High Frequency Excitation Research Articles

Effects of Cobalt Nanoisland Geometry on Terahertz Negative Refraction: a Numerical Analysis

The architectural framework plays a major role in determining the optical properties of left-handed materials. Most metamaterial so far designed have been based on complicated resonant elements such as split rings and hyperbolic metamaterials. Unfortunately, magnetic response of most of the materials tails off at higher frequencies. Higher frequency excitation in metamaterials requires more complicated designs or toroidal moment. In the proposed magnonic metamaterials, the toroidal excitations are naturally maintained and make it suitable for THz resonances. Magnetism and left-handed behavior are the two extreme phenomena that do not seem to be compatible and the coexistence of both cooperative effects was not foreseen. Terahertz spintronics is an emerging field that bridges the boundary between magnetism and photonics, which make the magnetic material suitable for photonic applications. Here, we present the first numerical realization of a magnonic metamaterial artificial spin ice (ASI), designed to operate at terahertz frequency. The two-dimensional square arrays exhibit a rich spin wave band structure that is tunable both by external electromagnetic fields and the magnetization configuration of individual elements. The ultrafast spin wave resonances have provided a direct gateway to manipulate magnetic field of free-space terahertz pulses. The proposed system consists of two-dimensional square arrays of cobalt nanoislands in four different geometries and the micromagnetic simulations of these systems are carried out using mumax3. From the numerical results, the theoretical prediction of negative permeability is done using Schloemann model.

Effect of cavity parameters on the propagation of shock wave generated by underwater pulsed discharge

In order to optimize the shock wave generated by underwater pulsed discharge, the relationship between cavity parameters and shock wave propagation is further studied by three-dimensional numerical simulation. According to the sound pressure field distribution obtained by the simulation, the reflection of the shock wave by the reactor wall can be clearly observed. The reflected pressure wave will reach its maximum value and then gradually attenuate. The study also found that when the deposition energy is constant, when the initial radius of the arc channel increases from 0.1 mm to 2.5 mm, the maximum amplitude of the shock wave will increase from 0.22 × 105 Pa to 1.70 × 105 Pa. When the initial radius of the arc channel is constant, as the deposition energy increases, the time to radiate the shock wave becomes earlier, and the maximum amplitude of the shock wave will increase. This means that a higher pressure can be generated by increasing the input of the deposition energy. When the deposition energy is constant, a higher-pressure level can be obtained by increasing the initial radius of the channel. The excitation frequency also affects the shock wave amplitude. Higher excitation frequency can obtain higher pressure amplitude. These methods will increase the efficiency of underwater pulse discharge treatment of bacteria.

An Ultrasensitive Debris Microsensor for Oil Health Monitoring Based on Resistance–Inductance Parameter

An ultrasensitive debris sensor based on microfluidic technology is introduced in this article. The microsensor not only achieves the traditional inductance detection, but also proposes a resistance detection method based on coil. The magnetic field distribution in the detection region was obtained by simulation, and the magnetization and eddy current effects of metal particles in harmonic magnetic field were simulated. The frequency characteristics and signal characteristics of inductance and resistance detection were studied through experiments. High-frequency excitation can enhance the eddy current effect inside the metal particles, thereby helping to improve the resistance detection sensitivity, enhance the inductance detection effect of nonferromagnetic metal particles, and reduce the inductance detection effect of ferromagnetic metal particles. The research results showed that inductance parameter has stronger detection ability for ferromagnetic metals, and resistance parameter has stronger detection ability for nonferromagnetic metals. According to the characteristics of inductance and resistance signals, the accurate distinction of metal debris property and size can be realized. And the size measurement error caused by the debris shape can be reduced by the judgment method based on two parameters. This impedance detection based on inductance and resistance parameters provides a new method for high-sensitivity monitoring of metal debris. The debris microsensor can be used as a sensing element of portable oil monitoring devices, and provide technical support for health assessment, state warning, and fault diagnosis of mechanical equipment.

