Range Of Oscillation Frequencies Research Articles

Modelling and small signal stability for islanded microgrids with hybrid grid-forming sources based on converters and synchronous machines

Grid-forming (GFM) technologies based on converters or synchronous machines (SMs) were proposed as emerging approaches for improving inertia, damping, and strength of power girds with increasing renewable energy penetration. Although the stability of GFM converters has been widely studied, the dynamics of a grid-connected generator can be dictated by the bulk power systems. Therefore, the stability of islanded microgrids with hybrid GFM sources should be investigated. To fulfill this gap, this paper adopts a general component connection method for modelling islanded microgrids integrated with multiple generators, and models for SMs, grid-following (GFL) and GFM converters are developed. Additionally, a new GFM technology known as the motor-generator pair (MGP) is investigated, and the model for small signal analysis is derived. Then, full-order state-space models for nine scenarios with hybrid GFM sources as well as simulation models which are used to verify their effectiveness are built. Finally, small-signal dynamics of different scenarios are fully examined. In the scenarios with SMs integrated, the range of oscillation frequency is wider than all converter-based scenarios, and mechanical dynamics can dominate stability; and in scenarios with only GFM converters, though the damping can be improved compared to GFL converters, low frequency oscillation may occur, and the dominant factor changes from the phase locked loop and power controller to voltage controller; then improvement in damping can be observed in MGP-based scenarios, but roots that dominate stability are sensitive to current controller and excitation. Further, critical ranges for control parameters, GFM converter penetration and electrical distance in different scenarios are investigated.

Sinusoidally Pitching Foils in Transonic Flow: Insights into Flutter from Time Accurate Simulations

The impact of shock formation and vortex dynamics on airfoil flutter in the transonic regime is investigated through a series of two-dimensional time-accurate simulations of a sinusoidally pitching airfoil at a Reynolds number of 10,000. To gain insight into transonic flutter in the fully nonlinear regime, an energy-based approach that employs prescribed kinematics is utilized. Flutter stability boundaries are identified as a function of the Mach number and the reduced velocity, the latter of which is a surrogate for structural stiffness or flight speed. The results show a subcritical instability near Mach numbers of M∞=0.7 for a range of oscillation frequencies. Additionally, three shock-induced mechanisms are identified that influence the transfer of energy from the flow to the airfoil. The primary mechanism is flow separation triggered by the generation of a lambda shock. The influence of the lambda-shock dynamics on airfoil flow separation is determined for different Mach numbers, providing insight into the well-known “transonic dip” observed in flutter boundaries.

Substantiation of parameters of unbalanced vibration exciter of berry harvester

The paper presents analysis of design of a vibration exciter of berry harvester, and dwells on substantiation of the main parameters of a two-shaft unbalanced exciter drive mechanism. As a result of theoretical studies, the type and design and technological layout is substantiated for a two-shaft vibration exciter with two unbalance unit rigidly connected to each other and rotating in the same direction with a displacement of 180°. Analysis of such a vibration exciter functioning made it possible to determine the rational values of the weights’ mass mа = 4.0 kg and the arms’ length r = 0.02 m, providing vibrations with an amplitude of A = 23…27 mm in the frequency range of oscillations of 8…20 Hz, which corresponds to agricultural and technical requirements for harvesting a wide range of berry crops. The results of the research were used for designing a trailed berry harvester TBH, created by Scientific and Practical Center of the National Academy of Sciences of Belarus for Agriculture Mechanization. It ensures high-quality harvesting with minimal damage to berries and shrubs of chokeberry, currant, gooseberry, dog rose and raspberry, that was determined by acceptance tests.

