Vortex Modes Research Articles

Spectral proper orthogonal decomposition of turbine tip clearance flow under low-Reynolds number conditions

Spectral proper orthogonal decomposition of turbine tip clearance flow under low-Reynolds number conditions

Origin of Chiral Phase Transition of Polar Vortex in Ferroelectric/Dielectric Superlattices.

Chiral vortices and their phase transition in ferroelectric/dielectric heterostructures have drawn significant attention in the field of condensed matter. However, the dynamical origin of the chiral phase transition from achiral to chiral polar vortices has remained elusive. Here, we develop a phase-field perturbation model and discover the softening of out-of-plane vibration mode of polar vortices in [(PbTiO3)m/(SrTiO3)m]n superlattices at a critical epitaxial strain or temperature. The softening of the mode leads to the appearance of the axial polarization at vortex cores, resulting in chiral phase transition. It is found that the local negative permittivity plays a crucial role in the enhanced oscillation of axial polarization near the phase transition. Our findings not only reveal the origin of the chiral phase transition of polar vortices but also provide considerable new insight into the dynamics of topological structures and topological phase transitions in ferroelectric systems.

Generation of tunable high-order vortex beams from a Hermite-Gaussian thin-disk laser.

We demonstrate the generation of high-order, high-power vortex modes from a Hermite-Gaussian (HG) Yb:YAG thin-disk oscillator, with tunable mode orders ranging continuously from one to ten. To the best of our knowledge, this is the highest order of HG and vortex modes obtained using a thin-disk module. The output power for most of these modes reaches up to 10 W, setting a new benchmark for intracavity high-order HG mode generation. The mode tunability is demonstrated experimentally and supported by simulations, realized by adjusting the angle and position of the output coupler to manipulate the intracavity resonance conditions. A cylindrical-lens mode converter is employed to transform the generated HG modes into Laguerre-Gaussian vortex modes. The properties of the resulting vortex beams are measured using a Mach-Zehnder interferometer and phase retrieval algorithms. Our results demonstrate significant progress in generating high-order structured light with an extended tunable range and high-power operation within thin-disk laser systems, providing new opportunities for applications in light-matter interaction, remote sensing, and optical manufacturing.

Influence of perpendicular uniaxial anisotropy on the switching of a magnetic vortex

Magnetic vortices are being considered for information storage in magnetic devices. In this study, we used micromagnetic simulations to explore the effect of a perpendicular uniaxial anisotropy (PUA) on switching the vortex core in Permalloy nanodisks. We studied how the presence of the perpendicular uniaxial anisotropy (PUA) changes the spatial profile of the magnetic vortex. We determined the diameters of the vortex core as the perpendicular uniaxial anisotropy constant Kz varied. Additionally, we determined the frequencies and spatial profiles of the radial modes of the spin waves. Our results show that the PUA affects the frequencies of the spin modes of a magnetic vortex in a nanodisk. We have also created phase diagrams demonstrating the areas where reversing the magnetic vortex core is possible by applying a sinusoidal field perpendicular to the nanodisk plane.

A computational fluid dynamics study of the impact of end effects on the Magnus effect and wake transition in the flow around a circular rotating cylinder with two free ends

This paper investigates the influence of end effects on the Magnus effect and the three-dimensional unsteady flow behavior of a circular rotating cylinder with two free ends. The large eddy simulation method, using the Smagorinsky–Lilly model, is employed with a fixed aspect ratio of 10. The Reynolds number (Re) varies from 50 to 500, and the spin ratio (α) ranges from 0 to 5. Within the examined parameter space, eight instantaneous wake vortex modes are identified, including two steady flow modes, three unsteady periodic vortex modes, and two unsteady nonperiodic vortex modes. As α increases, the mean drag coefficient initially changes gradually, followed by a rapid increase, and eventually stabilizes or decreases. Two critical spin ratios are identified. As Re increases, the first critical spin ratio increases from 0.5 to 1, while the second decreases from 4.5 to 3.5. For the mean lift coefficient, a critical spin ratio is identified, decreasing from 4 to 3.5 as Re increases. The results indicate that the influence range of end effects on dynamic coefficients is primarily governed by the rotation effects, while Re also affects the values and distributions of dynamic coefficients, particularly at low Reynolds numbers.

