Oscillation Phenomenon Research Articles

Piecewise rank‐one approximation of vector fields with measure derivatives

AbstractThis work addresses the question of density of piecewise constant (resp. rigid) functions in the space of vector‐valued functions with bounded variation (resp. deformation) with respect to the strict convergence. Such an approximation property cannot hold when considering the usual total variation in the space of measures associated to the standard Frobenius norm in the space of matrices. It turns out that oscillation and concentration phenomena interact in such a way that the Frobenius norm has to be homogenized into a (resp. symmetric) Schatten‐1 norm that coincides with the Euclidean norm on rank‐one (resp. symmetric) matrices. By means of explicit constructions consisting of the superposition of sequential laminates, the validity of an optimal approximation property is established at the expense of endowing the space of measures with a total variation associated with the homogenized norm in the space of matrices.

Micro-discharge in GIS/GIL induced by scratch defect on the surface of insulation pull rods

Abstract The insulation pull rod (IPR) plays a crucial role in gas insulated switchgear (GIS) and gas insulated transmission line (GIL), which must withstand high impulse voltages to cope with frequent switching operations under working conditions. During the manufacturing process, micro-defects may occur on the surface of insulation pull rods, which may cause micro-discharge or even breakdown. It is difficult to carry out experiments to study the microscopic characteristics of the micro-discharge induced by micro-defects on the surface of the insulation pull rod. In order to study the relationship between the micro-discharge and the micro-defects. Firstly, the characterization of the scratch defect based on Micro-CT (micro computed tomography) and white light interference are presented. Secondly, a coaxial DC SF6 corona discharge model is established to obtain the curve related to the density of charged particles in SF6 and the electric field. Finally, we combined the reconstruction of the defect model in the first step with the curve obtained in the second step to simulate the micro discharge around the scratch defect, and provided the micro-discharge in pure SF6 caused by the scratch defect. The phenomenon of micro-discharge induced by the scratch of insulation pull rod is explained. The oscillation phenomenon of internal micro-discharge in scratch defects under high electric field background was discovered.

Recovering detailed shape of phase response curves from the higher harmonics of singularity response

The phase response curve (PRC) is crucial in elucidating the synchronization characteristics of nonlinear oscillators under the influence of external forces. Oscillatory phenomena in nature often consist of an ensemble of multiple oscillators, which complicates the measurement of individual oscillator PRCs. A prior study introduced a technique for estimating individual PRCs from the singularity response (SR)—the reaction of an ensemble of desynchronized oscillators without couplings. However, the utility of this method is somewhat limited, as it considers only the first harmonic of the PRC. The current study demonstrates that the detailed contour of the PRC can be ascertained by employing the higher harmonics of the SR. Published by the American Physical Society 2024

Angular Bloch oscillations and their applications

Abstract To advance inertial navigation, we present the scheme for a compact quantum sensor which is based on the quantum phenomenon of the angular Bloch oscillations and measuring exclusively the angular acceleration of slow external rotation. We study the dynamics of ultra-cold atoms confined in a toroidal trap with a ring-lattice along the azimuth angle, realized with the superposition of two copropagating Laguerre–Gaussian beams. In the presence of external rotation with a small angular acceleration, or a prescribed linear chirp between the two beams, the measured angular momentum of the trapped atoms exhibits a specific periodic behaviour in time, which we call as the angular Bloch oscillations. This quantum phenomenon is shown to be a key element of fruitful applications for (i) an efficient transfer of quantized angular momentum from the light to the atoms by controlling the chirp, and (ii) the direct determination of the angular acceleration of external rotation by measuring the Bloch period.

Multi-parameter study of Aculeo lagoon (Chile)

Abstract. The Aculeo Lagoon, located in the Santiago Metropolitan Region, was one of the main water reservoirs of central Chile. It has experienced a decline in water levels since 2009 and completely dried up in 2018. It was unable to recover until 2023 due to sustained demand for water resources associated with population growth, changes in land use, and years of drought. The objective of this study is to estimate how anthropogenic factors and climate change have influenced the drying of the lagoon. Parameters considered include population growth, housing increases, changes in land use within the Aculeo basin, local temperature and precipitation variations, and global El Niño Southern Oscillation (ENSO) phenomena. Input data included multi-year Landsat images, census data, and a systematic review of the literature on the study area. Furthermore, applying geospatial data analysis techniques such as satellite image processing, supervised classification of land cover/use, and calculation of spectral indices, it is possible to conclude that the Aculeo Lagoon dried out due to a significant precipitation deficit over nearly a decade, exacerbated by the overexploitation of land for agricultural activities.

