Particle Tracking Thesis 2020

2D/3D PTV Application and Technique

Particle tracking velocimetry vortex ring thesis
Abstract: In this work, the interaction between model vortex structures observed in many flows, vortex rings, and a linearly stratified fluid is studied. A light homogeneous vortex ring is generated and penetrates by inertia into a density stratification. The resulting interaction depends on several control parameters which are: the dimensions of the vortex ring, its propagating speed, its orientation relative to the vertical, its initial density and the density gradient of the stratification. For short times, baroclinic vorticity is generated as the vortex ring pushes isopycnal during the penetration phase. The vortex ring dynamics is highly affected by its interaction with the stratified zone leading to reorganisation of the vorticity distribution depending on the control parameters. For long times, internal gravity waves are generated as the stratification relaxes. Several key points are addressed throughout the chapters. What is the maximum penetration depth a vortex ring can reach? What is the mechanism for the vortex ring recoil? What are the time scales of the vorticity reorganisation and generation of internal waves? What are the characteristics of the internal waves generated by the impact of such a localized fluid structure and how to quantify them? What is the influence of the angle of propagation of the vortex ring on the reorganisation of the flow? How is the flow modified when two vortex rings are launched consecutively? What is the role of the time delay between the generation of the vortex rings on the penetration depth reached and the internal waves generated? Qualitative (visualizations) and quantitative (2D-Particles Image Velocimetry and 4D-Particle Tracking Velocimetry) have been deployed to answer these questions.
Keywords: Dynamique tourbillonnaire - Milieu stratifié - Ondes internes - 4D-PTV - Mélange
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Thesis Author: Johan PINAUD
Lagrangian experimental exploration of turbulence in a rotating quantum fluid
Abstract: In this thesis, we describe a new cryogenic apparatus (called CryoLEM) used to study rotating helium II (or superfluid) flows by visualization of solid deuterium particles. Rotating helium II is a canonical flow that produces an organized and regular quantum vortex array. The CryoLEM is an experimental device whose implementation was completed during this thesis. Thus, the start-up and use protocols have been established, and the first tests in real conditions have been carried out. These tests allowed us to characterize the performance of the cryostat and to better understand the formation of solid deuterium particles. While the long term objective is to perform 3D particle motion measurements, we remained focused on performing 2D measurements in a plane containing the axis of. To perform Lagrangian particle tracking, an algorithm has been entirely written based on existing techniques.This cryostat was first used to study the transient regimes that occur during particle injections, and after the start (or stop) of the rotation of the device, in order to estimate their characteristic times and compare it to the Ekman time, and an empirical time constant obtained by Tsakadze. After a particle injection, a time proportional to the cryostat rotation rate must elapse to recover a steady state. For the spin-up problem, we found a very good agreement with an Ekman time based on the circulation quantum. In spin-down, a qualitative agreement with the Tsakadze model is found. This then allowed us to work with stationary data.During the stationary rotation of helium II an oscillating movement of the particles with an amplitude of the order of a millimetre is observed. We are not able to relate the amplitude of these oscillations to any of the physical parameters that we measured. Nevertheless, these oscillations are not observed in helium I (normal liquid), which shows the fundamentally different nature of these two fluids, and motivates further studies. Finally, a turbulent helium II flow caused by a counter-rotating turbine in the rotating CryoLEM has been studied. We have characterized this relatively simple flow, and understood the scalings of its physical parameters thanks to an energy balance between injection and dissipative mechanisms. We have defined a serie of surrogates to estimate the energy dissipation rate in this quantum turbulent and anisotropic flow. They use first to third order statistical quantities based on velocity increments. All these measurements lead to a coherent scaling of the dissipation rate as the cube of the rotation rate in the radial direction, with some discrepancies in the prefactor. The third order statistics led us to conclude that energy was being transported from the large to the small scales, while anisotropy grows with the rotation rate, as expected from the Taylor-Proudman theorem. This thesis ends by pointing out that some of the particles evolving in this flow show oscillations of smaller amplitude than their size. This could be compatible with a particle–quantum vortex interaction mediated by the Magnus force. This observation opens a new way to study quantum vortices dynamics.
