Weak Oscillations Research Articles

Internal dynamic detection of soliton molecules in Ti:sapphire femtosecond laser

Soliton is a universal format of nonlinear wave propagation in nature. Soliton can maintain its shape during propagation. This unique property has been widely observed in plasma physics, high energy electromagnetics, hydrodynamics, and nonlinear optics. Soliton interactions can reflect collective dynamic behaviors in complex nonlinear systems, showing significant basic research value. Passive mode-locked laser is an ideal platform for studying soliton interaction. The attraction and repulsion between two optical solitons can form soliton molecules. Their properties have been intensively studied by optical spectral analysis. However, conventional optical spectrum analyzers show low resolution and long average time. Time-stretched dispersive Fourier transformation (TS-DFT) is an emerging-powerful measurement technology, which can map the spectrum of an optical pulse to a temporal waveform under sufficient dispersion. The TS-DFT makes it possible to detect the dynamics of the solitons in real time. Based on TS-DFT, the internal dynamics of the solitons in Ti:sapphire femtosecond laser is studied in experiment. By changing the pump power, the stable soliton molecules with a separation of 180 fs and the weak phase oscillatory soliton molecules with a separation of 105 fs are observed. The amplitude in the weak oscillation state is merely 0.05 rad. We also find that the soliton molecules in stable state can transform into phase sliding state under environmental perturbation. These optical soliton molecules with a binding separation of 100 fs are of great significance for studying the short-range nonlinear interactions of solitons.

Modulation of the secondary Bjerknes force in multi-bubble systems.

The behaviours of insonated bubble clusters are regulated by the secondary Bjerknes force between bubble pairs. While the force has been investigated extensively for two-bubble systems, the modulation of the force by nearby bubbles remains unclear. This problem is investigated in this paper by theoretical analyses and numerical simulations of a three bubble system. For weak oscillations, the third bubble is found to have strong effects when its radius is close to the resonant radius. The equilibrium distance between the bubble pair is reduced when the radius of the third bubble is smaller than the resonant threshold, and increased when it is larger. For strong oscillations of bubbles with radii of a few microns, the third bubble reduces the magnitude of the force, hence increasing the equilibrium distance. The modulation effects depend strongly on the relative sizes of the bubbles. Stronger effects can be produced when the third bubble is placed closer to the smaller bubble in the bubble pair. The findings highlight the need for a more accurate parametrization of the secondary Bjerknes force in the simulation and manipulation of bubble clusters.

Experiments on self-sustained oscillations of leeward vortices over a hemisphere cylinder

Recent numerical studies have indicated the existence of a new type of vortex unsteadiness around a hemisphere cylinder from low to high angles of attack, characterized by large-scale alternate oscillations of leeward-vortex pairs. This investigation conclusively confirms the existence of vortex oscillations in experiment by directly measuring oscillatory vortex structures and explores their origin and evolution with increasing Reynolds number (Re = 957–6780, Re = U∞D/ν, where U∞ is freestream velocity, D is the diameter of the body, and ν is the coefficient of kinematic viscosity). The results indicate that the Reynolds number strongly influences the stability of the vortex pairs and oscillations of the vortices. As Re is less than a critical Re (Rec), only weak horizontal oscillations (antisymmetric modes) were observed downstream of the vortex pairs at low frequencies and a small amplitude. At Re > Rec, stronger vortex oscillations were observed with a mean dimensionless frequency of St = 0.11 (St = f D/U∞), where the amplitude of the oscillations increased with the value of Re. In this case, the oscillations of the vortex pairs consisted of antisymmetric and symmetric modes, where the antisymmetric modes were dominant and corresponded to alternate oscillations of the vortex pairs, and the symmetric modes were much weaker and corresponded to in-phase oscillations of the vortex pairs. The estimated wavelengths imply that the vortex oscillations originated in long-wave instabilities. However, the behaviors of these instabilities were significantly different from those of Crow-type long-wave instabilities in which symmetric modes that are insensitive to the Reynolds number are dominant.

