Waves Of Different Wavelengths Research Articles

Oblique Propagation and Refraction of Gravity Waves Over the Andes Observed by GLORIA and ALIMA During the SouthTRAC Campaign

AbstractGravity waves (GW) carry energy and momentum from the troposphere to the middle atmosphere and have a strong influence on the circulation there. Global atmospheric models cannot fully resolve GWs, and therefore rely on highly simplified GW parametrizations that, among other limitations, account for vertical wave propagation only and neglect refraction. This is a major source of uncertainty in models, and leads to well‐known problems, such as the late break‐up of polar vortex due to the “missing” GW drag around 60°S. To investigate these phenomena, GW observations over Southern Andes were performed during SouthTRAC aircraft campaign. This paper presents measurements from a SouthTRAC flight on 21 September 2019, including 3‐D tomographic temperature data of the infrared limb imager GLORIA (8–15 km altitude) and temperature profiles of the ALIMA lidar (20–80 km altitude). GLORIA observations revealed multiple overlapping waves of different wavelengths. 3‐D wave vectors were determined from the GLORIA data and used to initialize a GW ray‐tracer. The ray‐traced GW parameters were compared with ALIMA observations, showing good agreement between the instruments and direct evidence of oblique (partly meridional) GW propagation. ALIMA data analysis confirmed that most waves at 25–40 km altitudes were indeed orographic GWs, including waves seemingly upstream of the Andes. We directly observed horizontal GW refraction, which has not been achieved before SouthTRAC. Refraction and oblique propagation caused significant meridional transport of horizontal momentum as well as horizontal momentum exchange between waves and the background flow all along the wave paths, not just in wave excitation and breaking regions.

Application of wavy geometries for the reduction of trailing edge instability noise

Trailing edge instability noise can occur in a wide range of applications, including wind turbines and small aerial vehicles. It can be characterised by a high amplitude tonal or narrowband sound resulting from instability waves in the boundary layer scattering as they are convected over the trailing edge. In this study, a hybrid aero-acoustic model is used to demonstrate how wavy geometries can be used to successfully reduce trailing edge instability noise. The hybrid model uses incompressible large eddy simulations to compute acoustic source terms, which are then mapped onto a larger domain using radial basis functions. The acoustic perturbation equations are then solved on this larger domain to obtain the 3-dimensional acoustic field. The study uses a modified NACA0012 wing, where the surface contains spanwise waves of different wavelengths. The results show that there is an optimal wavelength that produces the greatest noise reduction for a given flow speed and Reynolds number, and that this relates to wavelengths of the dominant structures within the boundary layer. This shows that by understanding the spectral character of the boundary layer, one can design optimal geometries for reducing this important source of fluid-induced noise.

Prediction of the maneuvering behavior of a containership in a seaway

This study investigated the turning maneuver performance of a containership in regular waves using a two-time scale formulation. This article also presents a ship motion program, coupled with a 3-degree of freedom (3-DOF) maneuvering solver, whereby the seakeeping problem is solved with the forward speed considered in the computations. Captive tests were simulated in a computational fluid dynamics (CFD) environment to obtain the maneuvering derivatives. A near-field analysis was performed to obtain the effective wave force, which was the second-order mean drift force in this analysis. The proposed method was used to estimate the turning circle maneuver performance of the S-175 containership in both calm water and waves of different wavelengths. Ultimately, the simulation results showed high consistency with experimental data.

Airfoil trailing-edge noise and drag reduction at a moderate Reynolds number using wavy geometries

This study utilizes a hybrid aeroacoustic model to investigate how airfoils with spanwise wavy geometries can be used to reduce trailing-edge noise alongside improving the aerodynamic performance. A smooth airfoil is compared to four variants, which have spanwise surface waves of different wavelengths, at a Reynolds number of Re = 64 000 and an angle-of-attack of 1°. The first three variants have a geometry modified by a single wavelength, whereas the fourth has a surface composed of two wavelengths, which creates a more irregular surface variation. The results show that modest noise reductions of around 4 dB are achieved for the first three variants, but a much larger reduction of 17.7 dB is achieved for the fourth variant. The mechanisms behind the noise reduction are explored, and it is shown that the geometry reduces the spanwise correlation of the pressure fluctuations and also modifies the boundary layer dynamics, which contributes to the large reduction. It is further shown that a wavy geometry can reduce the drag force by reducing the shear stress over parts of the airfoil surface and by limiting the flow separation on the suction side. The fourth variant is also assessed across a wider range of angles (0°≤α≤4°) and is shown to produce less noise than the smooth wing across the entire range as well as a drag reduction for α≥1°.

