Mode Conversion Mechanism Research Articles

Study on the interaction mechanism of laser-generated Rayleigh waves and subsurface inclined cracks

Abstract In this paper, the finite element method was used to study the reflected and transmitted waves of laser-generated Rayleigh waves from subsurface inclined cracks, the propagation paths and mode conversion mechanisms of different characteristic waves are determined. The Rayleigh wave will interact with the crack top tip and propagate back and forth along the crack surface, and be converted to shear waves at the crack top tip. The shear waves will mode-convert to Rayleigh waves at the free surface when the incidence angle of the shear wave is larger than 60°. Moreover, for the Rayleigh wave interacting with the crack bottom tip, when the crack inclined angle is less than 60°, some Rayleigh waves will travel along the crack surface to the crack top tip. When the crack inclination angle is greater than 60°, in addition to the Rayleigh waves propagating upwards along the crack surface, some Rayleigh waves convert to shear waves at the crack bottom tip and then incident on the free surface of the workpiece. Experiments were carried out to validate some of the Rayleigh wave propagation paths. The experimental results matched the theoretical arrival time well, thus verifying the reliability of the analytical wave path. The results are helpful for the quantitative detection of subsurface inclined cracks using laser ultrasonic techniques.

Quantitative monitoring of icing on CFRP laminate with guided wave combining forward modeling and inverse characterization

Aircraft icing is one of the critical factors for flight safety. Timely and accurate monitoring of in-flight ice accretion is essential for flight systems to take immediate and effective action to significantly improve the efficiency of subsequent de-icing processes and ensure flight safety. Since carbon fiber reinforced polymer (CFRP) are now widely used in the aviation industry, the study of ice accretion on the surfaces of CFRP with anisotropic properties is of great importance. In this work, a three-dimensional frequency domain finite element model for icing CFRP laminate is first established, via which the mechanism of mode conversion of ultrasonic guided waves (UGWs) at different ice layer thicknesses is quantitatively analyzed. Then, the time-domain finite element simulation was performed, from which the time–frequency domain spectrogram of the acquired UGW signal is obtained by short-time Fourier transform (STFT). The extracted frequency–time-of-flight (ToF) curve of the signal via STFT shows high agreement with the theoretical results, which confirms the influence of icing on UGW propagation. Finally, numerous experiments with bonded lead zirconate titanate wafers to excite and acquire UGWs are conducted with different ice layer thicknesses and lengths on CFRP. The extracted frequency–ToF curves of UGW signals are used as input to a two-layer feedforward artificial neural network (ANN) to accurately evaluate the length and thickness of the ice layer. The high reliability of this method with ANN is confirmed, which shows the potential of the proposed UGW-based method for quantitative monitoring of icing length and thickness on aircraft CFRP laminate.

Enhanced performance of cutting angle cylinder piezoelectric energy harvester via coupling vortex-induced vibration and galloping

This paper proposes a novel flow-induced vibration piezoelectric energy harvester with various cutting angle cylinders (FIVPEH-C), for coupling vortex-induced vibration and galloping as well as improving the energy harvesting efficiency. The conceptual designing of the piezoelectric energy harvester with different cutting angle cylinders is conducted, the theoretical models of fluid–structure-electric multi-physical coupled fields are derived, the aerodynamic parameters are solved by three-dimensional computational fluid dynamics (CFD) simulation, and the experimental prototypes of the harvester system are fabricated. The accuracy of the theoretical model is verified by the experimental results. The flow field reveals the vortex shedding characteristics and mode conversion mechanisms. The cutting angle cylinder bluff body transforms the formation mode, initial shape, and intensity of the vortices at the microscopic level, which in turn affects the shedding mode and distance of the vortices at the macroscopic level. When α = 0° and β = 0°, the maximum output voltage of the FIVPEH-C is 13.36 V, and the enhancement ratio reaches up to 108.01 % over the conventional one, which verifies better harvesting performance. This work provides important guidance for designing more efficient piezoelectric energy harvesters via various cutting angle cylinders.

