Showing papers on "Longitudinal wave published in 2020"

Tailored elastic surface to body wave Umklapp conversion

Abstract: Elastic waves guided along surfaces dominate applications in geophysics, ultrasonic inspection, mechanical vibration, and surface acoustic wave devices; precise manipulation of surface Rayleigh waves and their coupling with polarised body waves presents a challenge that offers to unlock the flexibility in wave transport required for efficient energy harvesting and vibration mitigation devices We design elastic metasurfaces, consisting of a graded array of rod resonators attached to an elastic substrate that, together with critical insight from Umklapp scattering in phonon-electron systems, allow us to leverage the transfer of crystal momentum; we mode-convert Rayleigh surface waves into bulk waves that form tunable beams Experiments, theory and simulation verify that these tailored Umklapp mechanisms play a key role in coupling surface Rayleigh waves to reversed bulk shear and compressional waves independently, thereby creating passive self-phased arrays allowing for tunable redirection and wave focusing within the bulk medium

Realization of acoustic spin transport in metasurface waveguides.

Abstract: Spin angular momentum enables fundamental insights for topological matters, and practical implications for information devices. Exploiting the spin of carriers and waves is critical to achieving more controllable degrees of freedom and robust transport processes. Yet, due to the curl-free nature of longitudinal waves distinct from transverse electromagnetic waves, spin angular momenta of acoustic waves in solids and fluids have never been unveiled only until recently. Here, we demonstrate a metasurface waveguide for sound carrying non-zero acoustic spin with tight spin-momentum coupling, which can assist the suppression of backscattering when scatters fail to flip the acoustic spin. This is achieved by imposing a soft boundary of the π reflection phase, realized by comb-like metasurfaces. With the special-boundary-defined spin texture, the acoustic spin transports are experimentally manifested, such as the suppression of acoustic corner-scattering, the spin-selected acoustic router with spin-Hall-like effect, and the phase modulator with rotated acoustic spin. Spin angular momenta play a crucial role in topological phases of matter, in acoustic waves they have been demonstrated recently. Here, the authors present a symmetry-breaking metasurface waveguide that assists backscattering suppression of acoustic waves, because of tight spin-momentum coupling.

Symmetry selective directionality in near-field acoustics.

Abstract: Understanding unidirectional and topological wave phenomena requires the unveiling of intrinsic geometry and symmetry for wave dynamics. This is essential yet challenging for the flexible control of near-field evanescent waves, highly desirable in broad practical scenarios ranging from information communication to energy radiation. However, exploitations of near-field waves are limited by a lack of fundamental understanding about inherent near-field symmetry and directional coupling at sub-wavelengths, especially for longitudinal waves. Here, based on the acoustic wave platform, we show the efficient selective couplings enabled by near-field symmetry properties. Based on the inherent symmetry properties of three geometrically orthogonal vectors in near-field acoustics, we successfully realize acoustic Janus, Huygens, spin sources and quadrupole hybrid sources, respectively. Moreover, we experimentally demonstrate fertile symmetry selective directionality of those evanescent modes, supported by two opposite meta-surfaces. The symmetry properties of the near-field acoustic spin angular momenta are revealed by directly measuring local vectorial fields. Our findings advance the understanding of symmetries in near-field physics, supply feasible approaches for directional couplings, and pave the way for promising acoustic devices in the future.

Floquet engineering of interlayer couplings: Tuning the magic angle of twisted bilayer graphene at the exit of a waveguide

Abstract: We introduce an approach that allows one complete control over the modulation of the effective twist angle change in few-layer van der Waals heterostructures by irradiating them with longitudinal waves of light at the end of a waveguide. As a specific application, we consider twisted bilayer graphene and show that one can tune the magic angles to be either larger or smaller, allowing in situ experimental control of the phase diagram of this and other related materials. A waveguide allows one to circumvent the free-space constraints on the absence of longitudinal electric field components of light. We propose to place twisted bilayer graphene at a specific location at the exit of a waveguide, such that it is subjected to purely longitudinal components of a transverse magnetic mode wave.

Conformal gradient-index phononic crystal lens for ultrasonic wave focusing in pipe-like structures

Abstract: We explore a conformal gradient-index phononic crystal lens integrated within a pipe to amplify guided wave modes toward improved ultrasonic inspection of pipelines. The proposed conformal lens is composed of an array of cylindrical steel stubs attached to the outer surface of a steel pipe, which are tailored according to the hyperbolic secant profile of refractive index in the circumferential direction of the pipe. Hence, the ultrasonic guided wave energy is focused in the axial direction of the pipe and amplified at the focal point of the lens. Refractive indices are calculated using dispersion curves obtained from the finite element simulations of the stubbed unit cells, and the curved lens is designed for the second longitudinal wave mode of the pipe, which is commonly used in guided wave testing. The proposed lens design is implemented on a steel pipe, which is typically used in the distribution networks utilized in cities, and simultaneous focusing of longitudinal wave modes in a broad frequency range is verified through both numerical models and experimental measurements.

