10:30
Superfluids 1
Chair: Mickael Bourgoin
10:30
15 mins
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Spectral properties of Andreev reflection from quantum turbulence in 3He-B. What do they tell about turbulent fluctuations?
Yuri A. Sergeev, Carlo F. Barenghi, Viktor Tsepelin, Andrew W. Baggaley, Shaun N. Fisher, George R. Pickett
Abstract: One of the experimental techniques developed to measure quantum turbulence at low temperatures in 3He-B utilizes the Andreev reflection of thermal quasiparticle excitations from quantized vortices and vortex structures. We present the results of theoretical, numerical, and experimental study of Andreev scattering from quantum turbulence in 3He-B. We analyze the spectral properties of the Andreev reflection and compare these with the spectral properties of superfluid turbulence, and discuss the physical mechanisms responsible for the scaling of spectral densities. Finally, we discuss the relation between our findings and related observables in ordinary turbulence.
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10:45
15 mins
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Motion of quantum vortex lines near realistic rough boundaries
George Stagg, Nick Parker, Carlo F. Barenghi
Abstract: We numerically solve the three--dimensional, time--dependent Gross--Pitaevskii
equation to model a superfluid flowing over a realistic rough boundary. Our model for the boundary is based on the atom--force microscope image of a NbTi vibrating wire used to generate turbulence in actual experiments. We find that near the boundary a dense region of quantum vortices is created, which forms a kind of `superfluid boundary layer'.
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11:00
15 mins
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Particle trajectories in thermal counterflow of superfluid helium
Marco La Mantia, Ladislav Skrbek
Abstract: The motion of micrometer-sized particles in a quantum flow -- thermal counterflow of superfluid helium -- is studied experimentally by using the particle tracking velocimetry technique, at various values of temperature and applied heat flux. The trajectory appearance changes as the imposed thermal counterflow velocity increases, i.e., the particle tracks appear, at low velocities, straighter than at larger ones, suggesting thus the transition to turbulence of the investigated flow.
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11:15
15 mins
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LIQUID HELIUM FLOWS AROUND AN OSCILLATING CYLINDER
Daniel Duda, Patrik Svancara, Marco La Mantia, Miloš Rotter, Ladislav Skrbek
Abstract: The complementary flows of normal viscous liquid helium (He I) and of superfluid helium (He II) around an oscillating obstacle, of rectangular cross-section, have been studied experimentally by using the particle tracking velocimetry technique, with solid deuterium particles. The observed particle behaviour in He II is very similar to that seen in He I. It seems therefore that, without some kind of special forcing acting differently on each superfluid helium component, on length scales which the experiment can access, the oscillating quantum flow mimics the classical one.
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11:30
15 mins
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Dissipation of Quasiclassical Turbulence in Superfluid 4He
Andrei Golov, Dmitry Zmeev, Paul Walmsley, Peter McClintock, William Vinen
Abstract: We compare the decay of turbulence in superfluid 4He produced by a moving grid to the decay of turbulence in the same channel created by either an impulsive spin-down to rest or by intensive ion injection. In all cases the vortex line density L at late time t decays as L ~ t^-3/2. Below 0.8 K, the spin-down turbulence maintains initial rotation and decays slower than grid turbulence and ion-jet turbulence. This is presumably due to the decoupling of the superfluid component from the normal one at large length scales at low temperatures, thus changing its effective boundary condition from no-slip to slip.
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11:45
15 mins
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Mutual-Friction Coefficients in Two-Dimensional Superfluids: From the Gross-Pitaevskii equation to the Hall-Vinen-Bekharevich-Khalatnikov Two-fluid Model
Vishwanath Shukla, Marc Brachet, Anupam Gupta, Rahul Pandit
Abstract: We start from the two-dimensional Gross-Pitaevskii equation (GPE) and develop algorithms
for the ab-initio determination of the temperature ($T$) dependence of the
mutual-friction coefficients, $\alpha$ and $\alpha'$, and the normal-fluid density
$\rho_n$, which appear as parameters in the Hall-Vinen-Bekharevich-Khalatnikov (HVBK) two-fluid
model for a superfluid. In the second part of our study,
we elucidate the statistical properties of two-dimensional, homogeneous, isotropic
superfluid turbulence in the simplified HVBK model, with values for the mutual-friction
coefficients that are comparable to those we obtain from the first part of our study.
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12:00
15 mins
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Particles and Fields in Superfluids: Insights from the Two-dimensional Gross-Pitaevskii Equation
Rahul Pandit, Vishwanath Shukla, Marc Brachet
Abstract: We study the dynamics of active particles in two-dimensional superfluids at temperature $T=0$, for a
variety of initial configurations, by carrying out extensive direct-numerical-simulations
of the two-dimensional, Galerkin-truncated Gross-Pitaevskii equation. Our study elucidates
the interplay of particles and fields, in both simple and turbulent flows. We show
that particle collisions can be inelastic, if the repulsive interactions between particles
is weak, and elastic otherwise. We show that assemblies of many particles and vortices
yield turbulent spatiotemporal evolutions.
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