Pure Oscillation Research Articles

Coexistence of passive vortex-induced vibrations and active pitch oscillation triggered by a square cylinder attached with a deformable splitter plate

To understand passive vortex-induced vibrations (VIV) coexisting with active structure motions, this paper numerically investigates the use of pure pitch oscillation to control a square cylinder mounted with a deformable splitter plate at the Reynolds number of 333. The oscillation is enforced with an amplitude of 3° and different frequencies from 0 to 6 Hz. Direct numerical simulations using a partitioned method with a semi-implicit coupling algorithm are performed. According to the trajectories of the splitter-plate tip displacement with respect to the lift or drag force coefficient, a specific lock-in regime determined by the frequency of the enforced pitch oscillation is identified. Further spectral analyses of the tip displacement and lift force show that the lock-in frequencies are equal to the enforced frequencies. Next to the lock-in regime, semi-lock-in regimes with narrow bandwidths are distinguished, exhibiting both lock-in and non-lock-in features. In the non-lock-in regimes, the frequencies of the most predominant peaks in the spectra are found near the natural frequency of the splitter plate of 3.236 Hz, and the frequencies of the two secondary peaks are distributed along the characteristic lines following the ratios of these frequencies to the enforced frequency, which are ±1. Thus, the interaction is dependent on the combined effects of the passive VIV and the actively enforced pitch oscillations. Moreover, the intersection points of the characteristic lines are located close to the upper and lower frequency limits of the lock-in regime, inferring the conditions for the lock-in onset.

Wind tunnel measurements of dynamic aerodynamic coefficients using a freely rotating test bench

PurposeThis paper aims to propose a dedicated measurement methodology able to simultaneously determine the stability derivative Cmα̇ and the pitch damping coefficient sum Cmq + Cmα̇ in a wind tunnel using a single and almost non-intrusive metrological setup called MiRo.Design/methodology/approachTo assess the MiRo method’s reliability, repeatability and accuracy, the measurements obtained with this technique are compared to other sources like aerodynamic balance measurements, alternative wind tunnel measurements, Ludwieg tube measurements, free-flight measurements and computational fluid dynamics (CFD) simulations. Two different numerical approaches are compared and used to validate the MiRo method. The first numerical method forces the projectile to describe a pure oscillation motion with small amplitude along the pitch axis during a rectilinear flight, whereas the second numerical approach couples the one degrees of freedom simulation motion equations with CFD methods.FindingsMiRo, a novel and almost non-intrusive technique for dynamic wind tunnel measurements, has been validated by comparison with five other experimental and numerical methodologies. Despite two completely different approaches, both numerical methods give almost identical results and show that the holding system has nearly no impact on the dynamic aerodynamic coefficients. Therefore, it could be assessed that the attitude of MiRo model in the wind tunnel is very close to the free-flight one.Originality/valueThe MiRo method allows studying the attitude of a projectile in a wind tunnel with the least possible impact on the flow around a model.

Construction of a reduced-order model of an electro-osmotic micromixer and discovery of attractors for petal structure

The chaotic state of microfluidic devices such as electroosmotic micromixers has received extensive attention. Its unsteady flow and multi-physics mask low-dimensional structure and potential attractors. Based on the dynamic mode decomposition and the sparse identification of nonlinear dynamics, this study aims to construct a manifold equation with the minimum degree of freedom, reveal the mixing mechanism of micromixers, and discover the evolution of chaotic states. The attenuation degree of freedom was introduced to force the modal coefficients to be pure oscillations. The six, four, and two-dimensional minimum reduced-order models (ROMs) were constructed under different mixing conditions. The nonlinear dynamics evolves on attractors resembling a six-petal structure based on the amplitude-phase method. The attractor periodicity and decay map the evolution of the periodic oscillation and limit cycle of the active modes and are related to the appearance of the low-energy dominant non-axisymmetric modes. These results emphasize the significance of ROM technology in revealing the low-dimensional structure and attractor of the electroosmotic micromixer.

