Wave Focusing Research Articles

Optimised design of aviation stainless steel sheet SH guided wave focusing PPM EMAT

ABSTRACT Electromagnetic Acoustic Transducer (EMAT) is an ultrasonic testing technique that generates sound in the part inspected instead of the transducer. Electromagnetic ultrasonic guided wave detection technology has problems such as low energy conversion efficiency and the great influence of environmental noise, resulting in low echo signal amplitude. When the two rows of permanent magnets are arranged non-parallel, the sound beam will be focused at the intersection point of the axes of the two rows of permanent magnets. This article uses this rule to optimise the design parameters of PPM EMAT. Distance amplitude curve (DAC) is a method of compensating. However, large-scale detection requires a flat DAC curve. In addition, the increase in the angle and length of the permanent magnets and the number of pairs of permanent magnets will improve the intensity of the defect wave indication intensity. However, the increase in the angle of the permanent magnets will increase the signal attenuation. After simulation and experiments, when the angle, length and number of permanent magnet pairs are 4°, 25 mm and 8 pairs, the corresponding EMAT has the optimal axis radiation sound field and a smoother DAC, which is more suitable for stainless steel sheets, long-distance rapid guided wave detection.

Spatial evolution of the double-wave-group focusing influenced by the co- and counter-propagating current

Wave–current interaction has always been a challenging topic in fluid mechanics. The research on bimodal waves has received much more attention recently, but their evolutions influenced by underlying currents are not yet clear. This study aims to investigate the effects of co- and counter-propagating currents on spatial evolution using a fully nonlinear wave-current tank based on the high-order spectral method. The process of the wave focus is significantly shortened by the counter-propagating current, resulting in a sharper crest focus, followed by the trough focus. Concurrently, the decrease in the total envelope height and width is accelerated before wave focus and then the increase is decelerated, accompanied by a delay in the envelope profile transition from the backward-leaning to the forward-leaning. The co-propagating current exhibits the opposite phenomenon. The analysis of the spectral energy distribution aids in clarifying the variation of the envelope profile. The energy redistribution, characterized by a downshift of the frequency band, and a decreased energy distribution at the second peak, along with the slightly larger value of the root mean square frequency, indicates that the energy back-flow is obstructed by the counter-propagating current. These findings contribute to our understanding of the current effect on the focused double-wave-group, providing valuable insights for future research and applications in this field.

Resonant-pillar-type acoustic black hole for ultralow-frequency vibration reduction

Abstract This paper proposes a resonant-type acoustic black hole (ABH). The ABH is achieved with gradient refractive index varies from the center of the lens to its outer surface. The performance of wave focusing and vibration reduction via the ABH is explored theoretically and numerically. Results show that the designed ABH can achieve wave focusing inside the lens and provide good vibration reduction at low frequency.

Negative elastic wave refraction and focusing regulation of single-phase solid phononic crystals

This paper presents the design of a single-phase solid phononic crystal (PnC) structure featuring a regular hexagonal perforation pattern. The structure manifests three negative refraction bands, encompassing one for transverse waves and two for longitudinal waves, thereby enabling simultaneous control of shear and longitudinal waves. Due to the high symmetry of the triangular lattice, the equal frequency curves corresponding to the negative refraction band approach circular shapes, suggesting a nearly isotropic negative refraction effect. This negative refraction effect is achieved through specific mass resonance modes closely related to the porous structure designed in this paper. Initially, we analyze the band structure of the PnC, followed by designing the PnC plate structure to achieve negative refraction control for transverse waves at a frequency of 32.4 kHz, with a negative refraction index of −1. Additionally, negative refraction control for longitudinal waves is attained at frequencies of 44 and 64.54 kHz. Subsequently, we scrutinize the influence of various conditions on negative refraction, including different structural parameters, incident angles, and operating frequencies, while verifying the robustness of the designed phonon crystal structure. Leveraging the negative refraction characteristics of the structure, we construct an elastic wave lens to achieve perfect imaging of shear and longitudinal waves. Finally, employing finite element simulation and analyzing focusing imaging characteristics with different source positions, we validate that the results closely align with theoretical expectations. The solid PnC structure designed in this study holds significant potential for applications in the fields of elastic wave imaging.

