Direction Of Wave Propagation Research Articles

Alpha Traveling Waves during Working Memory: Disentangling Bottom-up Gating and Top-down Gain Control.

While previous works established the inhibitory role of alpha oscillations during working memory maintenance, it remains an open question whether such an inhibitory control is a top-down process. Here, we attempted to disentangle this issue by considering the spatio-temporal component of waves in the alpha band, i.e., alpha traveling waves. We reanalyzed two pre-existing and open-access EEG datasets (N = 180, 90 males, 80 females, 10 unknown) where participants performed lateralized, visual delayed match-to-sample working memory tasks. In the first dataset, the distractor load was manipulated (2, 4, or 6), whereas in the second dataset, the memory span varied between 1, 3, and 6 items. We focused on the propagation of alpha waves on the anterior-posterior axis during the retention period. Our results reveal an increase in alpha-band forward waves as the distractor load increased, but also an increase in forward waves and a decrease in backward waves as the memory set size increased. Our results also showed a lateralization effect: alpha forward waves exhibited a more pronounced increase in the hemisphere contralateral to the distractors, whereas the reduction in backward waves was stronger in the hemisphere contralateral to the targets. In short, the forward waves were regulated by distractors, whereas targets inversely modulated backward waves. Such a dissociation of goal-related and goal-irrelevant physiological signals suggests the co-existence of bottom-up and top-down inhibitory processes: alpha forward waves might convey a gating effect driven by distractor load, while backward waves may represent direct top-down gain control of downstream visual areas.Significance Statement When exploring the functional role of alpha band neural oscillations during working memory, mostly amplitude modulations have been considered so far, with relatively limited exploration of spatial-temporal dynamics of this rather global brain oscillatory signature. The present study seeks to address this gap by examining the directionality of alpha wave propagation during working memory retention. Our findings offer novel insights into the well-established inhibitory role of alpha waves, demonstrating that this function is manifested differently according to their propagation directions: forward waves seem to facilitate bottom-up gating, while backward waves might mediate top-down gain control.

Numerical research on flow field structure and droplets distribution of kerosene-fueled rotating detonation ramjet engine

In order to reveal the multiphase flow field structure and fuel droplets distribution under rotating detonation ramjet engine fueled by liquid kerosene, non-premixed simulations coupled with an Euler-Lagrangian approach is adopted. Supersonic air is used as oxidizer and the total pressure and total temperature at the entrance of isolation are set as 1.2 MPa and 1100 K, respectively, with a Mach number of 1.9. It is shown that a single-wave is formed and typical rotating detonation wave structures are established under two different orifice spacing conditions, namely 2 mm and 6mm. A "rich oil and poor oxygen band" is formed and attributed to the inconsistent supply of fuel and air after the passage of the detonation wave. When the orifices spacing is increased from 2 mm to 6 mm, both obvious strips after the detonation wave and “n-type” deflagration structures near the contact surface are observed. Besides, the detonation wave front becomes discontinuous, as well as from the deflagration heat release distribution. Despite of the effect of the circumferential propagation of detonation wave, kerosene droplets still propagate mainly along the downstream direction. However, Kerosene droplets distribution shows obvious difference along the detonation wave propagation direction.

On Velocity Stopbands in Electrostatic Solitary Wave Propagation in Magnetospheric Plasma in the Presence of an Ion Beam—Application in the Solar Wind

Satellite observations in the Earth’s magnetosphere suggest a coexistence of different types of ion populations, such as protons and heavier helium ions (alpha particles) transported by the solar wind. We have undertaken an investigation, from first principles, of the conditions for the occurrence of electrostatic solitary waves in such a multi-ion plasma environment. Nonlinear analysis has been employed to explore the effect of an ion beam on the occurrence of stopbands in a multicomponent plasma consisting of Boltzmann electrons and two ion species, modeled as cold and warm (adiabatic) ions. A “stopband” is an interval of pulse speed (mach (Mach number) values in which solitary waves cannot propagate. Such stopbands are known to exist in relation to fast ion-acoustic solitary waves (exclusively). Our parametric analysis reveals that the stopband widens over the range of cold ion charge density (values) upon increasing speed of a hot ion beam moving along the direction of wave propagation. However, the stopband becomes narrower in the case of a counterpropagating beam. Similarly, the streaming of a cold ion beam fluid along (or antiparallel) to the wave direction leads to the narrowing (or widening, respectively) of the stopband. The fast soliton existence (velocity) domain is characterized by two critical points (a crossover and a turning point), which are affected by the beam. Our study could be relevant to understanding energy dissipation processes in the fast solar wind associated with relative drifts between the major (protons) and minor (alpha particles) ions which results in the heating of both species.