The Oscillation in Evolution of Changing-look Blazar OQ 334

We investigate the evolution of a changing-look blazar (CLB) on long timescales and expect to trace the state change of a CLB. Three morphological types, including a flat spectrum radio quasar (FSRQ) state, transition state, and BL Lacertae (BL Lac) state, are classified according to the criteria proposed by analyzing the relationship between the equivalent width of the emission line and the γ-ray photon spectral index Γ γ . The multiwavelength light curves and spectral energy distributions corresponding to different epochs are obtained. The efforts found that Γ γ satisfy the relationships with Γ γ ≳ 2.2 for the FSRQ state, 2.0 < Γ γ < 2.2 for the transition state, and Γ γ ≲ 2.0 for the BL Lac state. We apply the criteria to the photon spectrum evolution of CLB OQ 334 during MJD 58678–60387. The evolution is subdivided into five FSRQ states, nine transition states, and four BL Lac states. Moreover, we use the model spectra parameters of each state epoch to test the reliability of subdivided morphological types. The result shows that: (1) the accretion rate parameter is consistent with our earlier research; and (2) there is an increasing trend in the epochs of the BL Lac states, even if there is not an obvious decreasing trend in epochs of the FSRQ states. We issue strong evidence that a CLB is an especial epoch in the evolution of blazars that could be obtained from the oscillation phenomenon in the CLB evolution.

Proximity‐Induced Unconventional Superconducting Quantum Oscillation in WTe2/NbSe2 Heterostructures

AbstractThe electron pairing mechanisms in topological superconductors are pivotal for understanding the emergent topology‐related quantum states and play key roles in condensed matter physics. Theoretically, the quantization of magnetic flux in nano‐hole arrays within topological superconductors can be studied for understanding the Bogoliubov quasiparticles via the Little–Parks oscillation. However, experimental evidence of the quasiparticle states in such structures of topological superconductors remains elusive. Here, the unconventional superconducting quantum oscillation phenomena of the proximity‐induced 2D topological superconductivity (TSC) is demonstrated in a nano‐hole array of WTe2/NbSe2 heterojunction. The unconventional oscillation period in WTe2 is substantially smaller than that in the s‐wave superconductor NbSe2 (corresponds to Cooper pairs), implying the probable presence of novel quasiparticle states associated with TSC. Interestingly, such phenomena of multi‐charge flux quanta might be related to the multi‐particle bound states of TSC. These observations provide a new approach for exploring the unconventional superconducting quantum states in topological superconductors.

Adversarial sample attack method based on loss smoothing

Deep neural networks (DNNs) are vulnerable to adversarial examples.Although the existing momentum-based adversarial example generation method can achieve a close 100%white-box attack success rate, it is still not ideal when attacking other models, and the black- box attack success rate is low. To address this, an adversarial example attack method based on loss smoothing is proposed to improve the transferability of adversarial examples. In the iterative process of calculating the gradient at each step, the current gradient is not used directly, but the local average gradient is used to accumulate momentum, so as to suppress the local oscillation phenomenon on the loss function surface, thereby stabilizing the update direction and escaping the local extreme point. A large number of experimental results on the ImageNet dataset show that compared with the existing momentum-based method, the average black-box attack success rate of the proposed method in single model attack experiments is improved by 38.07%and 27.77%, and the average black-box attack success rate in integrated model attack experiments is improved by and.Rising32.50%and28.63%

A comparative study on combustion characteristics of PPC and RCCI combustion modes in an optical engine with renewable fuels

Application of renewable fuels in compression ignition engines provides one solution to reduce greenhouse gas emission in heavy-duty transportation sector. However, it is a big challenge to find a single alternative fuel to replace the traditional diesel under full load conditions. In this study, the combustion characteristics of two kinds of renewable fuels with different reactivity, namely methanol and hydrogenated catalytic biodiesel (HCB), were investigated in an optical engine under partially premixed combustion (PPC) and reactivity-controlled compression ignition (RCCI) modes, respectively. A two-color (2C) method was employed to quantify the in-flame soot formation. Meanwhile, the flame oscillation phenomenon was captured and evaluated. The results indicate that the RCCI mode exhibits reduced peak cylinder pressure, resulting in smoother and more stable combustion compared to that of the PPC mode. Moreover, the more uniform fuel distribution in the RCCI mode further decreases soot formation from the blended fuel. Additionally, with increasing methanol proportion, the rise in oxygen content within the blended fuel significantly reduces soot formation. The flame oscillations are primarily related to the rate of cylinder pressure variation. Under PPC mode, multi-point autoignition causes a rapid increase in cylinder pressure, leading to larger oscillation amplitudes (M15: 0.268, M25: 0.772). Conversely, under the RCCI mode, the heat release process is more stable, resulting in weaken oscillations (M15: 0.161, M25: 0.064) compared to that of PPC.