Neutron-neutral particle mixing and its observable consequences

In this work, we explore the mixing between neutron ($n$) and elementary neutral particle ($\eta$), which violates both the baryon number ($\mathcal{B}$) and the lepton number ($\mathcal{L}$) by one unit but conserves their difference $(\mathcal{B}-\mathcal{L})$. Such mixing may give rise to non-trivial effects that are different from the Standard Model predictions. We organize our discussions based on two scenarios, roughly depending on whether an interference between oscillation and decay occurs, or whether the new-physics effects associated with the $n$-$\eta$ mixing contribute to the absorptive mixing amplitude. If an oscillation process is not accompanied by an interference between oscillation and decay, or the new-physics interactions do not contribute to the absorptive mixing amplitude, such a process can be classified as pure oscillation. Otherwise, it can be classified as impure oscillation. In the scenario of pure oscillation, CP-violation arising from the Majorana phase can manifest itself through the $n$-$\bar{n}$ oscillation process and may lead to observable effects. In the scenario of impure oscillation, we analyze the testable implications on the masses and lifetimes of the mass eigenstates formed as a result of the $n$-$\bar{n}$ oscillation mediated by $\eta$. In this scenario, we also suggest a unified interpretation of the neutron lifetime anomaly and the $n$-$\bar{n}$ oscillation measurements based on the $n$-$\eta$ mixing. In both scenarios, we present the lower bounds imposed by the experimental searches for $n$-$\bar{n}$ oscillations on the masses of the color multiplet bosons and point out that they could be within the reach of a direct detection at the LHC or future high-energy experiments.

Excitation spectra of strongly interacting bosons in the flat-band Lieb lattice

The strongly correlated bosons in flat band systems are an excellent platform to study a wide range of quantum phenomena. Such systems can be realized in optical lattices filled with ultracold atomic gases. In this paper we study the Bose-Hubbard model in the Lieb lattice by means of the time-dependent Gutzwiller mean-field approach. We find that in the Mott insulator phase the excitation modes are gapped and display purely particle or purely hole character, while in the superfluid phase the excitation spectrum is gapless. The geometry of the Lieb lattice leads to a non-uniform order parameter and non-uniform oscillation energy in the ground state. This results in additional anti-crossings between dispersive bands in the excitation spectra, while the flat bands remain insensitive to this effect. We analyze the oscillations of the order parameter on the sublattices as well as the particle-hole character of the excitations. For certain model parameters we find simultaneous pure phase and pure amplitude oscillations within the same mode, separated between the sublattices. Also, we propose a simple method to differentiate between the hole- and particle superfluid regions in the Lieb lattice by in-situ measurement of the atom population on the sublattices.

CONVOLUTIONAL NEURAL NETWORKS FOR DETECTING BRIDGE CROSSING EVENTS WITH GROUND-BASED INTERFEROMETRIC RADAR DATA

Abstract. This study focuses on detecting vehicle crossings (events) with ground-based interferometric radar (GBR) time series data recorded at bridges in the course of critical infrastructure monitoring. To address the challenging event detection and time series classification task, we rely on a deep learning (DL) architecture. The GBR-displacement data originates from real-world measurements at two German bridges under normal traffic conditions. As preprocessing, we only apply a low-pass filter. We develop and evaluate a one-dimensional convolutional neural network (CNN) to achieve a solely data-driven event detection. As a baseline machine learning approach, we use a Random Forest (RF) with a selected feature-based input. Both models’ performance is evaluated on two datasets by focusing on identifying events and pure bridge oscillations. Generally, the event classification results are promising, and the CNN outperforms the RF with an overall accuracy of 94.7% on the test subset. By relying on an entirely unknown second dataset, we focus on the models’ performances regarding the distinction between events and decays. On this dataset, the CNN meets this challenge successfully, while the feature-based RF classifies the majority of non-event decays as events. To sum up, the presented results reveal the potential of a data-driven DL approach concerning the detection of bridge crossing events in GBR-based displacement time series data. Based on such an event detection, a prospective assessment of bridge conditions seems feasible as an extension to previous structural health monitoring approaches.

