmeta-stable states. To describe such systems statistically, one mustunderstand how states are sampled,a difficult task in general when thermal equilibrium does not apply.This problem arises in various fields of science, and here I willfocus on a simple example, granular materials.These materials can flow until one jammed configuration (among theexponentially many possible ones) is reached. I will argue thatin simple models of granular materials, these dynamically-accessible configurations are atypical, implyingthatin its solid phase the material "remembers" that it was flowing just beforeit jammed. As a consequence, it is stable, but barely so. Such marginal stability offers a new perspective on the solid and liquid phase, in particular on dense suspension flows near jamming.

Talk 2: Plastic flow in soft amorphous solids

(Vendredi 25 juillet, 11h, à l’IUSTI, salle 250)

Yield stress materials flow if a sufficiently large shear stress is applied. Although such materials are ubiquitous and relevant for industry, there is no accepted microscopic description of how they yield, even in the simplest situations where temperature is negligible and where flow inhomogeneities such as shear bands or fractures are absent. Here I will discuss a scaling description of the yielding transition in amorphous solids made of soft particles at zero temperature. Our description makes a connection between the Herschel-Bulkley exponent characterizing the singularity of the flow curve near the yield stress Σc, the statistics and length scale of avalanches of plasticity at Σc, and the density P(x) of soft spots, or shear transformation zones, as a function of the stress increment x beyond which they yield. We propose three scaling relations, and argue that all exponents characterizing these properties can be expressed in terms of three exponents θ, dfand z, characterizing respectively the density of soft spots, the fractal dimension of the avalanches, and their duration. Our description shares some similarity with the depinning transition that occurs when an elastic manifold is driven through a random potential, but also presents some striking differences. I will discuss test our these relations in an elasto-plastic model, an automaton model similar to those used in depinning, but with a different interaction kernel.

**05/09/2014, Daniel Lecoanet**

Invité : Daniel Lecoanet, PhD, Physics and Astrophysics, UC Berkeley, 31/08/2014-13/09/2014

Action 3.3 "Milieux divisés". Volet "Instabilités et structures dans les disques d'accrétion" Responsable : M. Le Bars

Daniel Lecoanet - vendredi 5 septembre 2014 - 11:00 IRPHE UC Berkeley

Title : Simulations of Convective Excitation of Internal Waves in Water

Convectively excited internal waves play an important role in geophysical and astrophysical fluid dynamics : they are thought to drive the quasi-biennial oscillation (QBO), and are an important angular momentum transfer mechanism in stellar interiors. I will present a simulation of internal wave excitation in water, exploiting water’s density maximum at 4C. Next, I will describe ``simulations of the simulation,’’ where I use data from the full simulation to test two heuristic mechanisms of wave generation, the mechanical oscillator effect, and the deep excitation mechanism. The results suggest the deep excitation mechanism is a very good description of the wave excitation process.

**01/10/2014, Jens N. Sorensen**

Invité : Jens Norkaer Sorensen, Department of Wind Energy, Technical University of Denmark, 22/09/2014 – 06/10/2014

Action 1.1 " Aérodynamique ". Volet "Sillage de rotors" Responsable Th. Leweke

Jens N. Sorensen- mercredi 1er octobre 2014 - 11:00

Department of Wind Energy, Technical University of Denmark, Copenhague/Lyngby, Danemark

Title : Fluid mechanical challenges in wind energy

The modern development of wind power is a remarkable story of the combined effort of enthusiastic entrepreneurs and skilled engineers and scientists. Today, wind power forms the most rapid advancing renewable energy resource with an annual growth rate of about 30%. Within the last 20 years the size of wind turbines have increased from a rotor diameter of about 30 m to 150 m, corresponding to an increase in power by a factor of more than 25. In the same period of time, the knowledge and scientific level of the aerodynamic research tools to develop optimally loaded rotor blades have increased dramatically. Today, wind turbine aerodynamics forms one of the research frontiers in modern aerodynamics. The aerodynamics of wind turbines concerns modelling and prediction of the aerodynamic forces on the solid structures of a wind turbine, and in particular on the rotor blades of the turbine. Aerodynamics is the most central discipline for predicting performance and loadings on wind turbines, and it is a prerequisite for design, development and optimization of wind turbines. From an outsider’s point of view, aerodynamics of wind turbines may seem simple as compared to aerodynamics of e.g. fixed-wing aircraft or helicopters. However, there are several added complexities. Most prominently, aerodynamic stall is always avoided for aircraft, whereas it is an intrinsic part of the wind turbines operational envelope. Furthermore, wind turbines are subjected to atmospheric turbulence, wind shear from the atmospheric boundary layer, wind directions that change both in time and in space, and effects from the wake of neighbouring wind turbines. In the past year the development in numerical and experimental techniques dealing with rotor and wake aerodynamics of wind turbines has developed dramatically, and with appropriate simplifications it is possible to perform advanced simulations of single wakes of wind turbines as well as simulations and performance predictions of full wind farms. In the presentation I will give a review of the state-of-the-art of fluid mechanical aspects of wind energy, focusing mainly on wake aerodynamics and the fluid mechanics of wind farms, and show some examples from own research activities. Sørensen, J.N. (2011) “Aerodynamic Aspects of Wind Energy Conversion”. Annual Review of Fluid Mechanics, vol. 43, pp. 427-448.

