The Ranque effect as a self-organization phenomenon

The principle of integral Lagrangian minimization is used to explain the basic phenomena taking place in a vortex tube: separation of gas molecules in energy, tendency for an increase in the Mach number, and gas temperature nonadiabatic radial distribution. Reasons for turbulence are discussed.     

Mutual-friction induced instability of normal-fluid vortex tubes in superfluid helium-4

It is shown that, as a result of its interactions with superfluid vorticity, a normal-fluid vortex tube in helium-4 becomes unstable and disintegrates. The superfluid vorticity acquires only a small (few percents of normal-fluid tube strength) polarization, whilst expanding in a front-like manner in the intervortex space of the normal-fluid, forming a dense, unstructured tangle in the process. The accompanied energy spectra scalings offer a structural explanation of analogous scalings in fully developed finite-temperature superfluid turbulence. A macroscopic mutual-friction model incorporating these findings is proposed.     

涡流管能量分离物理机制研究进展

发生在涡流管内的流动规律以及由此所产生的能量分离效应异常复杂。至今没有一种精确的理论能够从本质上解释其效应。但仍有研究者提出了各种各样的理论模型,以期揭示其能量分离物理机制。就国内外对涡流管能量分离效应物理机制的研究动态及成果作了较全面的论述,根据今后涡流管的发展方向,对下一步的研究工作给予了前景性的展望,提出了进一步开展涡流管相关研究工作应解决的问题与途径。     

On the velocity and dissipation signature of vortex tubes in isotropic turbulence

The properties of vortex tubes are extracted and analyzed from a DNS database at various Re_λ, with the objective to characterize the associated distributions of induced velocity and kinetic energy dissipation. The induced velocity exhibits an inverse power-law scaling in the far field, different from Burgers’ r⁻¹ scaling, supporting the interpretation that tubes are the remnants of vortex sheets after roll-up, and suggesting a possible link with the Kolmogorov k^−5/3 spectral scaling. The energy dissipation signature is characterized by a local maximum near the edge of the vortex core, and an absolute peak at its center, which can be tentatively explained appealing to the occurrence of a bi-axial configuration of the strain-rate tensor.     

Peak effect in polycrystalline vortex matter

We report a study of the peak-effect phase diagram of a strongly disordered type-II superconductor V-21 at. %Ti using ac magnetic susceptibility and small-angle neutron scattering (SANS). In this system, the peak effect appears only at fields higher than 3.4 T. The sample is characterized by strong atomic disorder. Vortex states with field-cooled thermal histories show that both deep in the mixed state, as well as close to the peak effect, there exist no long-range orientationally ordered vortex lattices. The SANS scattering radial widths reveal vortex states ordered in the sub-mum scale. We conjecture that the peak effect in this system is a remnant of the Bragg glass disordering transition, but occurs on submicron length scales due to the presence of strong atomic disorder on larger length scales.     

Aharonov-Bohm effect in a draining bathtub vortex

We study planar waves in a circulating, draining fluid flow, which: (i) exhibit an analogue of the Aharonov-Bohm (AB) effect in Quantum Mechanics; (ii) obey a Klein-Gordon equation on an ‘effective spacetime’ which resembles the Kerr spacetime of General Relativity; and (iii) may be observed in the laboratory using gravity waves in a shallow basin. We describe a modified AB effect which depends on two dimensionless parameters, associated with the circulation α and draining β rates; we call this the ‘αβ effect’. We show that the αβ effect is inherently asymmetric even in the low-frequency limit, and that it leads to novel interference patterns which carry the signature of both rotation and absorption.     

Fractional quantum hall effect and vortex lattices

It is demonstrated that all observed fractions at moderate Landau-level fillings for the quantum Hall effect can be obtained without recourse to the phenomenological concept of composite fermions. The possibility of having the special topologically nontrivial many-electron wavefunctions is considered. Their group classification indicates the special values of the electron density in the ground states separated by a gap from the excited states. The text was submitted by the author in English.     

Proximity effect and spontaneous vortex phase in planar SF structures

The proximity effect in SF structures is examined. It is shown that, due to the oscillations of the induced superconducting order parameter in a ferromagnet, the critical temperature of an SF bilayer becomes minimal when the thickness of the ferromagnetic layer is close to a quarter of the period of spatial oscillations. It is found that the spontaneous vortex state arising in the superconductor due to the proximity of the magnetic domain structure of a ferromagnet brings about noticeable magnetoresistive effects.     

Instabilities in the vortex matter and the peak effect phenomenon

In single crystals of 2H-NbSe2, we identify for the first time a crossover from a weak collective to a strong pinning regime in the vortex state which is not associated with the peak effect phenomenon. Instead, we find the crossover is associated with an anomalous history dependent magnetization response. In the dc magnetic field (Bdc)-temperature (T) vortex matter phase diagram we demarcate this pinning crossover boundary. We also delineate another boundary which separates the strong pinning region from a thermal fluctuation dominated regime, and find that a peak effect appears on this boundary.     

