Oscillation Mechanism Research Articles

VLF oscillation mechanism analysis of underwater vehicle based on overlapping grid

ABSTRACT The present study aims to investigate the VLF oscillation mechanism of underwater vehicles motion by conducting numerical simulation on the AFF-1. The straight-line simulations of AFF-1 were performed using RANS equations with a Reynolds stress turbulence model at different speeds, and an initial flow field with converged resistance results was obtained. Meanwhile, the accuracy of the numerical model and input parameters were validated by the resistance results. The sinuous motion of AFF-1 was simulated based on dynamic fluid body interaction (DFBI) and overlapping grid. The results indicated that the inherent instability of the AFF-1 significantly impacts its motion dynamics. Specifically, when AFF-1was in straight-line motion at a constant forward velocity, it was subjected to the influence of pressure component of hydrodynamic force and moment, resulting in slow trajectory divergence motion of AFF-1. In scenarios where manual manipulation is employed, this instability leads to the emergence of VLF oscillations.

Analysis of low-frequency oscillation mechanism and optimal control of impedance on the DC side of a two-stage power conversion system

The two-stage power conversion system (PCS), a cascaded system consisting of bi-directional DC/DC and DC/AC modules, is extensively employed in battery storage systems. However, the cascading of modules may result in a less stable system which is vulnerable to disturbances, especially power fluctuations. In fact, the level of the power or the reversal of the power flow can adversely affect the stability of the cascaded system, resulting in a low-frequency oscillation of the bus voltage. Accordingly, to solve the problem of stability degradation after modules cascading, this paper proposes an active damping control strategy with coordinated control of the capacitor voltage. This strategy can reduce the resonant peak of the output impedance of the source converter and increase the phase of the input impedance of the load converter at the same time, so as to avoid the intersection of impedances, effectively enhance the stable operating range of the cascaded system. Finally, simulations and experiments have been carried out for the verification of the analysis and the effectiveness of the proposed control method.

The Unsteady Shock-Boundary Layer Interaction in a Compressor Cascade – Part 3: Mechanisms of Shock Oscillation

Abstract The shock-boundary layer interaction in transonic flows is known to cause strong unsteady flow effects that negatively affect the performance and operability of blade and cascade designs. Despite decades of research on the subject, little is still known about the physical mechanisms that drive the different oscillation frequencies observed with different designs. In the conclusion of this three-part series, the experimental and numerical data obtained with the Transonic Cascade TEAMAero are analyzed together in detail in order to test the main theories of continuous shock oscillation. This analysis exposes a main mechanism of shock oscillation, where pressure waves generated inside the passage of the cascade propagate upstream and interact strongly with the main shock when the latter is also in the passage. The interaction of these features causes a breakdown of the flow that is shown to propagate upstream, inevitably causing strong variations in the inflow angle and therefore on the operating conditions of the cascade. The high frequency content of these pressure waves is also shown to be responsible for weaker high-frequency variations of the shock movement throughout the cycle. Parallels are also drawn with previous experimental campaigns in order to search for a global understanding of the different observations made. Although various parts of the described interaction are not fully understood yet, and the dataset of experimental measurements compiled is still rather small, a good basis is provided on which to further study the underlying mechanisms of unsteady flows in transonic cascades.

Constraints on Schwarzschild-like black hole from QPOs data

The search for compact astrophysical objects, such as black holes and wormholes, along with the testing of gravity theories, are crucial topics in relativistic astrophysics. In this context, theoretical and observational studies of quasi periodic oscillations (QPOs) detected in (micro)quasars prove useful for investigating their central object. We explore the massive particle motion around black hole in Schwarzschild-like spacetime and using the equation of motion, the dependence of innermost stable circular orbits (ISCO) on the parameter l was obtained. The frequencies of radial and vertical oscillations of particles around stable circular orbits have been studied and used to explain the mechanism of quasi periodic oscillations in the relativistic precession (RP) model. Using observational data from GRO J1655-40 and XTE J1550-564, we have determined a constraint on the parameter l in Schwarzschild-like spacetime through Monte-Carlo-Markovian-Chain (MCMC) analyses.