Design and Application of a Self-Powered Dual-Stage Circuit for Piezoelectric Energy Harvesting Systems

This paper describes the design and practical application of a dual-stage H-Bridge (DSHBR) circuit to reduce the rectification losses and mitigate ripples in piezoelectric energy harvesting. The proposed DSHBR circuit integrates both AC-DC and DC-DC conversion processes using bidirectional switches and a step-up DC-DC converter, which applies to both positive and negative half cycles. One additional feature is that it does not require external power to turn on the bidirectional switches (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> <; 0.3 V). Such feature facilitates active rectification at very low AC voltages (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</sub> <; 0.5) generated by the piezoelectric device (PD). To validate the performance of the proposed circuit, a series of experimental tests were conducted. Firstly, the performance of circuit on rectifying the PD output was investigated using a shaker to generate high and low frequency excitations. Next, real-life testing was conducted with human arm motion as the source of excitation. Then, the ability of the entire system to charge solar batteries was investigated. The outcome shows that the DSHBR circuit prominently increased the rectified voltage and the output power while stabilising the DC voltage when compared with the conventional H-Bridge circuit.

Numerical Simulations on Unsteady Nonlinear Transonic Airfoil Flow

Large-amplitude excitations need to be considered for gust load analyses of transport aircraft in cruise flight conditions. Nonlinear amplitude effects in transonic flow are, however, only marginally taken into account. The present work aims at closing this gap by means of systematic unsteady Reynolds-averaged Navier-Stokes simulations. The RAE2822 airfoil is analyzed for a variety of sinusoidal gust excitations at different transonic Mach numbers. Responses are evaluated with respect to lift and moment coefficients, their derivatives and the unsteady shock motion. A strong dependency on inflow Mach number and excitation frequency is observed. Generally, amplitude effects decrease with lower Mach numbers or higher excitation frequencies. The unsteady nonlinear simulations predict lower maximum lift values and lower lift and moment derivatives compared to their linear counterparts for lower frequencies in combination with large-amplitude excitations. For the mid-frequency range, trends are not as clear. Additionally, it is shown that the variables of harmonic distortion and maximum shock motion might not be reasonable indicators to predict a nonlinear response.

Empirical Expression for AC Magnetization Harmonics of Magnetic Nanoparticles under High-Frequency Excitation Field for Thermometry.

The Fokker–Planck equation accurately describes AC magnetization dynamics of magnetic nanoparticles (MNPs). However, the model for describing AC magnetization dynamics of MNPs based on Fokker-Planck equation is very complicated and the numerical calculation of Fokker-Planck function is time consuming. In the stable stage of AC magnetization response, there are differences in the harmonic phase and amplitude between the stable magnetization response of MNPs described by Langevin and Fokker–Planck equation. Therefore, we proposed an empirical model for AC magnetization harmonics to compensate the attenuation of harmonics amplitude induced by a high frequency excitation field. Simulation and experimental results show that the proposed model accurately describes the AC M–H curve. Moreover, we propose a harmonic amplitude–temperature model of a magnetic nanoparticle thermometer (MNPT) in a high-frequency excitation field. The simulation results show that the temperature error is less than 0.008 K in the temperature range 310–320 K. The proposed empirical model is expected to help improve MNPT performance.

Kirigami auxetic structure for high efficiency power harvesting in self-powered and wireless structural health monitoring systems

Kirigami and auxetic topologies are combined to design an innovative metamaterial-based substrate (MetaSub) for piezoelectric energy harvesters. The proposed MetaSub piezoelectric energy harvester (MPEH) contains both advantageous metamaterial properties of negative Poisson’s ratio capability and enhanced planar stretchability. A computational parametric analysis is conducted to develop the optimum design for the MPEH to trap the maximum elastic energy. A finite element analysis (FEA) is employed to analytically and numerically validate the simulation model of the MPEH. Accordingly, two experimental results of conventional and auxetic strain energy harvesters are used to evaluate the power enhancement of the MPEH. The FEA results demonstrate the average power gained by the MPEH at a low level of frequency and strain excitation (10 Hz and 150 peak-to-peak) is 165 W which easily satisfies the minimum electric power amount required as a sensor node for self-powered wireless sensor networks. The harvested power output of the MPEH is 19.2 times more than power output produced by an equivalent conventional harvester with a plain substrate (8.6 W). The performance of the MPEH is investigated at different combinations of both low and high excitation frequencies. The creative design of the MetaSub can significantly improve the productivity of strain-induced devices whose efficiency is dependent on their deformation performance such as vibration energy harvesters, wearable sensors, flexible actuators, and micro electromechanical applications.

