Parasitic Waves Research Articles

Polarization-independent full mode-converting elastic metasurfaces

This investigation proposes a novel elastic metasurface tailored to convert a longitudinal wave into a transverse wave and vice versa at an arbitrary refraction angle with full (100 %) conversion efficiency. The previous mode-converting elastic metasurface based on Snell's critical angle can convert a longitudinal wave into a transverse wave, but the refraction angle of the transverse wave cannot be smaller than the critical angle. In addition, the previous metasurface cannot convert a transverse wave into a longitudinal wave without generating a parasitic transverse wave. Here, we propose an elastic metasurface that enables full mode-converting transmission at an arbitrary refraction angle, regardless of whether the incident wave's polarization is longitudinal or transverse. To this end, we present a novel strategy to design unit cells that yield total mode-conversion efficiency as well as the desired phase shifts covering the entire 2π range. Shape optimization and geometry-flipping strategies were employed to design the unit cells constituting the metasurface. The concept of the unit cell's effective length was useful for the unit cell design. The designed metasurfaces were then used for various applications, such as longitudinal-to-transverse and transverse-to-longitudinal wave steering, splitting, and focusing. The performance of our metasurface was verified by ultrasonic experiments performed on an aluminum plate with the designed metasurface embedded. The realized full mode-converting transmission applicable for an arbitrary angle is expected to be critically valuable for nondestructive testing and medical ultrasounds requiring a specific elastic wave mode of the desired arbitrary beam pattern.

Equal-strength beam design of acoustic wave accelerometers

Abstract Surface acoustic wave (SAW) based accelerometers have received significant attention due to their digital output, low cost, mass production and easy implementation of wireless passive function. However, conventionally rectangular cantilever-beam based SAW accelerometers often have non-uniform strains generated along the beams, which cause emergence of parasitic wave modes and measurement errors. In this paper, a simulation platform was developed to analyze and optimize designs of SAW accelerometers and variable-thickness and equal-strength beams were designed to solve the critical issue of non-uniform strain distribution along the beam. Frequency responses of SAW accelerometers under the acceleration were successfully obtained using the simulation platform, with the visualized strain/stress distribution and particle displacement field. The accuracy of this simulation platform was verified using the experimental result reported in literature. A highly sensitive and equal-strength beam SAW accelerometer was achieved with a sensitivity up to 1.40 kHz g−1, a linearity coefficient of ∼1, and a measurement range of 0∼15 g. Furthermore, a high-G accelerometer was designed, with the capability of enduring large shocks up to 11,500 g and a sensitivity of 6.96 Hz g−1.

Application of a Modification of the Grid-Characteristic Method using Overset Grids for Explicit Interface Description to Modelling the Relief of the Ocean Shelf

Introduction. The problem of modelling the propagation of elastic waves is of great practical importance when conducting seismic exploration. Based on it, a model of the environment under study is being built. At the same time, the quality of the constructed model is determined by the accuracy of solving the modelling problem, which ensures constantly increasing requirements for modelling accuracy. For accurate modelling, it is important to correctly describe and take into account the boundaries of the media. At the same time, the quality of the constructed model is determined by the accuracy of solving the modelling problem, which ensures constantly increasing requirements for modelling accuracy.Materials and Methods. We have studied a modification of the grid-characteristic method on rectangular grids using overset grids to describe the interface of media of complex shape. This approach has previously been used to describe the earth’s surface when conducting simulations on land. This paper describes its application in modelling the relief of the ocean shelf.Results. The use of the overset grid reduces the modelling error, the number of parasitic waves and artifacts and makes it possible to get a more visual picture.Discussion and Conclusions. Overset grids can be used to describe the interface of media in modelling seismic exploration of the ocean shelf. Their use makes it possible to increase the accuracy of modelling and reduce the number of artifacts compared to using only one grid.

Evaluation of the Efficiency of Generation of Terahertz Surface Plasmon Polaritons by the End-Fire Coupling Technique

One of the most important problems in the plasmonics of the terahertz (THz) range, which is actively developing now, is the efficient generation of surface plasmon polaritons (SPPs). The simplest and most promising technological technique of photon excitation of THz SPPs is through diffraction of radiation on the edge of the conducting surface of the sample (the end-fire coupling technique). In this paper, we experimentally evaluated the efficiency of the generation of monochromatic THz SPPs (λ0 = 141 μm) by this method with a sample in the form of a cylindrical segment, the convex surface of which has a gold layer coated by zinc sulfide (ZnS) with thickness d = 0–2 µm. Such configuration of the surface supporting the SPPs not only shields the detector from parasitic bulk waves arising during diffraction but also enables one to change the distribution of the SPP field in the air by varying the coating layer thickness d. On an uncoated gold surface, the SPP generation efficiency was η ≈ 20%. In the presence of a ZnS layer on the gold, the SPP generation efficiency gradually increased with d, reached the maximum (ηmax ≈ 60%) at d ≈ 1 μm, and then gradually decreased. Theoretical analysis showed that the efficiency of the SPP generation can be raised up to 80% due to the selection of an optimal SPP field profile via variation of the thickness of the dielectric layer on the metal surface, as well as with optimal incidence of the focused radiation on the edge of the sample.