Investigation on the integrated characteristics of n-decane/air rotating detonation combustor and supersonic turbine

In this study, the two-dimensional numerical simulations are conducted to study the flow field characteristics, turbine performance and loss mechanism of integrated system of rotating detonation combustor and supersonic turbine under two different directions of detonation wave propagation. The results indicate that the propagation direction of RDW affects the incident angle between OSW and guide vanes, resulting in different operating modes for aligned and unaligned modes. OSW in the turbine cascade undergoes the leading-edge shock, blade surface reflection and trailing edge diffraction. The backward-propagating rake-type shock envelope is formed due to leading-edge shock of the rotor. In misaligned mode, the stator has higher damping of both pressure and temperature. The stator significantly improves the circumferential uniformity of both velocity and pressure, particularly when operating in aligned mode. The total pressure loss in aligned mode is less, therefore the turbine achieves higher rim work and efficiency. The viscosity is one of the main sources of loss in the flow field. The reflected shock waves at the leading edge of the rotor and in the stator cascade are the primary factors contributing to the leading-edge vortices on the stator vanes.

Numerical study of wave resonance characteristics in gaps of a floating array

Wave resonance in the gaps formed by a four-float array for various drafts and incident wave frequencies is investigated using a numerical wave tank based on OpenFOAM. In the gap perpendicular to the direction of wave propagation, the resonant wave height is higher than that between two side-by-side floats under the same draft, and the resonant frequency is also different. Significant variations in wave height distribution are observed along the gap parallel to the wave propagation direction under different incident wave frequencies. When the incident wave frequencies are higher than the resonant frequency, the lateral force amplitude on the front floats increases, while the force amplitude on the rear floats does not show this effect. Using the dynamic mode decomposition method, we discover that the irregular distribution of wave heights across different frequencies leads to an increase in the lateral force amplitude on the front floats at non-resonant frequencies.

Detailed Occurrence of Feather Features in Quartz in Experimentally Shocked Granite

AbstractFeather features (FFs) in quartz consist of a planar fracture (PF) and associated fine lamellae (FF lamellae; FFL) and have been observed in various natural impact structures. However, the mechanisms and conditions of FF formation are poorly understood. We conducted shock recovery experiments on granite using decaying compressive pulses to investigate the formation conditions of FFs. We characterized the recovered samples using an optical microscope equipped with a universal stage, a scanning electron microscope combined with an electron back‐scattered diffraction detector, and a transmission electron microscope. We found that FFs are formed over a wide range of peak pressures (2–18 GPa) and that FFs can be divided into at least three types (I–III) based on the crystallographic orientation of the PFs and FFL, the angle between the orientation of the FFL and the propagation direction of the compression wave, and the presence/absence of amorphous silica in the FFL. The peak pressures that produce type I–III FFs are estimated to be <12, 12–14, and >16 GPa, respectively. We propose that FFs can be used as a shock barometer for quartz‐bearing rocks.

Theoretical framework for the response of barotropic shelf current to along-shelf bathymetric perturbations

Abstract The mechanism underlying the response of barotropic shelf current to along-shelf bathymetric perturbations is rationalized by a one-dimensional vorticity equation. Fourier analysis of the equation in the wavenumber domain suggests that bathymetric features with relatively short length scales in the alongshore direction drive steady cross-shelf velocity undulations when the shelf circulation is against the direction of Kelvin wave propagation (upwelling regime). In contrast, much weaker and non-oscillatory cross-shelf velocity is expected when the shelf circulation is in alignment with propagating Kelvin waves (downwelling regime). The velocity undulations in the upwelling regime result from a positive feedback mechanism associated with potential vorticity (PV) advection; however, the presence of bottom friction restricts such undulations within a few wavelengths. The characteristic wavelength of undulations is determined by when the normalized drag coefficient , where u is the along-shelf mean flow speed, Hs is shelf depth, αs is the bottom slope, f is the Coriolis parameter, and r is the linear drag coefficient. Using this formula, a scale of the cross-shelf displacement distance Yt is proposed, aligning with one-layer barotropic simulations over a large parameter space. The study provides a framework for investigating barotropic cross-shelf exchange caused by shelf circulation over irregular bottom bathymetries.