Guiding acoustic waves via a gradient index meta-layer

In this work, we propose an efficient approach for controlling acoustic waves through guided mode conversion within a single meta-layer comprised of gradient index metamaterials. By harnessing the subwavelength localization characteristics of acoustic surface waves, the meta-layer can be properly shaped to accommodate arbitrary and prospective trajectories, thus enabling flexible guidance and routing of acoustic waves. Utilizing such a mode conversion mechanism, the acoustic directional transmitter and beam splitter have been devised and numerically demonstrated based on Archimedean spiral structures. These acoustic devices can exhibit a broadband response, achieving transmission efficiency exceeding 80% over a specific bandwidth that can be controlled by the geometry design. The proposed gradient index meta-layer may open new possibilities for acoustic wave manipulation, enabling some promising applications in noise control, information transmission, and energy harvesting.

Modeling the Propagation of Fast Magnetosonic Waves and Their Conversion to Electromagnetic Ion Cyclotron Waves at Low L Shells

AbstractThe propagation of fast magnetosonic (MS) waves from high to extremely low L shells and their conversion into electromagnetic ion cyclotron (EMIC) waves is investigated with a ray tracing model and a full‐wave model. The ray tracing simulations show that MS waves in the vicinity of the local crossover frequency at low L shells have a latitudinal range from to with wave normal angles from to , both of which provide the opportunity for their mode conversion to EMIC waves. Results from ray tracing are fed into the full‐wave model as initial conditions. The full‐wave simulations show that the incoming MS waves with small () and intermediate () wave normal angles may be efficiently converted to right‐handedly polarized band EMIC waves, which may be further converted to right‐handedly polarized band EMIC waves or to guided left‐handedly polarized band EMIC waves through bi‐ion resonance reflection and polarization reversal. With intermediate () or larger wave normal angles, via polarization reversal and left‐handed polarization cut‐off reflection, the incoming MS waves may be efficiently converted to guided left‐handedly polarized band EMIC waves and the optimal wave normal angles corresponding to the maximum conversion efficiencies decrease with magnetic latitudes. Our study verifies the mode conversion mechanism of MS waves to EMIC waves proposed with a previous ray tracing study and examines the dependence of the efficiency of the wave energy transfer on the magnetic latitudes and wave normal angles, which may help to understand the wave properties and distribution of MS and EMIC waves observed at extremely low L shells.

Enhanced performance of piezoelectric energy harvester by two asymmetrical splitter plates

This paper proposes a novel vortex-induced vibration piezoelectric energy harvester attached to two asymmetrical splitter plates (VIVPEH-S), which aims at converting the vibration mode from vortex-induced vibration (VIV) to galloping and improving the energy harvesting efficiency. The conceptual designing of VIVPEH-S with two asymmetrical splitter plates under various installation angles is first conducted, the experimental prototypes are then fabricated and the wind tunnel experimental system is constructed, and the simulation model of the harvester system is finally established. The effects of the installation angles of two asymmetrical splitter plates on the vibration characteristics and harvesting performance of VIVPEH-S are experimentally investigated, and the vortex shedding characteristic and mode conversion mechanism are revealed by CFD simulation. The results demonstrate that the installation of the asymmetrical splitter plates changes the vortex shedding characteristics and transforms the vibration mode from VIV to galloping, which can significantly broaden the working bandwidth, and improve the energy harvesting performance. A maximum enhancement ratio of the output power of VIVPEH-S with α = 60° and β = 90° is up to 471.2% over the conventional VIVPEH. This work provides an important foundation for designing a more efficient piezoelectric energy harvester by using asymmetrical splitter plates.