ИССЛЕДОВАНИЕ ВИБРОАКТИВНОСТИ ЭЛЕКТРОШПИНДЕЛЕЙ ШЛИФОВАЛЬНЫХ СТАНКОВ

The purpose of this work is to support dynamic properties of spindle units in grinding machines. For this there are problems under solution for the definition of the origin of the constituents in the spindle unit vibratory activity by means of the linear increase of electric spindle rotation frequency, obtaining and analyzing a vibratory acceleration signal for the possibility to determine a preload. The vibratory acceleration signal was investigated through a spectrum analysis method. A scientific novelty of investigation consists in the substantiation of possibility to determine a preload by means of the spectrum analysis of a vibration acceleration signal at the linear increase of spindle rotation frequency that is at starting. It gives, in its turn, a possibility for the automated estimate of the spindle unit state before cutting beginning. In the experimental way there are obtained temporal realizations of the vibratory acceleration signal at different efforts of the preload. A high-speed grinding motor-spindle is as a basic element of the bench, which was investigated through the methods of testing diagnostics in the operation. In the bench design there were made some alterations. The bench was supplemented with the systems essential to support motor-spindle full operation, in particular: with systems of lubrication, cooling and drive control. There was revealed a large number of harmonics multiple to 50 Hz, which tells of the connection with the frequency of power supply circuit. Their coincidence with the own frequencies of the spindle unit results in the considerable increase of their amplitudes. To increase dynamic quality one should avoid the cases of the coincidence of switching frequencies and circuit harmonics with own frequencies of the electric spindle. It is also necessary to bring a form of power voltage to a pure harmonic oscillation to decrease the impact of a drive electromagnetic field upon dynamic characteristics of the spindle unit.

All-optical microwave oscillator based on feedback modulation within distributed feedback laser diode

An all-optical microwave oscillator is proposed and experimentally demonstrated, in which a distributed feedback laser diode (DFB-LD) acts as light source and optical-optical modulator simultaneously. By employing stimulated Brillouin scattering (SBS) process for oscillation frequency selection and active optical comb filter for side-modes suppression, single-mode photonic microwave signal generation can be achieved through a pure optical oscillation. In the experiment, a stable 10.87-GHz photonic microwave signal is obtained. The signal quality is evaluated by measuring the side-mode suppression ratio and single side-band phase noise of the signal. The corresponding values are 42 dB and −85 dBc / Hz at 10 kHz, respectively. This scheme explores the innovative application potentiality of DFB-LD, which will benefit on-chip photonic microwave systems.

Sound in a system of chiral one-dimensional fermions

We consider a system of one-dimensional fermions moving in one direction, such as electrons at the edge of a quantum Hall system. At sufficiently long time scales the system is brought to equilibrium by weak interactions between the particles, which conserve their total number, energy, and momentum. Time evolution of the system near equilibrium is described by hydrodynamics based on the three conservation laws. We find that the system supports three sound modes. In the low temperature limit one mode is a pure oscillation of particle density, analogous to the ordinary sound. The other two modes involve oscillations of both particle and entropy densities. In the presence of disorder, the first sound mode is strongly damped at frequencies below the momentum relaxation rate, whereas the other two modes remain weakly damped.

A coupled all-optical microwave oscillator with large tuning range based on SBS

A coupled all-optical microwave oscillator with large tuning range is proposed. Without optical–electrical–optical conversion components or any electrical microwave devices, microwave signals can be generated through a pure optical oscillation. In this system, a fiber ring laser provides a set of frequency references for frequency selection. Meanwhile, it also serves as a high-Q active resonator to optimize phase noise performance. By transferring the cavity modes to an optical carrier and selectively amplifying some of the modes via stimulated Brillouin scattering, the oscillation frequency is preliminarily selected. A semiconductor optical amplifier in the fiber ring laser is employed to accomplish the optical envelope detection and feedback modulation. Once the oscillation system is closed, good quality and high stability single-mode microwave signals are generated. By varying the wavelength of Brillouin pump light, a tuning range from 2 GHz to 26 GHz is demonstrated. Furthermore, the single side-band phase noise of these microwave signals are around −90 dBc/Hz@10 kHz.

