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2012

Eric Goncalves, LEGI, grenoble (28/03/2012)

(28/03/2012)
Modélisation et simulation de la cavitation

Le phénomène de cavitation dans les machines hydrauliques
est à l'origine de problèmes de chute de performance, de vibration et 
d'instabilités de fonctionnement. Le caractère diphasique, turbulent, 
compressible, instationnaire des écoulements cavitants, rend ardu 
l'étude numérique (et expérimentale) et la mise en place de codes de 
calculs efficaces.
La premiere partie de la présentation sera dédiée aux aspects modèles
(lois d'état, modèles de cavitation et modèles de turbulence)
et difficultés associées. Nous présenterons aussi la problématique liée
aux méthodes numériques pour des écoulements mixte 
compressible/incompressible.
Dans une seconde partie, nous présenterons des résultats de simulations
et retour d'experiences numériques: tube à détente 1D non visqueux avec 
cavitation, écoulements 2D turbulents en eau et fréon.
En savoir plus

Takahiro Tsukahara, Tokyo University of Science (29/02/2012)

(29/02/2012)
On the transition to/from turbulence in channel flows.

In this talk, we present a series of DNSs conducted in two types of
plane channel flows, i.e., plane Poiseuille flow and plane Couette
flows, considering the subcritical-transition regime with the use of
large aspect-ratio computational domains. Both flows give rise to
coexisting laminar and turbulent equilibrium regions in the form of
oblique stripes, so-called the turbulent stripe. In addition, through
the long-term tracking of an isolated turbulent spot, we confirmed
that the spot developed into the
turbulent stripes in the plane Poiseuille flow at transitional
Reynolds numbers. We have also found that, when subjected to
stabilizing rotation, the Couette turbulence is locally quenched and
exhibits the stripe pattern. In this context, the turbulence stripe
can be seen as an intrinsic phenomenon for the transition and the
reverse transition in channel flows.
En savoir plus

2011

Séminaire commun LAMSID-UME: thésards (23/06/2011)

(23/06/2011)

9h30-10h00 : Albert Alarcon Utilisation des méthodes d'assimilation de données et d'actualisation bayésienne pour l'identification et le diagnostic vibratoire.
10h-10h15 Assaf Ghanem Extension arlequin à la dynamique machines tournantes
10h15-10h45 Gilles Serre: Construction de modèles réduits numériques pour les problèmes aéroacoustiques et aéroélastiques linéarisés
11h00-11h30 CALEYRON Fabien Simulation numérique par la méthode SPH de fuites de fluide consécutives à la déchirure dans un réservoir sous impact
11h30-12h00 DESBONNETS Quentin Interaction Fluide Structure dans les réacteurs RNR-Na
12h00-12h15 MARCEL Thibaud Physique et modélisation de l'interaction fluide-structure dans un faisceau de tubes
12h15-12h45 Vladimir PAREZANOVIC : Experimental study of the sensitivity of a bluff body wake

En savoir plus

Benoît Theckes - LADHYX (25/05/2011)

(25/05/2011)

Damping by branching in slender structures: a bioinspiration from trees.

 Man-made slender structures are known to be sensitive to high levels of vibration, due to their flexibility, which often cause irreversible damages. Trees repeatedly endure large amplitudes of motion, mostly caused by strong climatic events, yet with minor or no damages in most cases.

A new damping mechanism inspired from the architectures of trees is here identified and charaterised in the simplest tree-like structure, a Y-shape branched structure. Through analytical and numerical analyses of a simple two-degree-of-freedom model, branching is shown to be the key ingredient in this protective mechanism.

This damping-by-branching originates in the geometrical nonlinearities and is therefore specifically efficient to damp out large amplitudes of motion. A more realistic model of the Y-shape, using beam finite-element approximation, shows that this mechanism is robust.

Finally, two bioinspired architectures are analysed, showing significant levels of damping, typically 4%, brought via branching. This concept of damping-by-branching is of simple practical use in the design of slender flexible structures.

 1) de Langre, E. Effects of wind on plants Annu. Rev. Fluid Mech., Annual Reviews, 2008, 40, 141-168

2) Spatz, H.; Bruchert, F. & Pfisterer, J. Multiple resonance damping or how do trees escape dangerously large oscillations? American Journal of Botany, Botanical Soc America, 2007, 94, 1603

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Philippe Meliga - Ecole Polytechnique Fédérale de Lausanne (18/05/2011)

(18/05/2011)

An asymptotic framework for the control of separated flows
Flow separation is a widespread phenomenon that affects a variety of flows of practical interest. Its
occurrence is usually detrimental to a satisfactory operation, for instance in the case of automotive
vehicles, separation yields an increase in drag, and thereby an increase in fuel consumption.
Alleviation and control is thus required to improve performances and reliability.
There exist various tractable strategies to prevent flow separation, including surface discharges or
wall blowing and suction. I will focus on open-loop methods, which rely on the basic idea that the
dominant dynamical processes can be altered by a prescribed modification of the flow conditions.
This yields simple practical implementation, the challenge being to choose appropriate actuators
and control laws enabling to achieve a given objective (e.g. decrease drag). Quite often, the choice
results from a simple trial & error process. This makes the design procedure extremely timeconsuming
in case the number of degrees of freedom is large, since the effectiveness of the control
must be estimated for each individual setting from either experimental measurements or numerical
simulations.
The presentation will be devoted to a systematic optimization framework combining asymptotic
analyses and adjoint-based receptivity methods. This technique applies to flows where the
detrimental effects of separation can be related to an instability process. Namely, it aims at
computing analytically the receptivity of the dominant instability mode with respect to the control
variables, which requires to undertake an asymptotic expansion of the governing equations close the
instability threshold. In return, one can quantify the effect of the control beforehand, in terms of the
linear growth rate of the flow disturbances, but also in terms of their nonlinear saturation amplitude.
 

En savoir plus

S. Arzoumanian - University of Cambridge (04/04/2011)

(04/04/2011)

Structural Acoustics with Mean Flow: Over-Scattering & Unstable Resonance

X. Chen - ENSTA UME (30/03/2011)

(30/03/2011)

Modélisation des matériaux à mémoire de forme ferromagnétique - Modèle en 1D

O. Doaré - F. Baj - P. Moussou (07/03/2011)

(07/03/2011)

Séminaire commun UME-LAMSID

Thème: interractions fluides-structures.

Sébastien Neukirch - Institut Jean le Rond d'Alembert (02/03/2011)

(02/03/2011)

Sauts de fréquences dans les vibrations d'une poutre encastrée.

Mickaël Bourgoin - Laboratoire des écoulements géophysique et industriels (02/02/2011)

(02/02/2011)

Particules inertielles en écoulements turbulents