Programmable Meta‐Reflector for Multiple Tasks in Intelligent Connected Environments

AbstractIn the face of the challenges posed by the advent of 5G, Internet of Things (IoT) and requirements of future communication generations, the urgent need for cutting‐edge technologies capable of adapting their operation to a variety of dynamic tasks for efficient signal propagation management is clear. Connected intelligent metasurfaces, distinguished by their ability to dynamically and autonomously adapt their responses, and able to interact with an external uncontrollable and fast‐changing communication environment to benefit from collective intelligence so as to determine the required action, are emerging as an innovative solution in this field. Herein, an intelligent connected system based on a programmable metasurface reflector, is presented and assessed in two scenarios simulating dynamic uplink and downlink communications. For the uplink configuration, the metasurface is exploited for the estimation of the direction of arrival (DOA) of an incoming wave, while for the downlink scenario, it enables the automated tracking of mobile targets using global positioning system (GPS) data updated from a real‐time database. Experimental results highlight the responsiveness of the metasurface and demonstrate efficiency in wave focusing and mobile target tracking.

Modal analysis of a submerged elastic disk: A hypersingular integral equation approach

A method based on the hypersingular integral equation approach and the modal analysis is presented to consider the effects of the motion of a submerged elastic disk on the incoming waves. Initially, the governing boundary value problem is reduced to a two-dimensional integral equation with a hypersingular kernel. This integral equation is further reduced to a one-dimensional Fredholm integral equation of the second kind with the help of Fourier series expansions and a newly defined function. As a part of modal analysis, eigenfunction expansion based on natural modes of structural motion is considered to describe the motion of a thin circular elastic disk. Physical quantities, such as hydrodynamic force, added mass, damping coefficient, and surface elevation, are numerically evaluated. The computed numerical results are verified by comparing them with those for the rigid disk horizontally submerged in deep water. Apart from this, as a part of the analytical verification of our present analysis, the reciprocity relation has been included. The effects of different parameters (disk's rigidity, radius, submergence depth, and mode of vibrations) on the aforementioned physical quantities have been studied. The maximum hydrodynamic force occurs around Ka = 0.5, while the maximum added mass and damping coefficient occur around the wavenumber Ka = 0.3 and Ka = 0.5, respectively. The peaks of the hydrodynamic force and free surface elevation become sharper with the increasing values of the disk's size. The numerical results emphasize that the wave focusing can be controlled by changing the submergence depth, size, and rigidity of the disk.

Experimental study on flow kinematics of breaking wave impinging and overtopping on a deck structure

The present study experimentally investigates the flow kinematics due to breaking waves impinging and overtopping on a simplified deck structure. Several breaking-wave impinging scenarios in relation to the model deck are conducted. The resulting flow velocities in the aerated regions are measured using bubble image velocimetry (BIV). A wave focusing method is employed to generate plunging breakers. Repeated measurements are performed to obtain mean flow and turbulence characteristics. Two distinguished flow behaviors are observed among all seven scenarios. For the cases of a fully developed plunging breaker interacting with the deck structure, the maximum horizontal velocities above and below the structure are both around 1.2C, with C being the phase speed of the breaking wave; for the cases of a non-fully-developed plunging breaker impingement, the flow velocity below the deck is surprisingly large, with a value of 1.8C below the deck against 1.2C above the deck. Furthermore, the use of a dam-break flow to model the overtopping horizontal velocities on the deck is performed with satisfactory agreements for all the impinging scenarios. Moreover, the temporal and spatial varying prediction equation proposed by Ryu et al. (2007b) is used to model overtopping flow velocities, showing the applicability of this simplified methodology.