Is it really chaos? The complexity of transient dynamics of double pendula

In this paper, we re-examine the dynamics of double pendulum in numerical simulations and experimental observations. Typical types of behaviors of the parametrically excited double pendula are presented, including chaos, rotations and periodic oscillations, and the bifurcation analysis is performed, exhibiting complex transitions from one type of motion into another. The character of the observed dynamics is analyzed using Lyapunov exponents, which confirms the hyperchaotic nature of the system. Particular attention is paid to the transient behaviors, showing that the length of the irregular motion can be extremely sensitive to both parameters and initial conditions. Apart from the single double pendulum, we consider also the case of two coupled double pendula, connected by a typical linear scheme. Our results show that depending on the network’s parameters, one can observe the phenomenon of a transient chaotic synchronization, during which the units spontaneously synchronize and desynchronize. The loss of coherence is strictly related to the motion of the pendula around the unstable equilibrium of the system, which has been confirmed in the scenario of pure chaotic oscillations. We determine the regions of the occurrence of transient synchronization in the coupling parameters’ plane, as well as study the statistical properties of the observed patterns. We show that the problem of determining the final dynamical attractor of the system is not straightforward.

The influence of leading-edge deflection on the stability of the leading-edge vortices

To examine the effect of leading-edge deflected angle [Formula: see text] on the stability of the leading-edge vortex, the three-dimensional flow field of a flapping wing is simulated by a numerical method. The multi domain mesh generation, dynamic mesh and large eddy simulation technology are employed to capture the finer flowfield structure. The wings perform pure periodic oscillations, and the Reynolds number ( Re) is 4527 based on the chord length c. The folding line formed after the deflection coincides with the pitch axis and is located at the 1/4 c from the leading edge. The results show that the increase of [Formula: see text] maintains the strength of the leading-edge vortex for longer time, and weakens the influence of the motion of the wing on the leading-edge vortex intensity. The flowfield topological analysis shows that the increase of [Formula: see text] also prevents the formation of secondary vortices between the wing surface and the leading-edge vortices, which indirectly contributes to the attachment of the leading-edge vortices to the wing. Moreover, the vortex dynamics equations have been analyzed, and the results indicate that the increase of [Formula: see text] will delay the occurrence of spanwise convection of vorticity and weaken its intensity. In addition, it can also suppress the spanwise flow behind the leading-edge vortices toward the symmetric plane. As a result, increasing [Formula: see text] stabilizes the boundary layer in this region and thereby stabilizes the leading-edge vortices indirectly. Finally, a new parameter is introduced to quantitatively evaluate the proximity of the leading-edge vortex to the surface of the plate. Our method comprehensively considers the influence of the leading-edge vortex scale and the core motion on the approaching of the leading-edge vortex to the wing, and some important conclusions on the developing law of the leading-edge vortex, which are agreement with the experimental measurement, are obtained.

Decomposing alpha and 1/f brain activities reveals their differential associations with cognitive processing speed

Research in cognitive neuroscience has extensively demonstrated that the temporal dynamics of brain activity are associated with cognitive functioning. The temporal dynamics mainly include oscillatory and 1/f noise-like, non-oscillatory brain activities that coexist in many forms of brain activity and confound each other’s variability. As such, observed functional associations of narrowband oscillations might have been confounded with the broadband 1/f component. Here, we investigated the relationship between resting-state EEG activity and the efficiency of cognitive functioning in N = 180 individuals. We show that 1/f brain activity plays an essential role in accounting for between-person variability in cognitive speed – a relationship that can be mistaken as originating from brain oscillations using conventional power spectrum analysis. At first glance, the power of alpha oscillations appeared to be predictive of cognitive speed. However, when dissociating pure alpha oscillations from 1/f brain activity, only the 1/f predicted cognitive speed, whereas the predictive power of alpha vanished. With this highly powered study, we disambiguate the functional relevance of the 1/f power law pattern in resting state neural activities and substantiate the necessity of isolating the 1/f component from oscillatory activities when studying the functional relevance of spontaneous brain activities.