Nonlinear dynamics of vortex-induced vibration of a nonlinear beam under high-frequency excitation

The present article studies the nonlinear dynamics and effects of high-frequency excitations (HFE) on a forced 2-D coupled beam and wake oscillator model ascribing vortex-induced vibrations. Oscillatory strobodynamics (OS) theory is employed for studying the characteristics of the system in slow time-scale. Linear stability analysis is performed near the equilibrium point of the system for both with and without sinusoidal high-frequency excitation. The method of multiple scales (MMS) is implemented to get the approximate periodic solutions of both the beam and wake responses. It is observed that for pure self-excitation the vortex induced instabilities are suppressed by the high-frequency excitation. However, shifting of primary resonance curve and changing of quasi-periodic attractor to periodic attractor are observed under the influence of high-frequency excitation in the simultaneous self-excited and forced excited system. Furthermore, for the existing system the quasi-periodic and transient routes to chaos are discussed. Numerical results show that the chaotic responses are changed into periodic responses for the higher strength of high-frequency (HF) excitation (product of amplitude and frequency of high-frequency excitation). Direct numerical simulations are carried out by MATLAB SIMULINK to validate the analytical results. Overall, an appropriately chosen high-frequency excitation can be beneficial in reducing the response amplitude as well as suppressing the complex instabilities in the system.

Experimental Statistical Method Predicting AC Losses on Random Windings and PWM Effect Evaluation

Nowadays, one of the challenges in transport electrification is the reduction of the components' size and weight in order to improve the power density. This is often achieved by designing electrical machines with higher rotational speeds and excitation frequencies. In addition, the converter needs to control the machine over a wide speed range given by the mission profile. Therefore, copper losses can significantly increase due to the combination of high frequency excitation and the harmonics introduced by the converter. The winding arrangement design plays a key role in the minimization of the copper losses, thus towards a higher efficiency and/or an improved power density. Different winding topologies can be adopted for high speed electrical machines and amongst them random windings are still one of the most widespread types. This paper presents an in depth study on AC losses in random windings for high frequency motor applications. An analytical method is compared against 2-D Finite Element (FE) simulation results. These are then compared to experimental measurements taken on a custom motorette. Importantly, in order to take into account the random positions of each strand within the machine slots, an Experimental Statistic Method (ESM) is proposed. The ESM allows to define the probability distribution which is useful to evaluate the winding copper losses at the design stage. The contribution of the Pulse Width Modulation (PWM) effect is also considered and experimentally evaluated.

Physical modeling and experimental verification of magneto-rheological damper under medium and high frequency excitation

As a promising semi-active device, magneto-rheological damper has been widely used in low-frequency vibration isolation fields (within 20 Hz) such as bridge damping and building seismic resistance. Recently, the application of magneto-rheological damper has extended to medium and high frequency fields such as satellite and power engine vibration control, accompanied with an urgent need of detailed understanding of its output characteristics. In this paper, a comprehensive physical model is established to analyze dynamic performance of the magneto-rheological damper. The model, derived from both Poiseuille and Couette flow, aims to describe the relationship between the flow rate and pressure difference. The compressibility of the magneto-rheological fluid, the inertia of both the fluid and piston assembly, and the friction are involved to capture the medium and high frequency dynamics of the damping force. Theoretical calculation and simulation verification of magnetic circuit are conducted. Then the experiment based on a self-made prototype is carried out. The results show that the damping force calculated by proposed physical model matches well with the experimental results across the predefined range of frequency and coil current levels.

Curvilinear One-Dimensional Antiferromagnets.

Antiferromagnets host exotic quasiparticles, support high frequency excitations and are key enablers of the prospective spintronic and spin-orbitronic technologies. Here, we propose a concept of a curvilinear antiferromagnetism where material responses can be tailored by a geometrical curvature without the need to adjust material parameters. We show that an intrinsically achiral one-dimensional (1D) curvilinear antiferromagnet behaves as a chiral helimagnet with geometrically tunable Dzyaloshinskii-Moriya interaction (DMI) and orientation of the Néel vector. The curvature-induced DMI results in the hybridization of spin wave modes and enables a geometrically driven local minimum of the low-frequency branch. This positions curvilinear 1D antiferromagnets as a novel platform for the realization of geometrically tunable chiral antiferromagnets for antiferromagnetic spin-orbitronics and fundamental discoveries in the formation of coherent magnon condensates in the momentum space.