Parasitic waves and micro-breaking on highly nonlinear gravity–capillary waves in a convergent channel

Gravity–capillary waves are waves influenced by both the effects of surface tension and gravity; these waves are at small scales with wavelength range from approximately 10 cm to less than 1 cm. Gravity–capillary waves play a significant role in air–sea interactions, and they exhibit much different features compared with gravity waves. They can be observed widely on the sea surface. Parasitic waves (capillary waves generated by and that ride on gravity and/or gravity–capillary waves) and micro-breaking can be observed on the water surface with winds; however, the presence of wind makes it difficult to analyse the mechanisms of the wave itself. In this paper, parasitic waves and micro-breaking on gravity–capillary waves are examined experimentally, both in the absence of wind. Parasitic waves and axisymmetric micro-breaking waves are generated mechanically in a convergent channel, where energy density increases due to spatial convergence. Three experimental techniques are used to measure wave properties: planar laser-induced fluorescence, particle image velocimetry and shadowgraphs. The wave profile evolution and vortices beneath the parasitic waves are studied. The micro-breaking of gravity–capillary waves is observed on a surface with added surfactant. The surfactant increases the Bond number, and makes breaking possible in these small-scale waves. Energy dissipation of parasitic waves and micro-breaking is quantified, and the enhanced dissipation caused by parasitic waves is identified through the experiments. In this study, mechanically generated breaking waves with wavelengths less than 10 cm are studied for the first time, without the effect of wind. The results yield insight into wave characteristics and energy dissipation on the air–sea interface at small scales.

Exponential asymptotics for steady parasitic capillary ripples on steep gravity waves

In this paper, we develop an asymptotic theory for steadily travelling gravity–capillary waves under the small-surface tension limit. In an accompanying work (Shelton et al., J. Fluid Mech., vol. 922, 2021) it was demonstrated that solutions associated with a perturbation about a leading-order gravity wave (a Stokes wave) contain surface-tension-driven parasitic ripples with an exponentially small amplitude. Thus, a naive Poincaré expansion is insufficient for their description. Here, we develop specialised methodologies in exponential asymptotics for derivation of the parasitic ripples on periodic domains. The ripples are shown to arise in conjunction with Stokes lines and the Stokes phenomenon. The resultant analysis associates the production of parasitic ripples to the complex-valued singularities associated with the crest of a steep Stokes wave. A solvability condition is derived, showing that solutions of this type do not exist at certain values of the Bond number. The asymptotic results are compared with full numerical solutions and show excellent agreement. The work provides corrections and insight of a seminal theory on parasitic capillary waves first proposed by Longuet-Higgins (J. Fluid Mech., vol. 16, issue 1, 1963, pp. 138–159).

Klystron-like Cyclotron Amplification of a Transversely Propagating Wave by a Spatially Developed Electron Beam

A klystron-like gyro-amplifier based on the excitation of a wave propagating across a spatially developed (in the transverse direction) electron beam is described within the simplest 2-D model. Such a configuration is attractive as a way of implementation of a short-wavelength source with a relatively high level of output power and with the possibility of quasicontinuous frequency tuning. We study the peculiarities of the 2-D process (developing in both the axial and transverse directions) of electron bunching and “free” wave emission from the electron beam in the open drift space, as well as the excitation of the output cavity used to provide formation of a compact and powerful output wave signal. The main problem of this 2-D process is that different fractions of the electron beam (located at different points of its cross-section) move in different wave fields. In addition, excitation of the parasitic wave propagating in the opposite direction relative to the operating wave is possible. However, we show that it is possible to organize effective electron–wave energy exchange for almost all fractions of the electron beam.

Spurious Fundamental-Harmonic Oscillations in the Horn Section of a High-Harmonic Gyrotron

In the experiments with a 30 keV/0.7 A sub-terahertz (THz) large-orbit gyrotron operating at the third cyclotron harmonic in the continuous-wave regime, it was found that the presence of reflections of the output signal from an external load leads to the excitation of parasitic oscillations at the fundamental cyclotron harmonic. Theoretical analysis shows that the excitation of parasitic oscillations occurs in the smooth-irregular output horn section of the microwave system, where the parasitic wave is close to the cyclotron resonance with the electron beam. Moreover, the fundamental-harmonic parasitic wave excited first in the output horn section can “penetrate” into the operating cavity, which can lead to suppression of the operating oscillation excited there at a high cyclotron harmonic.

