Exploration Expérimentale Lagrangienne de la Turbulence Dans Un Fluide Quantique en Rotation PDF Download

Author: Emeric Durozoy
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Languages : en
Pages : 0

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Book Description
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 & 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.