Effects of liquid compressibility on the dynamics of ultrasound contrast agent microbubbles

Ultrasound contrast agents (UCA) have been widely used in ultrasonic medical applications. The dynamics of UCA should be simulated accurately to maximize the application effects. The acoustic radiation of UCA oscillations is affected by the liquid compressibility which has to be considered. This paper deduces a simple equation based on Keller-Miksis equation to describe the dynamics of UCA with a polymer shell, taking consideration of compressibility of the liquid. The volume of UCA shell is constant. The shell is described by the Kelvin-Voigt viscoelasticity model.The extended compressible model is verified by comparing with experimental data and Hoff model in predicting the variations of bubble radius with time, resonance frequency, damping constants and acoustical scattering cross section of the UCA microbubbles. The physical explanations of the differences between the extended compressible model and Hoff model in predicting resonance frequency and acoustical scattering cross section are listed. The liquid compressibility correction in UCA model can be better used to predict the bubble dynamics at large radius with high excitation frequency, and it will give us some valuable references in ultrasonic medical applications.

Nonlinear dynamic characteristics of planetary gear transmission system considering squeeze oil film

Based on the planetary gear transmission system considering the coupling effects of friction and elastohydrodynamic lubrication, a torsional dynamic model considering friction, oil film, time-varying meshing stiffness, meshing damping, and gear backlash is established. The Runge–Kutta numerical method is used to solve the vibration equation of the system. The bifurcation diagram and largest Lyapunov exponent are used to analyze the dynamic characteristics of the system under different bifurcation parameters such as the excitation frequency, lubricant viscosity, sun–planet backlash, and planet–ring backlash. The numerical results demonstrate that with the increase of excitation frequency, the system exhibits rich nonlinear dynamic characteristics such as short-period motion, long-period motion, and chaotic motion. With the increase of lubricant viscosity, the chaotic motion of the system is suppressed at low excitation frequency and the periodic motion of the system increases at high excitation frequency. With the increase of sun–planet backlash, the chaotic motion of the system increases at low excitation frequency, and the bifurcation characteristics become complicated at high excitation frequency and enters chaotic motion in advance. With the increase of ring–planet backlash, the system delays into chaotic motion at low excitation frequency and bifurcates from single-period motion to multi-period motion in advance at high excitation frequency.

Enhanced Magnon-Photon Coupling at the Angular Momentum Compensation Point of Ferrimagnets.

We theoretically show that the coupling between magnons in an antiferromagnetically coupled ferrimagnet and microwave photons in a cavity is largely enhanced at the angular momentum compensation point (T_{A}) when T_{A} is distinct from the magnetization compensation point. The origin of the enhanced magnon-photon coupling at T_{A} is identified as the antiferromagnetic spin dynamics combined with a finite magnetization. Moreover, we show that strong magnon-photon coupling can be achieved at high excitation frequency in a ferrimagnet, which is challenging to achieve for a ferromagnet due to low magnon frequency and for an antiferromagnet due to weak magnon-photon coupling. Our results will invigorate research on magnon-photon coupling by proposing ferrimagnets as a versatile platform that offers advantages of both ferromagnets and antiferromagnets.

Experimental verification of the percussive drilling model

This paper focuses on the experimental verification of a dynamical model describing percussive drilling. The design of the experimental apparatus is introduced firstly, and then the dynamic interactions between rock and indenter are studied experimentally by varying three control parameters, including frequency and amplitude of excitation, and pneumatic pressure applied on the indenter. Additionally, the influence of rock stiffness is analyzed by testing different rock-indenter combinations. Meanwhile, the corresponding numerical simulations were conducted showing good agreements with the experimental results. According to the detailed bifurcation analyses, the dynamic responses of indenter mainly display period-one motions which can further develop into period-two motions via period-doubling bifurcation scenarios, or even chaotic motions by period-doubling cascade. The chaotic vibration of indenter can also return to periodic motions via inverse period-doubling bifurcation or fold bifurcation. Although the period-one motion is the main dynamic response in the considered parameter ranges, its impact number per excitation period varies with parameters, which directly affects the maximal impact acceleration of indenter. This results in more energy being dissipated during repeated weak impacts. Therefore, strong impacts are obtained if the indenter is exhibiting the period-one with one-impact motion. For such a purpose, high excitation frequency and amplitude are suggested, however, the high pneumatic pressure should be avoided since it can trigger chattering of the indenter.

An efficient method for calculating the frequency response of a proportional damping system over a given frequency interval

The computational cost of the frequency response within the frequency interval may be high for large-scale structures. An efficient method for calculating the frequency response of a proportional damping system over a given frequency interval which starts from zero is proposed. The Sturm sequence number is used to adaptively determine the number of low-order modes that need to be calculated. The complementary part of the contribution of the calculated low-order modes is approximated by the partial sum of the power series that is proven to converge. The number of terms in the sum is adaptively determined only by the highest excitation frequency. The resulting frequency response expression is valid for the entire frequency interval. One numerical example is presented to demonstrate the high accuracy and effectiveness of this method.