Parasitic Capillary Waves on Small-Amplitude Gravity Waves with a Linear Shear Current

This paper describes a numerical investigation of ripples generated on the front face of deep-water gravity waves progressing on a vertically sheared current with the linearly changing horizontal velocity distribution, namely parasitic capillary waves with a linear shear current. A method of fully nonlinear computation using conformal mapping of the flow domain onto the lower half of a complex plane enables us to obtain highly accurate solutions for this phenomenon with the wide range of parameters. Numerical examples demonstrated that, in the presence of a linear shear current, the curvature of surface of underlying gravity waves depends on the shear strength, the wave energy can be transferred from gravity waves to capillary waves and parasitic capillary waves can be generated even if the wave amplitude is very small. In addition, it is shown that an approximate model valid for small-amplitude gravity waves in a linear shear current can reasonably well reproduce the generation of parasitic capillary waves.

Parasitic excitation of fundamental-cyclotron-harmonic waves in high-harmonic gyrotrons

A typical problem of the development of high-harmonic short-wavelength gyrotrons, namely, the competition between the operating wave and a parasitic fundamental-harmonic wave, is studied. A specific case of the excitation of a far-from-cutoff parasitic wave in the backward-wave oscillator regime is considered.

An Epidemiological Analysis of 28 Vivax Malaria Cases in Gimpo-si, Korea, 2020.

Since 1993, vivax malaria has been recognized as a public health burden in Korea. Despite of pan-governmental malaria-control efforts and the dramatic reduction in the burden of this disease over the last 10 years, vivax malaria has not been well controlled and has remained continuously endemic. We focused interviewed and examined the charts of 28 confirmed vivax malaria patients given malarial therapy for whom daily records were kept from Gimpo-si, Gyeonggi-do of Korea. Various epidemiological characteristics of vivax malaria, including the incubation period, medication used, and recurrence, and an evaluation of the parasitic characteristics from the focused interviews of patients from this region are described here. Most of the participants indicated the 3 most common symptoms of malaria (headache, chills and fever). Of the 28 cases, 2 experienced a second attack and there were 17 and 11 cases with short- and long-term incubation periods, respectively, yielding a short-term to long-term ratio of 1.5. Based on the parasitemia stages, most of the participants were tested at 5 to 7 days (11 cases) and 7 to 15 days (11 cases) after initial wave of asexual parasites. In conclusion, public health authorities should consider developing management measures to decrease the time lag for diagnosis and drafting unified and robust guidelines for drug use for malaria and drawing up unified and robust guidelines on the use of medication for malaria. It also suggests that routine monitoring, surveillance, and precise medical surveys in high-risk vivax malaria endemic areas are pivotal to controlling this persistent public disease and finally eliminating it from Korea.

On the structure of steady parasitic gravity-capillary waves in the small surface tension limit

Abstract

Improved Simulation of Flows with Free-Surface Waves by Optimizing the Angle Factor in the HRIC Interface-Sharpening Scheme

In finite-volume-based simulations with free-surface waves, it is usually desired to obtain a sharp interface between both fluid phases. A widely used approach for interface-capturing and sharpening is the combination of the volume-of-fluid method with the HRIC scheme. The HRIC scheme contains a user-defined parameter, the angle factor, which influences the magnitude of the interface-sharpening. The present work demonstrates that the optimum value for the angle factor is case dependent: too small values can cause substantial flow disturbances, such as vorticity production within the wave and the occurrence of parasitic wave components, whereas too large values can cause excessive dissipation of wave energy. The optimum value for the angle factor was found to depend on the wave steepness, the aspect ratio of the grid cells, on the cell size and to a lesser degree on the time step size. Results from an extensive parameter study are presented, which can provide guidance for optimizing the angle factor for flow simulations of free-surface wave propagation. Further, two methods are presented which can be used to determine the optimum value of the angle factor. The magnitude of errors that can occur due to improper choices of the angle factor are discussed and recommendations are given to increase the accuracy of flow simulations with free-surface waves.

