SUPERFLUID TURBULENCE AND DYNAMICS OF QUANTIZED VORTICES

Chapter 1 Energy Spectra of Quantum Turbulence

We review numerical studies of quantum turbulence. Quantum turbulence is currently one of the most important problems in low temperature physics and is actively studied for superfluid helium and atomic Bose--Einstein condensates. A key aspect of quantum turbulence is the dynamics of condensates and quantized vortices. The dynamics of quantized vortices in superfluid helium are described by the vortex filament model, while the dynamics of condensates are described by the Gross--Pitaevskii model. Both of these models are nonlinear, and the quantum turbulent states of interest are far from equilibrium. Hence, numerical studies have been indispensable for studying quantum turbulence. In fact, numerical studies have contributed in revealing the various problems of quantum turbulence. This article reviews the recent developments in numerical studies of quantum turbulence. We start with the motivation and the basics of quantum turbulence and invite readers to the frontier of this research. Though there are many important topics in the quantum turbulence of superfluid helium, this article focuses on inhomogeneous quantum turbulence in a channel, which has been motivated by recent visualization experiments. Atomic Bose--Einstein condensates are a modern issue in quantum turbulence, and this article reviews a variety of topics in the quantum turbulence of condensates e.g. two-dimensional quantum turbulence, weak wave turbulence, turbulence in a spinor condensate, $etc.$, some of which has not been addressed in superfluid helium and paves the novel way for quantum turbulence researches. Finally we discuss open problems.

Fermionic superfluids provide a new realization of quantum turbulence, accessible to both experiment and theory, yet relevant to phenomena from both cold atoms to nuclear astrophysics. In particular, the strongly interacting Fermi gas realized in cold-atom experiments is closely related to dilute neutron matter in neutron star crusts. Unlike the liquid superfluids 4He (bosons) and 3He (fermions), where quantum turbulence has been studied in laboratory for decades, superfluid Fermi gases stand apart for a number of reasons. They admit a rather reliable theoretical description based on density functional theory called the time-dependent superfluid local density approximation that describes both static and dynamic phenomena. Cold atom experiments demonstrate exquisite control over particle number, spin polarization, density, temperature, and interaction strength. Topological defects such as domain walls and quantized vortices, which lie at the heart of quantum turbulence, can be created and manipulated with time-dependent external potentials, and agree with the time-dependent theoretical techniques. While similar experimental and theoretical control exists for weakly interacting Bose gases, the unitary Fermi gas is strongly interacting. The resulting vortex line density is extremely high, and quantum turbulence may thus be realized in small systems where classical turbulence is suppressed. Fermi gases also permit the study of exotic superfluid phenomena such as the Larkin–Ovchinnikov–Fulde–Ferrell pairing mechanism for polarized superfluids which may give rise to 3D supersolids, and a pseudo-gap at finite temperatures that might affect the regime of classical turbulence. The dynamics associated with these phenomena has only started to be explored. Finally, superfluid mixtures have recently been realized, providing experimental access to phenomena like Andreev–Bashkin entrainment predicted decades ago. Superfluid Fermi gases thus provide a rich forum for addressing phenomena related to quantum turbulence with applications ranging from terrestrial superfluidity to astrophysical dynamics in neutron stars.

We study theoretically and numerically the dynamics of quantized vortices in superfluid helium and rotating Bose-Einstein condensates. After reviewing briefly the recent motivation, we discuss these topics with the emphasis on the research done by our group. One of the modern important problems is how superfluid turbulence relates to classical turbulence. First, we show that superfluid turbulence consisting of a vortex tangle has an energy spectrum consistent with the Kolmogorov law. Second, we discuss the vortex states that appear in a rotating channel with counterflow. In the field of atomic-gas Bose-Einstein condensation, the dramatic observations of quantized vortices were made for rotating condensates. By solving numerically the Gross-Pitaevskii equation, we found a whole story of the vortex lattice formation consistent with the observations.

Chapter 3 focuses on the physics of quantized vortices in superfluid helium. We begin by introducing the unique properties of 4He, including its phase diagram, the superfluid transition (lambda transition), and the phenomenon of superfluidity. Following this, we will discuss the two-fluid model, a foundational and widely used phenomenological framework. Next, we delve into thermal counterflow, a crucial area that spurred the initial research on quantum turbulence, superfluid turbulence, and quantized vortices. We will then explore the dynamics of quantized vortices, often described by the Vortex Filament Model (VFM), which plays a central role in understanding quantum turbulence. The equations of motion and basic dynamics of vortices will be presented, and since the vortex motion is both nonlinear and nonlocal, numerical simulations are essential for accurately describing quantum turbulence. We will cover the computational methods used in these simulations, including both “two-way” and “one-way” approaches. In addition to quantum turbulence, we will examine the quantized vortex lattice under rotation, another key phenomenon in superfluidity. Early experiments on vortex lattice formation, the realization of rigid-body rotation, and simple examples of vortex dynamics under rotation will be introduced. Finally, we will discuss recent advancements in visualization experiments in superfluid helium, highlighting the results of visualizing quantized vortices, which have greatly enhanced our understanding of vortex dynamics.

The quantum turbulence is a collection of the interacting quantum vortex loops and filaments. The main goal of this work is to systematize the information on the current state of the research on the reconnecting quantized vortex loops in the superfluid helium: the geometry, the dynamics, the properties of the vortex loops, the energy spectrum before and after the reconnections. The paper discusses the possible role of the reconnections in the formation of the turbulent spectrum. In addition, in this paper the main methods and approaches to the study of the reconnecting vortex loops and quantum turbulence are discussed.

