Vortex Wave Research Articles

Shock wave structures and vortex unsteadiness in the tip region of a transonic turbine cascade under different conditions

The tip region of transonic turbine blades (exit Mach number of 0.95) exhibits complex flow characteristics with the coexistence of multiple shock wave systems and multiscale vortices. Based on a validated detached Eddy simulation method, unsteady flow features such as the spatial-temporal dynamic evolution of tip leakage vortices (TLV) and the periodic buffet/oscillation of shock trains inside the tip gap are revealed and discussed systematically under different incidence angles (i) and heights of tip clearance. Moreover, the distinction of tip flow structures between the subsonic and transonic conditions is also manifested. Results indicate that the wandering behavior of the TLV is influenced by both the swirling strength of the vortex itself and the interaction of adjacent secondary vortices. The TLV under a tip gap of 5% blade height (h) exhibits “binary and bimodal” wandering characteristics both along the pitchwise and spanwise direction, whereas under the 1%h case, only the pitchwise wandering is prominent. The dominant characteristic frequency of the TLV wandering under different conditions falls within the spectrum range of 2.2–2.4 kHz. As for the shock trains inside the tip clearance (τ), coherently movement back and forth along the pitchwise direction with varying amplitudes can be observed, where the magnitude within the τ=1%h exceeds that observed in the τ=5%h, depending on the intensity of the shock waves. Notably, significant shock wave oscillations are present throughout the range of the chord length (c) within the τ=1%h, whereas within the tip gap of 5%h, shock wave systems exhibit more pronounced oscillations predominantly near the trailing edge.

Phase and amplitude simultaneously coding metasurface with multi-frequency and multifunctional electromagnetic modulations

The integration of multiple functionalities into a single, planar, ultra-compact metasurface has presented significant opportunities for enhancing capacity and performance within compact 5G/6G communication systems. Recent advances in multifunctional metasurfaces have unveiled comprehensive wavefront manipulations utilizing phase, polarization transmission/reflection, and coding apertures. Despite these developments, there remains a critical need for multifunctional metasurfaces with expanded channel capabilities, including multiple operational frequencies, minimal crosstalk, and high-efficiency computable array factors. This study introduces a multifunctional metasurface that integrates phase- and amplitude simultaneous coding meta-atoms at dual frequencies. By altering the polarization of electromagnetic (EM) waves, it is possible to reshape the wave-fronts of reflected waves at these frequencies. The coding metasurface proficiently manipulates both x and y linearly polarized waves through phase and amplitude coding at dual frequencies, thereby enabling distinct functionalities such as anomalous reflection, reflection imaging, and vortex wave beam generation. Both theoretical analysis and full-wave simulation confirm the anticipated functionalities of the designed devices, paving the way for advancements in integrated communication systems with diverse functionalities.

A reconfigurable ultra-wideband high-purity multimode terahertz OAM antenna array based on graphene and vanadium dioxide

A reconfigurable ultra-wideband multimode terahertz OAM antenna array based on graphene and vanadium dioxide (VO2) is presented in this paper. The uniform circular array (UCA) composed of circularly polarized (CP) antennas, which include a ring radiation patch, a parasitic patch, a perovskite substrate and an irregular VO2 ground plane. The radiation patch is covered with a layer of graphene, by changing the chemical potential of graphene, the working bandwidth and axial ratio bandwidth of the antenna can be changed. By changing the phase state of VO2 (metal state / insulation state) to control the working state of the antenna. The impedance bandwidth (IBW) of the CP antenna proposed is 115.09% (1.73–6.42 THz), the axial ratio bandwidth (ARBW) is 43.97% (2.36–3.69 THz). Using the CP antenna unit can simplify the complex feed network, and only feeding the equal amplitude and equal phase excitation can realize the vortex wave. Changing the number and rotation angle of the antenna unit can realize multimode vortex waves with the l= 0, ±1, ±2, ±3, the purity is approximately 1, respectively. The proposed UCA has excellent application prospect in the 6G communication.

Generation of Quantum Vortex Electrons with Intense Laser Pulses.