Zeroth radial modes of azimuthal surface waves in dense plasma-loaded, coaxial helix traveling-wave-tube-like waveguides

An analytical model of coaxial traveling-wave-tube-like waveguides with plasma filling has been justified and utilized to analyze the eigenmodes. Very often, introducing plasma into vacuum electronic devices leads to essential advantages as compared with evacuated tubes. The cylindrical structure under the present consideration consists of a central dielectric rod, placed inside a plasma coaxial layer with a metallic helix sheath on its outer interface, and a metal screen separated from the plasma by another dielectric layer. The dispersion properties of electromagnetic waves propagating across the external axial static magnetic field in such traveling-wave-tube-like waveguides are studied and summarized. The presence of a dense plasma coaxial layer makes the media nontransparent for waves in the electron cyclotron frequency range. However, surface type electromagnetic waves can propagate in this case. These waves are called azimuthal surface waves (ASWs). The helix sheath causes coupling of ordinarily and extraordinarily polarized ASWs. The zeroth radial ASW modes have been found to be most dangerous for parasitic wave excitation in dense plasma-loaded, coaxial traveling-wave-tube-like waveguides.

Effect of liquid depth on dynamics and collapse of large cavities generated by standing waves

The effect of fluid depth on the collapse of large cavities generated by over-driven axisymmetric gravity waves in a 10 cm diameter cylindrical container has been studied. At a large fluid depth in a viscous glycerine–water solution, the collapse of the cavities is inertia dominant at the initial phase with the time-dependent cavity radius (rm) obeying rm ∝ τ1/2; τ = t − t0 being the time remaining for collapse, with t0 being the time at collapse. However, enhanced damping at a low liquid depth turns the late stage of the transition into the viscous regime (rm ∝ τ) at some critical depth beyond which a singular collapse (transition from non-pinch-off and pinch-off collapse) is impossible. At a shallow depth, the change in cavity radius follows a flip of the power law, i.e., rm ∝ τ at the initial stage of collapse followed by a transition to rm ∝ τ1/2, suggesting a viscous–inertial transition. For fluids with relatively lower viscosity but similar surface tension, here water, a smoother cavity with damped parasitic waves at a small liquid depth collapses at a smaller radius. The surface jet velocity due to the collapse of the cavity monotonically decreases with the decrease in the depth, whereas in the case of water, it increases with the depth reaching a maximum at a critical depth followed by a decrease again. The self-similarity, exhibited by the cavity up to the critical depth, is lost due to the axial movement restriction by the bottom wall.

Intestinal Parasites and Theta Brainwave Changes in Children

Background: Parasitic diseases are one of the health problems in all societies, especially in developing countries. Infection with parasites, especially in children, can be associated with learning disorders, malnutrition, anaemia, and physical disorders. On the other hand, an abnormal increase in the theta brainwave amplitude may also be one of the causes of mental slowness and disability in children’s learning, decentralization, or attention deficit hyperactivity disorder (ADHD). Objectives: In this study, we assessed the possible effects of intestinal parasites on brainwave changes in children in Karaj Health centers. Methods: A stool examination was done to separate healthy and infected children. Then, using a diagnostic interview and electroencephalography, the theta-wave was recorded in both groups. Results: From 69 children who participated in this study, 8.7% were infected with one or more intestinal parasites. Around 14.49% of all children had a high theta wave, but the interesting point was that 100% of infected children had a high theta brainwave (P value = 0.001). In contrast, only 6.34% of healthy people had high theta waves. Conclusions: In this study, there was a significant relationship between parasitic infection and brain wave changes. Parasitic diseases may alter brainwaves with unknown mechanisms and affect the ability and activities of the brain.

New information technologies for detecting adverse compromising electromagnetic radiations from electronic computer equipment

The article deals with the problem of TEMPEST - unintentional radiation of electronic equipment, which can be intercepted by malefactors in the form of radiation of an electromagnetic parasitic wave, and which, from a security point of view, is compromising radiation because they can have compromising information. Today, the term TEMPEST is the name of a technology that includes various methods of analyzing electromagnetic compromising radiation in such a way that they can be used to recover intercepted data. The task was completed: It is known that electronic equipment creates electromagnetic fields that can interfere with radio and television reception at a considerable distance. But interference is not the only problem caused by stray electromagnetic radiation. In some cases, you can get information about the signals that are used inside the equipment, when the radiation signals are intercepted by intruders and these signals are decoded. This capability poses a problem, especially in the case of digital equipment, since remote signal recovery within the equipment can allow the reconstruction of the data that the equipment is processing. Therefore, the problem arises of determining technologies for detecting spurious electromagnetic radiation from electronic computers, which may have compromising information. The content of TEMPEST-technology is considered as a physical phenomenon of capturing and recovery of electromagnetic radiation emitted by digital equipment, and which can have compromising information. TEMPEST hardware analysis is performed, which includes various types of sensitive receivers that can monitor a wide range of frequencies, as well as a combination of hardware and software capable of processing the received signals. The analysis of various types of TEMPEST-technology is carried out, namely: Tempest-attack, Soft TEMPEST, as well as a variety of technologies for intercepting information by receiving spurious radiation of a monitor signal, searching for the necessary information on a disk, outputting information to an unused serial port; reflections of the luminous flux from the monitor screen on the walls; modeling the luminous flux in LED indicators and others. Recommendations for protection against TEMPEST technologies for companies and individuals are given.