We describe the first measurements of line-density fluctuations and spatial correlations of quantum turbulence in superfluid 3He-B. All of the measurements are performed in the low-temperature regime, where the normal-fluid density is negligible. The quantum turbulence is generated by a vibrating grid. The vortex-line density is found to have large length-scale correlations, indicating large-scale collective motion of vortices. Furthermore, we find that the power spectrum of fluctuations versus frequency obeys a -5/3 power law which verifies recent speculations that this behavior is a generic feature of fully developed quantum turbulence, reminiscent of the Kolmogorov spectrum for velocity fluctuations in classical turbulence.

Comments to ‘Vortex clustering, polarisation and circulation intermittency in classical and quantum turbulence’

The term quantum turbulence denotes the turbulent motion of quantum fluids, systems such as superfluid helium and atomic Bose-Einstein condensates, which are characterized by quantized vorticity, superfluidity, and, at finite temperatures, two-fluid behavior. This article introduces their basic properties, describes types and regimes of turbulence that have been observed, and highlights similarities and differences between quantum turbulence and classical turbulence in ordinary fluids. Our aim is also to link together the articles of this special issue and to provide a perspective of the future development of a subject that contains aspects of fluid mechanics, atomic physics, condensed matter, and low-temperature physics.

Quantum turbulence: Theoretical and numerical problems

It is now 50 years since the discovery of turbulence in the superfluid component of liquid 4He, a form of turbulence that is strongly influenced by quantum effects and therefore often described as quantum turbulence. Early work focussed on turbulence associated with thermal counterflow in 4He. The pioneering computer simulations of Schwarz led to considerable understanding of this type of turbulence, which has no classical analogue, but important aspects of it, both experimental and computational, remain unexplored. More recently, attention has turned to types of quantum turbulence, in both 4He and 3He‐B, that have classical analogues, investigations (experimental, theoretical and computational) serving to emphasize similarities and differences between the classical and quantum cases and serving therefore to add to our understanding of both cases. It will be argued that progress is now hampered by the shortage of experimental techniques with which to probe quantum turbulence over a wide temperature range in the kind of detail possible in the classical case, and proposals are explored for addressing this shortage.

Motivated by current experiments, quantum turbulence in the presence of active and finite size particles is investigated numerically. We show that particles do not affect the development of the Kolmogorov regime, and the large scale Lagrangian observables are also compatible with a classical picture. Particles stay trapped inside superfluid vortices with occasional detachment and recapture. At small scales, particle dynamics is dominated by a fast precession frequency from the Magnus effect. Moreover, we observe that particle acceleration decorrelates much faster than in classical turbulence.

Turbulence plays a crucial role in contexts ranging from galaxy formation to heavy atomic nuclei, from jet engines to arterial blood flow, challenging engineers, physicists, and mathematicians. Recently, turbulence of quantum fluids displaying superfluidity has emerged as an exciting area of interdisciplinary research that spans fluid dynamics, low-temperature physics, and Bose-Einstein condensation. The first book on quantum turbulence, this work describes state-of-the-art results and techniques, stressing analogies and differences with classical turbulence. The authors focus in particular on low temperature phases of liquid helium, drawing on evidence from recent experiments, theory, and numerical simulations. Written by leading figures in the field, this is a go-to reference for students and researchers at all levels.

The problem of quantum turbulence in a channel with an inhomogeneous counterflow of superfluid turbulent helium is studied. \ The counterflow velocity $V_{ns}^{x}(y)$ along the channel is supposed to have a parabolic profile in the transverse direction $y$. Such statement corresponds to the recent numerical simulation by Khomenko et al. [Phys. Rev. B \textbf{91}, 180504 (2015)]. The authors reported about a sophisticated behavior of the vortex line density (VLD) $\mathcal{L}(\mathbf{r},t)$, different from $% \mathcal{L}\propto V_{ns}^{x}(y)^{2}$, which follows from the naive, straightforward application of the conventional Vinen theory. It is clear, that Vinen theory should be refined by taking into account transverse effects and the way it ought to be done is the subject of active discussion in the literature. In the work we discuss several possible mechanisms of the transverse flux of VLD $\mathcal{L}(\mathbf{r},t)$ which should be incorporated in the standard Vinen equation to describe adequately the inhomogeneous quantum turbulence (QT). It is shown that the most effective among these mechanisms is the one that is related to the phase slippage phenomenon. The use of this flux in the modernized Vinen equation corrects the situation with an unusual distribution of the vortex line density, and satisfactory describes the behavior $\mathcal{L}(\mathbf{r},t)$ both in stationary and nonstationary situations. The general problem of the phenomenological Vinen theory in the case of nonuniform and nonstationary quantum turbulence is thoroughly discussed.

Transition from laminar to turbulent states of classical viscous fluids is complex and incompletely understood. Transition to quantum turbulence (QT), by which we mean the turbulent motion of quantum fluids such as helium II, whose physical properties depend on quantum physics in some crucial respects, is naturally more complex. This increased complexity arises from superfluidity, quantization of circulation, and, at finite temperatures below the critical, the two-fluid behavior. Transition to QT could involve, as an initial step, the transition of the classical component, or the intrinsic or extrinsic nucleation of quantized vortices in the superfluid component, or a simultaneous occurrence of both scenarios—and the subsequent interconnected evolution. In spite of the multiplicity of scenarios, aspects of transition to QT can be understood at a phenomenological level on the basis of some general principles, and compared meaningfully with transition in classical flows.