scholarly journals Controllable vortex loops in superconducting proximity systems

2019 ◽  
Vol 100 (21) ◽  
Author(s):  
Eirik Holm Fyhn ◽  
Jacob Linder
Keyword(s):  
Vortex Loops

The formation mechanism and shedding frequency of vortices from a sphere in uniform shear flow

1995 ◽  
Vol 287 ◽  
pp. 151-171 ◽  
Author(s):  
Hiroshi Sakamoto ◽  
Hiroyuki Haniu
Keyword(s):  
Reynolds Number ◽  
Shear Flow ◽  
Formation Mechanism ◽  
Vortex Shedding ◽  
Uniform Flow ◽  
Sphere Diameter ◽  
Uniform Shear ◽  
Uniform Shear Flow ◽  
Vortex Loops ◽  
Shear Parameter

Experiments to investigate the formation mechanism and frequency of vortex shedding from a sphere in uniform shear flow were conducted in a water channel using flow visualization and velocity measurement. The Reynolds number, defined in terms of the sphere diameter and approach velocity at its centre, ranged from 200 to 3000. The shear parameter K, defined as the transverse velocity gradient of the shear flow non-dimensionalized by the above two parameters, was varied from 0 to 0.25. The critical Reynolds number beyond which vortex shedding from the sphere occurred was found to be lower than that for uniform flow and decreased approximately linearly with increasing shear parameter. Also, the Strouhal number of the hairpin-shaped vortex loops became larger than that for uniform flow and increased as the shear parameter increased.The formation mechanism and the structure of vortex shedding were examined on the basis of series of photographs and subsequent image processing using computer graphics. The range of Reynolds number in the present investigation, extending up to 3000, could be classified into three regions on the basis of this study, and it was observed that the wake configuration did not differ substantially from that for uniform flow. Also, unlike the detachment point of vortex loops in uniform flow, which was irregularly located along the circumference of the sphere, the detachment point in shear flow was always on the high-velocity side.

Three-dimensional wake transition

1996 ◽  
Vol 328 ◽  
pp. 345-407 ◽  
Author(s):  
C. H. K. Williamson
Keyword(s):  
Shear Layer ◽  
Three Dimensional ◽  
Streamwise Vortex ◽  
Physical Structure ◽  
Small Scale ◽  
Half Cycle ◽  
Vortex Loops ◽  
Wake Transition ◽  
Mode A ◽  
Flow States

It is now well-known that the wake transition regime for a circular cylinder involves two modes of small-scale three-dimensional instability (modes A and B), depending on the regime of Reynolds number (Re), although almost no understanding of the physical origins of these instabilities, or indeed their effects on near-wake formation, have hitherto been made clear. We address these questions in this paper. In particular, it is found that the two different modes A and B scale on different physical features of the flow. Mode A has a larger spanwise wavelength of around 3–4 diameters, and scales on the larger physical structure in the flow, namely the primary vortex core. The wavelength for mode A is shown to be the result of an ‘elliptic instability’ in the nearwake vortex cores. The subsequent nonlinear growth of vortex loops is due to a feedback from one vortex to the next, involving spanwise-periodic deformation of core vorticity, which is then subject to streamwise stretching in the braid regios. This mode gives an out-of-phase streamwise vortex pattern.In contrast, mode-B instability has a distinctly smaller wavelength (1 diameter) which scales on the smaller physical structure in the flow, the braid shear layer. It is a manifestation of an instability in a region of hyperbolic flow. It is quite distinct from other shear flows, in that it depends on the reverse flow of the bluff-body wake; the presence of a fully formed streamwise vortex system, brought upstream from a previous half-cycle, in proximity to the newly evolving braid shear layer, leads to an in-phase stream-wise vortex array, in strong analogy with the ‘Mode 1’ of Meiburg & Lasheras (1988) for a forced unseparated wake. In mode B, we also observe amalgamation of streamwise vortices from a previous braid with like-sign vortices in the subsequent braid.It is deduced that the large scatter in previous measurements concerning mode A is due to the presence of vortex dislocations. Dislocations are triggered at the sites of some vortex loops of mode A, and represent a natural breakdown of the periodicity of mode A instability. By minimizing or avoiding the dislocations which occur from end contamination or which occur during wake transition, we find an excellent agreement of both critical Re and spanwise wavelength of mode A with the recent secondary stability analysis of Barkley & Henderson (1996).Wake transition is further characterized by velocity and pressure measurements. It is consistent that, when mode-A instability and large-scale dislocations appear, one finds a reduction of base suction, a reduction of (two-dimensional) Reynolds stress level, a growth in size of the formation region, and a corresponding drop in Strouhal frequency. Finally, the present work leads us to a new clarification of the possible flow states through transition. Right through this regime of Re, there exist two distinct and continuous Strouhal frequency curves: the upper one corresponds with purley small- scale instabilities (e.g. denoted as mode A), while the lower curve corresponds with a combination of small-scale plus dislocation structures (e.g. mode A*). However, some of the flow states are transient or ‘unstable’, and the natural transitioning wake appears to follow the scenario: (2D→A*→B).

scholarly journals Probing of quantum turbulence with the emitting vortex loops

2014 ◽  
Vol 40 (12) ◽  
pp. 1116-1118 ◽  
Author(s):  
S. K. Nemirovskii
Keyword(s):  
Quantum Turbulence ◽  
Vortex Loops

Interpreting Vortex Interactions With a Free Surface

1994 ◽  
Vol 116 (1) ◽  
pp. 91-94 ◽  
Author(s):  
E. P. Rood
Keyword(s):  
Free Surface ◽  
Surface Condition ◽  
Viscous Forces ◽  
Vortex Interactions ◽  
Free Surface Condition ◽  
Vortex Loops ◽  
Physical Experiments ◽  
Vorticity Flux ◽  
Tangential Acceleration

An understanding of the process by which vorticity interacts with a free surface is sought by analytical examination of the free-surface condition for the vorticity flux. A novel mechanism is suggested that permits closed vortex loops to evolve into open loops terminating at the free surface. It is hypothesized that abrupt vortex “disconnection,” observed in physical experiments, arises from a smooth diffusion of vorticity through the interface, with a necessary coincident tangential acceleration of the interface attributed to viscous forces.

The nucleation of vorticity by ions in superfluid 4 He I. Basic theory

1984 ◽  
Vol 311 (1518) ◽  
pp. 433-467 ◽  
Keyword(s):  
High Pressure ◽  
Potential Barrier ◽  
Superfluid Helium ◽  
Recent Observation ◽  
Critical Value ◽  
Vortex Loop ◽  
Quantum Tunnelling ◽  
Vortex Loops ◽  
Very High ◽  
Vortex Nucleation

A theory is developed of vortex nucleation by an ion moving in superfluid helium at a low temperature. It is shown that production of a vortex loop attached to the side of the ion becomes energetically possible when the velocity of the ion exceeds a critical value, but that nucleation is impeded by the presence of a small potential barrier. The predicted critical velocity is close to that observed experimentally, at least at high pressure. Nucleation of an encircling vortex ring, considered some years ago by Schwarz & Jang (Phys.Rev. A8,3199 (1973)), probably becomes possible only at a higher velocity, and it is impeded by a large potential barrier. It is shown that for vortex loops the potential barrier can probably be overcome at a rate consistent with experiment either by quantum tunnelling at the lowest temperatures or thermally, by absorption of a single roton, at higher temperatures. Possible explanations of the recent observation by Bowley et al . {Phil. Trans. R. Soc. Lond. A307, 201 (1982)) that at high pressure the rate of vortex nucleation falls off at very high velocities are discussed.

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