Wave Speeds for a Time-Periodic Bistable Three-Species Lattice Competition System

In this paper, we consider propagation direction (which can be used to predict which species will occupy the habitat or win the competition eventually) of a bistable wave for a three-species time-periodic lattice competition system with bistable nonlinearity, aiming to address an open problem. As a first step, by transforming the competition system to a cooperative one, we study the asymptotic behavior for the bistable wave profile and then prove the uniqueness of the bistable wave speed. Secondly, we utilize comparison principle and build up two couples of upper and lower solutions to judge the sign of the bistable wave speed which partially provides the answer to the open problem. As an application, we reduce the time-periodic system to a space–time homogeneous system, we obtain the corresponding criteria and carry out numerical simulations to illustrate the availability of our results. Moreover, an interesting phenomenon we have found is that the two weak competitors can wipe out the strong competitor under some circumstances.

Momentum, energy and vorticity balances in deep-water surface gravity waves

The particle trajectories in irrotational, incompressible and inviscid deep-water surface gravity waves are open, leading to a net drift in the direction of wave propagation commonly referred to as the Stokes drift, which is responsible for catalysing surface wave-induced mixing in the ocean and transporting marine debris. A balance between phase-averaged momentum density, kinetic energy density and vorticity for irrotational, monochromatic and spatially periodic two-dimensional water waves is derived by working directly within the Lagrangian reference frame, which tracks particle trajectories as a function of their labels and time. This balance should be expected as all three of these quantities are conserved following particles in this system. Vorticity in particular is always conserved along particles in two-dimensional inviscid flow, and as such even in its absence it is the value of the vorticity that fundamentally sets the drift, which in the Lagrangian frame is identified as the phase-averaged momentum density of the system. A relationship between the drift and the geometric mean water level of particles is found at the surface, which highlights connections between the geometry and dynamics. Finally, an example of an initially quiescent fluid driven by a wavelike pressure disturbance is considered, showing how the net momentum and energy from the surface pressure disturbance transfer to the wave field, and recognizing the source of the mean Lagrangian drift as the net momentum required to generate an irrotational surface wave by any conservative force.

Far field ring beam generation based on 3-bit encoded metasurface

Abstract The encoding metasurface establishes a bridge between the physical and digital worlds, ushering in a new era of manipulating electromagnetic waves and realizing programmable metamaterials through digital coding sequences. This "digital metasurface," relying on binary logic, significantly simplifies the design process, thereby enhancing the flexibility and efficiency of controlling electromagnetic waves. While most encoding metasurfaces control beamforming for pencil-shaped beams, we propose a 3-bit encoding metasurface with a "well structure" in the microwave band.The 3-bit encoding metasurface features a more extensive encoding sequence, offering increased degrees of freedom and flexibility in manipulating the direction of electromagnetic wave propagation. Its symmetric design features polarization-insensitive characteristics, suitable for generating annular beams by varying gradient-encoded numbers radially. This approach enables the production of linearly polarized omnidirectional radiation within the desired elevation angle range. Additionally, adjustments in quantity and orientation of the produced annular beams are achieved through Fourier convolution theorem. This type of annular beams holds promise for applications in various fields, including wireless radio broadcasting and wireless local area networks.

Toggling near-field directionality via manipulation of matter's anisotropy.

Near-field directional excitation of dipolar sources is crucial for many practical applications, such as quantum optics, photonic integrated circuits, and on-chip information processing. Based on theoretical analyses and numerical simulations, here we find that the near-field directionality of circularly polarized dipoles can be flexibly toggled by engineering the anisotropy of the surrounding matter, in which the dipolar source locates. To be specific, if the circularly polarized dipole is placed close to the interface between a hyperbolic matter and air, the main propagation direction of excited surface waves would be reversed when the location of the dipolar source is changed from the air region to the hyperbolic-matter region. The underlying mechanism is that the spatial-frequency spectrum of evanescent waves carried by the dipolar source in a homogeneous surrounding matter could be flexibly reshaped by the matter's anisotropy, especially when the isofrequency contour of the surrounding matter changes from the circular shape to the hyperbolic one.