Design of patch-shaped lens with thickness variations for elastic wave focusing in thin-plate structures

In mechanical engineering, focusing on elastic waves is pivotal for applications, such as energy harvesting, shock mitigation, and wave manipulation. While phononic crystals have historically been a key method for managing wave propagation, this study explores a novel technique. This method introduces gradient refractive-index (GRIN) lenses by altering the plate thickness and creating localized high-refractive-index zones. Unlike traditional methods, this localized GRIN approach aims to overcome the fabrication and structural limitations, particularly in thin structures. The patch-shaped lenses offer the potential for elastic wave focusing in thinner structures without any degradation of structural performance. Through numerical analysis, we established design principles and examined the elastic wave propagation and focusing characteristics across various thickness variation profiles. This study conducts a thorough analytical and experimental evaluation of these lenses to confirm their effectiveness, structural robustness, and suitability for optimizing wave concentration in various mechanical engineering applications. The research represents an alternative, innovative, and promising pathway in the field of wave focusing, transcending the traditional constraints of thin plate structures.

Active Tunable Elastic Metasurface for Abnormal Flexural Wave Transmission

An active elastic metasurface has more flexibility than a passively modulated elastic metasurface, owing to the manipulation of the phase gradient that can be realized without changing the geometrical configuration. In this study, a negative proportional feedback control system was employed to provide positive active control stiffness for adaptive unit cells, with the aim of achieving the active modulation of the phase gradient. The relationship between the control gain and the phase velocity of the flexural wave was derived, and the transfer coefficients and phase shifts of the flexural wave through the adaptive unit cells were resolved using the transfer matrix method. Finite element simulations for wave propagations in the adaptive unit cells were conducted, and they verified the analytic solutions. Based on this theoretical and numerical work, we designed active elastic metasurfaces with adaptive unit cells with sub-wavelength thicknesses according to the generalized Snell’s law. These metasurfaces show flexibility in achieving abnormal functions for transmitted waves, including negative refraction and wave focusing, and transforming guided waves at different operating frequencies by manipulating the control gain. Therefore, the proposed active metasurface has great potential in the fields of the tunable manipulation of elastic waves and the design of smart devices.

Timoshenko-Ehrenfest Beam-Based Reconfigurable Elastic Metasurfaces for Multifunctional Wave Manipulation.

Herein, a Timoshenko-Ehrenfest beam-based reconfigurable elastic metasurface is introduced that can perform multifunctional wave phenomena within a single substrate, featuring high transmission in the ultrabroadband frequency range. Conventional elastic metasurfaces are typically limited to specific purposes and frequencies, thereby imposing significant constraints on their practical application. The approach involves assembly-components with various geometries on a substrate for reconfigurability, enabling to easily control and implement multifunctional wave phenomena, including anomalous-refraction, focusing, self-acceleration, and total-reflection. This is the first study on elastic metasurfaces to theoretically analyze the dispersion relation based on the Timoshenko-Ehrenfest beam theory, which considers shear deformations and rotational inertia. The analytical model is validated by demonstrating an excellent agreement with numerical and experimental results. The findings include full-wave harmonic simulations and experimentally visualized fields for measuring various wave modulations. Furthermore, the practicality of the system is verified by significantly enhancing the piezoelectric energy harvesting performance within the focusing configuration. It is believed that the reconfigurable elastic metasurface and analytical model based on the Timoshenko-Ehrenfest beam theory have vast applications such as structural health monitoring, wireless sensing, and Internet of Things.