The effect of turbulence damping on acoustic wave propagation in tubes

The attenuation of sound due to the interaction between a low Mach number turbulent boundary layer and acoustic waves can be significant at low frequencies or in narrow tubes. In a recent publication by the present authors the acoustics of charge air coolers for passenger cars has been identified as an interesting application where turbulence attenuation can be of importance. Favourable low-frequency damping has been observed that could be used for control of the in-duct sound that is created by the engine gas exchange process. Analytical frequency-dependent models for the eddy viscosity that controls the momentum and thermal boundary layers are available but are restricted to thin acoustic boundary layers. For cases with cross-sections of a few millimetres a model based on thin acoustic boundary layers will not be applicable in the frequency range of interest.In the present paper a frequency-dependent axis-symmetric numerical model for interaction between turbulence and acoustic waves is proposed. A finite element scheme is used to formulate the time harmonic linearized convective equations for conservation of mass, momentum and energy into one coupled system of equations. The turbulence is introduced with a linear model for the eddy viscosity that is added to the shear viscosity. The proposed model is validated by comparison with experimental data from the literature.     

Cyclostrophic adjustment in swirling gas flows and the Ranque-Hilsch vortex tube effect

A theoretical analysis of cyclostrophic adjustment is presented; i.e., adjustment to balance between pressure gradient and centrifugal force in axisymmetric flow of an inviscid gas is examined. The solution to the problem is represented as the sum of a time-independent (balanced) and time-dependent (wave) components. It is shown that the wave component of the flow in an unbounded domain decays with time, and the corresponding solution reduces to the balanced component. In a bounded domain, the balanced flow component exists against the background of undamped acoustic waves. It is found that the balanced flow is thermally stratified at Mach numbers close to unity, with a substantial decrease in gas temperature (to between ?50 and ?100°C) in the axial region. This finding, combined with the results of special experiments, is used to explain the Ranque-Hilsch vortex tube effect.     

Experimental adiabatic vortex ratchet effect in Nb films with asymmetric pinning trap

Nb films grown on top of an array of asymmetric pinning centers show a vortex ratchet effect. A net flow of vortices is induced when the vortex lattice is driven by fluctuating forces on an array of pinning centers without reflection symmetry. This effect occurs in the adiabatic regime and it could be mimiced only by reversible DC driven forces.     

Peak effect and dynamic melting of vortex matter in NbSe2 crystals

We present a mode locking (ML) phenomenon of vortex matter observed around the peak effect regime of 2H-NbSe2 pure single crystals. The ML features allow us not only to trace how the shear rigidity of driven vortices persists on approaching the second critical field, but also to demonstrate a dynamic melting transition of driven vortices at a given velocity. We observe the velocity dependent melting signatures in the peak effect regime, which reveal a crossover between the disorder-induced transition at small velocity and the thermally induced transition at large velocity. This uncovers the relationship between the peak effect and the thermal melting.     

Anomalous behavior of the transverse acoustoelectric effect in superconductors with a moving vortex structure and the formation of acoustic vortex domains

It is shown that a longitudinal ultrasonic wave in superconductors with a moving vortex structure creates an additional transverse acoustoelectric field proportional to the power of this wave. This field changes its sign when the velocity of the vortex structure is lower than the velocity of the ultrasonic wave. This behavior qualitatively distinguishes this effect from a similar effect on the conduction electrons in semiconductors. Such a behavior of the acoustoelectric field, together with the effect of the amplification of the ultrasonic wave, can give rise to the negative differential conductivity and, therefore, to the formation of acoustic vortex domains in the superconductor.     

离轴涡旋光束弱走离条件下的倍频效应

利用耦合波截断方程推导了无走离及弱走离条件下离轴涡旋光束经过负单轴晶体后产生倍频光的归一化电场及光强表达式. 主要分析了离轴量、弱走离角及晶体长度对于输出倍频光的影响. 研究结果表明: 不考虑走离效应时, 倍频光两暗核重合于轴上点, 而具有弱走离效应时, 暗核位置沿走离方向有所偏移, 并且两重合暗核产生分离, 截面上发生分离的方向垂直于存在走离的方向, 其中产生的偏移量与离轴量、走离角及晶体长度有关, 而分离的距离只与走离角及晶体长度相关. 具体表现为: 当离轴量增大, 倍频光两暗核沿存在离轴方向所产生的偏移增大, 而暗核间的分离距离不受离轴量影响, 当走离角、晶体长度增大时, 两暗核沿存在走离方向所产生的偏移增大, 同时, 暗核间的分离距离也有所增大. 此外, 通过对比发现, 传输晶体长度的缩小, 可以减小暗核沿存在走离方向或是存在离轴方向所产生的偏移, 同时也可以减小由离轴量与走离角所产生的暗核分离距离, 一定程度上对输出的倍频光起到校正作用.     

The effect of Dirac phase on acoustic vortex in media with screw dislocation

We study acoustic vortex in media with screw dislocation using the Katanaev–Volovich theory of defects. It is shown that the screw dislocation affects the beam?s orbital angular momentum and changes the acoustic vortex strength. This change is a manifestation of topological Dirac phase and is robust against fluctuations in the system.     

Generation of optical vortex array based on the fractional Talbot effect

A phase-only diffractive optical element (DOE) for generating array of optical vortices with high compression ratio is presented. Since it is designed according to the fractional Talbot effect, we name the DOE optical vortex Talbot array illuminator (OVTAI). As an example, an OVTAI for generating a hexagonal array of optical vortices is designed, and demonstrated through displaying the OVTAI on a programmable liquid crystal spatial light modulator. The diffraction properties of the vortex array generated by the OVTAI are observed and analyzed, and an optimal distance for generating sharp ringed vortex arrays is given.