Tunable Acoustic Tweezer System for Precise Three-Dimensional Particle Manipulation

This study introduces a tunable acoustic tweezer system designed for precise three-dimensional particle trapping and manipulation. The system utilizes a dual-liquid-layer acoustic lens, which enables the dynamic control of the focal length through the adjustable curvature of a latex membrane. This tunability is essential for generating the acoustic forces necessary for effective manipulation of particles, particularly along the direction of acoustic wave propagation (z-axis). Experiments conducted with spherical particles as small as 1.5 mm in diameter demonstrated the system’s capability for stable trapping and manipulation. Performance was rigorously evaluated through both z-axis and 3D manipulation tests. In the z-axis experiments, the system achieved a manipulation range of 33.4–53.4 mm, with a root-mean-square error and standard deviation of 0.044 ± 0.045 mm, which highlights its precision. Further, the 3D manipulation experiments showed that particles could be accurately guided along complex paths, including multilayer rectangular and helical trajectories, with minimal deviation. A visual feedback-based particle navigation system significantly enhanced positional accuracy, reducing errors relative to open-loop control. These results confirm that the tunable acoustic tweezer system is a robust tool for applications requiring precise control of particles with diameter of 1.5 mm in three-dimensional environments. Considering its ability to dynamically adjust the focal point and maintain stable trapping, this system is well suited for tasks demanding high precision, such as targeted particle delivery and other applications involving advanced material manipulation.

Hydrodynamic response characteristics of curved shape trash intercepting net under wave–current combined conditions

Under the influence of waves and ocean currents, the mesh panels of trash nets at nuclear power plants exhibit a curved shape. The combined action of waves and currents creates a Doppler effect that significantly influences the hydrodynamic response of these nets. This paper used OpenFOAM to establish a porous media macroscopic computational fluid dynamics model for analyzing the forces on curved trash nets. Wave–current combined conditions were simulated by introducing a uniform current in the direction of wave propagation. Forces on the nets under wave–current combined conditions were analyzed and compared with forces under separate uniform current and wave conditions. The results indicate that wave run-up on the nets is more significant under wave–current combined conditions than under wave action alone. The dimensionless horizontal force decreases more rapidly with increasing wave steepness (kA) and wave number (kd) under combined conditions than with linear superposition. Specifically, the rate of increase in dimensionless horizontal force with current speed, at the same blockage ratio, is about twice that of linear superposition. Additionally, the tension in the main cable of the nets is 17% higher under combined conditions than with linear superposition.

Constitutive modelling and wave propagation through a class of anisotropic elastic metamaterials with local rotation

Elastic metamaterials are typically periodic materials possessing unit cells endowed with engineered architecture much smaller than the typical phenomenological length scale. The development of continuum models capable of accurately representing the effects of this aforementioned architecture is extremely challenging, and hence a sparsely developed area. This paper develops a novel 2-D continuum model capable of capturing the dynamic behaviour of a class of anisotropic elastic metamaterials with local rotational elements in the long wavelength limit. A constitutive relation incorporating these local rotational effects is proposed, and ratified using a representative discrete model using linear Hookean springs and identical rigid disks. The new continuum model is used to generate a dispersion relation for harmonic plane waves propagating in an arbitrary direction, which is subsequently compared to the dispersion behaviour of the original discrete model. The general behaviour of this continuum when subjected to 2-D planar harmonic wave propagation in the anisotropic medium is then analyzed, with specific attention given to the effect of material anisotropy and wave propagation direction. This work is the first of its kind to create a new continuum model of a class of anisotropic elastic metamaterials with local rotational effects.

Structural Quality Factor of Flo-TENG under Stochastic Wave Excitation.