Microwave probing of relaxation oscillations related to terahertz power detection in superconducting hot electron bolometers

Relaxation oscillations related to terahertz (THz) power detection in superconducting niobium nitride hot electron bolometers (HEBs) have been studied using microwaves as a probe. The reflected microwave signal, which is related to the impedance changes of the HEB irradiated by an incident THz signal, can be used to characterize relaxation oscillations. The frequency of relaxation oscillation switching between the superconducting and resistive states increases linearly with incident THz power but nonlinearly with bias voltage or bath temperature. The observed strong periodic pulse trains of relaxation oscillations have a maximum duty cycle of ∼50%. These strong switching pulses indicate that DC Joule heating, THz radiation or thermal energy is absorbed in the bi-stable region of the HEB intermittently. In addition to relaxation oscillations, we also see weaker oscillations when the device is in the resistive state, which we call intrinsic oscillations. These have a constant frequency at the order of 10 kHz which is independent of external stimuli. The weak periodic pulse trains at a constant frequency can be divided into mainly two types—one is the pulse train with a small duty cycle and the other is the quasi-square wave. Measurement of these oscillations in HEBs has some practical applications. Digital THz power measurement with frequencies below and above the gap frequency can be made based on the linearity between relaxation oscillation frequency and incident THz power. The relaxation oscillation frequency can be used to estimate the maximum speed of a THz direct detection system especially for a multi-pixel HEB array. In particular, extra noise induced by relaxation oscillations and intrinsic oscillations can be determined for the HEB operating as a THz direct detector.

Health Assessment and Fault Detection System for an Industrial Robot Using the Rotary Encoder Signal

In an industrial robot, rotary encoders have been extensively used for dynamic control and positioning. This study shows that the encoder signal, after appropriate processing, can also be efficiently utilized for the health observation of energy performance of industrial robots system. Singular spectrum analysis (SSA) and Hilbert transform (HT) is proposed in this work, for detecting weak position oscillations to estimate the instantaneous amplitudes (IA) and the instantaneous frequencies (IF) of an industrial robot based on the encoder signal. Compared with empirical mode decomposition (EMD) and HT, the singular spectrum analysis and Hilbert transform (SSAHT) outperforms empirical mode decomposition Hilbert transform (EMDHT) in terms of ability and precision to determine source noise, and it can accurately catch the weak oscillations without signal deformation in both position and speed introduced via mechanical flaws. Combined with SSA, the IA and IF of both oscillations and residual are extracted by HT. They are obtained from the robot arm movement. These features play an important role in improving the performance detecting weak oscillations and the residual, essential information to evaluate the health conditions and fault detection to serve the energy performance for the industrial robot. The efficiency of the proposed system has been verified both numerical simulation and experimental data. The outcomes prove that the proposed SSAHT can detect flaw indications and additionally, it can also identify faulty components. Thus, the study presents a promising tool for the health monitoring of an industrial robot instead of the vibration-based monitoring scheme.

Discussion on the accuracies of He’s frequency–amplitude formulation and its modification with average residuals

A complete numerical comparison is given between the He’s frequency–amplitude formulation and its modification with average residuals for both weak and strong nonlinear oscillators, showing both me...

Vibrational antiresonance in nonlinear coupled systems.

We examine the response of a system of coupled nonlinear oscillators driven by a rapidly varying field, to a low frequency weak periodic excitation of one of the oscillators. The response amplitude of the weak field-driven oscillator at an optimal strength of the rapidly varying field exhibits a strong suppression accompanied by a large negative shift in its oscillation phase. The minimum can be identified as vibrational antiresonance in between the two maxima corresponding to vibrational resonance. This vibrational antiresonance can be observed only in nonlinear coupled systems and not in linearly coupled systems or in a single nonlinear oscillator, under similar physical condition. We discuss the underlying dynamical mechanism, the role of nonlinearity and high frequency in characterizing this counter-resonance effect. Our theoretical analysis is corroborated by detailed numerical simulations.

Shear stress relaxation and diffusion in simple liquids by molecular dynamics simulations: Analytic expressions and paths to viscosity.

