Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session E18: Superfluids |
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Chair: William Irvine, University of Chicago Room: 2004 |
Sunday, November 23, 2014 4:45PM - 4:58PM |
E18.00001: Approach and separation of quantum vortices with balanced cores Robert M. Kerr, C. Rorai, J. Skipper, K.R. Sreenivasan Using two innovations, smooth but different, scaling laws for the reconnection of pairs of initially orthogonal and anti-parallel quantum vortices are obtained using the three-dimensional Gross-Pitaevskii equations. For the anti-parallel case, the scaling laws just before and after reconnection obey the dimensional $\delta\sim|t-t_r|^{1/2}$ prediction with temporal symmetry about the reconnection time $t_r$ and physical space symmetry about $x_r$, the mid-point between the vortices, with extensions forming the edges of an equilateral pyramid. For all of the orthogonal cases, before reconnection $\delta_{in}\sim(t-t_r)^{1/3}$ and after reconnection $\delta_{out}\sim(t_r-t)^{2/3}$, which are respectively slower and faster than the dimensional prediction. In these cases, the reconnection takes place in a plane defined by the directions of the curvature and vorticity. [Preview Abstract] |
Sunday, November 23, 2014 4:58PM - 5:11PM |
E18.00002: The Effect of Quantized Vortices on Particle Motion in Superfluid Helium Akira Hirano, Daiki Kato, Ryouhei Ohtaka, Akifumi Iwamoto, Takahiro Ito, Yoshiyuki Tsuji Superfluid $^{4}$He (HeII) exits as liquid phase below 2.17K and indicates peculiar flow structure such as no viscosity and super heat conduction. Therefore, HeII is used as refrigerant in superconducting magnet. HeII property is well understood by so called two-fluid model that is composed of superfluid and normalfluid component. Quantized vortices are generated in superfluid component when the heat flux is larger than the critical value in a thermal counter flow. In this study, we use solid hydrogen particles as tracer and visualize tracer particle motions. The particles are forced by Stokes drag with the normalfluid and trapped by the quantized vortex with superfluid. The particle motions differ depending on the interaction between particle and quantized vortex. In order to analysis the particle trajectory, we adopt Pparticle Tracking Velocimetry. We identified two distinct types of particle trajectories moving straightly with normal fluid and moving irregularly with superfluid and apparently trapped by quantized vortex. They are compared with previous studies. The distribution of the vertical velocity component of particle motion was bimodal, which are consistent with theoretical values. We discuss in detail how the particle moves trapped by quantized vortices. [Preview Abstract] |
Sunday, November 23, 2014 5:11PM - 5:24PM |
E18.00003: Three Dimensional Quantized Vortex Dynamics in Superfluid Helium David Meichle, Peter Megson, Daniel Lathrop Vorticity is constrained to line-like topological defects in quantum superfluids, such as liquid Helium below the Lambda transition temperature of 2.17 Kelvin. A tangle of vortices exists in a dissipative dynamical state called quantum turbulence, which has quantitative features distinct from classical turbulence. To study the vortex dynamics, we have invented a novel method to disperse fluorescent nanoparticles directly into the superfluid which become trapped on the vortex cores. Using a newly constructed multi-camera stereographic microscope, we present new data showing vortex reconnections and Kelvin waves with fully three-dimensional particle trajectories. These events are of scientific interest as they play a key role in the dissipation of quantum turbulence. [Preview Abstract] |
Sunday, November 23, 2014 5:24PM - 5:37PM |
E18.00004: Knots and Coils in Superfluid Vortices Dustin Kleckner, Davide Proment, Martin Scheeler, William T.M. Irvine Recent work has demonstrated that linked and knotted vortices will spontaneously unknot or untie in both classical fluids and superfluids. This effect would appear to jeopardize any notion of conservation of fluid topology (helicity), but this need not be the case: vortices can transfer their knottedness to helical coils, preserving some measure of the original topology. By simulating superfluid vortices in the Gross-Pitaevskii equation, we find a geometric mechanism for efficiently transferring helicity in exactly this manner. Remarkably, the same transfer of topology to geometry also appears in viscous fluid vortices, suggesting it is a generic feature of non-ideal fluids. [Preview Abstract] |
Sunday, November 23, 2014 5:37PM - 5:50PM |
E18.00005: Quantum analogues of classical wakes in Bose-Einstein condensates George Stagg, Nick Parker, Carlo Barenghi We show that an elliptical obstacle moving through a Bose-Einstein condensate generates wakes of quantum vortices which resemble those of classical viscous flow past a cylinder or sphere. Initial steady symmetric wakes, similar to those observed in classical flow at low Reynolds number, lose their symmetry and form clusters of like-signed vortices, in analogy to the classical B\'enard--von K\'arm\'an vortex street. The key ingredient to produce classical-like wakes is that vortices are generated at a sufficiently high rate that they undergo strong interactions with their neighbours (rather than being swept away). The role of ellipticity is to facilitate the interaction of the vortices and to reduce the critical velocity for vortex nucleation. Our findings, demonstrated numerically in both two and three dimensions, confirm the intuition that a sufficiently large number of quanta of circulation reproduce classical physics. The effects which we describe (dependence of the critical velocity and cluster size on the obstacle's size, velocity and ellipticity) are also relevant to the motion of objects (such as vibrating wires, grids and forks) in superfluid helium, as the obstacle's ellipticity plays a role which is analogous to rough boundaries. [Preview Abstract] |
Sunday, November 23, 2014 5:50PM - 6:03PM |
E18.00006: Acoustics of the Lambda Transition in Superfluid Helium Peter Megson, David Meichle, Daniel Lathrop Liquid Helium undergoes a phase transition and becomes a quantum superfluid when cooled below the Lambda transition temperature of 2.17 Kelvin. The superfluid, which is a partial Bose Einstein Condensate, exhibits unique macroscopic properties such as flow without viscosity and ballistic temperature propagation. We have recorded striking audio-frequency sounds using a micro electromechanical microphone (MEMS) present as the Helium goes through the Lambda transition. Characterization of this sound, as well as its relevance to theories of the Lambda transition will be presented. [Preview Abstract] |
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