Keywords: Experiment, Superfluid, Turbulence,
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Thesis Author: Emeric Durozoy
Dynamics of freely transported fibers in confined viscous flows_thesis
Abstract: Studying confined situations of fluid structure interactions in viscous flows is important to understand locomotion of micro-organisms in soils or medical conducts as well as the movement of long fibers in fractures, where they are used as in-situ probes in oil recovery. Here we look at the dynamics of a model system, constituted by a fiber freely transported in a Hele-Shaw cell by pressure-driven flows. The fiber height is comparable to the channel height and the confinement plays a fundamental role in fiber transport, which shows specific characteristics, as for example lateral drift for fibers not aligned with the flow. Due to viscous friction with top and bottom walls particles act like moving ob- stacles and induce strong flow perturbations. These perturbations are at the origin of anisotropy in the friction forces leading to lateral drift and oscillatory movement between lateral walls. In this PhD we study how the transport dynamics are perturbed when the particle becomes more complex than a straight and rigid fiber. Two degrees of complexity have been studied in parallel: we either add flexibility to the fiber or we change its shape and focus our investigations on their transport. Flexible fibers perpendicular to the flow bend while parallel fibers deform in a sine shape that flatten out at the edges. We show that the bending of the perpendicular fiber is proportional to an elasto-viscous number and we fully characterize the influence of the confinement on the deformation of the fiber. Experiments on parallel flexible fibers reveal the existence of an instability threshold. Complementary, we change the shape of the fiber by adding an additional arm, forming an L-shaped fiber. This induces fiber rotation until a stable equilibrium orientation. Lateral drift is subsequently observed until the interaction with side walls becomes important. Tuning the fiber asymmetry allows for a precise control of particle trajectories, including the approach of side walls, robust even against small perturbations. We investigate these with a combination of well-controlled microfluidics experiments and simulations using modified Brinkman equations. We control the shape, orientation and mechanical properties of our particles using micro-fabrication techniques. To characterize the Young’s modulus and the Poisson’s ratio of the hydrogels we develop two independent novel in-situ measurement methods.
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Thesis Author: Jean Cappello
Abstract: The annual plastic input in the ocean is estimated to be 200 000 tons. To the best of current knowledge, only 1 % of this quantity is retrieved. Where is the rest? For logistical reasons, scientists have essentially investigated the plastic pollution at the sea surface. However, some mixing at the surface is induced by wind and waves, which moves this pollution in depth. In this work, we focus on the vertical transport of micro-plastics offshore, which are more sensitive to this vertical transport due to their low buoyancy. Their dynamics is described by a balance between an upward flux due to plastic buoyancy and a downward flux due to turbulence. We study their vertical distribution thanks to samples collected at sea and laboratory experiments, in which, micro-plastics are modelled by particles with regular shapes (spheres and disks). The surface turbulence, which mixes them in the water column, is generated by an oscillating grid system to mimic the turbulence decay with depth. First, we propose a model for the rise velocity (buoyancy) of the micro-plastics, taking into account their properties (density, size and shape). Given the wide range of plastic properties, we propose to encompass their rise velocity by an upper and a lower bound for each range of size. These bounds come from a choice of two densities and regular shapes, based on a statistical analysis of the properties of 400 samples collected in the Atlantic North Gyre in 2015.Second, the characterization of the flow used in laboratory experiments is done using Particle Image Velocimetry (PIV) technique. The oscillating grid turbulence is studied in both homogeneous and two-layer fluids. A parametric law for the eddy viscosity, taking into account the turbulence decay with depth, is proposed. This approach, giving a general description of the flow, is new. Indeed, oscillating grid turbulence is usually described in terms of turbulent kinetic energy and dissipation rate. Third, the vertical distribution of buoyant particles is investigated in oscillating grid turbulence experiments. It is obtained thanks to measurements in two dimensions unresolved in time (in a laser sheet) and measurements in volume resolved in time (Particle Tracking Velocimetry and stereoscopy). Coupling them with the characterization of the flow, we describe the vertical transport of plastic using the " k − ? " turbulent model. We provide the first estimation of the turbulent Schmidt number for finite size particles with low inertia; the turbulent Schmidt number being the key parameter for the fluid-particles coupling in such approaches.
Keywords: Stratification - Particules Micro-Plastiques - Turbulence - Flottabilité - Transport vertical
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Thesis Author: MARIE POULAIN