Accelerating a free electron to high-energy forms the basis for studying particle and nuclear physics. Here it is shown that the wave function of such an energetic electron can be further manipulated with the femtosecond intense lasers. During the scattering between a high-energy electron and a circularly polarized laser pulse, a regime is found where the enormous spin angular momenta of laser photons can be efficiently transferred to the electron orbital angular momentum (OAM). The wave function of the scattered electron is twisted from its initial plane-wave state to the quantum vortex state. Nonlinear quantum electrodynamics (QED) theory suggests that the GeV-level electrons acquire average intrinsic OAM beyond at laser intensities of 1020Wcm-2 with linear scaling. These electrons emit γ-photons with two-peak spectrum, which sets them apart from the ordinary electrons. The findings demonstrate a proficient method for generating relativistic leptons with the vortex wave functions based on existing laser technology, thereby fostering a novel source for particle and nuclear physics.

Efficient approach for generating vortex sources with arbitrary orbital angular momentum in acoustic experiments

Abstract In theoretical research framework of acoustics or optics, how to provide stable and efficient experimental vortex sources with arbitrary orbital angular momentum (OAM) (especially with larger OAM) is a highly challenging research topic. Here, we propose and demonstrate the general principle of two different methods to generate vortex sources with arbitrary OAM, based on the point-sources array and acoustic metamaterials, respectively. Specifically, the general synthetic law is summarized from the analytical perspective behind generating two-dimensional vortex waves using different point sources with different phases, and the design flexibility of acoustic metamaterials is also utilized to provide an ideal solution for generating vortex sources with larger OAM. Besides, we qualitatively and quantitatively determine the OAM of generated vortex waves through simple formulas, and briefly discuss the applicability and stability of two different methods with complementary advantages. The principles of vortex sources generation revealed in this work provide direct theoretical support for the experimental exploration of interactions between multiphysics fields and complex media, with potential applications in vortex fields manipulation and OAM detection.

Effects of Mach number on space-time characteristics of wall pressure fluctuations beneath turbulent boundary layers

Wall pressure fluctuations beneath turbulent boundary layers are a fundamental source of aerodynamic noise by exciting the wall structure, with their space-time characteristics serving as the basic ingredient for predicting the wall structural response. To this end, direct numerical simulations of fully developed compressible turbulent boundary layers at Mach numbers of 0.5, 1.2, and 2.0 are conducted to investigate wall pressure fluctuations comprehensively. The effects of Mach number on the single-point statistics of wall pressure fluctuations, such as the root mean square, skewness and flatness factors, probability density function, and frequency spectrum, are assessed to be very weak. Regarding the space-time characteristics, the convection velocity Uc determined by the space-time correlation of wall pressure fluctuations increases slightly with the Mach number, which only reflects the convective behavior of turbulent vortices. On the wavenumber–frequency spectrum, characteristic peaks of both the acoustic wave and convective vortices are identified. At Mach 0.5, the peaks of the fast (Uc+c) and slow (Uc−c) acoustic waves are unattached to others with c denoting acoustic speed, while only the peak of the fast acoustic wave is distinguishable from the convective peak at Mach 1.2 and 2.0. Due to the aerodynamic heating at supersonic conditions, the thermal effect on acoustic speed should be taken into account in determining the acoustic wavenumber. By introducing a convective Prandtl–Glauert parameter, a refined relation is proposed to provide a more accurate depiction of the acoustic domain in the wavenumber–frequency spectrum.

Multi-field coupling in the scrape-off layer of tokamak plasma

Abstract We study a reduced electrostatic fluid model for the tokamak scrape-off layer (SOL), which incorporates temperature gradient and vorticity gradient as two free energy fields. Two scenarios of field coupling are addressed: (1) sheath condition; (2) vortex wave coupling. For the sheath condition induced field coupling, the poloidal E×B flow shear is coupled with the temperature gradient. Combining an eigenmode analysis and the nonlinear phase dynamics approach, our findings indicate that in the absence of a vorticity gradient, the overall effect of the sheath condition induced flow shear can either stabilize or destabilize the interchange mode, depending on the competition between the flow shear suppression and the temperature gradient driving. This is different from the case where the gradient drive and shear damping are decoupled. When the field coupling is mediated by wave interactions, by setting an idealized step-like temperature and vorticity profiles, a joint mode forms through resonant interaction between the interfacial waves driven by the temperature and vorticity gradients, respectively. Near the phase locking condition, the joint mode can be more unstable than pure temperature gradient driven mode.