Sound wave propagation in rarefied molecular gases

Sound wave propagation in rarefied flows of molecular gases confined in micro-channels is investigated numerically. We first validate the employed kinetic model against the experimental results and then systematically study the gas damping and surface force on the transducer as well as the resonance/anti-resonance in confined space. To quantify the impact of the finite relaxation rates of the translational and internal energies on wave propagation, we examine the roles of bulk viscosity and thermal conductivity in depth over a wide range of rarefactions and oscillation frequencies. It is found that the bulk viscosity only exerts influence on the pressure amplitude and its resonance frequency in the slip regime in high oscillations. In addition, the internal degree of freedom is frozen when the bulk viscosity of a molecular gas is large, resulting in the pressure amplitude of sound waves in the molecular gas being the same as in a monatomic gas. Meanwhile, the thermal conductivity has a limited influence on the pressure amplitude in all the simulated flows. In the case of the thermoacoustic wave, we prove that the Onsager–Casimir reciprocal relation also holds for molecular gases, i.e. the pressure deviation induced by the temperature variation is equal to the heat flux induced by the plate oscillation. Our findings enable an enhanced understanding of sound wave propagation in molecular gases, which may facilitate the design of nano-/micro-scale devices.

A miniature tunable quadrature shadow oscillator with orthogonal control

This article presents a new design of a quadrature shadow oscillator. The oscillator is realized using one input and two outputs of a second-order filter cell together with external amplifiers in a feedback configuration. The oscillation characteristics are controlled via the external gain without disturbing the internal filter cell, following the concept of the shadow oscillator. The proposed circuit configuration is simple with a small component-count. It consists of, two voltage-different transconductance amplifiers (VDTAs) along with a couple of passive elements. The frequency of oscillation (FO) and the condition of oscillation (CO) are controlled orthogonally via the dc bias current and external gain. Moreover, with the addition of the external gain, the frequency range of oscillation can be further extended. The proposed work is verified by computer simulation with the use of 180 nm complementary metal–oxide–semiconductor (CMOS) model parameters. The simulation gives satisfactory results of two sinusoidal output signals in quadrature with some small total harmonic distortions (THD). In addition, a circuit experiment is performed using the commercial operational transconductance amplifiers LM13700 as the active components. The circuit experiment also demonstrates satisfactory outcome which confirms the validity of the proposed circuit.

Noninvasive material characterization of biomaterials: Measuring viscosity and elasticity using ultrasound

The advent of tissue engineering has led to increased interest in the viscoelastic characterization of biomaterials. Gelatin Methacrylate (GelMA) is a particularly promising biomaterial, largely due to its tunability, yet the impact of different preparation parameters on the material’s viscoelasticity is not well understood. We characterized an array of GelMA scaffolds, fabricated by varying both GelMA concentration and ultraviolet (UV) lightexposure time. Pulse-echo ultrasound techniques were used to non-invasively determine the sound speed and attenuation of the scaffolds, revealing significant dependence on GelMA concentration. Steady shear rate tests in addition to strain- and frequency-controlled oscillatory shear tests were performed using a rotational rheometer (Model: DHR-2, TA Instruments) to ascertain the levels of shear-thinning and viscoelasticity at a wide range of strain rates, oscillation frequencies, and amplitudes. The rheological tests show moduli dependence on both GelMA concentration and curing time. Together, this acoustic and rheological characterization can be used to inform the selection of GelMA scaffolds in tissue engineering applications, and this method can be used as a guide for characterizing other polymeric hydrogels.

Spectral analysis of IAR oscillations to determine the value and variability of the peak electron density NmF2

This methodical paper explores the possibility of estimating the peak electron density of the F2-region of the ionosphere (NmF2) under different conditions, using data on the frequency of spectral bands (harmonics) of the ionospheric Alfvén resonator (IAR) oscillation. We describe a simple technique for tracking the frequency of spectral bands during the day by measuring their position on the plot of the IAR daily dynamic spectrum. Through calculations within the framework of the global ionospheric model IRI-2016, we verify the correctness of the comparison of the frequencies of resonant bands, measured at one point, with data from radio sounding, performed at other points remote from IAR frequency measurement sites at a distance. We propose an algorithm for comparing NmF2, measured by a radiosonde, with frequencies of spectral lines by precalculating the evaluation factor. It is formed on the basis of a nonlinear combination of the frequencies of the three observed harmonics. Then the time series of this factor is compared with the results of radio sounding, and correlation and regression coefficients, as well as estimation errors are calculated. Using the material on rare cases of round-the-clock observation of IAR oscillations in the winter months of 2011–2012, we trace the dependence of the average error in determining the peak electron density on local time. We present the data on the most favorable local time intervals for determining NmF2 from IAR harmonic frequencies depending on season. Some additional factors are discussed which affect the accuracy of estimates and determine the frequency range of IAR oscillations.