Spatio-temporal tendencies of urban land surface temperature on the Andean piedmont under climate change: A case study of Metropolitan Lima, Peru (1986–2024)

The increase in land surface temperature (LST) in megacities has contributed to global warming due to poor urban planning and high population concentration. This study analyzes spatio-temporal trend patterns of LST in the city of Metropolitan Lima, Peru, during the summers of 1986–2024. The Mann-Kendall tests and the Theil-Sen Slope method were used to analyze the spatio-temporal trends of LST, relating R2 and the Mann-Kendall p-value of the annual average LST in each district. A hierarchical cluster analysis was performed to group districts according to their average yearly LST. Urban heat islands were identified based on basin configuration and distance to the sea at 1 km intervals. The results reveal a significant increase in LST (p < 0.001) related to El Niño-Southern Oscillation phenomena, in addition to urban growth and land cover change. The significant positive trend of LST showed a heterogeneous distribution in all districts. The districts were grouped into three clusters with statistically significant differences in LST (p < 0.001), spatially configured radially from the sea to the foot of the Andes, up to 1179 m a.s.l. Urban heat islands did not correlate with significant positive trends in basins. Still, they showed a considerable increase in LST in areas of high economic activity, including dense commercial, industrial, and residential areas. This information is crucial to managing climate change adaptation and mitigation measures and contributing to sustainable urban planning focused on the population's well-being.

Numerical Investigation of the transient process of direct collision between two droplets

Abstract The use of spray systems finds widespread applications in fields such as national defense and industry. These systems are employed for spray cooling, spray forming, spray coating, and inkjet painting. Within spray systems, atomized droplets play a crucial role, either impacting surfaces or colliding with each other. Understanding the impact process of these droplets is essential for optimizing spray system performance. In this study, computational simulations focus on direct collision of two droplets. Numerical methods are employed, including structured single-block and staggered grid systems for discretizing the spatial domain. The finite volume method iteratively solves the governing equations of mass and momentum. The coupling between velocities and pressure is handled using SIMPLER methods. During droplet collision, phenomena like deformation, breakup, and merging occur at the boundary. To accurately capture these movements, a level set function approach is utilized. Numerical simulations of a water droplet impinging on a flat surface are compared with experimental results for verification. When two droplets collide head-on with the same Reynolds number and Weber number, they merge into a single oscillating droplet at a fixed position. Increasing the Reynolds number of the two colliding droplets intensifies the oscillation phenomenon in the merged droplet. Even when two droplets collide at an angle or have different sizes, they still merge into a single oscillating droplet. The merged droplet moves in a direction where the original two droplets’ momentum cannot be fully offset.

Pressure oscillation and suppression method of large-aspect-ratio solid rocket motors

A two-dimensional large eddy simulation numerical model is proposed to study the transient vortex flow and pressure oscillation of a large-aspect-ratio solid rocket motor. The numerical model is validated through experimental data, finite element analysis and cumulative error analysis. The numerical simulations are executed to obtain the characteristics of the vortex-acoustic and pressure oscillation. The results show that the burning surface regression decreases the motor aspect ratio, increasing the corresponding natural frequency from 260 Hz to 293 Hz. The pressure oscillation phenomenon is formed due to the vortex-acoustic coupling. Decreasing the corner vortex shedding intensity shows negative effects on the dimensionless amplitude of the pressure oscillation. The head cavity without the injection can decrease the vortex-acoustic coupling level at the acoustic pressure antinode. The modified motor with head cavity can obtain a lower dimensionless oscillating pressure amplitude 0.00149 in comparison with 0.00895 of the original motor. The aspect ratio and volume of the head cavity without the injection have great effects on the pressure oscillation suppression, particularly at the low aspect ratio or large volume. The reason is that the mass in the region around the acoustic pressure antinode is extracted centrally, reducing the energy contribution to the acoustic system. With the volume increasing, the acoustic energy capacity increases.