The effect of swirling injector geometry on the flow dynamics of precessing vortex core

Precessing vortex core (PVC), a typical vortex structure, universally exists in swirling flow fields and originates from the Kelvin–Helmholtz instability of the swirling shear layer. In the combustion chamber, pressure pulsations from PVC might resonantly couple with acoustic modes, leading to thermoacoustic oscillations which is a significant challenge in the development of rocket engines and gas turbines. The present work focuses on the time-averaged and unsteady flow characteristics of different swirling flow fields in the premixed swirling combustor. The PVC observed in the swirlers adopted presents a single helical structure and the PVC frequency is proportional to the incoming flow velocity. Moreover, another low-frequency helical mode located in the downstream region of the flow fields has been identified and categorized as PVC type II to be distinguished with common PVC mode in this study, which originates from the precession of the central recirculation zone. The experimental results further emphasize the effects of the swirling injector geometry on PVC and the low-frequency helical modes. This indicates that as the diameter of the bluff body or the divergence cup angle increases, the energy of the PVC mode decreases while that of the helical mode increases significantly. In addition, the PVC mode would be gradually suppressed as the tail size of the bluff body increases. These results, especially the coexistence of the PVC mode and the low-frequency helical mode, are rarely reported in previous studies, and they may provide an effective guidance for investigating the mechanism and control of thermoacoustic oscillations.

Multi-timescale neural dynamics for multisensory integration.

Carrying out any everyday task, be it driving in traffic, conversing with friends or playing basketball, requires rapid selection, integration and segregation of stimuli from different sensory modalities. At present, even the most advanced artificial intelligence-based systems are unable to replicate the multisensory processes that the human brain routinely performs, but how neural circuits in the brain carry out these processes is still not well understood. In this Perspective, we discuss recent findings that shed fresh light on the oscillatory neural mechanisms thatmediate multisensory integration (MI), including power modulations, phase resetting, phase-amplitude coupling and dynamic functional connectivity. We then consider studies that also suggest multi-timescale dynamics in intrinsic ongoing neural activity and during stimulus-driven bottom-up and cognitive top-down neural network processing in the context of MI. We propose a new concept of MI that emphasizes the critical role of neural dynamics at multiple timescales within and across brain networks, enabling the simultaneous integration, segregation, hierarchical structuring and selection of information in different time windows. To highlight predictions from our multi-timescale concept of MI, real-world scenarios in which multi-timescale processes may coordinate MI in a flexible and adaptive manner are considered.

Effect of self-excited oscillation of three-dimensional rectangular shock-induced thrust vector nozzle jet

The fluidic thrust vector nozzles including the shock-induced thrust vector nozzles stand out from traditional mechanical thrust vector nozzles used in aeronautics and astronautics due to their simplicity and potential for higher efficiency. However, a significant challenge in the transition from theoretical studies to practical applications is the phenomenon of self-excited oscillation of the nozzle jet, particularly in ramjet and scramjet engines. This oscillation can notably impact the jet control stability, which is critical for the operational reliability, accuracy, and safety of these engines. To investigate the effects of self-excited oscillation of the jet in the three-dimensional rectangular shock-induced thrust vector nozzles, a large eddy simulation approach has been utilized to examine various nozzle pressure ratios and secondary pressure ratios. The simulation data are in good agreement with the experimental data of National Aeronautics and Space Administration Langley Research Center, lending credibility to the simulation results. The research sheds light on the formation and evolution of self-excited oscillation. It does so by examining the interactions between shock waves and boundary layers, as well as bubble dynamics, offering a comprehensive view of the oscillation mechanism in a three-dimensional context. The results demonstrate that the self-excited oscillation of jet mainly belongs to low-frequency oscillation. With the increase in nozzle pressure ratio, the self-excited oscillation of the jet is suppressed because the shock system is pushed out of the shock-induced thrust vector nozzle exit. The variation of secondary pressure ratio only affects the amplitude of jet self-excited oscillation and does not transform the motion pattern.

