Interaction particule-fluide : du pendule simple aux effets collectifs en turbulence

Abstract : This PhD thesis covers many features of fluid-particle interactions, ranging from a simple pendulum inmersed in a flow to the presence of superclusters of water droplets in a wind tunnel.The simplest case studied was a pendulum with a pendulum-blob facing the wind in the wind-tunnel. As the pendulum-blob was a plate, the aerodynamic coefficients as a function of the angle between the plate and the streamwise velocity present a non-trivial behavior, resulting in an hysteresis cycle. We also investigate the influence of turbulence on the equilibrium of the pendulum in general and on the observed bi-stability in particular.Then, different instabilities of towed systems has been studied. In chapter 4 we have seen that the wake of a sphere can produce helicoidal motion of a sphere towed by a wire. We found that there exists a particle Reynolds number Rep threshold for activating this unstable motion. A three-dimensional trajectory was reconstructed with an extremely simple experimental setup, used for characterizing the shape of particle's trajectory. In chapter 5 we investigate experimentally the equilibrium and the stability of the trajectory of a sphere towed at constant velocity in the wind tunnel at the tip of a cable with unprecedented large length-to-diameter aspect ratio. In thist chapter we study the instabilities developped in the wire for a laminar flow.Flutter and divergence instabilities has been found in this experiment.In chapter 6 the same system is studied, but the surrounding flow is turbulent. In this chapter we focus on a comparison with this towed system with freely advected particles in turbulence. Our results are consistent with a filtering scenario resulting from the viscous response time of an inertial particle whose dynamics is coupled to the surrounding fluid via the dragforce.Therefore, depending on several parameters such as the Reynolds number of the particle, the wire or the fluctuations level of the flow, a whole family of instabilities can appear, with no trivial dependencies and important consequences considering different applications of such systems.Concerning the collective effects, three different flows have been studied: a water tunnel, a von Karman flow and a wind tunnel. A broad range of Reynolds numbers, dissipation scales and particles diameters and densities has been covered. Using Voronoi diagrams, we have quantified preferential concentration as a function of the Stokes number and the Reynolds number. In chapter 7 and 8 simultaneous PIV measurements complemented the inertial particles acquisitions. The goal was to analyze if the particles tend to stick into special regions of the flow.In the last chapter also DNS have been performed for comparing with experimental results. A sweep-stick mechanism, in which inertial particles tend to have the same statistics as zero-acceleration points has been proved to be consistent with our results.Finally, a promising new technique has been presented. Based on the standard measurements, a spatial field has been reconstructed allowing us to acquire a several meters long image of particles. The enormous amount of structures present in the image has evidenced that the clusters are grouped at the same time in bigger clusters (i.e. clusters form clusters, that we call superclusters). This new result is still being studied and presents a new and fascinating field for studying particle-flow interactions.
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Martín Obligado. Interaction particule-fluide : du pendule simple aux effets collectifs en turbulence. Mécanique des fluides [physics.class-ph]. Université de Grenoble, 2013. Français. ⟨NNT : 2013GRENI108⟩. ⟨tel-01454681v2⟩

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