Спектральный анализ излучения ИАР для определения величины и изменчивости максимума электронной концентрации NmF2

This methodical paper explores the possibility of estimating the peak electron density of the F2-region of the ionosphere (NmF2) under different conditions, using data on the frequency of spectral bands (harmonics) of the ionospheric Alfvén resonator (IAR) oscillation. We describe a simple technique for tracking the frequency of spectral bands during the day by measuring their position on the plot of the IAR daily dynamic spectrum. Through calculations within the framework of the global ionospheric model IRI-2016, we verify the correctness of the comparison of the frequencies of resonant bands, measured at one point, with data from radio sounding, performed at other points remote from IAR frequency measurement sites at a distance. We propose an algorithm for comparing NmF2, measured by a radiosonde, with frequencies of spectral lines by precalculating the evaluation factor. It is formed on the basis of a nonlinear combination of the frequencies of the three observed harmonics. Then the time series of this factor is compared with the results of radio sounding, and correlation and regression coefficients, as well as estimation errors are calculated. Using the material on rare cases of round-the-clock observation of IAR oscillations in the winter months of 2011–2012, we trace the dependence of the average error in determining the peak electron density on local time. We present the data on the most favorable local time intervals for determining NmF2 from IAR harmonic frequencies depending on season. Some additional factors are discussed which affect the accuracy of estimates and determine the frequency range of IAR oscillations.

Online SSO stability analysis-based oscillation parameter estimation in converter-tied grids

Due to the wide frequency range of sub-synchronous oscillations (SSOs) in converter-tied grids, existing SSO parameter estimators generally use a long window to pre-identify the SSO frequency, and thus generally have slow responses in SSO parameter estimation. To deal with this problem, this paper proposes an online SSO stability analysis-based SSO parameter estimator. The operating condition of the grid is updated online by measuring the fundamental voltage/current phasor. Then the impedance of the converter is updated based on the identified operating condition, and the SSO stability is analyzed online. If an unstable SSO mode is detected, the identified SSO frequency is used to build the imaginary exponential function-based signal model, and the SSO parameters are estimated based on the least square method. Case studies show that the proposed method has a much faster response than the methods recently published in the literature, and can obtain accurate SSO parameter estimates.

Altered neural oscillations underlying visuospatial processing in cerebral visual impairment.

Visuospatial processing deficits are commonly observed in individuals with cerebral visual impairment, even in cases where visual acuity and visual field functions are intact. Cerebral visual impairment is a brain-based visual disorder associated with the maldevelopment of central visual pathways and structures. However, the neurophysiological basis underlying higher-order perceptual impairments in this condition has not been clearly identified, which in turn poses limits on developing rehabilitative interventions. Using combined eye tracking and EEG recordings, we assessed the profile and performance of visual search on a naturalistic virtual reality-based task. Participants with cerebral visual impairment and controls with neurotypical development were instructed to search, locate and fixate on a specific target placed among surrounding distractors at two levels of task difficulty. We analysed evoked (phase-locked) and induced (non-phase-locked) components of broadband (4-55 Hz) neural oscillations to uncover the neurophysiological basis of visuospatial processing. We found that visual search performance in cerebral visual impairment was impaired compared to controls (as indexed by outcomes of success rate, reaction time and gaze error). Analysis of neural oscillations revealed markedly reduced early-onset evoked theta [4-6 Hz] activity (within 0.5 s) regardless of task difficulty. Moreover, while induced alpha activity increased with task difficulty in controls, this modulation was absent in the cerebral visual impairment group identifying a potential neural correlate related to deficits with visual search and distractor suppression. Finally, cerebral visual impairment participants also showed a sustained induced gamma response [30-45 Hz]. We conclude that impaired visual search performance in cerebral visual impairment is associated with substantial alterations across a wide range of neural oscillation frequencies. This includes both evoked and induced components suggesting the involvement of feedforward and feedback processing as well as local and distributed levels of neural processing.

