Direction Of Wave Propagation Research Articles

Dumbbell-Shaped Damage Effect of Closed Cylindrical Shell Subjected to Far-Field Side-On Underwater Explosion Shock Wave

In naval warfare, underwater explosion (UNDEX) shock waves significantly influence the stability and safety of the pressure hull structure of the equipment. This study investigated the unique dynamic buckling of a closed cylindrical shell subjected to a far-field side-on UNDEX shock wave using a three-dimensional numerical simulation based on acoustic–structural arithmetic. In particular, the flow-field response characteristics, plastic deformation, and yield characteristics of the cylindrical shell were determined under the influence of the UNDEX shock wave. Subsequently, the failure mode of the cylindrical shell was analyzed to propose the dumbbell-shaped damage effect. The results revealed that when the UNDEX shock wave encounters a finite cylindrical shell, the fluid exhibits a perturbation such as pressure division, stress wave deflection, and flow in the surroundings of the circular cylinder. However, the fluid cannot produce a sizeable instantaneous displacement that yields certain strong constraints at both ends of the cylindrical shell. These constraints generate an irregular distribution of the flow field pressure, and the cylindrical shell tends to exhibit an “arch” deformation along the direction of shock wave propagation. Owing to the flow surrounding the circular cylinder, a negative pressure zone is generated in the flow field at both ends of the cylindrical shell, which induces a “sucking disc” shape at both ends of the cylindrical shell and ultimately produces a dumbbell-shaped damage effect. The present findings will aid in the structural design and impact resistance of submarines, unmanned undersea vehicles, and additional equipment under the impact load of the UNDEX.

Surface Wave Suppression for Improved Isolation and Bore-Sight Gain Based on Dipole Moment Conversion

This article proposes a novel approach to surface wave suppression based on dipole moment conversion for isolation and bore-sight gain improvement. To transform the power associated with the surface wave into the source of the radiating wave, an inverted-L shape composed of a thin strip with a via post is adopted. Unlike the conventional mushroom structure, the via post is placed at the leading edge of the strip, so that the longitudinal edge of the strip is aligned with the propagation direction of the surface wave. This inverted-L structure plays a key role in converting the vertical electric dipole moment to the horizontal component, and the conversion efficiency can be further improved by arranging the structure with a narrower interval of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$0.057\lambda $ </tex-math></inline-formula> in the axis perpendicular to the propagation direction. For validation, multipole moments are calculated from induced currents, and the proposed metasurface is fabricated for two array configurations. Scattering parameters and near- and far-fields are measured to demonstrate the proposed approach experimentally. The fabricated metasurface shows an operating frequency band from 9.74 to 10.62 GHz, and its average isolation and bore-sight gain are improved by 6.3 and 1.6 dB, respectively, without any severe distortion on impedance matching.

Study on the band gap and directional wave propagation mechanism of novel single-phase metamaterials

In this paper, a new type of single-phase metamaterial is proposed. Based on the finite element method, the band gap characteristics of the structures with different branches are comprehensively analyzed, and the existence of the band gap is further verified by the frequency response transmission spectrum. Then, the propagation characteristics of elastic waves of different frequencies in different combinations of structures are investigated respectively. The results show that the proposed single-phase metamaterial structure has an excellent band gap range, and under the action of relatively low-frequency elastic waves, the wave propagation in the structure has a certain direction and regionality, while the elastic waves at higher frequencies are applied, the propagation direction of the wave tends to propagate in all directions. This paper provides a new idea for studying the structural design of metamaterials with specific wave propagation directions.