The results are reported of an equilibrium molecular dynamics simulation study of the shear viscosity, η, and self-diffusion coefficient, D, of the Lennard-Jones liquid using the Green-Kubo (GK) method. Semiempirical analytic expressions for both GK time correlation functions were fitted to the simulation data and used to derive analytic expressions for the time dependent diffusion coefficient and shear viscosity, and also the correlation function frequency transforms. In the case of the shear viscosity for a state point near the triple point, a sech function was found to fit the correlation function significantly better than a gaussian in the ballistic short time regime. A reformulation of the shear GK formula in terms of a time series of time integrals ("viscuits") and contributions to the viscosity from components based on the initial stress ("visclets") enable the GK expressions to be recast in terms of probability distributions which could be used in coarse grained stochastic models of nanoscale flow. The visclet treatment shows that stress relaxation is statistically independent of the initial stress for equilibrium and metastable liquids, suggesting that shear stress relaxation in liquids is diffusion controlled. By contrast, the velocity autocorrelation function is sensitive to the initial velocity. Weak oscillations and a plateau at intermediate times originate to a greater extent from the high velocity tail of the Maxwell-Boltzmann velocity distribution. Simple approximate analytic expressions for the mean square displacement and the self Van Hove correlation function are also derived.

Interactions of Coherent Structures on the Surface of Deep Water

We numerically investigate pairwise collisions of solitary wave structures on the surface of deep water—breathers. These breathers are spatially localised coherent groups of surface gravity waves which propagate so that their envelopes are stable and demonstrate weak oscillations. We perform numerical simulations of breather mutual collisions by using fully nonlinear equations for the potential flow of ideal incompressible fluid with a free surface written in conformal variables. The breather collisions are inelastic. However, the breathers can still propagate as stable localised wave groups after the interaction. To generate initial conditions in the form of separate breathers we use the reduced model—the Zakharov equation. We present an explicit expression for the four-wave interaction coefficient and third order accuracy formulas to recover physical variables in the Zakharov model. The suggested procedure allows the generation of breathers of controlled phase which propagate stably in the fully nonlinear model, demonstrating only minor radiation of incoherent waves. We perform a detailed study of breather collision dynamics depending on their relative phase. In 2018 Kachulin and Gelash predicted new effects of breather interactions using the Dyachenko–Zakharov equation. Here we show that all these effects can be observed in the fully nonlinear model. Namely, we report that the relative phase controls the process of energy exchange between breathers, level of energy loses, and space positions of breathers after the collision.

Spatial-temporal characteristics of the air quality in the Guangdong−Hong Kong−Macau Greater Bay Area of China during 2015–2017

Spatial-temporal characteristics of the air quality including size-segregated particulate matter (PM) portions and ozone (O3) in the Guangdong−Hong Kong−Macao Greater Bay Area (GBA) of China are analyzed using daily observational data during 2015–2017. A big picture of multiple years' in situ observational air quality in GBA is provided. A collection of three neighboring cities Foshan (FS), Guangzhou (GZ), and Dongguan (DG), referred to as FGD, on average has the worst air quality in GBA, mostly due to the heavy primary emissions and high chance of secondary pollutant productions and accumulations. The air quality in FGD generally has a significant seasonal cycle which is influenced by the East Asian Monsoon and modulated by ENSO. Different cities have different prominent air quality issues and there are special multi-scale characteristics in PM2.5 at FS, O3 at DG, and PM2.5-10 at DG. The inter-city differences of multi-scale air quality variations in FGD could be attributed to both sources and meteorology. For example, DG has extremely high O3 (especially in 2015) and PM2.5/PM10 but relatively low PM, and PM2.5-10 at DG has strong 2–3 days’ oscillation but weak seasonal oscillation. The special air quality characteristics at DG might be attributed to combined effects of the different ventilation and dilution consequences of the sea-land breeze to different air quality components which have different three-dimensional source distributions. Through a combined multi-scale lag correlation analysis of air quality, some prior air quality components (with different leading time on different scales) are identified as potential indicators for statistic air quality prediction.