Mode conversion of Lamb waves in a composite phononic crystal plate: Numerical analysis and experimental validation

The mode manipulation of Lamb waves plays an important role in damage detection and identification of damage types, location, and size. In this paper, we propose a composite phononic crystal (PC) plate with antisymmetric and symmetric PCs for realizing mode conversion from A0 to S0 mode of Lamb waves. The theoretical analysis, numerical simulations, and experimental validation are introduced and the mode conversion mechanism of the composite PC plate is systematically investigated. The effect of geometrical parameters on band structures of antisymmetric and symmetric PCs is first discussed. Then multi-physics field simulation models are developed and in-plane displacement fields are obtained in numerical simulations, which shows that the mode conversion is enhanced when the period number of the antisymmetric PC decreases and that of symmetric the PC increases. The composite PC plate specimens are fabricated with precision wire cutting technology for experimental measurements and the self-designed polyvinylidene fluoride (PVDF) comb transducer is used to stimulate the Lamb waves. The experimental results are consistent with the numerical simulations, which demonstrate that the proposed composite PC plate can achieve the mode conversion from A0 to S0 mode of Lamb waves. Our proposed structures have applicable values for the mode manipulation of Lamb waves in damage detection.

Lamb wave mode conversion and multiple-reflection mechanisms for simply and reliably evaluating delamination in composite laminates

Lamb wave propagation must be understood comprehensively for simply evaluating delamination during ultrasonic testing. However, the difference between wave propagation, visualized using laser Doppler vibrometer and pulsed-laser scanners, has not been sufficiently investigated, and knowledge of optimal conditions for evaluating delamination is limited. Thus, in this study, the mode conversion and multiple reflections of Lamb waves propagating in a delaminated cross-ply laminate were visualized using different laser scanners, delamination depths, and wave incident angles. Delamination was characterized using maximum-amplitude map postprocessing under specific conditions. Further numerical analysis revealed that owing to multiple reflections of the antisymmetric mode in incident and mode-converted waves, standing waves were generated in the delaminated sublaminate. Dispersion curve and flexural stiffness calculations confirmed the conditions required for high-amplitude standing waves, thereby providing guidelines for simply and reliably evaluating delamination during inspections.

Conservation of Total Wave Action in the Expanding Solar Wind

The conservation of wave action in moving plasmas has been well known for over half a century. However, wave action is not conserved when multiple wave modes propagate and coexist close to the degeneration condition (where the sound speed equals the Alfvén speed, i.e., plasma β ∼ 1). Here, we show that the violation of conservation is due to wave mode conversion, and that the total wave action summed over the interacting modes is still conserved. Though the result is general, we focus on MHD waves and identify three distinctive mode-conversion mechanisms, i.e., degeneracy, linear mode conversion, and resonance, and provide an intuitive physical picture for the mode-conversion processes. We use one-dimensional MHD simulations with the Expanding Box Model to simulate the nonlinear evolution of monochromatic MHD waves in the expanding solar wind. The simulation results validate the theory; total wave action therefore remains an interesting diagnostic for studies of waves and turbulence in the solar wind.

Fabrication error tolerant broadband mode converters and their working principles.

Computational inverse design techniques have shown potential to become reliable means for designing compact nanophotonic devices without compromising the performance. Much effort has been made to reduce the computation cost involved in the optimization process and obtain final designs that are robust to fabrication imperfections. In this work, we experimentally demonstrate TE0-TE1 and TE1-TE3 mode converters (MCs) on the silicon-on-insulator platform designed using the computationally efficient shape optimization method. These MCs have mode conversion efficiencies above 95%, and the insertion loss ranges from 0.3 dB to 1 dB over a wavelength span of 80 nm ranging from 1.5 µm to 1.58 µm. Maximum modal crosstalk found experimentally in the C-band is -19 dB. The conversion efficiency drops at most by 2.2% at 1.55 µm for 10 nm over/under etch, implying good robustness to dimensional variations. We present the mode conversion mechanism of these MCs by studying the simulated electromagnetic field patterns and validate with supportive data. We also demonstrate their performance in the time domain with a 28 Gbps OOK and a 20 GBaud PAM-4 payload transmissions, which supports their utility for high throughput data communications. The open eye diagrams exhibit Q-factors of 8 dB.