Non-reciprocal piezoelectric metamaterials with tunable mode shapes

The mode shapes of piezoelectric metamaterials are tuned by manipulating spatially the electrical boundary conditions of the piezo-elements, in a desired and controlled manner, in order to tailor the wave propagation characteristics through these metamaterials. The boundary conditions of the piezo-elements are controlled by using inductive shunting networks. With appropriate tuning and optimization of the spatial distribution of these inductive boundary conditions, it would be possible to alter the mode shape characteristics of the metamaterial in order to control the magnitude and direction of wave propagation. This enables also breaking the reciprocity characteristics of the metamaterial in a controlled manner. A finite element model (FEM) is developed to model the mode shape characteristics and the wave propagation in a one-dimensional piezo-metamaterial. The effect of various shunting strategies on the spatial control of the mode shapes, energy flow, and reciprocity characteristics of the piezo-metamaterial are investigated. The presented work lays down the foundation for two and three-dimensional metamaterial with tunable mode shape characteristics.

Separation of the Icelandic Coastal Current from the Reykjanes Peninsula

The Icelandic Coastal Current (ICC) is a buoyant flow fueled by multiple rivers. Satellite observations during the October–November 2019 study period indicate the ICC often separates from the southwest tip of the Reykjanes Peninsula; it subsequently has large offshore excursion during west-southwestward winds. Results from a high-resolution realistic simulation exhibit this ICC behavior and provide detail on the associated currents and salinities. Sensitivity tests with simplified bathymetry, no winds, or no local buoyancy inputs are compared to the standard run to isolate contributions of those factors to flow separation. Analysis indicates that ICC separation occurs because the coast turns more tightly than the inertial radius; this behavior is consistent with inviscid theory. Partial flow deflection over the shelf expression of the Reykjanes Ridge and the widening shelf both increase ICC offshore excursion. The ICC is strongly influenced by the barotropic wind response to west-southwestward winds that includes a downshelf jet (flowing in the direction of Kelvin wave propagation) along the coast upshelf of the peninsula's tip where winds are downwelling-favorable, the downshelf extension of the jet that bends around the peninsula and progresses farther offshore with isobaths, a band of offshore and upshelf currents along the coast downshelf of the peninsula's tip, and surface flow aligned with Ekman transport located offshore beyond the downshelf jet. Similar wind-influence scenarios are discussed for other plumes. There are likely many such hotspots for offshore freshwater transport around the world.

Propagation of generalised Rayleigh wave at the surface of piezoelectric medium with arbitrary anisotropy

AbstractMathematical model for mechanical and electrical dynamics is solved for three‐dimensional propagation of harmonic plane waves in a piezoelectric medium with arbitrary anisotropy. A system of modified Christoffel equations is derived to explain the existence and propagation of bulk waves or decaying phases in the considered medium. At the free plane boundary, a superposition of decaying phases form a generalised Rayleigh wave, which is governed by a linear system of four homogeneous equations. A complex determinantal secular equation ensures a solution to this system. True surface wave at the boundary of the considered medium demands a real solution of this complex secular equation. The linear system of four equations is then transformed to replace the complex secular equation with a real one, which can be solved by standard numerical methods. A real solution of this real secular equation provides the phase velocity for generalised Rayleigh wave at the boundary of piezoelectric medium with arbitrary anisotropy. This phase velocity defines a complex slowness vector, which is used to calculate the motion of material particles and the wave‐induced electric field. A numerical example is considered to compute the phase velocity as well as group velocity for given (arbitrary) propagation directions of Rayleigh wave at the boundary. Variations in particle motion and electric field, induced by Rayleigh wave, are analysed at different depths for different propagation directions at the boundary.

Analysis of low frequency vibration attenuation and wave propagation mechanism of graded maze structure

In this paper, a new graded maze metamaterial with a single phase is proposed. Based on the finite element method and Bloch's theorem, the bandgap characteristics of graded maze structures in different regions are analyzed, and the mechanism of bandgap opening and closing is explored by vibration patterns. Subsequently, the mechanism of elastic wave propagation within different structures is comprehensively analyzed from four perspectives, such as group velocity and phase velocity. And the damping performance is further verified by frequency response spectra. The results show that the new structure has excellent low-frequency bandgap and damping performance, the local resonance of the graded maze plays an important role in the expansion of the bandgap, and there is directionality and regionality of wave propagation within the structure. This paper provides a new idea for wave propagation control and analysis, and low frequency damping of single-phase materials.