**15/04/2015 et 21/04/2015, Stefan Bilbao**

Invité : Stefan Bilbao, Université d'Edimbourg, 22/09/2014 – 06/10/2014

Action 1.2 " Dynamique non linéaire en acoustique et vibrations ". Volet "Vibrations non-linéaires et instruments de musique" Responsable : Ch. Vergez

**15/04/2015**, 12h30 à l'ECM à destination des élèves 1A et 2A.

Title : NESS : Synthèse Sonore par Modèles Physiques à Grande Échelle - Stefan Bilbao, Université d'Edimburgh

Résumé :NESS (Next Generation Sound Synthesis), qui se déroule actuellement à l’Université d’Edimbourg, est un projet ERC qui concerne la synthèsesonore par modèles physiques. En particulier, on s'intéresse à la synthèse à grande échelle - c’est-à-dire pour des systèmes pour lesquels unimportant effort est inévitable du point de vue de coût de calcul. Des exemples de tels systèmes sont : les vibrations à grande amplitude (etdonc fortement non linéaires) des structures minces, que l’on trouve dans plusieurs instruments de percussion ; la modélisation détaillée del’interaction de collision pour des instruments à corde et la caisse claire ; des cuivres modélisés comme des réseaux de tuyaux acoustiques,avec des positions de pistons variables au cours du jeu ; des réseaux modulaires d’éléments canoniques ; et, surtout, la modélisation en 3D, soitpour simuler les effets de réverbération dans les grandes salles, soit pour effectuer de la synthèse spatiale sur plusieurs canaux audio.

Plusieurs enjeux et questions seront abordés, au niveau du choix de modèle, de la construction des schémas numériques, et de laparallélisation sur GPU.

Plusieurs exemples sonores et visualisations seront présentés, accompagnés par un extrait d’une pièce synthétiquerécemment générée sur canaux multiples avec le système GPU à Edimbourg.

**21/04/2015**, 11h, au LMA

Title : The Changing Picture of Nonlinearity in Musical Instruments: Modeling and Simulation

Abstract:The standard model of the functioning of a musical instrument, namely that of a linear resonator (such as a string, acoustic tube, bar, membrane or plate) subject to a nonlinear excitation mechanism (such as a hammer, bow, or lip/reed) is a powerful one indeed. Such a formalism, through the application of the apparatus of linear system theory to the resonator, leads not only to ease in terms of analysis of the instrument as a whole, but also to efficient simulation designs, where the resonator may be modelled in terms of modes, or travelling waves, or in an input- output sense. In the past 20 or so years a more detailed picture of the resonator has emerged, involving nonlinear refinements to the model of the resonator, and leading to various perceptual effects: pitch glides in strings and membranes, phantom partial generation in strings, crashes in plate/shell based percussion instruments, brassiness in high-amplitude brass instrument playing, and finally, effects due to distributed collision as in the case of the sitar and snare drum. The range of phenomena to be investigated is thus greatly increased—and yet simulation becomes a much more challenging undertaking. In this talk, unifying features of (and distinctions among) such nonlinearities are discussed, particularly with regard to the problem of simulation, in the context of both model validation and physically-based sound synthesis.

**18/06/2015 et 23/06/2015, Jonathan Aurnou**

Invité : Jonathan Aurnou, Department of Earth, Planetary, and Space Sciences, UCLA, 05/2015 – 06/2015

Action 3.3.2 " Milieux divisés". Volet "Instabilités et structrues dans les disques d'accrétion". Responsable : M. Le Bars

**18/06/2015, 11h-12h, salle de séminaire d'IRPHE**

Title : Turbulent Core Weather: Flux Patchy with a Chance of Reversals

Abstract: The geomagnetic field is powered by turbulent flow in the Earth’s liquid metal outer core. In this seminar, I will first present the essential observations of the geomagnetic field. Then different scenarios will be presented on how best to explain the observations. Lastly, I will evaluate these scenarios based upon our present understanding of rapidly-rotating convective turbulence, and discuss how best to move forward in generating next-generation models of planetary core dynamics.