Experimental Study on Vortex-Induced Vibration Coupling Wake Interference of Multi-Riser Groups with Sensitive Spacing

The “riser group—fluid between risers” is taken as the carrier, and the experiment on vortex-induced vibration of tandem riser groups coupling interference effect under sensitive spacing is performed. The least-square method is used to linearly fit the reduced velocity and main frequency, and the rule of Strouhal numbers is analyzed. Each mode is separated based on the mode decomposition theory, and the mode conversion mechanism is also explored. The concept of “interference efficiency” is introduced to study the dynamic characteristics and response evolutions of different riser groups. The results show that the wake shielding effect widely exists in tandem riser groups, and the interference effect of midstream and downstream risers on their upstream risers is significantly lower than that of upstream risers on midstream and downstream risers. The trajectories of midstream and downstream risers lag behind their upstream risers due to multiple shadowing effects, the vibration frequency range of downstream riser is widened and the dominant frequency is extremely unstable. Compared with the isolated riser, wake interference suppresses the vibration displacement of the midstream and downstream risers in the in-line direction, and enhances the displacement of upstream and midstream risers in the cross-flow direction. The interference effect of the fluid between risers at low velocities is stronger than that at higher velocities, and the cross-flow displacements of upstream risers are always in the interference enhancement region. It is urgent to pay attention to the cross-flow displacement of upstream and midstream risers in tandem riser groups considering the safety design.

Reflective Metasurfaces with Multiple Elastic Mode Conversions for Broadband Underwater Sound Absorption

Unlike their electromagnetic and acoustic counterparts, elastic waves involve different wave modes. The interplay and the coupling among them increase the complexity of the problem while also offering a larger space for wave manipulation. Elastic bulk wave conversion in an elastic metamaterial has recently shown great promise in medical ultrasound and nondestructive testing. Unlike the transmission-type conversion, however, reflective elastic mode conversion has been explored less in terms of analysis and design, despite the enormous possibilities that it might offer for energy trapping and dissipation. In this work, we develop a theoretical framework for constructing elastic anisotropic metasurfaces that can enable reflective longitudinal-to-transverse (L-to-T) and transverse-to-longitudinal (T-to-L) wave conversions. We capitalize on the mechanism of multiple reflective mode conversion to achieve broadband, subwavelength, and near perfect sound absorption in the underwater environment. The reflective scattering properties of the metasurfaces are systematically exploited for incident longitudinal or transverse waves. The conversion mechanism is rooted in reflective Fabry-Perot (FP) resonance, whose occurrence conditions and features are predicted for prescribed effective parameters of the metasurface. We then establish an inverse-design framework for conceiving an underwater coating system formed by a viscoelastic rubber layer and the metasurface. A series of metasurfaces allowing for customized mode conversions are realized for delivering broadband low-frequency and high-efficiency underwater sound absorption. Specifically, an ultrathin rubber-metasurface layer in which the metasurface with a thickness of approximately \ensuremath{\lambda}/70 can lead to nearly 100% sound absorption. Furthermore, we demonstrate that a persistently high absorption (over 80%) can be obtained in a rather robust manner within a wide range of wave incidence angle from \ensuremath{-}60\ifmmode^\circ\else\textdegree\fi{} to 60\ifmmode^\circ\else\textdegree\fi{}. More importantly, high-efficiency acoustic absorption exceeding 75% can be readily achieved through multiple mode conversions within the ultrabroadband range featuring a relative bandwidth of 119%. We reveal the combined FP resonance mechanism of underwater sound absorption, i.e., the FP resonance of the metaconverter, which determines the L-to-T and T-to-L conversion ratio, and the FP resonance of the rubber-metasurface layers, which enhances the wave attenuation inside the rubber. The proposed reflective multiple mode-conversion mechanism and metasurface design methodology open a route towards a class of elastic-wave-based devices with promising potential for underwater applications.