Vortex light bullets in rotating Quasi-Phase-Matched photonic crystals with quadratic and cubic nonlinearity

Vortex light bullets in rotating Quasi-Phase-Matched photonic crystals with quadratic and cubic nonlinearity

Transition of counter-rotating plasma-induced vortices into a jet

Two low-frequency-modulated plasma actuators are symmetrically positioned on either side of a thin flat plate to continuously generate pairs of counter-rotating vortices, with velocity fields captured using time-resolved particle image velocimetry (PIV). Convolutional neural networks with a U-Net architecture are adopted to generate the phase-averaged velocity field of plasma-induced vortices from a single PIV snapshot to eliminate the requirement of multiple measurements. The influence of the number of input features and samples on the model accuracy is examined. The model-predicted results match well with the measurements, accurately restoring the vortex dynamics and capturing the strength variation. The proposed model is then utilized to reconstruct the phase-averaged results of the starting vortex for continuous plasma jets, revealing that the evolution of plasma-induced vortices and their transition into a jet are characterized by four distinct stages: formation, boosting, distortion, and jetting, all governed by the vortex convection velocity. The vortices exhibit a marked increase in circulation after a delayed development, moving linearly downstream while diverging from the centerline. Consequently, the vortex-induced effect significantly enhances the centerline velocity. The vortices are subsequently stretched and distorted from a circular into a chain-like shape due to the large velocity gradient between the vortex pair, leading to the vortex breakdown and transition into a jet, accompanied by a collapse in the velocity magnitude. Insights into vortex-to-jet transition inform the optimal placement of plasma actuators, thereby enhancing control efficiency in active flow control applications.

Nonlinear interaction of an acoustical wave with a counter-propagating weak shock.

During its propagation, a shock wave may come across and interact with different perturbations, including acoustical waves. While this issue has been the subject of many studies, the particular acoustic-acoustic interaction between a weak shock and a sound wave has been very scarcely investigated. Here, a theory describing the encounter of those two waves is developed, up to second- and third-order. According to the incidence angle and shock strength, several regimes of acoustic transmission through the shock are identified. The generation of entropy as well as vorticity modes are determined, while the perturbation of the shock front by the acoustic wave is quantified. The theory predicts strongly different behaviors between air and water, and preliminary results are coherent with recent experimental observations in solids. It paves the way to both an acoustic monitoring of shock wave as well as a method to determine the quadratic and cubic nonlinear parameters of material.

Waves and non-propagating mode in stratified and rotating magnetohydrodynamic turbulence

In this study, we consider a freely decaying, stably stratified, and rotating homogeneous magneto-hydrodynamic (MHD) turbulent plasma with a vertical background magnetic field (B0=B0ẑ), aligned with the density gradient (with a constant Brunt–Váisálá frequency N) viewed in a frame rotating uniformly around the vertical axis (Ω0=Ω0ẑ). Quasi-linear theory is used to analyze the flow dynamics for an inviscid and non-diffusive Boussinesq fluid. We perform a normal mode decomposition emphasizing three types of motions: a non-propagating (NP) mode, which is no longer a vortex mode, and slow and fast magneto-inertia-gravity waves. The total energy as well as the L2 norm, say Γ, of the magnetic induction potential scalar (MIPS), which remains similar to the potential enstrophy for non-magnetized rotating and stratified flows, are inviscid invariants. In contrast with the potential vorticity for non-magnetized rotating and stratified flows, the MIPS is not affected by system rotation in the quasi-linear limit, and this is the effect of rotation which presumes an inverse cascade of energy in the equilibrium statistical mechanics. We characterized the system setting up our investigation from the point of view of equilibrium statistical mechanics in the limit of small Froude number and small Alfvén–Mach number. In this limit, the non-propagating quantity Γ can be approximated by its quadratic part that explicitly depends only on the vertical component of the fluctuating magnetic field and the density fluctuations. We demonstrate that the partition function restricted to the non-propagating manifold does not indicate an inverse cascade of energy.

MEMS-metasurface-enabled mode-switchable vortex lasers.