The new structure of analytical and semi-analytical solutions of the longitudinal plasma wave equation in a magneto-electro-elastic circular rod

Abstract: This research paper extracts novel analytical and semi-analytical wave solutions of the longitudinal wave equation in a magneto-electro-elastic circular rod by using the modified Khater method as o...

Numerical Investigation of Aerodynamic Drag and Pressure Waves in Hyperloop Systems

Abstract: Hyperloop is a new, alternative, very high-speed mode of transport wherein Hyperloop pods (or capsules) transport cargo and passengers at very high speeds in a near-vacuum tube. Such high-speed operations, however, cause a large aerodynamic drag. This study investigates the effects of pod speed, blockage ratio (BR), tube pressure, and pod length on the drag and drag coefficient of a Hyperloop. To study the compressibility of air when the pod is operating in a tube, the effect of pressure waves in terms of propagation speed and magnitude are investigated based on normal shockwave theories. To represent the pod motion and propagation of pressure waves, unsteady simulation using the moving-mesh method was applied under the sheer stress transport k–ω turbulence model. Numerical simulations were performed for different pod speeds from 100 to 350 m/s. The results indicate that the drag coefficient increases with increase in BR, pod speed, and pod length. In the Hyperloop system, the compression wave propagation speed is much higher than the speed of sound and the expansion wave propagation speed that experiences values around the speed of sound.

Slowing of acoustic waves in electrorheological and string-fluid complex plasmas

Abstract: The PK-4 laboratory consists of a direct current plasma tube into which microparticles are injected, forming a complex plasma. The microparticles acquire many electrons from the ambient plasma and are thus highly charged and interact with each other. If ion streams are present, wakes form downstream of the microparticles, which lead to an attractive term in the potential between the microparticles, triggering the appearance of microparticle strings and modifying the complex plasma into an electrorheological form. Here we report on a set of experiments on compressional waves in such a string fluid in the PK-4 laboratory during a parabolic flight and on board the International Space Station. We find a slowing of acoustic waves and hypothesize that the additional attractive interaction term leads to slower wave speeds than in complex plasmas with purely repulsive potentials. We test this hypothesis with simulations, and compare with theory.

Elastic Wave Energy Entrapment for Reflectionless Metasurface

Abstract: This paper presents an elastic metasurface that can entrap the elastic wave energy of both longitudinal and shear wave modes in a lossy region, resulting in almost no reflection environment, which we call a reflectionless metasurface. Unlike previous attempts in acoustics, elastic waves are governed by different physics, such as mode conversion, and different behavior of shear waves. Accordingly, achieving a reflectionless elastic metasurface requires a different approach to that of previous studies on acoustic metasurfaces for sound absorption. Here, physics for entrapping the elastic wave energy is proposed by combining two phenomena: the local resonance of metasurface unit cells and the surface wave conversion. As a result, the elastic wave energy originating from an incident longitudinal wave can be highly concentrated in the metasurface, and energy dissipation can be significantly increased if any damping is implemented in the metasurface region. To realize the idea, a reflectionless metasurface, consisting of rod-shaped unit cells with various lengths, is designed. Numerical and experimental investigations show that the proposed reflectionless metasurface, with a small amount of loss, can effectively reduce both the reflected longitudinal and shear waves under longitudinal wave incidence with broad angles. We expect that the proposed idea, based on a thin artificial elastic layer, will provide an alternative route to design ultrasonic absorbers.

Broad-angle refractive transmodal elastic metasurface

Abstract: Achieving total mode conversion from longitudinal to shear waves for a broad incident angle has been a big scientific challenge in elastic fields, which was impossible to be achieved in classical elastic wave theory. In this paper, we propose and realize a refractive transmodal elastic metasurface that can convert an incident longitudinal wave to a shear wave for a broad incident angle. Here, the total mode conversion is achieved via a sufficiently large phase gradient, while the full transmission is achieved with the impedance-matched single-layered metasurface. Numerical and experimental investigations show that the proposed metasurface can provide almost total mode conversion for a broad incident angle from −20.4 ° to 22.3 °. We expect that the proposed refractive transmodal metasurface can be applied in various ultrasonic systems.