Characteristics analysis of flame propagation and its coupling effect with other parameters in LPG pipeline

AbstractTo study the flame propagating characteristic and its coupling effect with other parameters in the LPG pipeline, a typical pipeline model with two equal‐length branches perpendicular to each other is designed for experiment and simulation. Then, gas explosion scenarios are experimentally tested and numerically simulated, which is followed by the analysis of flame shape changing with time and peak temperature changing with space. Results show that when passing through the bifurcation, flame propagates to vertical branch B in a sharp knife shape affected by the strong vortex, reflected airflow, and compressed pressure wave in the pipeline with a diameter of 0.125 m. At the monitoring point that is 0.4 m away from the bifurcation point, the peak temperature of the vertical branch B is 57.87% bigger than that of the horizontal branch C, and its arrival time is 80% longer than that of the horizontal branch C, due to the existence of flame in vertical branch B. What's more, in both branches, the coupling results between peak temperature and peak velocity agree very well with the growth function, while the coupling results between peak temperature and peak pressure agree well with the decay function, providing aids to the optimal layout design of industrial pipeline branches as well as to the explosion suppression measures.

Broadband vortex beam generation by a reflective meta-surface based on a metal double-slit resonant ring

Recently, the meta-surface (MS) has emerged as a promising alternative method for generating vortex waves. At the same time, MS also faces the problem of a narrow bandwidth; in order to obtain a broad bandwidth, the MS unit cell structure becomes more and more complex, which will bring many inconveniences to the preparation process of MS devices. Therefore, we want to design a simple MS unit cell with a multi-frequency selection. In this paper, based on the principle of geometric phase, we design a simple reflective MS unit cell based on a metal double-slit resonant ring. We elaborate on the resonance mechanism of the MS unit cell. Under the normal incidence of circularly polarized (CP) waves, the reflection coefficient of the same polarization was greater than 85%. By rotating the orientation angle of the resonator on the MS unit cell, the continuous 2π phase coverage was satisfied in the frequency range of 0.52–1.1 THz, and the relative bandwidth becomes 71.6%. Based on this, we construct a vortex generator by using a 15×15 MS unit array. The right-handed circularly polarized (RCP) waves and left-handed circularly polarized (LCP) wave are separately incident on MS with topological charges of l=+1,+2,+3 under multiple resonant frequencies. The generated RCP vortex wave has topological charges of l=−1,−2,−3 and the generated LCP vortex wave has topological charges of l=+1,+2,+3. The numerical simulation results demonstrate that our designed MS, capable of achieving multiple resonance outcomes, can effectively operate in a multi-broadband mode and produce a wide-band vortex beam. In addition, we also calculate the pattern purity. Through theoretical analysis and numerical simulation, we prove that our designed MS can generate a broadband vortex wave.

Switchable dual-functional optical vortex manipulation via cylindrical phase-change metagratings

Subwavelength artificial engineering microstructures, such as metasurfaces and metagratings, can realize superior optical properties that many natural materials cannot finish. However, most functionalities achieved are mostly single and fixed. The appearance of phase-change materials makes it possible to extend the functions of metagratings. Here, we propose and demonstrate a structural design of cylindrical phase-change metagratings based on VO2, which can achieve switchable dual-functional optical vortex manipulation in the terahertz range. Specifically, by excitation of the temperature-sensitive metallic and insulating states of VO2, the simple two-dimensional cylindrical gear-like structure can achieve dual-functionality for vortex waves with orbital angular momentum, namely efficient retroreflection and near-perfect absorption. In addition, we also discussed the influence of the gear-like structure dimensions on the transmissivity and absorptivity of dual-function vortex wave manipulation. This work provides a simple and effective approach for the tunable and multifunctional control of vortex waves in subwavelength artificial devices, with potential applications in automated device design and terahertz (THz) communications.