Recognitions of colored fabrics/laser-patterned metals based on photothermoelectric effects.

Color is the mapping of electromagnetic waves of different wavelengths in human vision. The electronic color recognition system currently in use is mainly based on the photoelectric effect. Here, we demonstrate a color materials’ recognition system based on photothermoelectric effects. The system uses a microfabricated thermoelectric generator (TEG) as a platform, which is covered with dye-colored fabric pieces or structure-colored laser-patterned metal sheets. Under light irradiation, the fabrics/metals selectively absorb light and convert it into heat, which flows through the underlying TEG arrays and then converted into electrical signal output to realize the distinction of color and materials. This previously unidentified high-sensitivity TEG detection method provides a potential approach for precise color materials’ detection over wide areas and may help understand the mechanism of bionic color recognition.

Background-Oriented Schlieren technique using Fast Checkerboard Demodulation for extending measurement range of pressure of underwater shock waves

We have successfully extended the measurement range by applying Fast Checkerboard Demodulation (FCD) to Background-Oriented Schlieren (BOS) technique, which is a contactless measurement technique. In the experiment, the apparent displacement field on the background image distorted by underwater shock waves were measured using FCD-BOS and contact measurement pressure (hydrophone). The results show that the measurement limitation of FCD is higher than the methods of previous studies which used Optical Flow (OF) and Digital Image Correlation (DIC) algorithms. Since the displacement is detected from the frequency change due to the distortion of the background image, FCD that combines fast Fourier transform (FFT) and a background image with a two-dimensional periodic pattern, can be applied to the large distortion which is difficult to be detected using OF and DIC. The numerical approach in our previous study(Shimazaki et al., 2022) showed that the measurement range of FCD-BOS is strongly influenced by the background pattern. Therefore, this study also performed numerical investigation on the use of synthetic background images with the frequency of the resultant wave of two sine waves of different wavelengths for FCD-BOS. The numerical results show that the measurement range using the synthetic background images with wavelengths of (60,60/12) μm is three times larger than the experimental measurement range using the lattice grid (width: 20 μm) background. In addition to the synthetic background of wavelengths of 60,60/12, the measurement accuracy, phase wrapping, overlap, and aliasing artifacts of those of (40,40/12) and (30,30/12) were investigated. Such results show the superiority of the background with the frequency of sine wave over that of square wave (lattice grid).

Electromagnetic-acoustic biphysical cloak designed through topology optimization.

Various strategies have been proposed to achieve invisibility cloaking, but usually only one phenomenon is controlled by each device. Cloaking an object from two different waves, such as electromagnetic and acoustic waves, is a challenging problem, if not impossible, to be achieved using transformation theory and metamaterials, which are the major approaches in physics. Here, by developing topology optimization for controlling both electromagnetic and acoustic waves, we present a multidisciplinary attempt for designing biphysical cloaks with triple-wave cloaking capabilities, specifically for Ez- and Hz-polarized waves and acoustic wave. The topology-optimized biphysical cloak cancels the scattering of the three waves and reproduces the original propagating waves as if nothing is present, thus instilling the desired cloaking capability. In addition, we describe cloaking structures for multiple incident directions of the three waves and structures that work for both electromagnetic waves and sound waves of different wavelengths.

The numerical study on active control strategies of T-foil

Objectives An active control T-foil can effectively improve the longitudinal anti-rolling motion of the ship, but there is still no conclusion on what control strategy has the best anti-rolling effect. So the paper has studied and analyzed the active control strategies of the T-foil. Methods The paper uses the slender body theory to calculate the longitudinal motion of Wigley III in regular waves of different wavelengths. Then, the damping effect of the ship with the passive and active T-foil is calculated and analyzed under the same speed and wave conditions. Results The results show that the passive control T-foil has a good anti-rolling effect in the resonance region, however, the anti-rolling effect in the long and short wave region is relatively limited. The control with a pitch velocity signal is very effective in reducing the pitch motion and the acceleration of bow motion, but it increases the heave motion in the longwave area. The anti-rolling effect of bow motion velocity signal on-pitch motion and acceleration of bow motion is 10%-20% higher than that of a pitch velocity signal, and there is no reduction effect on heave motion. Conclusions For the active control strategies of T-foil, the bow motion velocity signal can achieve a better anti-rolling effect by comprehensively considering both pitch and heave motions.

Development of Nanoporous AAO Membrane for Nano Filtration Using the Acoustophoresis Method.