ION cyclotron resonance: Geomagnetic strategy for living systems?

ABSTRACTExcept for relatively few polarity reversals the magnitude of the magnetic dipole moment of the earth has remained constant since life first began, allowing evolutionary processes to integrate the geomagnetic field (GMF) into several biological functions. One of these, bearing the classical signature of an ion cyclotron resonance (ICR)-like interaction, results in biological change associated with enhanced proton transport. The wide range of cation masses over which this effect is found suggest a fundamental biological dependence on the GMF, one that functions equally well for electric as well as magnetic fields. Such generalization of ICR requires two things: transparency of tissues to the GMF and suitably tuned ELF resonant magnetic or electric fields. To complement the widely reported ICR responses to applied AC magnetic fields, we hypothesize the existence of weak endogenous ICR electric field oscillations within the cell. This equivalence implies that even in the absence of applied AC magnetic fields, biological systems will exhibit intrinsic GMF-dependent ion cyclotron resonance intracellular interactions. Many ICR effects that have been reported appear as antagonist pairs suggesting that the characteristics of the GMF have not only been incorporated into the genome but also appear to function in an endocrine-like manner.

Design of Memory Test System for Measuring Transmission Lines Galloping

A mechine in order to warn of transmission lines galloping, reduce the loss caused by transmission lines galloping. Aiming at a transmission system, a set of lines galloping storage and testing device is developed. The device takes STM32 as the control core to realize signal conditioning, data storage, serial communication and other functions. In the design of the software, the method of single external trigger and multiple internal trigger is adopted to effectively avoid the false trigger phenomenon in weak oscillation, so as to test and store the acceleration of the wire when it gallopings violently. The acceleration sensor and the storage test device directly install on electric wire, and it can detect and record the acceleration data in the process of wire galloping.After the test, the recovery device uses the upper computer to read the detected data and display the waveform, which can provide important experimental basis for the theoretical analysis and parameter setting of the lines galloping.

Atmospheric Circulation Response to Anomalous Siberian Forcing in October 2016 and its Long‐Range Predictability

AbstractThe warm Arctic‐cold continent pattern was of record strength in October 2016, providing the opportunity to test its proposed influence on large‐scale atmospheric circulation. We find a record weak polar stratospheric vortex and negative North Atlantic Oscillation in November–December 2016 and link them to increased planetary wave generation associated with cold Siberian anomalies followed by troposphere‐stratosphere dynamical coupling. At the same time the warm Arctic anomalies, in particular those over the Barents‐Kara Seas, do not appear to play an important role in forcing the atmospheric circulation. Long‐range forecasts initialized on 1 October 2016 reproduced both the weak polar vortex and negative North Atlantic Oscillation, as well as their link with the Siberian temperatures. Our results support the stratospheric pathway for atmospheric circulation forcing associated with Siberian surface anomalies and uncover a source of skill for subseasonal forecasts from October to December.

Revised cloud and convective parameterization in CFSv2 improve the underlying processes for northward propagation of Intraseasonal oscillations as proposed by the observation-based study

The performance of revised climate forecast system version 2 (CFSv2) are evaluated on the simulation of the underlying cloud and convective processes associated with the strong and weak boreal summer intraseasonal oscillations (BSISOs) events. The revised version of the CFSv2 consists of a six-class Weather Research Forecasting single moment (WSM6) cloud microphysics scheme and the default version has Zhao and Carr (ZC) cloud microphysics scheme. Both the version uses revised simplified Arakawa-Schubert (RSAS) convection scheme. The study reveals that the revised version of CFSv2 (RSAS-WSM) is able to better simulate the northward propagation of BSISOs and associated dynamical and thermodynamical mechanism put forward by earlier observation-based studies. It is found that the large-scale organized northwest-southeast tilted structure of rain band is better captured in RSAS-WSM simulation as compared to the default version of CFSv2 (RSAS-ZC) during strong BSISO events. Further, the reasonable large-scale or stratiform rainfall associated with the northward propagating strong BSISO events is seen in RSAS-WSM while it is completely missing in RSAS-ZC simulation. The pressure-latitude profiles of cloud liquid water (CLW) and cloud ice (CLI) show more realistic steady northward propagation in RSAS-WSM simulation. Consistent with the CLW and CLI distribution and their influence on the large-scale heating structure, the large-scale condensation heating shows quasi-periodic northward propagation in RSAS-WSM whereas such type of distribution is not captured in RSAS-ZC simulation. The realistic representation of cloud processes in WSM leads to simulate reasonable dynamical and thermodynamical processes associated with the strong BSISO events which follows the observation-based hypothesis proposed by earlier studies.