On the determination of vertical gravity gradients by corner-cube absolute gravimeters

This paper describes a new approach for the determination of vertical gravity gradients (VGGs) from the measurements of corner-cube absolute gravimeters based on laser interferometry. The new approach has been obtained by the modification of the equation for linear least squares solution used in the determination of the g-values. It enables one to establish a clear mathematical relation between the gravity residuals and the VGG estimates. As it is shown, in the determination of gradients with an accuracy approaching 10 µGal m−1 (1 µGal = 1 × 10−8 m s−2), it is necessary to correct directly the measured quantities (time and distance pairs) for ‘known’, or sufficiently well modelled, perturbations as e.g. self-attraction or cable effects. Generally, if the gradients are determined from the data of absolute gravimeters, systematic low frequency perturbation in the residuals have to be analysed carefully. In this way, hidden perturbations in the residuals can be identified, which might be important also for the correct estimation of the g-values and associated uncertainties. This is shown on the example of a parasitic wave detected in the residuals of the FG5X-251 gravimeter, reaching a period of about 0.13 m and amplitudes up to 40 pm. Finally, we compare the gravity gradients determined by the new approach with a more accurate method using relative gravimeters. This comparison shows that the gradient estimates from the FG5X-251 absolute gravimeter are biased by about 10 µGal m−1 and reach reproducibility of 14 µGal m−1. The effort to determine VGGs from absolute gravimeters enhanced the analyses of the measured data, identified still existing artefacts in the gravity residuals, the explanation of which should lead to improvements in the measurement model in order to obtain more accurate estimates of the g-values, VGGs and their uncertainties.

Zinc oxide nanoparticles based passive saturable absorber for pulse generation in fiber laser

A stable passive Q-switched pulsed generation in Erbium doped fiber laser by Zinc Oxide nanoparticles embedded in polyvinyl alcohol (ZnONP-PVA)-based saturable absorber is demonstrated in this paper. The surface morphology and thickness profile of the fabricated film were observed using FESEM and 3D measuring laser microscope with the measured thickness of 12 µm. Meanwhile the its optical properties is characterized using Raman spectroscopy. The developed ZnONP-PVA film, has modulation depth of 7.8% and intensity saturation of 88.97 MW/cm2. The threshold input pump power to generate Q-switched pulse is at 45.4 mW and can be tuned until 92.4 mW before the pulse diminished. The operating wavelength of generated pulse is at 1535 nm with 3 dB bandwidth approximately of 2 nm with exclusion of parasitic continuous wave lasing. As the input pump power was tuned from threshold to maximum value, the recorded pulse train of repetition rate is tunable from 73.53 kHz to 103.10 kHz while the pulse width decreases from 6.8 µs to 4.8 µs. The calculated maximum output power and pulse energy at maximum input pump power was 5.14 mW and 49.85 nJ, respectively. The measured signal to noise ratio was 56 dB indicated that the generated pulse by ZnO NP based passive saturable absorber was stable.

Instability of sheared density interfaces

Of the canonical flow instabilities (Kelvin–Helmholtz, Holmboe-wave and Taylor–Caulfield) of stratified shear flow, the Taylor–Caulfield instability (TCI) has received relatively little attention, and forms the focus of the current study. First, a diagnostic of the linear instability dynamics is developed that exploits the net pseudomomentum to distinguish TCI from the other two instabilities for any given flow profile. Second, the nonlinear dynamics of TCI is studied across its range of unstable horizontal wavenumbers and bulk Richardson numbers using numerical simulation. At small bulk Richardson numbers, a cascade of billow structures of sequentially smaller size may form. For large bulk Richardson numbers, the primary nonlinear travelling waves formed by the linear instability break down via a small-scale, Kelvin–Helmholtz-like roll-up mechanism with an associated large amount of mixing. In all cases, secondary parasitic nonlinear Holmboe waves appear at late times for high Prandtl number. Third, a nonlinear diagnostic is proposed to distinguish between the saturated states of the three canonical instabilities based on their distinctive density–streamfunction and generalised vorticity–streamfunction relations.