A concept of a nanoporous anodic aluminum oxide (AAO) membrane as a vibro-active micro/nano-filter in a micro hydro mechanical system for the filtration, separation, and manipulation of bioparticles is reported in this paper. For the fabrication of a nanoporous AAO, a two-step mild anodization (MA) and hard anodization (HA) technique was used. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to analyze the surface morphology of nanoporous AAO. A nanoporous structure with a pore diameter in the range of 50–90 nm, an interpore distance of 110 nm, and an oxide layer thickness of 0.12 mm with 60.72% porosity was obtained. Fourier-transform infrared spectroscopy (FTIR) and energy-dispersive X-ray spectroscopy (EDS) were employed to evaluate AAO chemical properties. The obtained results showed that the AAO structure is of hexagonal symmetry and showed where Al2O3 is dominant. The hydrophobic properties of the nanoporous surface were characterized by water contact angle measurement. It was observed that the surface of the nanoporous AAO membrane is hydrophilic. Furthermore, to determine whether a nanomembrane could function as a vibro-active nano filter, a numerical simulation was performed using COMSOL Multiphysics 5.4 (COMSOL Inc, Stockholm, Sweden). Here, a membrane was excited at a frequency range of 0–100 kHz for surface acoustics wave (SAW) distribution on the surface of the nanoporous AAO using a PZT 5H cylinder (Piezo Hannas, Wuhan, China). The SAW, standing acoustic waves, and travelling acoustic waves of different wavelengths were excited to the fabricated AAO membrane and the results were compared with experimental ones, obtained from non-destructive testing method 3D scanning vibrometer (PSV-500-3D-HV, Polytec GmbH, Waldbronn, Germany) and holographic interferometry system (PRISM, Hy-Tech Forming Systems (USA), Phoenix, AZ, USA). Finally, a simulation of a single nanotube was performed to analyze the acoustic pressure distribution and time, needed to center nanoparticles in the nanotube.

Effective Fluid Bulk Modulus in the Partially Saturated Rock and the Amplitude Dispersion Effects

AbstractFrequency dispersion is a well‐known effect in geophysics, which means that waves of different wavelengths propagate at different velocities. Amplitude dispersion is a less‐known effect, which means that waves of different amplitudes propagate at different velocities. Herewith, we consider the alteration of the interfacial energy during wave‐induced two‐phase fluid flow in a partially saturated rock and demonstrate that this leads to a nonlinear amplitude dispersion effect. When the wave amplitude is small, seismic waves cause bending of the interface menisci between immiscible fluids at the pore scale. However, when the wave amplitude is sufficiently large, the interface menisci will slip at the pore scale, causing attenuation of the elastic energy by the contact line friction mechanism. At the zero frequency limit, all viscous dissipation models predict zero attenuation of the elastic wave energy, while this approach predicts a nonzero attenuation due to a static contact angle hysteresis effect. Herein, we extend the Gassmann's theory with three extra terms, which can be obtained from standard laboratory tests: pore‐size distribution and interfacial tension between immiscible fluids and rock wettability (advancing and receding contact angles). We derive closed‐form analytical expressions predicting the effective fluid modulus in partially saturated rock, which falls between Voigt and Reuss averages. Next, we demonstrate that the nonlinear amplitude dispersion effect leads to energy transfer between different frequencies. This may explain the low‐frequency microtremor anomalies, frequently observed above hydrocarbon reservoirs, when the low‐frequency energy of ocean waves (0.1–1 Hz) is converted to higher frequencies (2–6 Hz) by partially saturated reservoirs.

Single-shot optical multiple-image encryption by jointly using wavelength multiplexing and position multiplexing.

A single shot large-capacity optical multiple-image encryption method based on wavelength multiplexing and position multiplexing is proposed. In the encryption process of the proposed method, multiple plane waves of different wavelengths are adopted to illuminate secret images that are placed at different positions along the optical axis. All the secret images are encoded into a single grayscale intensity-only image that is recorded by a monochromic camera by applying a diffractive-imaging based double random phase encoding (DRPE) system. In the decryption process, high accuracy images are decrypted without crosstalk from the intensity image through a multimode phase retrieval algorithm and a two-step iterative shrinkage/thresholding (TwIST) algorithm. The feasibility of the proposed method is demonstrated by numerical simulations.

Active PZT Composite Microfluidic Channel for Bioparticle Manipulation.

