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16:15   Quantum and Superfluid Turbulence 16:15 15 mins #53 Investigation on the occurrence of flight-crash events in turbulent flows of superfluid helium-4 Marco La Mantia, Petra Hrubcová, Patrik Švančara Abstract: We experimentally investigate by visualization the occurrence of flight-crash events in turbulent flows of superfluid helium-4 (He II). We find that, in the range of investigated parameters, the flow-probing particles suspended in the liquid do not tend on average to decelerate faster than they accelerate. The outcome can be regarded as experimental evidence that energy transport mechanisms in turbulent flows of He II are significantly different from those occurring in similar flows of viscous fluids, due to the presence of quantized vortices. 16:30 15 mins #60 Three-dimensional numerical simulations of two-fluid coupled dynamics in thermal counterflows of superfluid 4He Hiromichi Kobayashi, Satoshi Yui, Makoto Tsubota Abstract: In a duct flow filled with the superfluid 4He connected to a large helium bath, when the duct end opposed to the helium bath is heated, the normal fluid flow goes to the helium bath and the superfluid flow moves to the heated end to satisfy the total mass conservation. This experimental setup is termed as a thermal counterflow. As increasing a heat flux at the heated end, a relative velocity of the two fluids increases and mutual friction emerges in the thermal counterflow [1]. The mutual friction is due to the tangle of the quantized vortex in the superfluid flow, namely, quantum turbulence. The tangle structure of the vortex lines was numerically demonstrated by using the vortex filament method (VFM) in a computational box under the periodic boundary condition [2-4]. A solid boundary effect was considered in channel flows [5] and in square duct flows [6]. In the simulations, although velocity profiles of the normal fluid were prescribed as parabolic and tail-flattened flows, vortex line density distributions become bimodal in the wall-normal direction. On the other hand, the experimental visualization exhibited the modulation of the normal fluid velocity distribution like a tail-flattened flow [7]. Recently, the VFM for the superfluid and Navier-Stokes equations for the normal fluid were fully two-way coupled by the mutual friction. Figure 1 shows the distributions of quantized vorticities (blue lines) and normal fluid velocity (color contour) for an initial state (left) and a developed state (right). As increasing the vortex line density from the initial state to the developed state, the mutual friction becomes strong and it flattens the normal fluid velocity profiles [8]. In the presentation, we show the coupling method and the effect of the mutual friction on the two-fluid distributions in detail. References [1] W.F. Vinen. Mutual Friction in a Heat Current in Liquid Helium II. I. Experiments on Steady Heat Currents. Proc. R. Soc. Lond. A 240: 114–127, 1957. [2] K.W. Schwarz. Three-dimensional vortex dynamics in superfluid 4He: Homogeneous superfluid turbulence. Phys. Rev. B 38: 2398–2417, 1988. [3] C.F. Barenghi, R.J. Donnelly, W.F. Vinen. Quantized Vortex Dynamics and Superfluid Turbulence. Springer, Berlin 2001. [4] H. Adachi, S. Fujiyama, M. Tsubota. Steady-state counterflow quantum turbulence: Simulation of vortex filaments using the full Biot-Savart law. Phys. Rev. B 81: 104511, 2010. [5] A.W. Baggaley, J. Laurie. Thermal Counterflow in a Periodic Channel with Solid Boundaries. J. Low Temp. Phys. 178: 35–52, 2015. [6] S. Yui, M. Tsubota. Counterflow quantum turbulence of He-II in a square channel: Numerical analysis with nonuniform flows of the normal fluid. Phys. Rev. B 91: 184504, 2015. [7] A. Marakov, J. Gao, W. Guo, S.W. Van Sciver, G.G. Ihas, D.N. McKinsey, W.F. Vinen. Visualization of the normal-fluid turbulence in counterflowing superfluid 4He. Phys. Rev. B 91: 094503, 2015. [8] S. Yui, M. Tsubota, H. Kobayashi. Three-Dimensional Coupled Dynamics of the Two-Fluid Model in Superfluid 4He: Deformed Velocity Profile of Normal Fluid in Thermal Counterflow. Phys. Rev. Lett. 120: 155301, 2018. 