Recent experiments have demonstrated a remarkable progress in implementing and use of the Particle Image Velocimetry (PIV) and particle tracking techniques for the study of turbulence in 4 He. However, an interpretation of the experimental data in the superfluid phase requires understanding how the motion of tracer particles is affected by the two components, the viscous normal fluid and the inviscid superfluid. Of a particular importance is the problem of particle interactions with quantized vortex lines which may not only strongly affect the particle motion, but, under certain conditions, may even trap particles on quantized vortex cores. The article reviews recent theoretical, numerical, and experimental results in this rapidly developing area of research, putting critically together recent results, and solving apparent inconsistencies. Also discussed is a closely related technique of detection of quantized vortices negative ion bubbles in 4 He.

This report addresses an important question discussed by the quantum turbulence community during the last decade: do quantized vortices form, in zero-temperature superfluids, coherent structures similar to vortex tubes in ordinary, viscous turbulence? So far the evidence provided by numerical simulations is that bundles of quantized vortices appear in finite-temperature superfluids, but from the interaction with existing coherent structures in the turbulent (viscous) normal fluid, rather than due to the intrinsic superfuid dynamics. In this report we show that, in very intense quantum turbulence (whose simulation was made possible by a tree algorithm), the vortex tangle contains small coherent vortical structures (bundles of quantized vortices) which arise from the Biot-Savart dynamics alone, and which are similar to the coherent structures observed in classical viscous turbulence.

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We present a numerical, computational, and physical analysis of particle-vortex collisions in thermal superfluids. Our method allows fully self-consistent, dynamic computation of particle-vortex collisions within the vortex dynamical formalism. The algorithm is described in detail and is shown to be both accurate and efficient. The method is applied to the collision of a solid particle with a straight vortex at finite temperature. It predicts that the smallest velocity that the approaching particle must have in order to escape the vortex after being captured by it increases as the temperature approaches the superfluid transition temperature. A comparative study of particle-vortex collisions at various temperatures reveals the contributions of viscous damping, inertial, and boundary-induced effects on the dynamics of the system, as well as different particle-vortex interaction behaviors. The findings corroborate the possibility of direct measurement of the normal fluid velocity in thermal superfluids via appropriately designed particle image velocimetry experiments.

We address the question which, after a decade-long discussion, still remains open: what is the nature of the ultraquantum regime of decay of quantum turbulence? The model developed in this work reproduces both the ultraquantum and the quasiclassical decay regimes and explains their hydrodynamical natures. In the case where turbulence is generated by forcing at some intermediate lengthscale, e.g. by the beam of vortex rings in the experiment of Walmsley and Golov [Phys. Rev. Lett. 100, 245301 (2008)], we explained the mechanisms of generation of both ultraquantum and quasiclassical regimes. We also found that the anisotropy of the beam is important for generating the large scale motion associated with the quasiclassical regime.PACS numbers: 67.25.dk, 67.30.he, 47.27.Gs The existence of a macroscopic complex order parameter in superfluid helium ( 4 He and 3 He) constrains the vorticity to vortex lines, each line carrying one quantum of circulation κ. This is in sharp contrast to ordinary fluids, where vorticity is continuous. An important question is how quantum turbulence compares to classical turbulence [1]. Experiments in helium have revealed two regimes [2-4] of turbulent decay characterized by L ∼ t −1 (ultraquantum) and L ∼ t −3/2 (quasiclassical) behaviour, where t is time and the vortex line density (vortex length per unit volume) L measures the turbulence's intensity. In these two regimes the kinetic energy (per unit mass) decays as E ∼ t −1 and E ∼ t −2 , respectively. (Here it seems appropriate to point to the detailed theoretical analysis [5] of energy decay in classical, viscous, uniform and isotropic turbulence.) The second regime is thought to be associated with the classical Kolmogorov distribution of kinetic energy over the length scales, but the nature of the first regime is still a mystery. Here we show that the first regime, associated entirely with the Kelvin wave cascade along individual vortex lines, takes place when the energy input at some intermediate lengthscale is insufficient to induce the large-scale motion which is associated with quasiclassical, "Kolmogorov" turbulence. In other words, the first regime is a transient turbulent state which decays before energy can be transferred to large scales by vortex reconnections, which play a key role in this reverse energy transfer.Theoretical and experimental studies have revealed analogies between superfluid turbulence and classical turbulence, notably the same Kolmogorov energy spectrum in continually forced turbulence [6][7][8][9][10], as well as many dissimilarities and new effects. Our concern is the decay of pure superfluid turbulence at temperatures small enough that thermal excitations are negligible; in the absence of viscous forces, in 4 He the only mechanism [1] to dissipate kinetic energy is phonon emission at length scales much shorter than the average intervortex distanceHe-B, which is a fermionic superfluid, the dissipation is thought to be associated with the CaroliMatricon mechanism [11] of energy loss from short Kelvin w...