Pyrolysis behaviors of model compounds with representative oxygen-containing functional groups in coal over calcium

Calcium has great catalytic potentials to regulate volatiles from low-rank coal pyrolysis to form high-valued products. Due to the complexity of coal structure, it's still no consensus on the catalytic mechanism over calcium catalyst. In this paper, coal-related model compounds with representative O-containing functional groups [1,4-naphthalenedicarboxylic acid, poly (4-vinylphenol) and poly (2,6-dimethyl-1,4-phenylene oxide)] were chosen to study the primary pyrolysis product distribution and composition over calcium components by FTIR, TGA and in-situ pyrolysis vacuum ultraviolet photoionization time-of-flight mass spectrometry. The results show that calcium could be ion-exchanged with the O-containing acidic groups (carboxyl functional group or phenolic hydroxyl group) presented in the model compounds to significantly improve the thermal stability, and the pyrolysis of carboxylic and phenolic compounds in the presence of calcium will not only promote a complex cross-linking reaction for the formation of polymers, but also lead to the formation of ketone compounds. The addition of calcium has no effect on the decomposition modes of ether bond connecting two aromatic rings. In addition, the interaction mode and thermal conversion mechanism between carboxyl groups and calcium component were studied. The benzoic acid, 2-naphthoic acid, anthracene-9-carboxylic acid, and 1-pyrenecarboxylic acid were used to investigate the effect of aromatic ring number on the transformation of carboxyl functional groups under the action of calcium. The results showed that the difference of substitution positions may have a certain effect on the formation of products, but the effect of calcium on the main transformation mechanism of carboxyl functional groups in different aromatic structures is universal.

Manipulating Evanescent Waves in a Gradient Waveguide

Manipulation of evanescent waves in a waveguide with gradient-index metamaterials is analyzed. It is shown that evanescent waves excited by a cutoff waveguide can be efficiently transmitted over a long distance by a mode-conversion mechanism. The experimental results are in good agreement with simulated results, and the manipulation is of broadband frequencies. The results suggest potential applications in subwavelength microwave transmission.

Compensating Mode Conversion Due to Bend Discontinuities Through Intentional Trace Asymmetry

In this letter, a comparative analysis is carried out between the mechanism of mode conversion in differential microstrip lines due to bend discontinuities on one side and trace asymmetry on the other side. With the help of equivalent modal circuits, a theoretical basis is provided for the idea to compensate the undesired common mode (CM), due to the presence of the bend, by intentionally designing asymmetric traces. As an application example, the proposed CM-reduction strategy is used in conjunction with another recently-presented wideband CM suppression filter for differential microstrip lines. It is shown that the proposed solution enhances the overall CM-reduction performance of the filter by some decibels, while preserving its transmission properties.

Mode Conversion of the Edge Modes in the Graphene Double-Ribbon Bend.

In this paper, a new kind of graphene double-ribbon bend structure, which can support two edge graphene surface plasmons (EGSPs) modes, is proposed. In this double-ribbon bend, one edge mode can be partly converted into another one. We attribute the mode conversion mechanism to the interference between the two edge plasmonic modes. Based on the finite element method (FEM), we calculate the transmission and loss of EGSPs propagating along this graphene double-ribbon bend in the mid-infrared range under different parameters.

Influence of Resonant Absorption on the Generation of the Kelvin-Helmholtz Instability