Wide-area measurement-based modal decoupling for power system oscillation damping

Inter-area oscillation modes usually fall within the same frequency range of 0.1–1.0Hz; thus, power oscillation damping (POD) control methods should avoid unnecessary excitation of weakly damped modes. This paper presents a decoupled control strategy to damp inter-area oscillations in the power grid. Pure oscillation mode signals are extracted from wide-area measurements and used as feedback input for POD actuators such as power system stabilizers or high-voltage direct current transmission. The decoupled control strategy is able to increase the damping of the concerned oscillation modes without deteriorating others. This proposed strategy could be implemented independently, or serve as a complement and work along with conventional POD controllers to damp concerned inter-area modes. The proposed method has been tested on a modified MinniWECC system. Extensive results indicate that inter-area oscillations can be successfully and precisely damped using the proposed decoupled strategy.

Tunable single-mode microwave signal generation utilizing an all-optical coupled microwave oscillator.

A tunable all-optical coupled microwave oscillator is proposed and experimentally demonstrated. This system has no any electrical microwave devices as well as photoelectric conversion and electro-optic modulation. In this scheme, a fiber ring laser not only provides a set of frequency references but also acts as an active optical resonator. The oscillation frequency selection is carried out by cavity modes transfer and injection locking process. Due to the mode pulling effect, a self-adapting oscillation frequency locking is achieved. Through the microwave envelope detection and feedback modulation in a semiconductor optical amplifier (SOA), a microwave signal can be generated by a pure optical oscillation. In the experiment, good quality and high stability single-mode microwave signals are obtained, whose frequencies can be tuned from 6.93 GHz to 25.54 GHz by simply adjusting the wavelength of the master laser. Under different operation frequencies, the measured single-sideband phase noises are approximately -95 dBc/Hz at a 10-kHz offset from the carriers.

High-precision Asteroseismology in a Slowly Pulsating B Star: HD 50230

Abstract The slowly pulsating B star HD 50230, which is in fact a hybrid B-type pulsator, has been observed by CoRoT for at least 137 days. Nearly equidistant period spacing patterns are found among eight modes that are extracted from the oscillation spectrum with more than 500 frequencies. However, it is thought to be most likely accidental by Szewczuk et al. In the present work, we analyze the eight modes in depth with the χ 2-matching method. Based on the best-fitting model (model MA), we find that they can be well explained as sequences of consecutive dipolar (l, m) = (1, 0). The period discrepancies between observations and the best-fitting model are within 100 s except for the outlier, which is up to 300 s. Based on the calculated χ 2-minimization models, we find that, for pure g-mode oscillations, the buoyancy radius, Λ0, can be precisely measured with the χ 2-matching method between observations and calculations. It represents the “propagation time” of the g-mode from the stellar surface to the center. It is of Λ0 = 245.78 ± 0.59 μHz with a precision of 0.24%. In addition, we also find that HD 50230 is a metal-rich (Z init = 0.034–0.043) star with a mass of M = 6.15–6.27 M ⊙. It is still located in the hydrogen-burning phase with central hydrogen of X C = 0.298–0.316 (or ); therefore, it has a convective core with a radius of R cc = 0.525–0.536 R ⊙ (or ). In order to interpret the structure of the observed period spacing pattern well, the convective core overshooting (f ov = 0.0175–0.0200) and the extra diffusion mixing ( ) should be taken into account in theoretical models.