Effects of doors on an open cavity flow at Mach number 6

Open cavities with different door-opening angles are investigated using high-speed schlieren visualization and dynamic pressure measurements in hypersonic flow with a freestream Mach number of 6. With the help of numerical simulations, the shear layer deformation and pressure increase in the cavities due to the impingement of the door-leading-edge shocks are identified via comparison with those in the cavity without doors. As the door-opening angle decreases from 90°, the shear layer above the forepart of the cavity is gradually raised by the high pressure in the cavity. When the door-opening angle decreases to 30°and 15°, the boundary layer on the upstream flat plate of the cavity separates, and separation shock is observed. The doors enhance the instability of the cavity flow and increase the pressure fluctuations in the cavities. A new oscillation pattern, referred to as coupled oscillation, is observed in the cases with separation on the upstream flat plate, in which the separation shock oscillates at the same dominant frequency as the flow inside the cavity. Compared with the cavity without doors, this coupled oscillation causes a lower oscillation frequency and a larger overall sound pressure level. Cross-correlation analyses between pressure signals indicate that the disturbances generated at the trailing edge of the cavity can propagate to the separation on the upstream flat plate and cause coupled oscillation of the separation shock. The fundamental frequencies of the coupled oscillations can be normalized to approximately the same Strouhal number as that of the cavity without doors. These findings support that the oscillation mechanisms of hypersonic cavities without and with doors are primarily dominated by acoustic feedback.

A topological mechanism for robust and efficient global oscillations in biological networks

Long and stable timescales are often observed in complex biochemical networks, such as in emergent oscillations. How these robust dynamics persist remains unclear, given the many stochastic reactions and shorter time scales demonstrated by underlying components. We propose a topological model that produces long oscillations around the network boundary, reducing the system dynamics to a lower-dimensional current in a robust manner. Using this to model KaiC, which regulates the circadian rhythm in cyanobacteria, we compare the coherence of oscillations to that in other KaiC models. Our topological model localizes currents on the system edge, with an efficient regime of simultaneously increased precision and decreased cost. Further, we introduce a new predictor of coherence from the analysis of spectral gaps, and show that our model saturates a global thermodynamic bound. Our work presents a new mechanism and parsimonious description for robust emergent oscillations in complex biological networks.

A mechanism for slow rhythms in coordinated pancreatic islet activity

Insulin levels in the blood oscillate with a variety of periods, including rapid (5–10 min), ultradian (50–120 min), and circadian (24 h). Oscillations of insulin are beneficial for lowering blood glucose and disrupted rhythms are found in people with type 2 diabetes and their close relatives. These in vivo secretion dynamics imply that the oscillatory activity of individual islets of Langerhans are synchronized, although the mechanism for this is not known. One mechanism by which islets may synchronize is negative feedback of insulin on whole-body glucose levels. In previous work, we demonstrated that a negative feedback loop with a small time delay, to account for the time required for islets to be exposed to a new glucose concentration in vivo, results in small 3–6 islet populations synchronizing to produce fast closed-loop oscillations. However, these same islet populations could also produce slow closed-loop oscillations with periods longer than the natural islet oscillation periods. Here, we investigate the origin of the slow oscillations and the bistability with the fast oscillations using larger islet populations (20–50 islets). In contrast to what was observed earlier, larger islet populations mainly synchronize to longer-period oscillations that are approximately twice the delay time used in the feedback loop. A mean-field model was also used as a proxy for a large islet population to uncover the underlying mechanism for the slow rhythm. The heterogeneous intrinsic oscillation periods of the islets interferes with this rhythm mechanism when islet populations are small, and is similar to adding noise to the mean-field model. Thus, the effect of a time delay in the glucose feedback mechanism is similar to other examples of time-delayed systems in biology and may be a viable mechanism for ultradian oscillations.

Beta and theta oscillations track effort and previous reward in the human basal ganglia and prefrontal cortex during decision making

Choosing whether to exert effort to obtain rewards is fundamental to human motivated behavior. However, the neural dynamics underlying the evaluation of reward and effort in humans is poorly understood. Here, we report an exploratory investigation into this with chronic intracranial recordings from the prefrontal cortex (PFC) and basal ganglia (BG; subthalamic nuclei and globus pallidus) in people with Parkinson's disease performing a decision-making task with offers that varied in levels of reward and physical effort required. This revealed dissociable neural signatures of reward and effort, with BG beta (12 to 20 Hz) oscillations tracking effort on a single-trial basis and PFC theta (4 to 7 Hz) signaling previous trial reward, with no effects of net subjective value. Stimulation of PFC increased overall acceptance of offers and sensitivity to reward while decreasing the impact of effort on choices. This work uncovers oscillatory mechanisms that guide fundamental decisions to exert effort for reward across BG and PFC, supports a causal role of PFC for such choices, and seeds hypotheses for future studies.