The inhomogeneous solar corona is continuously disturbed by transverse MHD waves. In the inhomogeneous environment of coronal flux tubes, these waves are subject to resonant absorption, a physical mechanism of mode conversion in which the wave energy is transferred to the transition boundary layers at the edge between these flux tubes and the ambient corona. Recently, transverse MHD waves have also been shown to trigger the Kelvin-Helmholtz instability (KHI) due to the velocity shear flows across the boundary layer. Also, continuous driving of kink modes in loops has been shown to lead to fully turbulent loops. It has been speculated that resonant absorption fuels the instability by amplifying the shear flows. In this work, we show that this is indeed the case by performing simulations of impulsively triggered transverse MHD waves in loops with and without an initially present boundary layer, and with and without enhanced viscosity that prevents the onset of KHI. In the absence of the boundary layer, the first unstable modes have high azimuthal wavenumber. A boundary layer is generated relatively late due to the mixing process of KHI vortices, which allows the late onset of resonant absorption. As the resonance grows, lower azimuthal wavenumbers become unstable, in what appears as an inverse energy cascade. Regardless of the thickness of the initial boundary layer, the velocity shear from the resonance also triggers higher order azimuthal unstable modes radially inwards inside the loop and a self-inducing process of KHI vortices occurs gradually deeper at a steady rate until basically all the loop is covered by small-scale vortices. We can therefore make the generalisation that all loops with transverse MHD waves become fully turbulent and that resonant absorption plays a key role in energising and spreading the transverse wave-induced KHI rolls all over the loop.

Broad bandwidth waveguide polarizer via grating mediated mode conversion

A polarization beam splitter (PBS) based on a four-layer slab waveguide is proposed, where a sub-wavelength grating is embedded between the waveguide core and the cladding. This grating not only affords Bragg momentum to tune the propagation constant of guiding modes but also converts the forward zero-order waveguide mode to the backward first one for a specific polarization. Thus, the incident light with polarization that satisfies the phase-matching condition is highly reflected in the waveguide, while other light with orthogonal polarization keeps intact and passes through it efficiently. Numerical simulations show that one can make the compact PBS for both polarizations with an extinction ratio higher than 35 dB, a waveband larger than 80 nm, a grating period tolerance of 20 nm, and a waveguide height tolerance of 80 nm. The revealed mode conversion mechanism via the sub-wavelength grating enriches the design of PBSs for integrated silicon waveguide chips.

Characterisation of complex fluid media by an angle-scanning ultrasound diffractometer

The characterisation of complex fluid media (particles in liquids) by ultrasound can provide information on the particle size distribution, concentration, and aggregation of the particles. In the Rayleigh scattering regime, systems of liquid particles are dominated by monopole scatter (potentially producing thermal waves), whereas solid particle suspensions are dominated by dipole scatter (producing shear waves); thus the angular dependence of the diffracted field is expected to be different in these cases. Since mode conversion mechanisms are affected by the proximity of particles and thus by the degree of aggregation, the effect of particle type and degree of aggregation is expected to be observed in the angle-dependent signals. We report an experimental investigation of the angle- and frequency-dependent diffracted signal from cylindrical samples (in tubing) of emulsions and suspensions. The experimental results are compared with theoretical predictions using effective homogeneous properties for the complex fluids derived from multiple scattering models. We identify the differences in the diffracted signals for complex media in which the scattering mechanisms are monopole or dipole-dominated.

Engineered metabarrier as shield from seismic surface waves

Resonant metamaterials have been proposed to reflect or redirect elastic waves at different length scales, ranging from thermal vibrations to seismic excitation. However, for seismic excitation, where energy is mostly carried by surface waves, energy reflection and redirection might lead to harming surrounding regions. Here, we propose a seismic metabarrier able to convert seismic Rayleigh waves into shear bulk waves that propagate away from the soil surface. The metabarrier is realized by burying sub-wavelength resonant structures under the soil surface. Each resonant structure consists of a cylindrical mass suspended by elastomeric springs within a concrete case and can be tuned to the resonance frequency of interest. The design allows controlling seismic waves with wavelengths from 10-to-100 m with meter-sized resonant structures. We develop an analytical model based on effective medium theory able to capture the mode conversion mechanism. The model is used to guide the design of metabarriers for varying soil conditions and validated using finite-element simulations. We investigate the shielding performance of a metabarrier in a scaled experimental model and demonstrate that surface ground motion can be reduced up to 50% in frequency regions below 10 Hz, relevant for the protection of buildings and civil infrastructures.