Metamaterials with Reprogrammable Reciprocity

Metamaterials composed of asymmetric elements have been shown to break reciprocity; permitting the propagation of waves in one direction. The configuration of these metamaterials typically limits the propagation of the wave to a single direction, speed, or occurrence with little or no tunability. Here we present---theoretically, numerically, and experimentally---a simple design approach for reprogrammable metamaterials, which allows for all of these parameters to be tuned, enabled, disabled, or reset. We present a map of different geometries that allow for reciprocal and nonreciprocal wave propagation and attenuation. We show how a single design can be programmed on demand to support wave propagation and attenuation in both directions (reciprocal) or wave propagation in only one direction (nonreciprocal). In addition, we demonstrate real-time control of the wave velocity, both in amplitude and direction, as it propagates through the metamaterial. We show speeding up, slowing down, or complete reversal of the propagating wave direction. Furthermore, we show that the waves maintain a constant velocity regardless of the impulse magnitude, and even along a curved path. Our metamaterial could open exciting possibilities for designing nonlinear materials with exotic topological properties.

Low and ultra-wide frequency wave attenuation performance and tunability of a new cruciate ligament structure

This paper presents two novel cruciate ligament structures with outstanding contributions in lightweight design of metamaterials, band gap tuning, and ultra-wide and ultra-low frequency vibration and noise reduction, as well as providing a new design idea. The novelty of this paper is manifested in various aspects, combining Bloch's theorem and finite element method to calculate the band gap, and using the transfer function to verify the evaluation; using structural optimization and applying active strain to test the structural tunability; and illustrating the wave propagation and band gap opening mechanism from the vibration mode and energy perspectives. The results show that the widest band gap can reach 5061.9 Hz after structural optimization and tuning, while the lowest can be reduced to 274.3–285.6 Hz, and the vibration attenuation peak is around −620, so the structure has excellent vibration and sound insulation effect, while the band gap can be widened and reduced under active strain. The opening of the band gap is closely related to the directionality and regionality of wave propagation, and the comprehensive analysis method of wave propagation provides innovative research ideas for wave propagation, and also provides theoretical and technical support for structure and band gap optimization.

Thermoviscous hydrodynamics in nondegenerate dipolar Bose gases

We present a hydrodynamic model of ultracold, but not yet quantum condensed, dipolar Bosonic gases. Such systems present both $s$-wave and dipolar scattering, the latter of which results in anisotropic transport tensors of thermal conductivity and viscosity. This work presents an analytic derivation of the viscosity tensor coefficients, utilizing the methods established in [Wang et al., arXiv:2205.10465], where the thermal conductivities were derived. Taken together, these transport tensors then permit a comprehensive description of hydrodynamics that is now embellished with dipolar anisotropy. An analysis of attenuation in linear waves illustrates the effect of this anisotropy in dipolar fluids, where we find a clear dependence on the dipole orientation relative to the direction of wave propagation.

High-fidelity multiplexing meta-hologram for information display, storage and encryption

Holography has been continuously spurring tremendous interest of researchers attributed to its unparalleled capability of photographically reconstructing the desired images. Based on the metasurfaces with multiple degrees of freedom, multiplexing meta-holograms are experiencing the research upsurge because of enormous application potential. However, existing multiplexing meta-holograms cannot tailor the amplitude information, and possess limited imaging channels and high channel crosstalk, which hinder the improvement of information capacity and imaging quality of multiplexing meta-holograms. Herein, an innovative multiplexing meta-hologram based on the helicity-assisted meta-atoms is proposed, which can perfectly reconstruct four independent images by altering the working frequency, polarization state and propagation direction of incident waves, and theoretically realize the holography in sixteen different channels by easily rotating, mirroring, and arranging the structures of meta-atoms. Numerous simulations and experiments collectively corroborate the feasibility of our methodology, and the quantitative analyses verify the high fidelity of the reconstructed images, which results from the low crosstalk between the amplitude modulation and phase modulation in different imaging channels. This inspiring strategy of multiplexing meta-hologram with large channel capacity and high imaging quality will have promising application prospects in information display, storage and encryption, and provide an alternative path for the design of multifunctional microwave devices.