Compared to conventional lasers limited to generating static modes, mode-switchable lasers equipped with adjustable optics significantly enhance the flexibility and versatility of coherent light sources. However, most current approaches to achieving mode-switchable lasers depend on conventional, i.e., inherently bulky and slow, optical components. Here, we demonstrate fiber lasers empowered by electrically actuated intracavity microelectromechanical system (MEMS)-based optical metasurface (MEMS-OMS) enabling mode switching between fundamental Gaussian and vortex modes at ~1030 nm. By finely adjusting the voltage applied to the MEMS mirror, high-contrast switching between Gaussian (l = 0) and vortex (l = 1, 2, 3, and 5, depending on the OMS arrangement) laser modes is achieved, featuring high mode purities (>95%) and fast responses (~100 microseconds). The proposed intracavity MEMS-OMS-enabled laser configuration provides an at-source solution for generating high-purity fast-switchable laser modes, with potential applications ranging from advanced optical imaging to optical tweezers, optical machining, and intelligent photonics.

Topological orbital angular momentum extraction and twofold protection of vortex transport

Vortex phenomena are ubiquitous in nature. In optics, despite the availability of numerous techniques for vortex generation and detection, topological protection of vortex transport with desired orbital angular momentum (OAM) remains a challenge. Here, by use of topological disclination, we demonstrate a scheme to confine and guide vortices featuring arbitrary high-order charges. Such a scheme relies on twofold topological protection: a non-trivial winding in momentum space due to chiral symmetry, and a non-trivial winding in real space due to the complex coupling of OAM modes across the disclination structure. We unveil a vorticity-coordinated rotational symmetry, which sets up a universal relation between the vortex topological charge and the rotational symmetry order of the system. As an example, we construct photonic disclination lattices with a single core but different Cn symmetries and achieve robust transport of an optical vortex with preserved OAM solely corresponding to one selected zero-energy vortex mode at the mid-gap. Furthermore, we show that such topological structures can be used for vortex filtering to extract a chosen OAM mode from mixed excitations. Our results illustrate the fundamental interplay of vorticity, disclination and higher-order topology, which may open a new pathway for the development of OAM-based photonic devices such as vortex guides, fibres and lasers.

Correction: Mariaccia et al. Impact of Polar Vortex Modes on Winter Weather Patterns in the Northern Hemisphere. Atmosphere 2024, 15, 1062

In the original publication [...]

Competing effects of temperature and mechanical stress on polar vortex transition in oxide superlattices

Competing effects of temperature and mechanical stress on polar vortex transition in oxide superlattices

Wake characteristics of near-wall submerged bluff bodies with varying streamwise length

This study aims to investigate the effect of streamwise length on the wake characteristics of submerged sharp-edged bluff bodies in the presence of an underbody gap using large eddy simulation. To this end, three bodies with identical width (W) and height (h), but varying only in their streamwise lengths (L) were employed resulting in streamwise elongation ratios of L/h = 1, 2, and 3, respectively. The underbody gap between the bottom face of the body and the wall was fixed at 0.14 h for all cases. A fully developed turbulent boundary layer with a thickness of 3.6 h was used as the approaching flow. It was noted that the mean flow and turbulent stresses were significantly affected by the streamwise length. Premultiplied frequency spectra of the velocity fluctuations were utilized to examine the fluctuating properties of the wake. A single dominant vortex shedding frequency was observed for L/h = 1 and 3, whereas dual mode vortex shedding was noted for L/h = 2. The latter case exhibited an intermittent reattachment on the top surface of the body. The fluid structures evaluated using the λ2 criterion, indicated that they were strongly influenced by L/h. Interestingly, even with the presence of a gap, a weak horseshoe vortex which occurred intermittently was captured close to the bed for the three cases.

Nonlinear evolution of vortical disturbances entrained in the entrance region of a circular pipe

The nonlinear evolution of free-stream vortical disturbances entrained in the entrance region of a circular pipe is investigated using asymptotic and numerical methods. Attention is focused on the low-frequency disturbances that induce streamwise elongated structures. A pair of vortical modes with opposite azimuthal wavenumbers is used to model the free-stream disturbances. Their amplitude is assumed to be intense enough for nonlinear interactions to occur inside the pipe. The formation and evolution of the perturbation flow are described by the nonlinear unsteady boundary-region equations in the cylindrical coordinate system, derived and solved herein for the first time. Matched asymptotic expansions are employed to construct appropriate initial conditions and the initial–boundary value problem is solved numerically by a marching procedure in the streamwise direction. Numerical results show the stabilising effect of nonlinearity on the intense algebraic growth of the disturbances and an increase of the wall-shear stress due to the nonlinear interactions. A parametric study is carried out to evince the effect of the Reynolds number, the streamwise and azimuthal wavelengths, and the radial length scale of the inlet disturbance on the nonlinear flow evolution. Elongated pipe-entrance nonlinear structures (EPENS) occupying the whole pipe cross-section are discovered. EPENS with $h$ -fold rotational symmetry comprise $h$ high-speed streaks positioned near the wall, and $h$ low-speed streaks centred around the pipe core. These distinct structures display a striking resemblance to nonlinear travelling waves found numerically and observed experimentally in fully developed pipe flow. Good agreement of our mean-flow and root mean square data with experimental measurements is obtained.