16:45 15 mins #283 PERSISTENCE-TIME PROBLEM IN THE THREE-DIMENSIONAL HVBK MODEL FOR SUPERFLUID TURBULENCE Akhilesh Kumar Verma, Vishwanath Shukla, Akshay Bhatnagar, Rahul Pandit Abstract: We study the persistence-time problem with tracers and heavy inertial particles in the three-dimensional (3D) Hall-Vinen-Bekharevich-Khalatnikov (HVBK) model of homogeneous and isotropic superfluid turbulence via direct nu- merical simulations (DNS). We examine the dependence of persistence times on the Stokes numbers, on temperature, and the mutual-friction parameters that couple the superfluid and normal-fluid components. In particular, we calculate the Q n , R n and Q s , R s invariants of velocity-gradient tensors along the particles trajectories for normal-fluid and superfluid components, respectively. We classify the topology of the flow into four regions, two vortical-dominated regions and two 2 3 strain-dominated regions, based on the values of R α and ∆ α in the Q − R plane, where ∆ α = 27 4 R α + Q α , and where α stand for n (normal) and s (superfluid). Furthermore, we calculate the cumulative probability distribution functions (CPDFs) of time (t per ) spent by particles in the each region of the Q − R planes and find that CPDFs of t per have expo- nential tails. We extract time scales from these CPDFs to measure the time spent by particles in the vortical-dominated regions and strain-dominated regions of the flow. 17:00 15 mins #294 LAGRANGIAN STUDY OF ISOTHERMAL TURBULENCE IN NORMAL AND SUPERFLUID HELIUM Bernard Rousset, Mickael Bourgoin, Diribarne Pantxo, Mathieu Gibert, Fatimata Sy Abstract: This study aims at comparing the isothermal turbulence in normal Helium (HeI) and superfluid Helium II (HeII) through a Lagrangian analysis. Analogies and/or differences between the classical and superfluid behaviors are expected to shed new light on the intrinsic multi-scale properties of turbulence. The flow is generated by an oscillating grid, which produces isotropic and homogeneous (per plane) turbulence, with no mean velocity (hence allowing long observation times). The tracers are hollow glass microspheres, which are almost neutrally buoyant in liquid Helium. The cryostat hosting the experiment is fully transparent, allowing visualization at multiple angles. Measurements were performed using high speed imaging in backlight illumination. Particle trajectories were reconstructed using Lagrangian tracking, from which we performed analysis of single particle statistics as well as study of relative dispersion of particle pairs. These diagnoses allowed us to compare the rate of kinematic energy at all scales, from production at large scales, transfer along the inertial scales and dissipation at smallest scales (see figure 1 below). While the spatial and temporal resolution of our measurements give us access to the dynamics of the flow in the range of inertial scales, dissipative scales are marginally resolved. Besides, in these conditions, as expected turbulence results in HeI are in agreement with previous results obtained in similar flows using classical fluids. In the range of currently accessible scales, our study doesn’t reveal any difference of turbulence properties between HeI and HeII. Further studies aim at achieving even higher resolution measurements to explore possible differences between classical and superfluid turbulence at and below dissipative scales. 17:15 15 mins #540 How well do particles track superfluid vortices? Insights from the Gross-Pitaevskii model Umberto Giuriato, Giorgio Krstulovic, Vishwanath Shuckla, Sergey Nazarenko Abstract: Nowadays, particles are the main experimental tool used to visualise quantum vortices and to investigate their dynamics. We use a model based on the three-dimensional Gross-Pitaevskii equation to study how the dynamics of a collection of active particles trapped inside quantum vortices reflect the motion of the vortices themselves. We perform a series of numerical simulations with vortices and particles in different configurations, and varying the relative mass and the radius of particles. We also develop a simple hydrodynamic model that is able to predict analytically the reaction of particles to the presence of Kelvin waves on the vortex filament. This model is in good agreement with the numerics and it reproduces how the frequencies excited on the particle-vortex system differ from the ones of the bare Kelvin-wave dispersion relation. 