13:30
Superfluids 2
Chair: Marco La Mantia
13:30
15 mins
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Wave excitations in adjacent vortex filaments
Niklas Hietala, Risto Hänninen, Hayder Salman, Carlo F. Barenghi
Abstract: The interactions of the nearest neighbour vortices are argued to play a significant role in the crossover range of scales that lies between the Kolmogorov-Richardson cascade and the Kelvin wave driven cascade in superfluid turbulence. In this work, we study how a wave excitation (a Kelvin wave or a soliton) on a vortex affects a nearby straight vortex. Our numerical simulations reveal that coherent excitations can hop from one vortex filament to another whilst retaining their coherent properties.
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13:45
15 mins
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Three-dimensional structure of quantized vortices in rotating Bose-Einstein condensates
Ionut Danaila, Philippe Parnaudeau, Atsushi Suzuki
Abstract: Bose-Einstein condensates (BEC) are ideal superfluid systems to realize quantum turbulence (QT): vortex cores in BECs are larger than in superfluid Helium, making easier their observation. Recent experimental and numerical studies reported that vortex states in BEC can evolve towards a turbulent regime when an oscillatory excitation is applied. We discuss in this work how to accurately prepare initial states with vortices before running numerical simulations of QT based on the Gross-Pitaevskii equation. The case of a dense Abrikosov lattice in a fast rotating BEC is presented. High resolution numerical simulations using parallel computing are used to accurately capture physically important features of the vortices (vortex radius, inter-vortex spacing, vortex density profile).
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14:00
15 mins
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Wave turbulence of a rotating array of quantized vortices in the $T \rightarrow 0$ temperature limit
Jere Mäkinen, Samuli Autti, Vladimir Eltsov, Petri Heikkinen, Jaakko Hosio, Matti Krusius, Victor L'vov, Paul Walmsley, Vladislav Zavjalov
Abstract: The dynamics of quantized vortices in the zero temperature limit $T \rightarrow 0$ is currently of great interest, particularly in the case of the Fermi superfluid $^3$He-B.
Here we study wave turbulence, generated by the librating motion of a rotating cylindrical container filled with $^3$He-B, in the limit of vanishing viscous forces at temperatures
$T \leq 0.2 T_{c}$. The polarization of the quantized vortices with respect to the axis of rotation is
measured using non-invasive NMR techniques. We observe a decrease of the polarization when the librating motion is
started, and a two-stage relaxation process when the modulation of the rotation velocity is stopped.
The first relaxation process is associated with the dissipation of large-scale flow stored in inertial
waves and the solid body rotation of the vortex array. From the decay of these energy reservoirs we determine the rate of energy dissipation of large-scale flow.
The later second process is related to the relaxation of Kelvin waves on individual vortices. This process is monitored by the
recovery of the polarization. The existence of a Kelvin wave cascade at the lowest temperatures is currently a central open question. We supply some evidence for the cascade.
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14:15
15 mins
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LOCAL VELOCITY MEASUREMENTS IN THE SHREK EXPERIMENT AT HIGH REYNOLDS NUMBER
Christophe Baudet, Michel Bon Mardion, Patrick Bonnay, Alan Braslau, Bernard Castaing, Francesca Chillà, Laurent Chevillard, François Daviaud, Pantxo Diribarne, Bérengère Dubrulle, Davide Durì, Davide Faranda, Basile Gallet de Saint-Aurin, Mathieu Gibert, Alain Girard, Bernard Hébral, Thierry Lehner, Iouri Moukharski, Jean-Paul Moro, Jean-Marc Poncet, Philippe-Emmanuel Roche, Bernard Rousset, Éléonore Rusaouën, Brice Saint-Michel, Julien Salort, Ewe-wei Saw, Konstantinos Steiros, Cécile Wiertel-Gasquet
Abstract: We report preliminary results obtained using new local velocity probes in the Superfluid Helium high REynold number von
Kármán flow (SHREK) experiment for different forcing conditions. The presentation will focus on the validation of the signals obtained
from a hot-wire and a total head pressure tube in both normal and superfluid phases of liquid helium.
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14:30
15 mins
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Inter-vortex spacing in superfluid turbulence : temperature and Reynolds number dependences
Philippe-E. Roche, Simone Babuin, Emmanuel Lévêque, Emil Varga
Abstract: The typical spacing between superfluid vortices in an isothermal turbulent tangle is proportional to the integral scale H rescaled by the quantum Reynolds number Re_K=H.V/K to the power of 3/4, where K is the quantum of circulation around of single vortex [Salort et al.,EPL 2011]. This empirical relation can be seen as the quantum-turbulence version of the corresponding well-know equation giving Kolmogorov dissipative scale in classical turbulence. In 2014, we studied the temperature dependence of the numerical factor (d/H).Re_K^{3/4} in 4He by joint numerical and experimental analysis of steady state turbulence over a wide temperature interval (1.2 - 2.16 K) [Babuin et al., EPL 2014]. Agreement between the two analyses was found good except at the very ends of this temperature interval. We will discuss this issue by presenting additional experimental data obtained by post-processing of superfluid experiments published between 1975 and 1998.
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14:45
15 mins
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UNIVERSAL STATISTICS OF POINT VORTEX TURBULENCE: THE DOUBLY-PERIODIC DOMAIN
Gavin Esler
Abstract: A new solution technique is used to obtain a statistical description
of the motion of $N$ point
vortices, evolving
in the usual two-dimensional doubly-periodic domain, in the limit $N
\to \infty$. In contrast to previous approaches such as \cite{joyce:73}, a
mean-field approximation is not used, meaning that the theory can
describe the full (divergent) stationary energy spectrum associated with
the vortex motion. An explicit
formula for this energy spectrum is obtained, which is compared with
direct numerical simulations with $N=50$ vortices, and excellent
agreement is found across a range of vortex interaction energies
(Hamiltonian of the point vortex system). The implications for
understanding related non-equilibrium systems such as 2D
classical turbulence and superfluid turbulence are discussed.
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15:00
15 mins
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OSCILLATING GRID HIGH REYNOLDS EXPERIMENTS IN SUPERFLUID
Ndeye Fatimata SY, Mickaël Bourgoin, Pantxo Diribarne, Mathieu Gibert, Bernard Rousset
Abstract: The study of particle preferential concentration in oscillating grid turbulence is described. Clustering in normal helium and superfluid helium is presented.
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