Chiral-assisted geometric phase metasurfaces: Manipulating orbital angular momentum for efficient and stable microwave attenuation

Chiral wave-absorbing metamaterials can enhance microwave attenuation performance by converting linearly polarized waves into circularly polarized waves. However, the mechanism for transforming incident waves into vortex waves to overcome the angular momentum mismatch to satisfy broadband stealth is unclear. This work proposes a chiral-assisted geometrically phase (CAGP) metasurfaces that utilizes the Pancharatnam-Berry (PB) phase concept to convert incident plane waves into reflected vortex waves. Such metamaterials demonstrate adaptive transformation capabilities under the incidence of electromagnetic (EM) waves with multi-polarization modes, providing an effective absorption bandwidth (≥90 % absorptivity, reflection loss ≤ −10 dB) reaches 6.92 GHz (9.16–16.08 GHz) at a thickness of 2.48 mm. Based on an orbital angular momentum (OAM) phase and amplitude analysis, the mode purity of the generated vortex waves is discussed in detail, and the mechanisms are analyzed and verified by simulation models to confirm their effectiveness and practicability. Additionally, the metamaterials exhibit excellent radar cross-section (RCS) reduction properties and stable polarization insensitivity properties. In short, this work presents a tailorable metasurface to manipulate the OAM spectrum and achieve broadband microwave attenuation, which holds great potential for various applications in chiral wave-absorbing metamaterials.

Experimental Study on the Sloshing of a Rectangular Tank under Pitch Excitations

Fluid sloshing within containers subjected to external motion is a crucial yet intricate phenomenon with implications across various industries. This study investigates sloshing in a rectangular liquid tank through a series of experiments examining pitch excitations with diverse excitation frequencies and amplitudes across different liquid carrying rates. By analyzing pressure data and imagery of the free liquid surface, statistical trends in peak pressure at measurement points within the tank are identified, revealing the nonlinear behavior of the fluid. Spectral analysis generates power spectrum curves that delineate frequency components and energy distribution within the sloshing dynamics. Key findings include the identification of resonance-induced violent sloshing at a 20% liquid-carrying rate and a resonant frequency shift at a 70% liquid-carrying rate due to nonlinearity, displaying a “soft spring” characteristic in the frequency response. The free liquid surface exhibits four distinct waveforms depending on frequency. Notably, at a 70% liquid-carrying rate and resonant frequency excitation, three-dimensional vortex waves emerge, highlighting a complex three-dimensional effect within the tank. The power spectrum shows that the dominant response frequency aligns with the excitation frequency and its multiples. This investigation enhances our understanding of the intricate nature of sloshing in various liquid-carrying conditions, offering insights valuable for diverse industrial applications.

Broadband spin-decoupled metasurface for independent wavefront manipulations of orthogonal circularly-polarized waves

Abstract Metasurfaces endowed with spin-decoupled functionalities offer the capability to meticulously customize the electromagnetic wavefronts of incident dual orthogonal circularly-polarized (CP) waves in a desirable manner, that holds immense potential for broadening their application fields. Nevertheless, a major lack that persists in most spin-decoupled metasurfaces is the limited bandwidth or the intricate design requirements. Herein, we propose a broadband spin-decoupled metasurface, consisted of weak-resonant mirror-symmetry unit structures, that enables independent and distinct wavefront manipulations under the incidence of orthogonal CP waves. As a demonstration, we present a dual-channel metasurface that integrates geometric and propagation phases to generate vortex waves with two distinct modes in a wide frequency range from 10 GHz to 16 GHz. Both simulated and experimental results are consistent and collectively confirm the validity of our proposed metasurface. The research provides a practical and efficient avenue for constructing spin-decoupled metasurface within a broad frequency band.

Spatiotemporal Airyprime complex-variable-function wave packets in a strongly nonlocal nonlinear medium.