The re-initiation mechanism of detonation diffraction in a weakly unstable gaseous mixture

Abstract: Numerical simulations were performed to investigate the re-initiation mechanism of a diffracted detonation wave near the critical channel width for a weakly unstable gas. Two scenarios were examined: diffraction of a planar detonation wave and of a cellular detonation wave inside the inlet channel. The results revealed that the critical channel width predicted using a cellular detonation wave is smaller than that predicted using a planar detonation wave. The re-initiation mechanisms are described in detail by tracing massless particles along both the plane of symmetry and the re-initiation path. For planar detonation diffractions, a compression wave is formed in the far field behind the diffracted shock. Re-initiation is closely related to the amplification of this compression wave and its coalescence with the diffracted shock. Depending on the inlet channel width, the strength of the reflected rarefaction wave is responsible for weakening the strength of the compression wave and its coalescence with the diffracted shock, consequently hindering the reaction of particles behind the diffracted shock wave. In cellular cases, the continuous collisions of transverse waves, which generate local explosion sites, sustain detonation wave propagation.

Tuning the magic angle of twisted bilayer graphene at the exit of a waveguide

Abstract: We introduce a new approach that allows one complete control over the modulation of the effective twist angle change in few-layer van der Waals heterostructures by irradiating them with longitudinal waves of light at the end of a waveguide. As a specific application, we consider twisted bilayer graphene and show that one can tune the magic angles to be either larger or smaller, allowing in-situ experimental control of the phase diagram of this and other related materials. A waveguide allows one to circumvent the free-space constraints on the absence of longitudinal electric field components of light. We propose to place twisted bilayer graphene at a specific location at the exit of a waveguide, such that it is subjected to purely longitudinal components of a transverse magnetic modes (TM) wave.

Full-wave modeling of micro-acoustofluidic devices driven by standing surface acoustic waves for microparticle acoustophoresis

Abstract: Surface acoustic wave (SAW)-based acoustofluidic systems are emerging as an important tool for acoustophoresis. In this paper, we present a full cross-sectional model of standing SAW acoustofluidic devices for obtaining full-wave results. Our model involves a piezoelectric substrate with interdigitated electrodes and a rectangular water channel enclosed in a finite soft elastic solid. This model accounts for piezoelectric SAWs with electromechanical coupling, simultaneous transverse and longitudinal wave fields in the elastic solid from SAW radiation, and acoustic and streaming fields in the enclosed water channel in an integrated system by solving the elastodynamic and Navier–Stokes field equations. Accordingly, the acoustic radiation force and streaming-induced Stokes drag force are obtained to analyze the acoustophoretic motion of microparticles of different sizes. Using the full-wave results, we reveal the influences of the channel wall displacements and acoustic and flow fields in the water domain. The full-wave field also allows us to determine the effects of the channel dimensions and its location in the finite elastic solid on the force strengths. We demonstrate that the critical diameter of the microparticles can be reduced by an order of magnitude by changing the channel location, while maintaining the same acoustic frequency. We note that the results, mechanisms, and method presented in this study can be usefully applied to the rational design of standing SAW acoustofluidic devices and for developing innovative acoustophoretic systems involving complex structure–fluid interactions.

A flexible laser ultrasound transducer for Lamb wave-based structural health monitoring

Abstract: Lamb waves are widely used in the structural health monitoring (SHM) for plate-like structures. In this paper, a new and flexible ultrasonic transducer with a high photo-acoustic conversion efficiency was proposed by using candle soot nanoparticles (CSNPs) and polydimethylsiloxane (PDMS). Experimental results demonstrate that the developed transducer can generate a longitudinal wave with a short duration of 0.28 μs under the illumination of a nanosecond laser pulse. The amplitude of the excited longitudinal wave is 10 times to that of the signal generated by the traditional laser ultrasound technique. Further, wedge shape transducers were developed to excite Lamb waves in a 1.5-mm thick aluminium substrate by the oblique incidence method. The specific A0 and S0 modes of the Lamb wave with the central frequency of 647 kHz were successfully excited in the aluminium plate. Based on the synthetic aperture focusing imaging technique, a delay-and-sum signal processing method was adopted for damage location in the plate by using the A0 mode Lamb. A 3.5-mm defect was well imaged and the results demonstrate that the developed flexible photo-acoustic transducer can be a good alternative method for the SHM.

Comparison of Experimental Measurements of Material Grain Size Using Ultrasound

Abstract: Material grain size is related to metallic material properties and its elastic behaviour. Measuring and monitoring material grain size in material manufacturing and service is an important topic in measurement field. In this paper, three materials, i.e., aluminium 2014 T6, steel BS970 and copper EN1652, were chosen to represent materials with small, medium and large grain size, respectively. Various techniques of measuring material grain size were demonstrated and compared. These techniques include the measurements from material microstructure images, backscattered ultrasonic grain noise using a conventional transducer, longitudinal wave attenuation using ultrasonic arrays and shear wave attenuation using a lead zirconate titanate (PZT) plate. It is shown that the backscattered ultrasonic noise measurement and material attenuation measurement are complementary. The former is pretty good for weak scattering materials, e.g., aluminium, while the latter for materials with large grains, e.g., steel and copper. Consistent measured grain size from longitudinal and shear wave attenuations in steel and copper suggests that shear wave attenuation can be calculated from the measured longitudinal wave attenuation integrated with Stanke–Kino’s model or Weaver’s model, if there is a difficulty to either excite or capture shear waves in practice. The outcome of the paper expects to provide a further step towards the industrial uptake of these techniques.