High-resolution, non-hydrostatic simulation of internal tides and solitary waves in the southern East China Sea

The slope area northeast of Taiwan Island in the southern East China Sea (ECS) is well known as a hotspot for both internal tides and internal solitary waves (ISWs) despite uncertainties regarding their specific source and generation mechanism. This study investigates the generation and evolution processes of internal tides and ISWs northeast of Taiwan Island in the southern ECS, based on a super-high-resolution and non-hydrostatic numerical simulation. ISWs create a complex spatial pattern in both satellite images and our simulation, with wave propagation in multiple directions, including onshore and offshore. These ISWs, with the largest amplitudes reaching ∼50 m, are mainly generated by two mechanisms: local tide-topography interactions and disintegration of internal tides remotely generated over I-Lan Ridge. Locally generated ISWs propagate in various directions owing to the complex topography, whereas ISWs generated by the disintegration of remote internal tides are more consistent in their pathways. Strong barotropic to baroclinic energy conversions occur over both Mien-Hua Canyon and North Mien-Hua Canyon. For the baroclinic tidal energy fluxes, the advection flux is comparable to the pressure-work term in shallow water, while the pressure-work flux dominates in deep water. The nonlinear energy flux terms in shallow water are more important for the supertidal than the tidal signals. The semidiurnal energy flux features a clockwise pattern over the double-canyon region, indicating the formation of partial standing waves.

Rayleigh waves in nonpolar Al0.7Sc0.3N(112¯0) films with enhanced electromechanical coupling and quality factor

This work reports on the growth of 1 µm nonpolar a-plane Al0.7Sc0.3N(112¯0) thin films on an r-plane sapphire Al2O3(11¯02) via magnetron sputter epitaxy. The electro-acoustic properties of the film structures were characterized using surface acoustic wave (SAW) resonators. Measured electrical responses were found to be strongly anisotropic in terms of the wave propagation direction. We identified a sagittal polarized Rayleigh wave mode with large coupling (keff2= 3.7%), increased phase velocity (v= 4825 m/s), as well as high quality factor (Q &amp;gt; 1000) for SAW propagation along the c-axis [0001] and normalized thicknesses h/λ=0.2. Finite element method simulations using electro-acoustic properties of Al0.7Sc0.3N obtained from the density functional theory reproduce our experimental results.

Polynomial eigenvalue solution for elastic wave prediction of piezoelectric shunting arrays

This paper presents a polynomial eigenvalue solution to predict the propagation behaviors of elastic wave in piezoelectric shunting arrays. Based on the Bloch theorem, one independent unit cell is selected to conduct the dynamic characteristic analysis instead of infinity. The reduced form of the discretized governing equations is first derived by the standard finite element procedures. To facilitate the subsequent acquisition of dispersion relationship, the dynamic stiffness matrix is then partitioned into a block matrix. Through applying the periodic boundary conditions, a polynomial eigenvalue equation concerning complex propagation constant is finally obtained. The wave propagation and attenuation characteristics in arbitrary directions are investigated using the above methodology. The results demonstrate that the present method can provide very accurate and reliable solutions for wave propagation prediction of piezoelectric shunting arrays.