A type of circular Airyprime function of complex-variable Gaussian vortex (AFCGV) wave packets in a strongly nonlocal nonlinear medium is introduced numerically, combining the properties of helicity states and abrupt autofocusing. We investigate the effects of the chirp factor, distribution parameter, and decay factor on the AFCGV wave packets in the strongly nonlocal nonlinear medium. Interestingly, by adjusting the distribution parameter, the AFCGV wave packets can exhibit stable rotational motions in various shapes, such as symmetric lobes and doughnuts. In addition, the Poynting vector and the gradient force of the AFCGV wave packets are also discussed. Our research not only explains the theoretical model for controlling AFCGV wave packets but also advances fundamental research on self-bending and autofocusing structured light fields.

Vortex waves in fluid–structure interaction with high Froude number and a damped structure

In this paper, we study the non-linear dynamic response generated as a result of a fluid–structure interaction between a flexible structure and a flowing fluid, when the structure is subjected to non-linear excitations. In first place, the use of semi-discrete approximations allowed us to show that the motion of a flexible structure coupled with a surrounding fluid flowing could be modelled and analysed via a coupled Complex Cubic Ginzburg–Landau equations (CCGLEs). Through the obtained CCGLEs, we were able to show that modulational instability (MI) is the main mechanism responsible for the generation of vortex shedding. Moreover, we showed that the stability of continuous wave depends on the coupling parameters between the fluid and the structure. Secondly, using a mathematical method, namely the G’/G expansion method, we found that vortex wave trains could be generated as cylindrical waves. These results are highly significant from a theoretical point of view and could be a plus to explain the process of generation of Kármán Vortex as a consequence of unstable coupling between two continuous wave in the fluid–structure system. Moreover, considering the industrial interest, such as floating wind turbines, this work aims to provide an additional understanding of the interactions between a flexible body and a surrounding flow.

Improving millimeter-wave imaging quality using the vortex phase method

This paper investigates a new vortex wave imaging approach to improve the imaging quality of small metal targets of size less than 1.5 mm. Antennas with different spiral phase plates are designed to efficiently transmit vortex beams with orbital angular momentums (OAMs). By analyzing the OAM spectrum of the target, it was discovered that the predominant reflection contains a particular OAM mode that carries abundant azimuthal information. This can be explained by the OAM selectivity of the target and the guidance of the vortex transmitting beam. A simple reflection vortex imaging system was designed to capture the phase information. Measurement results show that the high image contrast reaches 14.9%, which is twice as high as that of the imaging without OAM. Both of simulations and experiments demonstrate that the vortex phase imaging approach proposed in this paper can effectively improve the imaging quality at 80 GHz. This approach is suitable for other millimeter wave imaging systems and is helpful to improve the resolution in anti-terrorism security checks.

Numerical study of shock wave boundary layer interaction flows using a novel turbulence model

This study proposes a novel partial average fluctuation velocity (PAFV) turbulence based on the PAFV and the average strain rate tensor, which is used to calculate the shock wave boundary layer interaction flows, including cascade flow and transonic compressor flow. In order to illustrate the adequacy, the calculation results of several other turbulence models were also compared with PAFV, including k-ω, shear stress transport, SA-neg (negative Spalart-Allmaras), and stress-BSL (ω-based Reynolds stress model). The suitability of the PAFV turbulence model was initially tested by calculating the transonic flow in the L030-4 cascade. Subsequently, numerical computations were performed for the 3D (three-dimensional) National Aeronautics and Space Administration Rotor 67 transonic compressor rotor. In the numerical simulation, the flow control equations were transformed in a general curvilinear coordinate system for precision enhancement, and the spatial discretization of control equations was executed using the finite difference method. The convection term was processed using the Steger–Warming flux vector splitting method, the diffusion term was discretized using a second-order central difference scheme, and the time derivative term was discretized using the third-order Runge–Kutta method. A parallel computing strategy with multi-block partitioning was employed to speed up the calculations and facilitate faster convergence alongside a local time step method. All numerical computation procedures were written in Python, revealing the following: (1) The PAFV turbulence model aptly simulates the transonic flow within the cascade channel. The shock wave pattern in the cascade channel aligns well with the experimental schlieren images. (2) During the Rotor 67 transonic compressor rotor calculation, the flow–pressure ratio, flow efficiency, and inlet and outlet total temperatures and pressures align well with the experimental data. Furthermore, the resolution of the flow field structure related to the interaction between the tip leakage vortex and channel shock wave is high. (3) The turbulence model uses PAFV as the turbulence velocity scale, thereby suppressing the fluctuation velocity and turbulence viscosity near shock wave areas, preventing excessive turbulence viscosity. Moreover, PAFV inherently exhibits transport properties and anisotropic characteristics, making it well-suited for handling transonic compressor flows.