(270) LISWT Before Penile Prosthesis Implantation Might Facilitate Dilation of Fibrotic Corpora

Abstract Introduction Severe corporeal fibrosis presents significant challenges in penile implant insertion. LISWT studies in rats and humans have suggested its ability to rejuvenate cavernous bodies through various mechanisms and growth factors. Objective This prompted our evaluation of LISWT's effectiveness in facilitating prosthesis implantation in patients with penile fibrosis, which are typically considered difficult cases. Methods A 34-year-old male smoker with a history of sickle cell disease, bilateral hip replacements, and splenectomy due to sickle cell crisis sought penile prosthesis implantation. He presented with severe erectile dysfunction and corporeal fibrosis following two ischemic priapism episodes caused by sickle cell disease, which were treated with distal shunts 4 and 1 years prior, respectively (Fig. 1). After obtaining consent, the patient underwent four cycles of LISWT, spaced one month apart. Each cycle consisted of four weekly sessions using the PiezoWave2 device from Richard WOLF with settings of 0.16mj/mm2 energy flux density, 6Hz frequency, and 10mm wave focus penetration depth. In each session, the patient received a total of 6000 shocks, distributed between the perineum, dorsum penis, and the lateral aspect of the penis. Results A month post-LISWT, the patient's Erection Hardness Score (EHS) improved from 0 to 1 following an intracavernous injection of 20 mcg of alprostadil (Fig. 2). Subsequently, he underwent a malleable penile implant procedure through the penoscrotal approach. Cavernosal dilation was successful up to 10mm using a Hegar dilator, with bilateral corporal measurements of 8cm proximally and 9cm distally. A Tactra malleable penile prosthesis (9.5mm girth and 17cm length) from Boston Scientific was implanted without complications (Fig. 3.A.B) Conclusions In cases of severe corporeal fibrosis, pre-treatment with LISWT may aid in the dilation of the corpora during penile prosthesis implantation, potentially reducing complications. Randomized studies are warranted to validate this indication Disclosure No.

Rayleigh wave excitation with an elliptical reflector for high-power ultrasound

This study proposes a Rayleigh wave transducer with an elliptical reflector for high-power ultrasound (ELIPS). In contrast to conventional transducers using IDT with single crystal piezoelectric plates or wedge transducers, our ELIPS enables high-power Rayleigh wave excitation in the megahertz range, which will be useful for applications such as surface acoustic wave (SAW) motors, SAW tactile devices, and SAW atomizers. The driving principles of the transducer are Rayleigh wave excitation with a transverse wave source and transverse wave focusing with a dilatational wave source and an elliptical reflection surface. The effectiveness of this mechanism was confirmed with an FEM simulation, and the Rayleigh wave excitation was validated using a prototype transducer. Finally, the high-power characteristics were examined, and a maximum vibration amplitude of 15.5 nm at 1.0 MHz was successfully obtained. It should be noted that this vibration amplitude is not a saturated value, and a much higher value could be realized with higher input power.

(268) Intracavernous Prostaglandin Injection as a Predictor and Effectiveness Measure for LISWT Sessions in ED Patients: A Single Urologist's Experience

Abstract Introduction In recent years, therapeutic approaches for treating vasculogenic Erectile Dysfunction (ED) have evolved, with Low Intensity Shock Wave Therapy (LISWT) emerging as a promising modality. Objective To objectively evaluate the efficacy of LISWT for ED patients and to predict the required number of sessions, we examined the effects of administering up to 10 mcg of intracavernous alprostadil injections both before and after treatment Methods 85 vasculogenic ED patients (Average age: 57); 33 with mild to moderate ED and 52 with moderate or severe ED. Intracavernous injection (ICI) of ≤10 mcg of alprostadil was given before LISWT and one month post-treatment. Depending on ICI response, patients underwent either six (Erection Hardness Score (EHS) ≤3) or four (EHS=4) weekly LISWT sessions using PiezoWave2 from Richard WOLF. LISWT Parameters: 0.16mj/mm2 energy flux density, 8 Hz frequency, 10mm wave focus penetration depth, and 6000 shocks per session distributed among the two crura of the penis (2000 shocks), dorsum penis (2000 shocks), and lateral penis (2000 shocks). Results 33 patients showed a robust response to the 10mcg ICI and were treated with four LISWT sessions. 52 patients exhibited a partial response to the 10mcg ICI, necessitating six LISWT sessions. 78.6% (26 out of 33) of the patients who initially responded well to 10 mcg ICI had a consistent response to 8 mcg ICI; 63.6% (33 out of 52) of those with a partial initial response to 10 mcg ICI demonstrated an improved reaction to the same dosage post-treatment (table 1) (fig. 1). No complications were reported from the ICI. Conclusions Intracavernous prostaglandin injection can serve as a predictor for the required number of LISWT sessions and can objectively gauge the treatment's effectiveness in ED patients. Further randomized studies, incorporating parameters such as EHS, IIEF, and the ICI dosage, are recommended to validate this method's feasibility Disclosure No.