Penetration state recognition for tungsten inert gas welding via an alternating cusp-shaped magnetic field-assisted molten pool-oscillation

To ensure optimal penetration in direct current (DC) argon tungsten arc welding, a method was proposed to recognize the molten pool penetration during molten pool oscillation under alternating cusp-shaped magnetic field (ACSMF). An arc discharge model is established to analyze the oscillation mechanism of the molten pool under ACSMF. The periodic variation of arc shape induces the periodic oscillation to establish a mapping relationship with the fluctuation of arc voltage waveform. At the excitation current of 3 A and frequency of 10 Hz, the arc voltage waveform undergoes abrupt changes at 1.26 V and 1.37 V, corresponding to the critical and the full penetration states, respectively. The errors in identifying moving penetration are 0.8 % and 0.7 %, with a full penetration weld, demonstrating a penetration rate of 100 %, indicating effective penetration recognition. Arc sensing technology for tungsten inert gas (TIG) welding under ACSMF is poised for gradual application in single-side welding and double-side forming of medium and thick plates. It is anticipated to evolve towards real-time identification technology for molten pool penetration monitoring.

A quasi-harmonic voltage compensation control of current-controlled battery energy storage systems for suppressing mid-frequency oscillations and harmonics

In grid-connected mode, current-controlled battery energy storage systems (BESS) face the issues of harmonic caused by nonlinear loads and interactive instability under weak grids. Firstly, the mechanisms of mid-frequency oscillations (MFO) and mid-frequency harmonics (MFH) are revealed by the impedance network theory and the circuit principle. Because harmonic currents of nonlinear loads tend to interact with the mid-frequency impedance of BESS to produce harmonic voltage drops, the grid is susceptible to MFH. Worse still, the grid-connected system based on BESS tends to exhibit negative damping characteristics in the mid-frequency band as the grid stiffness decreases, which triggers MFO. Aiming at the above problems, this paper proposes a quasi-harmonic voltage compensation control strategy without any harmonic extractor and provides a detailed parameter design rule. The proposed control strategy can effectively suppress MFO by enhancing the damping between BESS and weak grids. Meanwhile, the disturbance resistance of BESS to nonlinear loads is significantly improved because the impedance magnitude of BESS in the mid-frequency band is obviously reduced. Finally, the effectiveness of the proposed strategy is verified by simulation and experiment.

Neurophysiological Oscillatory Mechanisms Underlying the Effect of Mirror Visual Feedback-Induced Illusion of Hand Movements on Nociception and Cortical Activation.

Mirror Visual Feedback (MVF)-induced illusion of hand movements produces beneficial effects in patients with chronic pain. However, neurophysiological mechanisms underlying these effects are poorly known. In this preliminary study, we test the novel hypothesis that such an MVF-induced movement illusion may exert its effects by changing the activity in midline cortical areas associated with pain processing. Electrical stimuli with individually fixed intensity were applied to the left hand of healthy adults to produce painful and non-painful sensations during unilateral right-hand movements with such an MVF illusion and right and bilateral hand movements without MVF. During these events, electroencephalographic (EEG) activity was recorded from 64 scalp electrodes. Event-related desynchronization (ERD) of EEG alpha rhythms (8-12 Hz) indexed the neurophysiological oscillatory mechanisms inducing cortical activation. Compared to the painful sensations, the non-painful sensations were specifically characterized by (1) lower alpha ERD estimated in the cortical midline, angular gyrus, and lateral parietal regions during the experimental condition with MVF and (2) higher alpha ERD estimated in the lateral prefrontal and parietal regions during the control conditions without MVF. These preliminary results suggest that the MVF-induced movement illusion may affect nociception and neurophysiological oscillatory mechanisms, reducing the activation in cortical limbic and default mode regions.