High-frequency breakdown of the integer quantum Hall effect in GaAs/AlGaAs heterojunctions

The integer quantum Hall effect is a well-studied phenomenon at frequencies below about 100 Hz. The plateaus in high-frequency Hall conductivity were experimentally proven to retain up to 33 GHz, but the behavior at higher frequencies has remained largely unexplored. Using continuous wave THz spectroscopy, the complex Hall conductivity of GaAs/AlGaAs heterojunctions was studied in the range of 69-1100 GHz. Above 100 GHz, the quantum plateaus are strongly smeared out and replaced by weak quantum oscillations in the real part of the conductivity. The amplitude of the oscillations decreases with increasing frequency. Near 1 THz, the Hall conductivity does not reveal any features related to the filling of Landau levels. Similar oscillations are observed in the imaginary part as well, this effect has no analogy at zero frequency. This experimental picture is in disagreement with existing theoretical considerations of the high-frequency quantum Hall effect.

Indirect-to-direct band gap transition and optical properties of metal alloying of Cs2AgMxBr6 (M = Bi, In, Sb): Insights from the first principles

Cs2AgBiBr6 halide double perovskite, which possesses stable light-absorbing and non-toxic properties, have recently been proposed as potential environmentally friendly alternatives to lead-based hybrid halide perovskites. However, its indirect band gap indicates weak oscillation strengths for optical absorption and radiative recombination in the photovoltaic field. In this work, we change the lattice structure of Cs2AgBiBr6 by metal alloying with In and Sb elements and calculate their electronic and optical properties of the metal alloying Cs2AgMxBr6 (M = Bi, In, Sb) double-perovskites using the first-principles methods. We find that the indirect band gap of Cs2AgBiBr6 (1.94 eV) turn into direct band gap of Cs2AgBi0.25In0.75Br6 (1.85 eV). Cs2AgBi0.25In0.5Sb0.25Br6 also have a small indirect band gap of 1.50 eV as well as a direct band gap of 1.67 eV, and its absorption spectra cover the entire visible range. Therefore, our work may open the way to the development of lead-free double perovskites with direct and tunable band gaps.

Chemical enhancement in the SERS spectra of indigo: DFT calculation of the Raman spectra of indigo-Ag14 complexes