Focused shear wave beam propagation through a 3D printed human rib cage

Transient elastography (TE) is used in clinical screenings for liver fibrosis. TE generates shear wave motion in the liver by vibration of a piston at the skin. The small, flat piston in current devices radiates shear waves that diverge from the liver, resulting in low SNR and high failure rates, especially in obese patients. Our group recently introduced focused shear wave beams for TE [Cormack et al., IEEE TBME (2024)], in which vibration of a concave piston generates shear waves that converge towards the focal region, thereby increasing SNR for liver stiffness estimation. However, because piston sizes needed for efficient shear wave focusing are larger than the typical intercostal space, the rib cage that lies between the skin and the liver may cause shear wave aberration and influence stiffness measurement. Here, we present measurements of broadband focused shear wave propagation in tissue-mimicking gel in which are embedded 3D printed human ribs. Both straight elliptical rods and anatomically realistic 3D printed ribs are used as aberrators. Measurements are compared to 3D simulations of shear wave beam propagation [Archer et al., JASA (2023)]. Effects of shear wave aberration by the rib cage depend on shear wavelength, piston-to-rib distance, and piston radius and curvature.

Influence of metal magnesium addition on detonation initiation in shock wave focusing Pulse Detonation Engine

Abstract The process of shock wave focusing can make the strength of shock waves be continuously accumulated and turned into detonation wave in Pulse Detonation Engine (PDE). However, its effective application needs the inlet jets be in high temperature and velocity, which is difficult to be satisfied under certain conditions. Therefore, in this paper, metal magnesium assisted detonation initiation is proposed and the effect of magnesium particle addition on the shock wave focusing process in a kerosene-fueled PDE with cavity configuration is investigated through numerical simulation. The result showed that when the temperature of the premixed fuel/air jets injected in opposite direction was set as 650 K, the collision of leading shock waves on the central axis was the main source of energy deposition and the shock wave focusing could make the detonation be initiated in the cavity. When the temperature of jets is reduced to 550 K, fuel ignition and detonation could not be achieved through shock wave focusing. Then adding metal magnesium particles into the combustor made the energy deposition be enhanced and the detonation be induced. The diffusion of metal particles can significantly change the structure, motion, merging and dissipation of vortices in the flow field. Generally, the shock wave focusing process is basically not affected with metal particles injection. Therefore, this method can be successfully employed for detonation initiation in the cavity when the fuel/air premixed jet temperature is not high for PDE.

A Waveguide Inline Binary Metasurface for Wavelength-Selective Transmission and Standing Wave Focusing.

This study presents an innovative inline metasurface design for selective wavelength transmission and focusing. When integrated into optical fibers, it improves the stability and compatibility with techniques like wavelength division multiplexing and phase modulation. Precise parameters, determined through analytical calculations and simulations, allow for the design of multifunctional lenses within the optical fiber platform. The numerical results demonstrate unmodulated transmission for specific wavelengths, while others exhibit standing wave focusing with a 0.67 μm beam radius and a 0.31 μm depth of focus. This technology holds promise for applications in quantum experiments, sensing, and optical communication.