17:30 15 mins #533 Quantum Vortex Reconnections: crossover from interaction to driven regimes Luca Galantucci, Andrew W Baggaley, Nick G Parker, Carlo F Barenghi Abstract: Quantum vortex reconnections play a fundamental role in quantum fluid dynamics. In particular, reconnecting events redistribute energy and helicity amongst scales, trigger a turbulent energy cascade generating the same Kolmogorov spectrum of classical turbulence and, in the low-temperature limit, reconnections represent the ultimate mechanism for the dissipation of incompressible kinetic energy via acoustic emission. In past studies, many authors have focused on the potential existence of a universal route to reconnections, which may take the form of a vortex ring cascade, a particular rule for the reconnecting cusp angles or, more promising, a special scaling with time of the minimum distance δ(t) between the reconnecting vortex strands [3, 7, 4, 6, 1]. In the present work [2], we focus on the last property showing that δ(t) can obey two distinct scalings. In addition to the already observed δ ∼ t 1/2 scaling, we predict and observe a new linear δ ∼ t behaviour 1. In our study we show how these two scalings arise from rigorous dimensional arguments and then demonstrate their existence in a vast campaign of numerical simulations performed with both the Gross-Pitaevskii (GP) model and the Vortex Filament (VF) method. Distinctive of our simulations are the larger initial distance compared to past numerical studies and the extension of the investigations to experimentally accessible trapped Bose-Einstein Condensates (BECs) (both box- and harmonically trapped), where vortex reconnections can now be studied with unprecedented resolution [5]. The numerical results obtained show that the δ(t) scaling is determined by the balance between the vortex mutual interaction (responsible for the δ ∼ t^1/2 scaling) and extrinsic factors (vortex curvature, density gradients, vortex images) driving the individual vortices towards reconnection (leading to the δ ∼ t behaviour). The relevance of these two scalings arises from their observation in both all experimentally accessible condensates and in the two limiting, yet fundamental, cases of vortex reconnections in homogeneous systems, namely the reconnection of orthogonal vortices and the vortex ring - vortex line interaction. [1]E Fonda, KR Sreenivasan, and DP Lathrop. Proc Nat Acad Sci, 116(6):1924–1928, 2019. [2]L. Galantucci, A. W. Baggaley, N. G. Parker, and C. F. Barenghi. arXiv, page 1812.00473, 2018. [3]S Nazarenko and R West. J Low Temp Phys, 132:1, 2003. [4]C Rorai, J Skipper, RM Kerr, and KR Sreenivasan. J Fluid Mech, 808:641, 2016. [5]S Serafini, L Galantucci, E Iseni, T Bienaime, R Bisset, CF Barenghi, F Dalfovo, G Lamporesi, and G Ferrari. Phys Rev X, 7:021031, 2017. [6]A Villois, D Proment, and G Krstulovic. Phys Rev Fluids, 2(4):044701, 2017. [7]S Zuccher, M Caliari, AW Baggaley, and CF Barenghi. Phys Fluids, 24:125108, 2012.. 17:45 15 mins #536 INTERACTION BETWEEN ACTIVE PARTICLES AND QUANTUM VORTICES AT LOW TEMPERATURES Giorgio Krstulovic, Umberto Giuriato Abstract: In this talk, we present theoretical and numerical results on the attractive interaction between particles and quantised vortices at very low temperature. The results are obtained from a simple self-consistent model based on the three-dimensional Gross-Pitaevskii coupled with particle dynamics. This model allows for deriving analytically a reduced central-force model that only depends on the classical degrees of freedom of the particle. Then, we show how this model can be generalised to include deformations of the vortex filament due to interactions with particles. The resulting long-range mutual interaction between particles and vortices, qualitatively reproduces the observed generation of a cusp on the vortex filament during the particle approach. Moreover, It is possible to show analytically and numerically that particles can excite Kelvin waves on the vortex filament through a resonance mechanism even if they are still far from it. Finally, the dynamics of a large number of active finite size particles in a quantum turbulent tangle will be briefly discussed. 18:00 15 mins #548 Flying in a superfluid Davide Proment, Seth Musser, Miguel Onorato, William Irvine Abstract: The lift in a subsonic flight is related to the presence of a velocity boundary layer which, in some conditions, allows for the roll up of the so called starting vortex at the trailing edge of the airfoil, as it is accelerated from rest in a fluid. The vortex induces a net circulation on the airfoil and a consequent lift. A superfluid is a fluid with zero viscosity and no velocity boundary layer can be developed. Here we investigate on the possibility of generating a lift in a superfluid described by the Gross--Pitaevskii equation. Using numerical simulations and phenomenological arguments, we are able to establish that in such superfluids a lift is possible and it has the feature of being quantised. Our results shed new light on vortex generation and manipulation mechanisms in superfluids and might have direct experimental applications.