Triboelectric nanogenerators (TENGs) have recently emerged as a promising technology for efficient water wave energy harvesting. However, there is a paucity of clear guidance regarding the optimal designs of TENGs and their shells to achieve efficient absorption and conversion of water wave energy in real random waves. Herein, from the perspective of wave-body interaction and energy transfer, this paper proposes a structural quality factor (Qunit) for the quantitative evaluation of both the motion of floating triboelectric nanogenerator (Flo-TENG) shells and their capability to absorb and convert water wave energy efficiently. The factor is further subdivided into the amplitude structural quality factor (Qacc), which characterizes shell motion amplitude, and the frequency structural quality factor (Qf), which describes shell motion frequency. This paper systematically investigates the impact of various shell parameters such as bow shapes, curvatures, inclinations, and immersion ratios on Qacc and Qf. The findings indicate that variations in shell shape result in distinct Qunit values along different axial directions of wave propagation. These variations directly influence energy absorption efficiency in these directions. These results provide fundamental guidance for the design of high-performance Flo-TENG shells and the selection of internal energy harvesting directions to enable more efficient energy conversion.

Low-frequency bandgap and vibration suppression mechanism of a novel square hierarchical honeycomb metamaterial

AbstractThe suppression of low-frequency vibration and noise has always been an important issue in a wide range of engineering applications. To address this concern, a novel square hierarchical honeycomb metamaterial capable of reducing low-frequency noise has been developed. By combining Bloch’s theorem with the finite element method, the band structure is calculated. Numerical results indicate that this metamaterial can produce multiple low-frequency bandgaps within 500 Hz, with a bandgap ratio exceeding 50%. The first bandgap spans from 169.57 Hz to 216.42 Hz. To reveal the formation mechanism of the bandgap, a vibrational mode analysis is performed. Numerical analysis demonstrates that the bandgap is attributed to the suppression of elastic wave propagation by the vibrations of the structure’s two protruding corners and overall expansion vibrations. Additionally, detailed parametric analyses are conducted to investigate the effect of θ, i.e., the angle between the protruding corner of the structure and the horizontal direction, on the band structures and the total effective bandgap width. It is found that reducing θ is conducive to obtaining lower frequency bandgaps. The propagation characteristics of elastic waves in the structure are explored by the group velocity, phase velocity, and wave propagation direction. Finally, the transmission characteristics of a finite periodic structure are investigated experimentally. The results indicate significant acceleration amplitude attenuation within the bandgap range, confirming the structure’s excellent low-frequency vibration suppression capability.

Nonreciprocity of surface acoustic waves coupled to spin waves in a ferromagnetic bilayer with noncollinear layer magnetizations

Nonreciprocity of propagation of surface acoustic waves (SAWs) in the microwave frequency band can be achieved using the magnetoelastic interaction of SAWs with spin waves (SWs) propagating in magnetic heterostructures. Recent works have shown that the ultimate isolation of a counterpropagating hybridized SAW/SW is achieved in heterostructures consisting of a synthetic antiferromagnet—a ferromagnetic (FM) bilayer with antiferromagnetic Ruderman-Kittel-Kasuya-Yosida interlayer coupling—placed on top of a piezoelectric acoustic waveguide. In this work, we study in detail a more practical and technologically simpler system based on an FM bilayer, where layers are coupled by only dipole-dipole interaction, and having noncollinear magnetizations of the FM layers. A weak in-plane anisotropy with noncollinear easy axes in the layers is shown to be the only essential factor for the realization of strongly nonreciprocal propagation of a hybridized SAW/SW. We formulate requirements for the relative orientation of the layer’s magnetizations and wave propagation direction necessary to realize an efficient SAW isolator, and demonstrate examples of SAW transmission characteristics which prove the possibility of achieving an isolation exceeding 50 dB for a submillimeter-long FM bilayer with insertion losses of just a few decibels more than those of a pure SAW device. In addition to relative fabrication simplicity, the proposed magnetoelastic heterostructure exhibits a reasonable robustness in respect to deviations in the anisotropy axes and/or bias field directions—an important benefit for device mass production. Published by the American Physical Society 2024