Dispersive Wave Focusing on a Shear Current: Part 1—Linear Approximations

We consider the evolution and kinematics during dispersive focussing, for a group of waves propagating atop currents varying with depth. Our analysis assumes long-crested linear waves propagating at arbitrary angles relative to the current. Although low steepness is assumed, the linear model is often a reasonable approximation for understanding rogue waves. A number of analytical approximate relations are derived assuming different sub-surface current profiles, including linearly varying current, exponentially varying current, and currents of arbitrary depth profile which are weakly sheared following the approximation of Stewart and Joy (Deep Sea Res. Abs. 21, 1974). The orbital velocities are likewise studied. While shear currents have modest influence on the motion of the envelope of the wave group, they significantly change wave kinematics. Horizontal orbital velocities are either amplified or suppressed depending on whether the shear is opposing or following, respectively. To illustrate these phenomena we consider a real-world example using velocity profiles and wave spectra measured in the Columbia River estuary. Near the surface at the point where focusing occurs, horizontal orbital velocities are, respectively, increased and decreased by factors of 1.4 and 0.7 for focusing groups propagating on following and opposing shear (respectively, upstream and downstream in the earth-fixed reference system). The implications for the forces a focusing wave group can exert on vessels and installations are profound, emphasising the importance of considering current profiles in maritime operations.

Numerical study of reflector configuration design on shock wave focusing detonation initiation process

Numerical simulation research was carried out to study the effect of the parabolic cavity shape on detonation initiation process for shock wave focusing engine with kerosene as fuel. The results showed that the peak pressure in the cavity was decreased significantly as the depth of the cavity was enhanced. An umbrella-shaped aerodynamic structure which is composed by the expansion of the fan and two vortices was generated under the K–H instability effect when the supersonic jet entered into the cavity. When the inlet jet temperature was at ambient temperature (300 K) or set as 500 K, nearly no ignition was generated for all the three cavity configurations. As the jet temperature was increased to 600 K, the flame can be formed and stay in the cavity for all of these three cavities. However, the coupling between the flame propagation and the reflected shock wave is not close that detonation was not generated as well. As the inlet jet temperature was further increased to 700 K, the hot spot at the bottom of the cavity could be coupled with the reflected shock wave and detonation wave was successfully formed for all of these three cavity configurations.

The sub-wavelength focusing of flexural waves achieved by splitting one plane lens into two halves with a controllable angle

The flexural wave focusing in plates and shells is of great importance for energy harvesting and vibration control. Traditional plane lens based on gradient index and metasurface usually exhibits normal focusing, i.e., the focusing spot size is larger than half wavelength. In order to improve its working performance, increase energy at the focal point, reduce the focusing spot size and further achieve subwavelength focusing, a novel design scheme is proposed, i.e., splitting one traditional plane lens into two halves with a controllable inner angle. Firstly, the transmission and phase are calculated by using the Timoshenko beam theory, utilizing which the lens design is finalized via changing beam thickness perpendicular to wave propagation. After that, the working performance of two sub-lenses is compared with that of one single lens via numerical simulations, including the focal position, energy distribution, and spot size, which are further validated through experimental measurements. It is demonstrated that the flexural wave can exhibit subwavelength focusing after traveling across two adjacent halves, with the focusing spot size smaller than half wavelength. Specifically, the subwavelength focusing is broadband, and still occurs in a frequency region centered at the designed frequency. The proposed design scheme is expected to provide an innovative approach for the lens design in acoustics and optics.

Observation of Plasmonics Talbot effect in graphene nanostructures

We report on the theoretical models of the plasmoincs Talbot effect in graphene nanostructure. The Talbot effect for the plasmonics applications in the IR range is theoretically studied and the respective Talbot effect for the novel advanced plasmonics structures are numerically investigated for the first time. It is shown that the metamaterial structures with periodic grating configuration represents a complex three-dimensional lattice of beamlet-like graphene plasmonics devices. The calculated results agree well with the experimental ones. The results obtained can be used to create and optimize the structures considering diffraction limit for a wide range of application areas. Effective focusing of plasmonic waves with exact focal spots and a subwavelength full width at half maximum can be obtained by using periodic graphene grating.