Spontaneous symmetry breaking and vortices in a tri-core nonlinear fractional waveguide

Spontaneous symmetry breaking and vortices in a tri-core nonlinear fractional waveguide

Vortex solitons in rotating quasi-phase-matched photonic crystals.

We present an approach to generate stable vortex solitons (VSs) in rotating quasi-phase-matched photonic crystals with quadratic nonlinearity. The photonic crystal is introduced with a checkerboard structure, which can be realized using available technology. The VSs are constructed as four-peak vortex modes of two types: rhombuses and squares. Control parameters, including the power, rotating frequency, and size of each square cell, affect the distribution and stability range of these VSs. The tightly binding rhombic VSs realize the system's ground state, which features the lowest value of the Hamiltonian. By introducing rotation, stable VSs with topological charges l = ±1 and ±2 are observed, and the VSs turn from a quadrupole to a vortex-like state. The generation and modulation of stable VSs in rotating quasi-phase-matched photonic crystals demonstrate promising applications in optical communication systems, optical tweezers, and quantum information processing, where precise control over light propagation and vortex states is crucial.

Orbital angular momentum superimposed mode recognition based on multi-label image classification.

Orbital angular momentum (OAM) multiplexing technology has great potential in high capacity optical communication. OAM superimposed mode can extend communication channels and thus enhance the capacity, and accurate recognition of multi-OAM superimposed mode at the receiver is very crucial. However, traditional methods are inefficient and complex for the recognition task. Machine learning and deep learning can offer fast, accurate and adaptable recognition, but they also face challenges. At present, the OAM mode recognition mainly focus on single OAM mode and ±l superimposed dual-OAM mode, while few researches on multi-OAM superimposed mode, due to the limitations of single-object image classification techniques and the diversity of features to recognize. To this end, we develop a recognition method combined with multi-label image classification to accurately recognize multi-OAM superimposed mode vortex beams. Firstly, we create datasets of intensity distribution map of three-OAM and four-OAM superimposed mode vortex beams based on numerical simulations and experimental acqusitions. Then we design a progressive channel-spatial attention (PCSA) model, which incorporates a progressive training strategy and two weighted attention modules. For the numerical simulation datasets, our model achieves the highest average recognition accuracy of 94.9% and 91.2% for three-OAM and four-OAM superimposed mode vortex beams with different transmission distances and noise strengths respectively. The highest experimental average recognition accuracy for three-OAM superimposed mode achieves 92.7%, which agrees with the numerical result very well. Furthermore, our model significantly outperforms in most metrics compared with ConvNeXt, and all experiments are within the affordable range of computational cost.

Single- and dual-tube He-Ne lasers for high-power excitation of LG01-mode vortex beam

Abstract In this study, aiming to extend the power of LG01-mode vortex light at 632.8nm, the commercial He-Ne laser was upgraded to dual-tube gain geometry with the assistance of an intracavity spot defect for spatial filtering. We investigated the impact of using single and double He-Ne gas gain tubes on the powers of vortex laser output , and achieved a maximum 4.7-mW power of LG01-mode vortex light, correspionding to an optical-optical conversion efficiency of 39.2% and making it the highest power of vortex light obtained in He-Ne laser.

Generating optical vortex beams using cylindrical waveguides

As an initial step toward exciting optical vortices using high-power millimeter waves, this study developed a method for inducing vortex modes within a cylindrical waveguide using Gaussian beams. By examining the coupling between tilted and offset Gaussian beams and specific waveguide modes, appropriate tilt angles and offsets were obtained. Numerical simulations demonstrated that the vortex modes could be efficiently excited using four Gaussian beams. Furthermore, the results revealed that increasing the number of Gaussian beams improves the excitation efficiency of vortex modes.