The appearance of the Raman and the SERS spectra of indigo (IND) at 785 and 514 nm laser wavelength excitations has prompted the study of the interactions between the molecule and the silver substrate to which the molecule is adsorbed. In particular, no change of appearance between Raman and SERS spectra, apart some intensification, is observed at 785 nm excitation whereas some changes in the number and intensity of the bands are observed at 514 nm excitation. In a previous study a simplified model has been adopted where the molecule is simply bonded to an Ag2 cluster. Structural models have now been devised with the molecule is sited either perpendicular (edge-on) or planar (surface-on) to the surface of a Ag14 pyramidal cluster. The structure of the two complexes, the energy of the electronic excitations, the Raman spectra of the molecule/metal complexes have been studied through a TD-DFT simulation. The appearance of the experimental SERS spectra have been confronted with the prediction of the Herzberg–Teller surface selection rules of Lombardi et al., which have been formulated for the SERS spectra of molecules adsorbed on metal nanoparticles, in the frame of the Albrecht's theory for Raman scattering. The spectral predictions for the edge-on geometry fully meet the observation that the SERS spectra are dominated by totally symmetric Ag (under C2h symmetry) bands or, taking into account the lost of the inversion center after adsorption on the metal surface, in-plane A′ (under Cs symmetry) modes. This fully justifies the pop up of IR modes of the molecule in the SERS spectra, modes which belong to Bu and Au species in the free molecule C2h symmetry group. A systematic assignment of the modes of the free molecule and of the molecule in the complex has been made through the inspection of the calculated vibrational coordinates of the single vibrations. Dynamic polarizability (pre-resonance) calculations have been performed at 475 and 690 nm wavelength excitations, corresponding to the experimental 514 and 785 nm wavelengths, respectively. A small spectral enhancement is predicted at 690 nm for both complex geometries which should depend on charge transfer (CT) states calculated, albeit with very weak oscillator strength, in that spectral range. A substantial enhancement is instead calculated for both the edge-on and the surface-on models at the excitation wavelength of about 475 nm. The calculated Raman spectra intensification originates from the proximity of electronic excitations of intra-cluster, intra-molecular and CT type. The plasmonic excitation cannot be accounted for by the present model calculation. Natural Transition Orbitals (NTO) have been generated for the electronic excitations of both geometries, by transforming the ordinary orbital representation into a more compact form in which each excited state is expressed, if possible, as single pair of orbitals.

Two-way fluid-structure interaction simulation for steady-state vibration of a slender rod using URANS and LES turbulence models

Abstract Anisotropic distribution of the turbulent kinetic energy and the near-field excitations are the main causes of the steady state Flow-Induced Vibration (FIV) which could lead to fretting wear damage in vertically arranged supported slender rods. In this article, a combined Computational Fluid Dynamics (CFD) and Computational Structural Mechanic (CSM) approach named two-way Fluid-Structure Interaction (FSI) is used to investigate the modal characteristics of a typical rod's vibration. Performance of an Unsteady Reynolds-Average Navier-Stokes (URANS) and Large Eddy Simulation (LES) turbulence models on asymmetric fluctuations of the flow field are investigated. Using the LES turbulence model, any large deformation damps into a weak oscillation which remains in the system. However, it is challenging to use LES in two-way FSI problems from fluid domain discretization point of view which is investigated in this article as the innovation. It is concluded that the near-wall meshes whiten the viscous sub-layer is of great importance to estimate the Root Mean Square (RMS) of FIV amplitude correctly as a significant fretting wear parameter otherwise it merely computes the frequency of FIV.

Wide-area optimal damping control for power systems based on the ITAE criterion

Wide-area damping control is one of the most used methods for improving stability in power systems. However, this type of control requires suitably tuning the main controller and wide-area controller under dependency and constrains. In this paper, we propose a method for wide-area optimal damping control based on the integral of the time-weighted absolute error (ITAE) to mitigate low-frequency oscillations in the interconnected power grid. First, we perform the modal analysis of the open-loop system to determine the weak damping-interval oscillation mode. Then, based on the geometric observability and controllability indices, we derive the wide-area optimal closed-loop control. Finally, we formulate an objective function using the ITAE and employ the bird swarm algorithm to optimize the structurally constrained controller parameters for the coordination control. Simulation results demonstrate that the proposed control strategy has a highly accurate dynamic response and effectively suppresses inter-area oscillations in the interconnected power systems.

Wide-Area Damping Controller Design Based on the Frequency Domain Self-Excitation Method

In this paper, a wide-area DC damping controller design method is proposed to damp the weak interarea oscillation modes. First, the low-order power system control model identification method is proposed based on the frequency domain self-excitation method to obtain the dominant modes and other related information. In what follows, the identified model is transformed into a low-order state space equation. By comparing the geometric metrics of different schemes, the various input signals and installation locations of the controller are selected. Furthermore, the damping controller parameter calculation is realized based on the self-excitation frequency domain identification method, which does not depend on the detailed model of the system. The design process is simple and it is easy to apply in the practical engineering of large-scale complex power grids. The applicability and effectiveness of the proposed controller design method are demonstrated through simulations of a two-area four-generator power system.