Séminaires à venir

Séminaires du LadHyX
(10h45-Bât. 67-Bibliothèque)
Séminaires Mécanique et Systèmes Vivants
(Bât. Turing)

Lieu : Bibliothèque LadHyX, 10h45

Résumé :

In the last decade, it has been shown that microsystems based on Surface Acoustic Waves (SAWs), i.e. waves propagating at the surface of a solid medium, are versatile tools to manipulate fluids at small scales for microfluidic and biological applications. A non-exhaustive list of operations which can be performed with SAWs includes sessile droplet displacement, atomization, division, and merging but also the actuation of fluids embedded in microchannels or the collective manipulation and sorting of suspended particles and cells. These operations are achieved with simple plane or focalized surface waves and are based on two nonlinear effects that enable to exert net forces on fluid and particles: the acoustic streaming and the acoustic radiation pressure. Nevertheless, there are still some operations which cannot be performed with these simple waves, such as (i) the 3D selective manipulation of micrometric particles with forces several order or magnitude larger than the optical tweezers, or (ii) the remote control of hydrodynamic vorticity. In this presentation, we will show that such operations can be achieved with some specific waves called acoustical vortices, some solutions of Helmholtz equation in cylindrical or spherical coordinates. Experimentally, these helical waves are synthesized with spiraling electrodes deposited at the surface of a piezoelectric medium, which encode the phase of the wave like a hologram. With this microsystem, we demonstrate the independent positioning of microparticles (see Fig. 1 a) but also compute the hydrodynamic vorticity field that they can synthesize (see Fig. 1 b). For applications, these “single beam SAW based acoustical tweezers” have many attractive features: they are selective, flat, cheap, easily integrable and compatible with disposable substrates. http://films-lab.univ-lille1.fr/michael

Lieu : Bibliothèque LadHyX, 10h45

Résumé :

In gas turbine industry, it is common practice to implement swirling jets and associated vortex breakdown to stabilize the flame and to enhance turbulent mixing. The flow field of such swirl-stabilized combustors features hydrodynamic instabilities that generate large-scale coherent flow structure. This seminar presents recent experimental studies targeting the impact of these instabilities on the combustion performance. Particular focus is placed on two types of instability: (i) a self-excited helical instability, typically known as the precessing vortex core, which affects mixing and flame anchoring; (ii) the axisymmetric Kelvin-Helmholtz instability, which is a key enabler of thermoacoustic oscillations. The experimental observations are correlated with results from hydrodynamic stability theory. We apply linear stability analysis to the time-averaged turbulent flow taken from experiments, contrasting the classical stability approach that is based on a stationary base flow. We benchmark this approach on fundamental flow problems and comment on the mean flow assumption and involved turbulence models. The results of the linear stability analysis of the combustor flow are discussed thereafter. We show that this methodology reveals the mechanisms that lead to the formation, saturation, and suppression of large-scale flow structures and how these mechanisms interact with the combustion process.