Evaluation of optimum length scale parameters in longitudinal wave propagation on nonlocal strain gradient carbon nanotubes by lattice dynamics

Abstract: Longitudinal wave propagation in carbon nanotubes has been investigated by using Rayleigh-Bishop rod model and nonlocal strain gradient elasticity theory in the present study. Size dependent govern...

Shock wave interaction with a polygonal bubble containing two different gases, a numerical investigation

Abstract: The interaction between a planar shock wave propagating in air and a polygonal bubble (composed of two triangles) containing two different gases is studied numerically. Studying the interaction between an oncoming shock wave with front- and rear-facing triangles containing light and heavy gases is of great importance in understanding the complex shock wave propagation, interaction and hydrodynamic instabilities as well as their effect on mitigating or enhancing the colliding shock/blast wave. Two different cases were studied: in the first case, the front triangle contained sulfur hexafluoride (SF6) and the rear one contained helium (He); while in the second case, He is in the front and SF6 in the rear triangle. As the speed of sound in He is significantly higher than that in SF6 and in air, different flow fields were evolved. When SF6 is placed in the front triangle, the shock wave transmitted through the SF6 is reflected back from the interface separating the two gases and starts propagating downstream; over the He segment of the bubble, the incident shock wave (in the open air) is already seen over the He section and it submits compression waves into the He gas. These compression waves travel upstream and downstream; in their upstream movement they generate compression waves into the ambient air ahead of the incident shock wave. The part moving downstream will hit the interface separating SF6 and He, resulting in a complex wave pattern. A completely different wave pattern is visible when He is placed in the front triangle. Now the fastest shock is the transmitted shock wave in the He section; it reaches the membrane separating the two gases well before the incident shock wave reaches this location. Unlike the previous case, now the resulting flow in the rear triangle of the bubble is affected not only by the incident shock wave but also by the transmitted compression waves from the helium section. Furthermore, when helium is placed in the front section of the bubble, the compression waves in the He impacts the rear triangle of the bubble (containing SF6) almost like a planar shock wave. This is different from the previous case where SF6 was in the front section; then the shock wave impacting on the rear bubble containing He had a completely different shape due to its propagation into the SF6 bubble. This resulted in completely different peak pressures.

S-band delay lines in suspended lithium niobate

Abstract: Thin-film lithium niobate is an attractive platform for GHz-frequency applications in low-power RF analog signal processing, optomechanics, and quantum devices due to its high coupling, low loss, excellent optical properties, and compatibility with superconducting quantum circuits. We demonstrate aluminum interdigitated transducers (IDTs) in this platform for horizontal shear (SH) waves between 1.2 and 3.3 GHz and longitudinal waves between 2.1 and 5.4 GHz. For the SH waves, we measure a piezoelectric coupling coefficient of 13 % and 6.0 dB/mm propagation losses in delay lines up to 1.2 mm with a 300 ns delay in air at room temperature. In these high k eff 2 transducers, electrical loading gives rise to large reflections and resonances. Finite element method models and an experimental finger-pair sweep are used to characterize the role of resonance in these transducers, illuminating the physics behind the large motional admittances of these small-footprint IDTs.

On the interactions between a propagating shock wave and evaporating water droplets

Abstract: One-dimensional numerical simulations based on hybrid Eulerian-Lagrangian approach are performed to investigate the interactions between propagating shock waves and dispersed evaporating water droplets in two-phase gas-droplet flows. Two-way coupling for interphase exchanges of mass, momentum and energy is adopted. Parametric study on shock attenuation, droplet evaporation, motion and heating is conducted, through considering various initial droplet diameters (5-20 {\mu}m), number densities (2.5 x 1011 - 2 x 1012 1/m3) and incident shock Mach numbers (1.17-1.9). It is found that the leading shock may be attenuated to sonic wave and even subsonic wave when droplet volume fraction is large and/or incident shock Mach number is low. Attenuation in both strength and propagation speed of the leading shock is mainly caused by momentum transfer to the droplets that interact at the shock front. Total pressure recovery is observed in the evaporation region, whereas pressure loss results from shock compression, droplet drag and pressure gradient force behind the shock front. Recompression of the region between the leading shock and two-phase contact surface is observed when the following compression wave is supersonic. After a critical point, this region gets stable in width and interphase exchanges in mass, momentum, and energy. However, the recompression phenomenon is sensitive to droplet volume fraction and may vanish with high droplet loading. For an incident shock Mach number of 1.6, recompression only occurs when the initial droplet volume fraction is below 3.28 x 10-5.