Mechanism of vortex oscillation around a hemisphere–cylinder body

Previous studies have shown that low-frequency vortex oscillations occur around a hemisphere–cylinder body at different angles of attack, but the underlying mechanism is still unclear. In this study, we examine the origin of the vortex oscillation using numerical simulations and global linear stability analysis. The vortex oscillation is reproduced using numerical simulations, and the oscillatory modes are computed through dynamic mode decomposition (DMD). We obtain the base flow through a selective frequency damping method, which exhibits a pair of steady leeward vortices over the body. The four unstable modes are computed using a modified Arnoldi iteration. The antisymmetric mode with a Strouhal number of 0.105 is discovered to be responsible for the alternate oscillation of the vortex pair, and the mode with a Strouhal number of 0.220 corresponds to the in-phase vortex oscillation. Their frequencies have good agreement with the modes of DMD. The other two unstable modes with higher frequencies, one antisymmetric and one symmetric, are harmonic frequencies of the above two modes. The study conclusively verifies that the vortex oscillation over a hemisphere–cylinder body originates from a global flow instability.

Study on the keyhole oscillation mechanism of laser welding based on electro-mechano-acoustical analogy theory

The acoustic emission phenomena generated by the laser welding process are closely related to the keyhole's forced oscillation and the dynamic pressure balance at the keyhole bottom. In this study, we constructed the mechanical equation of keyhole bottom pressure balance and used the “electro-mechano-acoustical” analogy theory to convert it into an acoustic equation, the acoustic resonant frequency as well as the magnification amplification factor of the keyhole oscillation in different penetration states were derived. A monitoring platform combining acoustic sensor and high-speed photography was established to obtain acoustic emission signals and lateral keyhole images. The acoustic and image features extracted from the monitoring data validated the derived formulas. Additionally, the acoustic phenomena of different frequency-energy components were explained according to the derived formulas in turn. The results demonstrated that the derived theory is compatible with experimental comparisons, showcasing its great interpretability and applicability. The introduced analogy theory effectively establishes a bridge between keyhole forced pressure and acoustic emission vibration, aiding in the understanding of keyhole oscillation characteristics. A novel model of keyhole oscillation was proposed and verified using process monitoring data in this study, which provides a new approach for the investigation of keyhole dynamic.

Mechanisms of middle-frequency oscillations in an offshore wind farm induced by the converter current inner loops

Abstract Middle-frequency oscillation instability is one of the important issues that threats the safe integration of offshore wind farms. However, analytical explanations to key affecting factors and laws for the middle-frequency oscillation are not clear yet, which makes it difficult to avoid this kind of oscillation effectively. Therefore, in this paper, mechanism for the middle-frequency oscillation induced by the dynamics of converter current inner loops is studied in-depth. First, small-signal transfer function model for a general offshore wind farm integrated via AC submarine cable is established. Then impact of the AC submarine cable and control parameters of the converter current inner loops on oscillation stability of the wind farm is analysed via eigenvalue analysis. It is found that with the length of AC submarine cable increases, frequency for the open-loop pole of the AC grid decreases. When open-loop pole of the AC grid gets close that of the PMSGs on complex plane, the near strong mode resonance can cause middle-frequency oscillation of the system. Finally, effectiveness of the conclusions drawn is evaluated through nonlinear simulations.

Overmodulation causes a variation in the number of jumps in the Duffing system

Amplitude modulation, involving normal and overmodulation scenarios, is crucial for information transmission. However, the complex dynamics of how modulation phase shift affects relaxation oscillations, especially under overmodulation conditions, has not been fully elucidated. Thus, this paper aims to explore the dynamical mechanism of relaxation oscillations affected by modulation phase shift under overmodulation conditions. The result shows that minor phase adjustments in low overmodulation phases can change the time series of the signal. Notably, at critical modulation phase thresholds, the number of transitions in each period of relaxation oscillations increases, and this phenomenon can be observed across a range of parameter values. However, further increase in phase will lead to the decrease in the number of transitions in relaxation oscillations, which demonstrates a clear correlation between phase adjustments and fold bifurcations affecting oscillation patterns. Based on the tri-parametric and bi-parametric bifurcation analysis, we explore the effect of overmodulation index on the number of transitions, and find that higher indices induce complex variations in it. These findings highlight the intricate interplay between modulation phase and modulation index in determining relaxation oscillation patterns, which are crucial for understanding amplitude modulation diversity and can serve as a reference for future research on other modulation scenarios.