Метаповерхности Патчаратнама-Берри с генерацией углового орбитального момента и комбинированным фазовым кодированием для широкополосного широкоугольного снижения ЭПР

The phase mechanism of a broadband wide-angle radar cross-section (RCS) reduction is examined, based on the use of non-absorbing Patcharatnam-Berry (PB) metasurfaces (MSs) with the generation of orbital angular momentum (OAM) and a combination of different phase profiles. MS unit cells contain a thin shielded single-layer substrate and meta-particles in the form of perforated rectangular patches with a coded rotation angle. The purpose of the work is to compare the scattering characteristics and scattered field cancelation efficiency for structural elements (modules) of a MS, consisting of the same number of meta-particles, but having different PB-phase profiles. A profile with an azimuthal gradient, which forms broadband scattering of vortex waves with OAM, profiles with radial and linear gradients and anomalous scattering, parabolic profiles with wide-angle scattering, and various combined phase profiles are analyzed. The purpose of the work is comparing the scattering characteristics. The MS modules scattering characteristics are studied using the HFSS (finite element method) for co-polarization and cross-polarization in the case of irradiation with a normally incident plane circular polarized wave. The simulation showed that the scattering diagrams of the co-polarized MS field in these cases have a funnel-shaped vortex character with an intense OAM minus the first order (the order of the OAM remains the same as for an MS with an azimuthal phase gradient). OAM generation significantly suppresses backscattering by co-polarization. The far-field phase in the vortex region depends not only on the azimuthal, but also on the meridional observation angles. The combination of OAM and parabolic profile increases the diffusion of broadband scattering, realizes a wider-angle scattering of the resulting field with a vortex opening angle of about 100° (at an average frequency of 14 GHz), which is important for RCS reduction.

Effects of wall transpiration on the supersonic boundary-layer oblique-type transition

The study of transpiration cooling is vital for the development of high-speed aircraft. In the current work, direct numerical simulation (DNS) is performed to investigate the impacts of wall transpiration on the boundary-layer oblique breakdown over a Mach 2 flat plate. The porous injection model is used to mimic the transpiration from the equally spaced circular pores. It has been observed from the numerical results that wall transpiration leads to the amplified growth rate of the imposed oblique mode waves, steady vortex waves, and other higher-harmonic waves. As a result, the occurrence of boundary-layer transition shifts upstream. Due to the presence of transpiration, the normal gradients of both streamwise velocity and temperature are decreased at the wall, which causes reduced skin friction and heat flux in the transpiration region. In addition, when upstream transpiration is present, reductions in skin friction and heat flux can also be observed within turbulent regions. This study provides insights into the DNS investigation on compressible boundary-layer natural transitions coupled with wall transpiration, and the results indicate that more systematic investigations addressing this problem are needed.

Mirror Symmetry Broken of Sound Vortex Transmission in a Single Passive Metasurface via Phase Coupling.

Asymmetric transmission in a passive vortex system is highly desirable, as it enables the development of compact vortex-based devices. However, breaking the mirror symmetry of transmission via a single metasurface poses challenges due to the inherent symmetric transmission properties in reciprocity. Here, we theoretically propose and experimentally demonstrate a novel transmission-reflection phase coupling mechanism to achieve the broken mirror symmetry of sound vortex transmission. This mechanism establishes a special coupling link between transmission and reflection waves, superimposing asymmetric reflection phases on the transmission phases. By utilizing a single passive phase gradient metasurface with asymmetric reflection phase twists, distinct transmission phase twists for mirror-symmetric incident vortices can be achieved within a cylindrical waveguide. This is typically difficult to imple-ment in a reciprocal system. Numerical and experimental results both demonstrate the broken mirror symmetry of vortex transmission and reflection. Our findings offer a new strategy for controlling vortex wave propagation, which may inspire new directional applications and extend to the field of photonics.