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Subject of the thesis

Vorticity-velocity cross diagnostic for comparative study of classical and superfluid turbulence

Published on 4 December 2020
Abstract:
The analytical resolution of the equations describing turbulence is currently impossible, is currently impossible and even the existence of regular solutions is not guaranteed (cf. AMS millennium prize). On the numerical side, the limitation comes from the number of scales involved, which increases almost to the cube as a function of the turbulent intensity (Reynolds number). In case of strong turbulence, the codes therefore model only the largest scales and assume subgrid model for smaller scales down to viscous dissipation. A better understanding of the mechanisms governing turbulence would allow us to obtain a more realistic "fit ". For this, one of the approaches chosen by a group of laboratories, including Dsbt, consists in the comparative study of classical/superfluid turbulence. It is within this framework that we propose a thesis based on the joint use of two diagnostics, visualization and attenuation of second sound, which will allow us to access the velocity field and average vorticity.

Presentation of the subject:
The analytical resolution of the equations describing turbulence is currently impossible and even the existence of regular solutions is not guaranteed (cf. AMS millennium prize). However, turbulence governs a large number of processes, often in a penalizing way for example the drag associated with the motion of cars, planes, ... but sometimes in an beneficial way especially for the transport of momentum and heat: mixers, heat exchangers ...
The understanding of turbulence is often associated with the study of the transfer of kinetic energy from its injection at large scales to its dissipation by viscosity at small scales. Superfluid helium (He II) is described as having a non-viscous component to which quantum vorticity is associated and it is interesting to study turbulence within this fluid in order to define the invariants (laws applicable independently of the nature of the fluid) and possible specific properties. The comparison of a standard fluid (He I) versus He II can thus shed new light on our understanding of turbulence and its properties.
The CEA (DSBT and SPEC), the Néel Institute, the Legi and the ENSLyon have been collaborating for many years on this topic. More recently, some of these actors have been interested in oscillating flows (thus without average velocity) to carry out this comparative study of classical turbulence/superfluid turbulence. One of the assets of this type of flow is the possibility to use visualization efficiently, for example for a Lagrangian description of the flow. The joint use of a sensor measuring the attenuation of the second sound propagated through the flow will allow to directly probe the turbulence associated with quantum vortices.
The objective of this thesis is to use and improve this type of measurements in an existing optical cryostat to probe the turbulence at small scales, in He I as well as in He II, which has never been achieved until now. Obtaining 3D measurements of the velocity field (by particle tracking or using on-line holography) coupled with the measurement of the quantum vortex density is an innovative approach that is at the heart of this project.

Host laboratory:
Laboratoire Réfrigération et Thermohydraulique Hélium (LRTH)
Département des Systèmes Basses Températures (DSBT)
CEA-Grenoble
17 Avenue des Martyrs
38 054 Grenoble cedex 9

Beginning of the thesis:
September/October 2021


Contacts:
Pantxo Diribarne Correspondent in charge of the supervision of the thesis at the CEA (French Atomic Energy Commission)
Bernard Rousset Thesis supervisor