TEMPORAL EVOLUTION OF THE PLASMA DENSITY CAVITY CAUSED BY INHOMOGENEOUS STOCHASTIC ELECTRIC FIELDS

The time evolution of the formation of a plasma density cavity caused by inhomogeneous stochastic electric fields is investigated. The Fokker-Planck equation, which governs the temporal evolution of the plasma electron density due to localized stochastic inhomogeneous electric fields in the frequency range of lower hybrid oscillations, is solved numerically. The spatial dependence of the plasma electron density for various times is obtained.

Optogenetic stimulation shapes dendritic trees of infragranular cortical pyramidal cells.

Spontaneous or experimentally evoked activity can lead to changes in length and/or branching of neocortical pyramidal cell dendrites. For instance, an early postnatal overexpression of certain AMPA or kainate glutamate receptor subunits leads to larger amplitudes of depolarizing events driven by spontaneous activity, and this increases apical dendritic complexity. Whether stimulation frequency has a role is less clear. In this study, we report that the expression of channelrhodopsin2-eYFP was followed by a 5-day optogenetic stimulation from DIV 5-10 or 11-15 in organotypic cultures of rat visual cortex-evoked dendritic remodeling. Stimulation at 0.05 Hz, at a frequency range of spontaneous calcium oscillations known to occur in the early postnatal neocortex in vivo until eye opening, had no effect. Stimulation with 0.5 Hz, a frequency at which the cortex in vivo adopts after eye opening, unexpectedly caused shorter and somewhat less branched apical dendrites of infragranular pyramidal neurons. The outcome resembles the remodeling of corticothalamic and callosal projection neurons of layers VI and V, which in the adult have apical dendrites no longer terminating in layer I. Exposure to 2.5 Hz, a frequency not occurring naturally during the time windows, evoked dendritic damage. The results suggested that optogenetic stimulation at a biologically meaningful frequency for the selected developmental stage can influence dendrite growth, but contrary to expectation, the optogenetic stimulation decreased dendritic growth.

Power-frequency performance of step-graded double Gunn diodes

Simulations of conventional Gunn diodes containing the double Gunn effect have shown a significant increase in oscillation frequency and frequency range. However, conventional Gunn diodes suffer from higher phase noise, lower efficiency, and reduced temperature stability due to the lack of a hot electron injector. A study on the performance of a Double Gunn diode with a graded gap injector is presented. The second region of negative differential mobility is gradually introduced in several temperature-dependent mobility models, which are implemented in an energy balance simulation. Devices with transit lengths of 0.7μm, 1.1μm, and 1.65μm are simulated. All devices experience an increase in oscillation frequency with increasing intensities of the double Gunn effect. The frequency range doubles at significant intensities of the effect in the devices with transit lengths of 0.7μm and 1.65μm.

Wavelet analysis of the flow field around an oscillating airfoil undergoing pure pitching motion at low Reynolds number

This paper investigates the flow field around a NACA (National Advisory Committee for Aeronautics) 0012 airfoil undergoing pure pitching motion using continuous wavelet transform. Wind tunnel experiments were performed with a test-stand that provides a wide range of oscillation frequencies (f = 0–10 Hz). Sinusoidal pure pitching motion was considered with respect to the quarter chord for five reduced frequencies (K = 0.05, 0.1, 0.15, 0.2, and 0.3) at a Reynolds number of Re = 6 × 104. Mean angle of attack and pitch amplitude for all the cases were considered 0° and 6°, respectively. Unsteady surface pressure measurement was conducted, and the lift coefficient was calculated based on the phase-averaged surface pressure coefficient. The unsteady velocity distributions in the airfoil wake have been measured employing a pressure rake. The results indicate that the maximum value of the lift coefficient decreases by increasing the reduced frequency due to the “apparent mass” effects. For K = 0.05, close to the quasi-steady regime, the cl-α loop approximately follows the trend of the static case. Wavelet transform was used as a tool to examine the surface and wake pressure time series. Surface pressure wavelet transform plots indicate the presence of oscillation frequency and its superharmonics. Moreover, surface pressure wavelet analysis shows that the third and higher superharmonic frequencies are sensitive to the airfoil pitch angle during the oscillation cycle. Wavelet transform on wake reveals that the effective wake width gets smaller by increasing the reduced frequency. Furthermore, the trailing edge vortices get weaker by increasing the reduced frequency.