The concept of active microchannel for precise manipulation of particles in biomedicine is reported in this paper. A novel vibration-assisted thermal imprint method is proposed for effective formation of a microchannel network in the nanocomposite piezo polymer layer. In this method, bulk acoustic waves of different wavelengths excited in an imprinted microstructure enable it to function in trapping–patterning, valve, or free particle passing modes. Acoustic waves are excited using a special pattern of electrodes formed on its top surface and a single electric ground electrode formed on the bottom surface. To develop the microchannel, we first started with lead zirconate titanate (PZT) nanopowder [Pb (Zrx, Ti1−x) O3] synthesis. The PZT was further mixed with three different binding materials—polyvinyl butyral (PVB), poly(methyl methacrylate) (PMMA), and polystyrene (PS)—in benzyl alcohol to prepare a screen-printing paste. Then, using conventional screen printing techniques, three types of PZT coatings on copper foil substrates were obtained. To improve the voltage characteristics, the coatings were polarized. Their structural and chemical composition was analyzed using scanning electron microscope (SEM), while the mechanical and electrical characteristics were determined using the COMSOL Multiphysics model with experimentally obtained parameters of periodic response of the layered copper foil structure. The hydrophobic properties of the PZT composite were analyzed by measuring the contact angle between the distilled water drop and the three different polymer composites: PZT with PVB, PZT with PMMA, and PZT with PS. Finally, the behavior of the microchannel formed in the nanocomposite piezo polymer was simulated by applying electrical excitation signal on the pattern of electrodes and then analyzed experimentally using holographic interferometry. Wave-shaped vibration forms of the microchannel were obtained, thereby enabling particle manipulation.

Structure and Implementation Principles of a Photonic Computer

A structure and implementation principles of a photonic computer are proposed. Its operation is based on the effects of interaction between coherent light wave systems generated by a laser source. The performance of photonic computers, their power consumption, and physical dimensions are estimated. These estimates indicate a great potential advantage of photonic computers over electronic ones. In particular, analysis carried out in this study and the obtained estimates demonstrate that a photonic computer working on the light with a wavelength of 1530 can operate 104 – 105 times faster than up-to-date electronic computers with the same power consumption. Light waves of different wavelengths do not interact with each other. With a proper OLG implementation, several computations represented by light waves of different wavelengths can be run on a single photonic computer.

Acousto-optic tunable filter chromatic aberration analysis and reduction with auto-focus system

An acousto-optic tunable filter (AOTF) displays optical band broadening and sidelobes as a result of the coupling between the acoustic wave and optical waves of different wavelengths. These features were analysed by wave-vector phase matching between the optical and acoustic waves. A crossed-line test board was imaged by an AOTF multi-spectral imaging system, showing image blurring in the direction of diffraction and image sharpness in the orthogonal direction produced by the greater bandwidth and sidelobes in the former direction. Applying the secondary-imaging principle and considering the wavelength-dependent refractive index, focal length varies over the broad wavelength range. An automatic focusing method is therefore proposed for use in AOTF multi-spectral imaging systems. A new method for image-sharpness evaluation, based on improved Structure Similarity Index Measurement (SSIM), is also proposed, based on the characteristics of the AOTF imaging system. Compared with the traditional gradient operator, as same as it, the new evaluation function realized the evaluation between different image quality, thus could achieve the automatic focusing for different multispectral images.

Influence of cross-sectional discontinuity on the damping characteristics of viscoelastically supported rectangular plates

The geometry of a rectangular plate used in a structural application is an important design parameter that influences the vibrational response of the plate when it is subjected to an impact. In this study, the influence of a cross-sectional discontinuity on the vibration characteristics of viscoelastically supported plates was investigated. The discontinuity was induced at a specific location in the length-wise span. Experimental studies were performed to identify the effect of the discontinuity on the plate vibration response. The mode shapes and damping ratios of the plates with and without discontinuities in the cross-section were measured and compared. Forced vibration responses and modal properties were predicted using a numerical model. The variation in cross-sectional geometry was modeled to determine the changes in bending stiffness. The translational and rotational viscoelastic stiffnesses at the plate edges were used for modeling the vibration damping at the boundaries. This damping occurred at the contact surface between the plate and the fixtures. To investigate the effect of support stiffness on the vibration damping, flexural wave propagation analysis was performed with different boundary conditions. The ratio between the incident and reflected waves from the boundaries was predicted for flexural waves of different wavelengths. The predicted reflection ratios of the plate with and without the discontinuities were compared to the predicted loss factors using numerical analysis. The vibration energy dissipation at the viscoelastic supports was proportional to the measured modal damping.