Study on pile cap effects of pile group foundations under linear wave action by a hybrid BEM and EEM method

For pile group foundations (PGFs) subjected to ocean wave action, diffraction waves from the large-scale pile cap can remarkably change wave loads on slender piles below the pile cap (i.e., pile cap effects). However, relevant influencing factors and the variation pattern of wave loads on piles are still unclear. To this end, this study develops a three-dimensional hybrid method for calculating wave loads on slender piles in PGFs under linear wave action. The presented method is implemented by combining the boundary element method (BEM) and the eigenfunction expansion method (EEM) and can consider the effect of diffracted waves from the pile cap. The proposed method is then applied to a truncated cylinder and a PGF with a square pile cap, and the calculated results are respectively compared with the analytical and numerical results. Excellent agreements verify the validity of the presented method. The effects of various factors, including cap shapes, cap dimensions, cap drafts, wave propagation directions, wave numbers, pile slopes, and pile tilt directions, on wave loads of piles are investigated using the presented method. The results show that wave numbers and cap horizontal dimensions have a very strong effect on wave forces on slender piles in PGFs, and wave forces on piles decrease substantially as they increase. However, the influence of cap drafts is weaker. The wave propagation direction determines the distribution pattern of wave load magnitudes at different pile locations, and this distribution pattern varies with different cap shapes. For tilted piles, the wave propagation direction and the pile tilt direction jointly affect (enhance or weaken) wave loads on piles, and the pile slope can amplify this effect. Wave loads on the external piles of the pile group are generally stronger and these external piles need to be reasonably strengthened. This research can assist with the design and optimization of PGFs for offshore bridges.

Relationship between surface residual stress and dislocation configuration after laser shock processing of TC4 titanium alloy

Through the combination of simulation and experiment, this paper explored the correlation between residual compressive stress and loading pressure in the range of 2800 MPa to 11,000 MPa. The experimental results showed that the surface residual compressive stress increased first and then decreased with the increase of loading pressure. When the surface residual compressive stress decreased, the surface nanocrystallization appeared. With the increase of loading pressure, the form of energy dissipation changed. When the loading pressure was greater than 2800 MPa, which is equal to the Hugoniot elastic limit, the residual compressive stress layer was formed on the surface of material. When the loading pressure was greater than 5600 MPa, which is slightly more than the double Hugoniot elastic limit, the residual compressive stress layer was formed and the dislocation configuration changed. When the loading pressure was greater than 8500 MPa, which is slightly more than the triple Hugoniot elastic limit, the residual compressive stress decreased and the surface nanocrystallization occurred, accompanied by surface damage. For the new grains and sub grains formed by surface nanocrystallization, the atomic arrangement direction in the grains was consistent with the propagation direction of laser shock wave.

Numerical Modeling of the Impact of Sea Level Rise on Tidal Asymmetry in Hangzhou Bay

The tidal asymmetry under the action of sea level rise in Hangzhou Bay. Coastlines dominated by the tide are not only directly affected by the rise of the mean sea level but also by the tidal dynamics. The computational domain of the hydrodynamic model covers the entire Hangzhou Bay and takes into account the feedback between the tidal motion and the erodible bottom. Its main application fields include: the simulation of different sea level rise (SLR) rates, the interaction between tidal duration and skewness and the interaction between tidal range and astronomical tide. The results on tidal asymmetry in Hangzhou Bay is a systematic process consisting of four aspects: Firstly, the tide increase, which is affected by the sea level rise, is between 25% and 50%. Secondly, the value of the sea level rise is about two times the added value of the tidal range, and the tidal range increased to the left side of the tidal wave propagation direction, which accelerated the propagation velocity. Thirdly, the sea level rise amplified the M2 tidal amplitude and delayed the M2 tidal phase in the inner bay, which reduced about 50%. Finally, the change of the tidal range caused by tide level had the same magnitude as the change caused by the mean sea level rise. The purpose of this study was to emphasize the importance of the predictions of the response area affected by tidal asymmetry based on the action of the sea level rise in Hangzhou Bay.