Investigation of the formation and evolution of over-tip shock waves in the pressure-driven tip leakage flow by time-resolved schlieren visualization

Time-resolved schlieren visualization and transonic wind tunnel are used to investigate tip leakage flows (TLFs) over several generic blade tip models. Focus is on the generation and evolution of the over-tip shock waves in the clearance region. A multi-cutoff superposition technique is developed to improve the schlieren system for better visualization. Unsteady flow structures, such as over-tip shock oscillation, shear-layer flapping, and vortex shedding, are revealed by Fourier analysis and dynamic mode decomposition. To predict the generation and decaying of over-tip shocks, a simplified model is proposed by analogizing the shock system to be an N-shaped sawtooth wave. The results show that (1) the proposed model is able to capture the main features of the generation and decaying of over-tip shock waves. The processes of shock generation, decaying, and fading-out are dominated by the mean background flow, the shock state, and the flow fluctuations, respectively. Adding extra coming flow fluctuations can be an efficient way to control the evolution of over-tip shock system. (2) The shock-oscillating frequency is kept the same with the shear-layer flapping, and shock waves with a given oscillating frequency range is constrained to a specific position range. This is termed the “lock-in effect,” which is also observed in TLFs over contoured blade tips. The non-uniformity generation and the nonlinear propagation of shock waves are responsible for this effect. Constrained by this effect, the evolution of over-tip shock waves is separated into four discrete phases. Thus, this effect can be applied for the control of TLFs.

Vortex shedding analysis of flows past forced-oscillation cylinder with dynamic mode decomposition

Direct numerical simulations are performed for flow past circular cylinders by the lattice Boltzmann method coupled with immersed moving boundary method. By analyzing the flows past a single cylinder at a wide range of cross-flow or in-line oscillation amplitude (0.25≤A/D≤1.5) and frequency (0.5≤fe/f0≤1.5), the results find that the vortex shedding modes inside and outside “lock-in” interval are of significant difference. The vortex shedding mode in the “unlock-in” state is 2S, but C(2S) and P + S shedding modes can be found in the lock-in state. Dynamic mode decomposition is used to analyze characteristic flow features, which shows that mode 1 is the main factor reflecting the flow field structure and mode 2 represents the vortex shedding mode in this work. The vortex shedding modes of flows past a tandem and side-by-side cross-flow double oscillating cylinders are systemically investigated. For tandem double oscillation cylinders, the results of modal decomposition suggest that the shear layer of upstream oscillating cylinder is separated behind the downstream cylinder at a space rate of L/D≤2, but separated behind the upstream cylinder at L/D≥3. Mode 2 at L/D=4 differs from other vortex shedding modes due to the strong inhibition effect by the downstream cylinder on the vortex formation of upstream cylinder. For side-by-side double oscillation cylinders, the wake of two cylinders is a single vortex street at H/D=1, a bistable flow at H/D=2 or 3, a coupled vortex street at H/D=4, and close to a single cylinder at H/D>4. The results of modal decomposition are disordered at H/D=2 due to the interaction between two cylinders and effect of gap flow.

Time-trend analysis of the center frequency of the intrinsic mode function from the Hilbert–Huang transform of electroencephalography during general anesthesia: a retrospective observational study

