QuantumVIW

When the most common isotope of helium is cooled to a temperature below 2.17K, it appears in an atypical fluid phase called He II. This fluid phase consists of a mixture of a normal, viscous fluid and a, non-viscous, superfluid which interact by mutual friction forces. Remarkably, the vorticity of the superfluid component of He II is localized in vortices of atomic size and whose circulation (the integral of the velocity over a closed loop) is quantified by the ratio between Planck's constant and the mass of the helium atom: we speak of quantum vortices. However, the description of the dynamics of He II in its entirety remains a challenge and there is not yet a unified theory capable of describing all the regimes and all the scales of He II flows. This project focuses on the interaction between quantum vortices, the building blocks of superfluid flows, and the normal component of the fluid when He II is submitted to a global rotation and to a heat flux. We will consider three configurations. In the first one, we will study a container of He II subjected to a global rotation, in which configuration a hexagonal array of quantum vortices is expected. We will slowly modulate the rotation rate of the He II bucket to study the associated dynamics of creation or disappearance of quantum vortices. We will then study in a second step a rotating He II flow forced by a moderate (possibly oscillating) heat flux, in which case we will characterize the instabilities leading to the propagation of Kelvin, Tkachenko and inertial wave modes in the quantum vortex lattice and their interaction with the inertial waves in the classical component of the He II. We will finally study the flow of rotating He II forced by an intense heat flux for which we will focus on the interactions and reconnections between quantum vortices and the state of quantum turbulence that we will finally reach. This will be done through innovative and coordinated experiments and numerical simulations aiming at describing the "transition to quantum turbulence" that the succession of these three configurations form together. In the experiments, we will use advanced techniques for visualizing quantum vortices in a rotating cryostat. In the numerical simulations, we will implement a new approach that relies on a self-consistent treatment of the coupling between quantum vortices and normal fluid. Thanks to these complementary approaches at the best current scientific and technological level, we will bring a new light on the dynamics of He II and will open the way to theoretical advances concerning the transition to quantum turbulence.