**23/06/2015, 11h-12h, salle de séminaire d'IRPHE**

Title : Laboratory Models of Planetary Jet Formation

Abstract: Strong east-west directed, banded jets have long been observed in the atmospheres of the gas planets. However, it is still unknown how they are driven and to what depth they extend down into the planets. Are they driven by insolation or from internal heat of planetary formation? Are they relatively shallow atmospheric structures or do they extend down to Mbar depths? In this talk, I will present end-member models of planetary jet formation and then discuss a unique experiment being developed at IRPHE that seeks to generate, for the first time in the laboratory, strong zonal jet flows comparable to those found on the gas planets.

**04/11/2015, Yuji Hattori**

Guest : Pr. Yuji Hattori, Insitute of Fluid Science, Tohoku University, Sendai, Japan, 15/10/2015-15/11/2015

Action 1.1 " Aérodynamique ". Volet "Sillage de rotors" Responsable Th. Leweke

Conference, 04 november 2015, 11-12h, IRPHE

Title : Concentration of Vorticity in a Destabilized Vortex due to Selective Decay

Formation of concentrated vortices like tornadoes and tropical cyclones in rotating fluids is of much interest in atmospheric flows. It is shown by direct numerical simulation that selective decay of inviscid invariants leads to concentration of vorticity in a destabilized vortex. Here by selective decay we mean that circulation of the mean flow decays much faster than the angular momentum or energy. Initially localized disturbances are super-imposed onto the two-dimensional flattened Taylor-Green vortices to trigger the elliptic instability. In the later stage of nonlinear evolution of the disturbance circulation decays much faster than angular momentum and energy, giving rise to a sharp peak in the vorticity distribution of the mean flow. The selective decay is due to reconnection and the subsequent annihilation of vortex pairs at the cell boundaries as shown by a simple model. Relevance of the mechanism to previous experiments and general cases is also discussed.

**05/11/2015, Alexander Chesnokov**

Guest : Pr. Alexander Petrov, Novosibirsk State University, 01/11/2015-26/11/2015

Action 2.3 "Hétérogénéité, Multi-échelles, changement d'échelles". Volet "Modélisation des systèmes hétérogènes" Responsable : R. Saurel

Conference, 05 november 2015, 11-12h and 14-15h, salle 250, IUSTI

Title : Shallow water theory for shear flows: stability and generalized hyperbolicity

I recall the main concepts and theorems of the linear theory of hydrodynamic stability for plane parallel flows of an inviscid fluid. These results are also applicable for shear flows within the framework of long wave approximation. Next, I formulate a nonlinear integro-differential model describing shallow shear flows with a free boundary (Benney equations) and give different mathematical formulations of the model (Eulerian, semi-Lagrangian and Vlasov-like). Simplified models (flows with linear velocity profile, weakly shear flows) are also considered.

Theoretical analysis of the hydrodynamic models is based on concept of hyperbolicity of equations with operator coefficients (V. M. Teshukov). I apply this approach to the Benney equations and formulate hyperbolicity conditions for the model. These conditions are necessary for well-posedness of the Cauchy problem. Finally, I show that the conditions of hyperbolicity of correspond to the stability criteria of shear flows.

**01-02/06/2016, Daniel Lecoanet**

Invité : Daniel Lecoanet, PhD, Physics and Astrophysics, UC Berkeley, 29/05/2016 - 08/08/2016

Action 3.3 "Milieux divisés". Volet "Instabilités et structures dans les disques d'accrétion" Responsable : M. Le Bars

Daniel Lecoanet - mercredi 1 juin 2016 - 11:00 IRPHE ; jeudi 2 juin 2016 - 11:00 IRPHE

An introduction to Dedalus

UC-Berkeley, USA

Dedalus is an open-source, python-based framework for solving nearly arbitrary partial differential equations (PDEs) using spectral methods. In two interactive lectures, I will describe and demonstrate the capabilities and limitations of Dedalus and spectral methods more generally. The participants will learn how to use Dedalus to solve PDEs important in their research. No prior knowledge of python or scientific computing is necessary.

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