Transition to detonation in inhomogeneous hydrogen-air mixtures: The importance of gradients in detonation cell size

This paper presents a numerical study of flame acceleration and deflagration-to-detonation transition (DDT) of hydrogen-air mixtures in a channel with obstacles, where the mixture composition has a spatial gradient perpendicular to the wave propagation direction. The mixture composition gradient corresponds to a detonation cell size gradient, which characterizes the detonability or the detonation sensitivity. We use a recently developed chemical-diffusive model (CDM) to investigate the influence of the cell size gradient on the detonability of an inhomogeneous mixture and examine characteristic behaviors of flame acceleration and DDT. The CDM is a parametric model for chemical reaction and diffusive transport processes in compressible Navier-Stokes simulations. When optimized, the CDM can reproduce the correct flame and detonation properties, particularly the detonation cell size. The simulation results show that the ease with which an inhomogeneous mixture can detonate is bounded by the detonation limits of the most and least detonable homogeneous mixture. That is, the run-up distance to DDT of inhomogeneous mixtures with 0.8<ϕ<1.0 is found to be greater and less than the most and the least detonable mixtures with ϕ=1.0 and ϕ=0.8, respectively. Other macroscopic observables in the inhomogeneous mixture such as the average combustion wave speed and the total flame surface area are also in between those in these two homogeneous mixtures. The simulation data showed that pressure waves generated by a deflagration play an important role to re-distribute the prescribed mixture composition. The combustion process occurs preferentially in mixtures that initially concentrate along the unobstructed channel centerline. The numerical results agree with prior experimental findings that the averaged equivalence ratio cannot adequately estimate the detonability of an inhomogeneous mixture.

Influence of wave height on round jet in regular waves: Direct numerical simulations

The influence of wave height on the dynamics of a turbulent round jet is analyzed using direct numerical simulation. The vortex dynamics associated with the interaction of the jet with waves of different heights is compared using the swirling strength criterion and the contours of vorticity. An analysis of the vortex roll-up frequency reveals that it increases with increasing wave height. With an increase in the wave height, the deflection of the jet elevates, and therefore, the span of the vortex rings in the near-field reduces in the direction of wave propagation. Further, an analysis of the braid region reveals that the number of lateral jets reduces to three with increasing wave height despite the number of counter-rotating vortex pairs remaining as four. The longer time evolution suggests that as the jet moves from the crest to the zero down-crossing, the fluid in the far-field gets detached from the main jet flow for the stronger waves. The evaluation of the preferred mode reveals that it remains helical for all wave heights. The proper orthogonal decomposition (POD) establishes that the highest energy is captured by the modes of the highest wave height. Also, as the wave height increases, a fewer number of modes capture 90% of the energy. Further, contours of POD modes represent the jet oscillations as well as the kinetic energy gain in the shear layer and the far-field. The time-averaged quantities quantitatively demonstrate an increase in mixing and entrainment in the jet as the wave height increases.

Broadband and thermally switchable reflective metasurface based on Z-shape InSb for terahertz vortex beam generation

In this paper, a broadband and thermally switchable reflective metasurface based on InSb Z-shape resonator (ZSR) structure was proposed for the vortex beam generation in terahertz (THz) region. Numerical simulation results indicate that the operation frequency range and efficiency will be basically unchanged when the external environment temperature increases from 300 K to 340 K, but gradually decreased when the temperature decreases from 300 K to 240 K. More specifically, the proposed InSb metasurface can transform the normal incident circular-polarization (CP) wave to its orthogonal component with conversion coefficient over 0.8 from 0.65 THz to 1.66 THz (relative bandwidth of 87.4%) after reflection when the temperature is changed from 300 K to 340 K. However, the bandwidth will be narrowed and conversion coefficient will be limited with the temperature decreases from 300 K to 240 K. The full 2π phase shift could be obtained at operation frequency by rotating the InSb ZSR structure along the wave propagation direction. Taking advantages of the above characteristics, the reflective THz vortex beams carrying orbital angular momentum (OAM) with topological charge of l = ±1, ±2, and ±3 at the temperature of 300 K over a broadband range can be achieved. In addition, thermal switchable effect of the vortex beams with topological charge of l = 2 at three different frequencies also can be observed. The further numerical results also indicate that the generated reflective THz vortex beams have high mode purity by the designed InSb metasurface. Our work provides a potentially efficient method for the development and integration of the broadband and switchable wavefront controlling devices.