Numerical investigations of the hydrodynamics of an oscillating water column device

An oscillating water column (OWC) device is a renewable energy device that is used to extract ocean wave energy through the action of waves on a partially submerged chamber consisting of an air and a water column. The operation of an OWC device involves complex hydrodynamic interactions between the waves and the device and a good understanding of these interactions is essential for the design of hydrodynamically efficient and structurally stable devices.In this paper, a two-dimensional numerical wave tank is utilized to simulate the interaction of an OWC device with waves of different wavelengths and steepnesses. The chamber pressure, provided by a turbine in a prototype, is simulated using porous media flow theory in the numerical model. The pressure in the chamber and the velocity of the free surface are calculated to evaluate the efficiency of the device and the model is validated by comparing the numerical results with experimental data. The performance of the device under a range of wavelengths for different wave steepnesses is evaluated. The effect of wave steepness on the device efficiency at a lower wave steepness was found to be low, but a large reduction in performance was found in the presence of steep non-linear waves.

Experiments on an Axial Fan Stage: Time-Resolved Analysis of Rotating Instability Modes

Rotating instability (RI) occurs at off-design conditions in axial compressors, predominantly in rotor configurations with large tip clearances. Characteristic spectral signatures with side-by-side peaks below the blade passing frequency (BPF) are typically referred to RI located in the clearance region next to the leading edge (LE). Each peak can be assigned to a dominant circumferential mode. RI is the source of the clearance noise (CN) and an indicator for critical operating conditions. Earlier studies at an annular cascade pointed out that RI modes of different circumferential orders occur stochastically distributed in time and independently from each other, which is contradictory to existing explanations of RI. Purpose of the present study is to verify this generally with regard to axial rotor configurations. Experiments were conducted on a laboratory axial fan stage mainly using unsteady pressure measurements in a sensor ring near the rotor LE. A mode decomposition based on cross spectral matrices was used to analyze the spectral and modal RI patterns upstream of the rotor. Additionally, a time-resolved analysis based on a spatial discrete-Fourier-transform (DFT) was applied to clarify the temporal characteristics of the RI modes and their potential interrelations. The results and a comparison with the previous findings on the annular cascade corroborate a new hypothesis about the basic RI mechanism. This hypothesis implies that instability waves of different wavelengths are generated stochastically in a shear layer resulting from a backflow in the tip clearance region.

Dual-channel rainbow trapping of terahertz surface plasmon-polariton in graded metallic grating structures

We numerically show that slowing down and even stopping Terahertz waves can be achieved in a one-dimensional graded-grating-depth metal structure. Since the dispersion curves are dependent on the graded grating depths, waves of different wavelengths can be localized at different spatial positions of the structure, leading to rainbow trapping. Especially, dual-channel trapped rainbows with different modes can be realized in a single structure as two bands appear in the dispersion curve within the analyzed frequency range. In addition, due to the fact that the bands with different order modes do not overlap one another in the frequency domain, for each mode, its single-mode propagation can be available by properly choosing operation frequency. Such a phenomenon never happens for conventional dielectric or metal waveguides.

Influence of cross-sectional discontinuity on the damping characteristics of viscoelastically supported rectangular plates

Cross-sectional shape of a structure is one of the important design parameter associated with the vibration responses and resulting sound radiation. In this study, the influence of cross-sectional discontinuity on the vibration characteristics of plate structures was investigated. The variation of cross-sectional geometry in the rectangular plate was modeled as the change of bending stiffness for flexural wave propagation analysis. The complex translational and rotational stiffness at plate edges were used for modeling of the damping at the boundaries. The ratio between the incident and reflected waves from the boundaries was predicted for the flexural waves of different wavelengths to analyze the effect of support stiffness on the vibration damping. Using the wave propagation model, the condition of the viscoelastic boundary properties and the discontinuous flexural stiffness of plate for minimum reflection ratio, i.e., maximum dissipation of the vibration was calculated. Modal damping characteristics of the plate with and without the discontinuity in the cross-section were measured and compared to the predicted reflection ratios. The measured damping ratios on the different boundary conditions showed similar pattern with the predicted vibration energy dissipation at the viscoelastic supports.

Local Resonance Broadband Gap in a Homogeneous Plate with Periodic Truncated Cones

We investigate the band structures in a homogeneous plate with periodic truncated cones for the square lattice based on finite element method (FEM). The radius of the truncated cone is gradually reduced from the lower base to the upper base. Compared to the classical stubbed phononic plates, a considerable enlargement of the bandwidth by a factor of 2.04 is obtained by varying the semiangle of the truncated cone, while the truncated cone's weight is only 65.19% of that of the cylinder of the stubbed case. Moreover the relative bandwidth is also enlarged by a factor of 1.68. We show that this band gap enlargement is due to the acoustic waves of different wavelengths is localized at the different parts of the truncated cone. These characteristics of elastic or acoustic waves suggest potential applications in aerospace.