The advent of resolved Lagrangian measurements has helped understand the dynamics of turbulence from the point of view of fluid particles. In the experiments, the fluid particles are tracked from the motion of solid tracers; it naturally raises the question of the understanding of the dynamics of finite size objects in turbulent flows – often called inertial particles. If the particles are quite small compared to the smallest fluid motion, arguments show that they behave as tracers of fluid motions. Observations have revealed a very intense intermittency in the motion of fluid tracers; they experience very strong accelerations, with a probability distribution that displays wide stretched exponential tails.
When the diameter of the advected particles is of the order of, or larger than the small scales of the flow their equation of motion is largely unknown. The following topics were examined during my thesis.
We developed a novel technique to track the 6–dimensional dynamics – position and absolute orientation – of a large sphere advected by the turbulent flow. The sphere movements are recorded with 2 high-speed cameras. Its orientation is tracked using an efficient algorithm based on the identification of possible orientation `candidates' at each time step. Although the sphere’s diameter is comparable to the integral length of the underlying flow, we find intermittency for both the translation and the rotation. Further we find a coupling between the rotation and the translation, which is in agreement with a lift force. Apart from the fact that the acceleration statistics are not gaussian, we find that the particle diameter has a surprisingly strong influence on how a particle samples the flow, and how the particles exchange energy with the carrying flow.
In cooperation with smartINST, a startup on the ENS Lyon campus, we worked on an instrumented particle, which embarks a 3D-accelerometer and a radio-transmission system to constantly emit the felt Lagrangian acceleration as it is advected in the flow.
We developed methods for the interpretation of the signals and characterized its performance by simultaneously tracking its orientation and position (with our novel tracking technique) while acquiring the acceleration signal measured by the instrumented particle.
Measuring the particle’s absolute orientation was a crucial step here to project the acceleration measured by the particle into the laboratory reference frame. This enabled us to compare the forces computed by the two independent measurements, and allowed for developing new signal analysis tools
Creating homogeneous and isotropic turbulence is difficult in a laboratory experiment as the large scale forcing influences small scales. To overcome this problem I participated in the development of two new apparatuses (Lagrangian Exploration Module), built and designed in cooperation with E. Bodenschatz (MPI Göttingen, Germany), which produces homogeneous and isotropic turbulence by driving the flow with twelve independently controlled motors.
We did further a feasibility test for an Eulerian pH measuring probe suitable for turbulent flows. This is highly interesting for the study of chemical mixing using instrumented particles.
Salle des Thèses - Site MONOD - ENS Lyon