A Stator-PM Resolver With Field Modulation Principle

The stator-PM variable reluctance (VR) resolver has been proposed as a new position sensor for a wide speed range. The high-frequency (HF) signal injection is only required to measure the rotor position at low-speeds. Alternatively, at higher speeds, without requiring the HF excitation, the position measurement can be achieved by the back-EMF generated by the stator-PM. This novel resolver has a simple rotor structure and the analog signals can be efficiently decoded at high-speeds. In the first proposed stator-PM VR resolver, the prototype must have even number of rotor salient poles. However, the resolvers with odd number of rotor poles are also useful in many scenarios. To complete the design scheme of this resolver category, this article practices a solution on the basis of the theory of field modulation. The design issue and solution are illustrated and addressed in this paper. The winding function approach is applied to analyze the signals. With the low-cost materials, the proposed resolver measurement system is prototyped.

Analytical Modelling and Optimization of a Piezoelectric Cantilever Energy Harvester with In-Span Attachment.

In this paper, the location of masses and of a piezoelectric patch for energy harvesting reported onto a vibrating cantilever beam is studied and optimized. To this aim, a genetic algorithm is adapted and utilized to optimize the voltage amplitude generated by the piezoelectric patches by choosing attachment mass, attachment mass moment of inertia, attachment location, piezoelectric patch location and force location on the beam as parameters. While an analytical approach is proposed to evaluate the voltage amplitude, a multi-layer perceptron neural network is trained by the derived characteristic matrix to obtain an approximate function for natural frequencies based on the attachment parameters. The trained network is then used in the core of genetic algorithm to find the best optimization variables for any excitation frequency. Numerical simulation by COMSOL Multiphysics finite element software validates the calculated voltage by analytical approach. The optimization method successfully matches the natural frequency of the beam with the excitation frequency which therefore maximizes the output energy. On the other hand, the superiority of the optimized design over the conventional configuration in harvesting the energy at high frequency excitation is also approved.

Research on Electromagnetic Flowmeter Based on Double-frequency Trapezoidal Wave Excitation

Electromagnetic Flowmeter is widely used in industrial production and life, because it is characterized by high precision and little environmental interference in electromagnetic flow measurement. However, the commonly used low frequency rectangular wave excitation greatly reduces the measurement accuracy and measurement range of electromagnetic flowmeter due to in-phase interference, differential interference and other factors. Double-frequency trapezoidal wave excitation has the advantages of both excellent zero-point stability of low frequency excitation and strong ability to suppress the noise of slurry fluid and fast response speed of high frequency excitation. In this paper, a double-frequency trapezoidal wave excitation method for electromagnetic flow measurement is proposed. Then the experimental platform is established to verify the measurement accuracy and measurement range. According to the comparison with the instant flow of the standard meter, the measurement accuracy of double-frequency trapezoidal wave excitation is significantly higher than that of low frequency rectangular wave excitation, and in phase interference and differential interference are significantly reduced.

High-frequency magnon excitation due to femtosecond spin-transfer torques

Femtosecond laser pulses can induce ultrafast demagnetization as well as generate bursts of hot electron spin currents. In trilayer spin valves consisting of two metallic ferromagnetic layers separated by a nonmagnetic one, hot electron spin currents excited by an ultrashort laser pulse propagate from the first ferromagnetic layer through the spacer reaching the second magnetic layer. When the magnetizations of the two magnetic layers are noncollinear, this spin current exerts a torque on magnetic moments in the second ferromagnet. Since this torque is acting only within the sub-ps timescale, it excites coherent high-frequency magnons as recently demonstrated in experiments. Here, we calculate the temporal shape of the hot electron spin currents using the superdiffusive transport model and simulate the response of the magnetic system to the resulting ultrashort spin-transfer torque pulse by means of atomistic spin-dynamics simulations. Our results confirm that the acting spin-current pulse is short enough to excite magnons with frequencies beyond 1 THz, a frequency range out of reach for current induced spin-transfer torques. We demonstrate the formation of thickness dependent standing spin waves during the first picoseconds after laser excitation. In addition, we vary the penetration depth of the spin-transfer torque to reveal its influence on the excited magnons. Our simulations clearly show a suppression effect of magnons with short wavelengths already for penetration depths in the range of 1 nm confirming experimental findings reporting penetration depths below $2\, {\rm nm}$.