Abyssal Slope Currents

Abstract Realistic computational simulations in different oceanic basins reveal prevalent prograde mean flows (in the direction of topographic Rossby wave propagation along isobaths; a.k.a. topostrophy) on topographic slopes in the deep ocean, consistent with the barotropic theory of eddy-driven mean flows. Attention is focused on the Western Mediterranean Sea with strong currents and steep topography. These prograde mean currents induce an opposing bottom drag stress and thus a turbulent boundary-layer mean flow in the downhill direction, evidenced by a near-bottom negative mean vertical velocity. The slope-normal profile of diapycnal buoyancy mixing results in down-slope mean advection near the bottom (a tendency to locally increase the mean buoyancy) and up-slope buoyancy mixing (a tendency to decrease buoyancy) with associated buoyancy fluxes across the mean isopycnal surfaces (diapycnal downwelling). In the upper part of the boundary layer and nearby interior, the diapycnal turbulent buoyancy flux divergence reverses sign (diapycnal upwelling), with upward Eulerian mean buoyancy advection across isopycnal surfaces. These near-slope tendencies abate with further distance from the boundary. An along-isobath mean momentum balance shows an advective acceleration and a bottom-drag retardation of the prograde flow. The eddy buoyancy advection is significant near the slope, and the associated eddy potential energy conversion is negative, consistent with mean vertical shear flow generation for the eddies. This cross-isobath flow structure differs from previous proposals, and a new one-dimensional model is constructed for a topostrophic, stratified, slope bottom boundary layer. The broader issue of the return pathways of the global thermohaline circulation remains open, but the abyssal slope region is likely to play a dominant role.

An Algorithm to Retrieve Range Ocean Current Speed under Tropical Cyclone Conditions from Sentinel-1 Synthetic Aperture Radar Measurements Based on XGBoost

In this study, a novel algorithm to retrieve the current speed along the range direction under extreme sea states is developed from C-band synthetic aperture radar imagery. To this aim, a Sentinel-1 (S-1) dual-polarized synthetic aperture radar (SAR) dataset consisting of 2300 images is collected during 200 tropical cyclones (TCs). The dataset is complemented with collocated wave simulations from the Wavewatch-III (WW3) model and reanalysis currents from the HYbrid Coordinate Ocean Model (HYCOM). The corresponding TC winds are officially released by IFRMER, while the Stokes drift following the wave propagation direction is estimated from the waves simulated by WW3. In this study, first the dependence of wind, Stokes drift, and range current on the Doppler centroid anomaly is investigated, and then the extreme gradient boosting (XGBoost) machine learning model is trained on 87% of the S-1 dataset for range current retrieval purposes. The rest of the dataset is used for testing the retrieval algorithm, showing a root mean square error (RMSE) and a correlation coefficient (r) of 0.11 m/s and 0.97, respectively, with the HYCOM outputs. A validation against measurements collected from two high-frequency (HF) phased-array radars is also performed, resulting in an RMSE and r of 0.12 m/s and 0.75, respectively. Those validation results are better than the 0.22 m/s RMSE and 0.28 r achieved by the empirical CDOP model. Hence, the experimental results confirm the soundness of the XGBoost, exhibiting a certain improvement over the empirical model.

Layered heterogeneous structures integrated device for multiplication, division arithmetic unit and multiple-physical sensing

A layered heterogeneous structure (LHS), consisting of silver, liquid crystal, and nonlinear dielectric layers, is proposed to realize functions of computing and sensing. By leveraging the optical Tamm state, the intrinsic absorption principle of liquid crystal, and nonlinear effects, the design of an integrated device capable of passive multiplication and division operations, along with high-performance multi-physical quantity sensing functionalities, is achieved. The given LHS exhibits Janus properties, with different physical functions manifested depending on the direction of electromagnetic wave (EW) propagation. During forward propagation of EWs, the LHS displays high and sharp absorptivity peaks at 774.8 and 1517.6 nm. The relationship between the two peaks approximates a frequency multiplication factor of 1.960, enabling signal multiplication. Furthermore, the two absorptivity peaks at different wavelengths facilitate the sensing of serum creatinine solution concentration and external pressure, with sensitivity (S), quality factors (Q), and figure of merit (FOM) of 266.76 μmol L−1/nm and 213.33 GPa/nm, as well as 248.76 and 348.22, 84.1 L (μmol)−1 and 49.06 GPa−1, respectively. During backward propagation of EWs, absorptivity peaks with distinct resolutions are observed at 1423 and 2809 nm, with a multiple relationship between them of 1.974, enabling frequency doubling for signal division. Additionally, the absorptivity curve facilitates temperature sensing over a wide range from 257 to 347 K. Owing to the unique temperature S of liquid crystal, different sensitivities and resolutions are observed at 257 to 297 K and 307 to 347 K, with S of 1.015 and 0.686 K/nm, and corresponding Q and FOM of 21.57 and 12.576, 0.076 K−1 and 0.003 04 K−1, respectively.