BackgroundAnesthesiologists are required to maintain an optimal depth of anesthesia during general anesthesia, and several electroencephalogram (EEG) processing methods have been developed and approved for clinical use to evaluate anesthesia depth. Recently, the Hilbert–Huang transform (HHT) was introduced to analyze nonlinear and nonstationary data. In this study, we assessed whether the changes in EEG characteristics during general anesthesia that are analyzed by the HHT are useful for monitoring the depth of anesthesia.MethodsThis retrospective observational study enrolled patients who underwent propofol anesthesia. Raw EEG signals were obtained from a monitor through a previously developed software application. We developed an HHT analyzer to decompose the EEG signal into six intrinsic mode functions (IMFs) and estimated the instantaneous frequencies (HHT_IF) for each IMF. Changes over time in the raw EEG waves and parameters such as HHT_IF, BIS, spectral edge frequency 95 (SEF95), and electromyogram parameter (EMGlow) were assessed, and a Gaussian process regression model was created to assess the association between BIS and HHT_IF.ResultsWe analyzed EEG signals from 30 patients. The beta oscillation frequency range (13–25 Hz) was detected in IMF1 and IMF2 during the awake state, then after loss of consciousness, the frequency decreased and alpha oscillation (8–12 Hz) was detected in IMF2. At the emergence phase, the frequency increased and beta oscillations were detected in IMF1, IMF2, and IMF3. BIS and EMGlow changed significantly during the induction and emergence phases, whereas SEF95 showed a wide variability and no significant changes during the induction phase. The root mean square error between the observed BIS values and the values predicted by a Gaussian process regression model ranged from 4.69 to 9.68.ConclusionsWe applied the HHT to EEG analyses during propofol anesthesia. The instantaneous frequency in IMF1 and IMF2 identified changes in EEG characteristics during induction and emergence from general anesthesia. Moreover, the HHT_IF in IMF2 showed strong associations with BIS and was suitable for depicting the alpha oscillation. Our study suggests that the HHT is useful for monitoring the depth of anesthesia.

High-precision nanosecond detection of a gas absorption spectrum based on optical frequency comb time-frequency mapping.

There is an increasing demand for high-precision gas absorption spectroscopy in basic research and industrial applications, such as gas tracking and leak warning. In this Letter, a novel, to the best of our knowledge, high-precision and real-time gas detection method is proposed. A femtosecond optical frequency comb is used as the light source, and a broadening pulse containing a range of oscillation frequencies is formed after passing through a dispersive element and a Mach-Zehnder interferometer. Four absorption lines of H13C14N gas cells are measured at five different concentrations within a single pulse period. A single scan detection time of only 5 ns is obtained along with a coherence averaging accuracy of 0.0055 nm. High-precision and ultrafast detection of the gas absorption spectrum is accomplished while overcoming complexities related to the acquisition system and light source that are encountered in existing methods.

Linear Voltage Controlled Oscillator Implementation in Electronically Variable Immittances

New realization schemes of electronically tunable inductor (L) and Frequency-Dependent Negative Resistance (D) type immittances using a Current Feedback Amplifier (CFA) and Multiplication Mode Current Conveyor (MMCC) composite active building block (ABB) are proposed. Applications of the immittances to the design of selective filters and LC-type linear voltage controlled quadrature oscillator (LVCQO) are presented. Experimental results based on PSPICE simulation and hardware design for a linear range of oscillation frequency (fo ~ 13.6MHz) with satisfactory phase-noise figure on the oscillator wave response had been verified. Effects of the Active Building Block (ABB)-nodal imperfections are analyzed to be insignificant. The new ideas in this article are two types of immittance functions realizable in the same topology; appropriate frequency-domain selective responses are also presented with experimentation results.

Underwater oscillations of rigid plates with H-shaped cross sections: An experimental study to explore their flow physics

In this work, we present a comprehensive experimental study on the problem of harmonic oscillations of rigid plates with H-shaped cross sections submerged in a quiescent, Newtonian, incompressible, viscous fluid environment. Motivated by recent results on the minimization of hydrodynamic damping for transversely oscillating flat plates, we conduct a detailed qualitative and quantitative experimental investigation of the flow physics created by the presence of the flanges, that is, the vertical segments in the plate cross section. Specifically, the main goal is to elucidate the effect of flange size on various aspects of fluid–structure interaction, by primarily investigating the dynamics of vortex shedding and convection. We perform particle image velocimetry experiments over a broad range of oscillation amplitudes, frequencies, and flange size-to-width ratios by leveraging the identification of pathlines, vortex shedding and dynamics, distinctive hydrodynamic regimes, and steady streaming. The fundamental contributions of this work include novel hydrodynamic regime phase diagrams demonstrating the effect of flange ratio on regime transitions, and in the investigation of their relation to qualitatively distinct patterns of vortex–vortex and vortex–structure interactions. Finally, we discuss steady streaming, identifying primary, and secondary structures as a function of the governing parameters.