Tunable optoelectronic oscillator based on a polarization-dependent phase modulator cascaded with a linear chirped FBG

A tunable optoelectronic oscillator (OEO) based on polarization controlled frequency response shifting of a dispersion-induced microwave photonic filter (MPF) is proposed and experimentally demonstrated. The MPF consists of a phase modulator (PM), a linear chirped fiber Bragg grating (LCFBG), and polarization multiplexed dual loops. Thanks to the polarization properties of the LiNbO3 crystal, the center frequency of the MPF can be tuned by simply adjusting the polarization state of the optical signal, and a tunable optoelectronic oscillator is established. Compared to the previous structure, the expensive wavelength-tunable laser is not needed in the proposed structure. In addition, the instability of oscillation frequency caused by the bias drifts of the modulator is avoided. A pure oscillation signal with a tunable frequency range from 3.3 to 7.3 GHz is generated. The single-sideband phase noise of the oscillation signal is about −110 dBc/Hz at 10-kHz offset. During more than 30 min observation, the oscillation frequency drift is about 7 kHz.

Definitive Conditions on Maxwell's Tangential E or H for Solutions to His Equations Inside a Bounded Region

AbstractLet U be a connected, closed, bounded region in ℝ3 with smooth boundary 𝛛U. Consider Maxwell's equations on U for smooth fields and smooth sources, which are time harmonic, i.e., the fields E,B,H,D and the current density J and charge density ρ on U temporally varying as e−iωt. We assume ω ≠ 0; when it is real, ω is the frequency of the pure oscillation; when ω is not real, one has exponentially decreasing or increasing oscillatory fields. We treat very general electric permittivity ɛω and magnetic permeability μω, which may both depend upon ω. Both can be arbitrary, time‐independent, smoothly varying with position and described by complex Hermitian tensors subject only to weak positivity conditions specified below at each point of U.In this paper necessary and sufficient conditions are found for the tangential components, ET or HT on 𝛛U, to be realized by solutions of Maxwell's equations on all of U with these given sources J, ρ in the time‐harmonic realm with ω ≠ 0. Also, all solutions E,H on U with specified ET, HT, respectively, satisfying the necessary finite‐dimensional conditions are derived by an effective natural algorithm.In the electric ET case, the positivity condition is this: the complex Hermitian matrix μω(p) is to be positive definite while only the real part of ɛω(p), i.e., Re(ɛω(p)), necessarily real symmetric, need be positive definite. In the magnetic‐type HT case, the two roles are reversed. These positivity conditions are more general than those commonly considered. In the final section Maxwell's equations and all the present results are reformulated yet more generally in a context of compact, smooth, Riemannian manifolds with boundary. Sample examples include periodic lattices and wave guides. © 2019 Wiley Periodicals, Inc.

Theoretical Analysis of Empirical Mode Decomposition

This work suggests a theoretical principle about the oscillation signal decomposition, which is based on the requirement of a pure oscillation component, in which the mean zero is extracted from the signal. Using this principle, the validity and robustness of the empirical mode decomposition (EMD) method are first proved mathematically. This work also presents a modified version of EMD by the interpolation solution, which is able to improve the frequency decomposition of the signal. The result shows that it can provide a primary theoretical basis for the development of EMD. The simulation signal verifies the effectiveness of the EMD algorithm. At the same time, compared with the existing denoising algorithm, it has achieved good results in the denoising of rolling bearing fault signals. It contributes to the development and improvement of adaptive signal processing theory in the field of fault diagnosis. It provides practical value research results for the rapid development of adaptive technology in the field of fault diagnosis.

Experimental verification of the adiabatic transfer in Wilberforce pendulum normal modes

A pendulum bob has been constructed to experimentally probe the adiabatic transfer in the normal modes of a Wilberforce pendulum. The rotational inertia of the pendulum bob can be changed during its motion in order to follow the gradual transition from a pure translational to a pure rotational oscillation. The reported extension of the conventional use of the Wilberforce pendulum helps understand and demonstrate the concept of normal modes, coupled oscillators, beating, and avoided crossing in undergraduate university classes.