Dampening long-period doppler shift oscillations using deep machine learning techniques in the solar network and internetwork

This study aimed to explore the Doppler shift at different wavelengths in the IRIS solar spectrum and analyze the evolution and statistical properties of longitudinal oscillations with damping in chromosphere and transition region bright points (BPs). Understanding the damping mechanism and statistical properties of oscillations in BPs is crucial for gaining insights into the dynamics and energy transport within the solar atmosphere.This study explores the Doppler shift at different wavelengths in the solar spectrum of the Interface Region Imaging Spectrograph (IRIS) and implements a comprehensive consideration of Doppler velocity oscillations in the IRIS channels. This comprehensive consideration reveals a propagating periodic perturbation in a large number of chromosphere and transition region (TR) bright points (BPs). To the best of our knowledge, this is the first investigation of the longitudinal oscillations with damping in BPs using comprehensive consideration of the Doppler velocity at various wavelengths. The phenomena of attenuation in the red and blue Doppler shifts of the solar wavelength range were seen several times during the experiments. We utilized deep learning techniques to examine the statistical properties of damping in network and internetwork BPs, as well as active, quiet areas, and coronal hole areas. Our results revealed varying damping rates across different regions, with 80% of network BPs exhibiting damping in quiet areas and 72% in coronal hole areas. In active areas, the figure approached 33%. For internetwork BPs, the values were 65%, 54%, and 63% for quiet areas, coronal hole areas, and active regions, respectively. The damping rate in active regions is twice as high at Internetwork’s BPs. The damping components in this study were computed, and the findings show that the damping at all points is underdamped. The observed damping process suggests the propagation and leaking of energetic waves out of TR bright points, potentially contributing to the energy transport from the bright magnetic footpoints to the upper chromosphere, transition region, and corona.

Propagation and Maintenance of the Quasi-Biweekly Oscillation over the Western North Pacific in Boreal Winter

Abstract This study investigates the propagation and maintenance mechanisms of the dominant intraseasonal oscillation over the western North Pacific in boreal winter, the quasi-biweekly oscillation (QBWO). The wintertime QBWO over the western North Pacific is characterized by the westward-northwestward movement from the tropical western Pacific to the western North Pacific and resembles the n = 1 equatorial Rossby wave. Its westward migration is primarily driven by the seasonal-mean zonal winds that advect vorticity anomalies in the lower–middle troposphere and moisture anomalies in the lower troposphere. Its northward movement is preconditioned by the vorticity dynamics of the beta effect, the low-level vertical moisture variation, and the local air–sea interaction. The latter involves the atmospheric forcing on the underlying ocean by changing the surface heat flux fluctuations and the sea surface temperature feedback on the low-level atmospheric instability. Its maintenance is primarily through atmospheric external energy sources from diabatic heating, which first generates eddy available potential energy and then converts it to eddy kinetic energy. Significance Statement Atmospheric quasi-biweekly oscillation (QBWO) is an important climate phenomenon over the western North Pacific in boreal winter. It can trigger significant influences both in the tropical and extratropical regions. Given the importance, it is necessary to investigate the propagation and maintenance mechanisms of the QBWO over the western North Pacific in boreal winter. The QBWO usually propagates northwestward from the tropical western Pacific to the western North Pacific. The westward propagation is driven by the seasonal mean zonal winds that advect vorticity anomalies in the lower–middle troposphere and moisture anomalies in the lower troposphere. The northward propagation is caused jointly by several processes, including the atmospheric forcing on the ocean via the surface heat flux fluctuations, the feedback of ocean on the low-level atmospheric instability, the vorticity dynamics of the beta effect, and the vertical variation of low-level moisture. In addition, converting from eddy available potential energy to eddy kinetic energy is essential in maintaining the QBWO’s development. The disclosure of these crucial processes deepens the dynamics of the QBWO over the western North Pacific in boreal winter.