Depressurization strategy of China's advanced pressurized water reactor under typical accidents

Advanced pressurized water reactor was simulated using system analysis program. The depressurization strategy for the automatic depressurization system (ADS) under typical ADS-triggering accidents (2-inch/10-inch cold leg break, inadvertent opening of the ADS, DVI pipeline break) was investigated. The results show that after these accidents, ADS can ensure the operation of the three types of safe injection tanks, with high redundancy and inherent safety. The multi-stage design of ADS aids the sparger in maintaining a stable critical jet state. In 10-inch cold leg break accident, the sparger cannot reached the critical jet state under all of pipeline fault conditions. From a system point of view, adjusting the pipeline design of ADS is necessary to avoid long-term condensation oscillation of the sparger in some accidents. From a design perspective of the local equipment, the nozzle structure of the sparger should attenuate the dynamic load caused by the condensation oscillation and back-attack phenomenon.

Future increase in extreme El Niño supported by past glacial changes

El Niño events, the warm phase of the El Niño–Southern Oscillation (ENSO) phenomenon, amplify climate variability throughout the world1. Uncertain climate model predictions limit our ability to assess whether these climatic events could become more extreme under anthropogenic greenhouse warming2. Palaeoclimate records provide estimates of past changes, but it is unclear if they can constrain mechanisms underlying future predictions3–5. Here we uncover a mechanism using numerical simulations that drives consistent changes in response to past and future forcings, allowing model validation against palaeoclimate data. The simulated mechanism is consistent with the dynamics of observed extreme El Niño events, which develop when western Pacific warm pool waters expand rapidly eastwards because of strongly coupled ocean currents and winds6,7. These coupled interactions weaken under glacial conditions because of a deeper mixed layer driven by a stronger Walker circulation. The resulting decrease in ENSO variability and extreme El Niño occurrence is supported by a series of tropical Pacific palaeoceanographic records showing reduced glacial temperature variability within key ENSO-sensitive oceanic regions, including new data from the central equatorial Pacific. The model–data agreement on past variability, together with the consistent mechanism across climatic states, supports the prediction of a shallower mixed layer and weaker Walker circulation driving more frequent extreme El Niño genesis under greenhouse warming.

Sequential Convex Programming for Reentry Trajectory Optimization Utilizing Modified hp-Adaptive Mesh Refinement and Variable Quadratic Penalty

Due to the strong nonlinearity in the reentry trajectory planning problem for reusable launch vehicles (RLVs), the scale of the problem after high-precision discretization can become significantly large, and the non-convex path constraints are prone to exceed limits. Meanwhile, the objective function oscillation phenomenon may occur due to successive convexification, which results in poor convergence. To address these issues, a novel sequential convex programming (SCP) method utilizing modified hp-adaptive mesh refinement and variable quadratic penalty is proposed in this paper. Firstly, a local mesh refinement algorithm based on constraint violation is proposed. Additional mesh intervals and mesh points are added in the vicinity of the constraint violation points, which improves the satisfaction of non-convex path constraints. Secondly, a sliding window-based mesh reduction algorithm is designed and introduced into the hp-adaptive pseudospectral (PS) method. Unnecessary mesh intervals are merged to reduce the scale of the problem. Thirdly, a variable quadratic penalty-based SCP method is proposed. The quadratic penalty term related to the iteration direction and the weight coefficient updating strategy is designed to eliminate the oscillation. Numerical simulation results show that the proposed method can strictly satisfy path constraints while the computational efficiency and convergence of SCP are improved.

Derivation of analytic formulae for several resonance frequencies of the SparkJet actuator

Building on previous research that emphasized the importance of focusing on resonance frequencies for a fundamental understanding of the thrust and complex internal flow phenomena of the SparkJet actuators, this study theoretically derives analytic formulae for several important resonance frequencies. The research addresses the typical configuration of the SparkJet actuators, which consists of a cylindrical cavity and orifice connected by a conical converging nozzle. While the resonance frequencies of the SparkJet actuators can be obtained by solving the eigenvalue problem presented in previous studies, this eigenvalue problem, despite being a typical boundary value problem in the form of an elliptic partial differential equation, is challenging to solve using conventional numerical methods such as iterative methods, because the eigenvalue is included as an unknown in the operator. Consequently, Boundary Element Method (BEM) or methods using the Green functions have been proposed to obtain numerical solutions, but these still require handling large matrix data, resulting in significant computational costs and memory consumption. To overcome this, the current study first simplifies the eigenvalue problem using the conformal mapping presented in previous research. Then, referencing prior studies that claim that three types of resonance frequencies (Helmholtz resonance frequency and resonance frequencies representing streamwise or radial directional oscillation) significantly affect the thrust of the SparkJet actuators, characteristics of these frequencies are mathematically defined. Using these mathematical characteristics, the study derives the asymptotic and approximate solutions of the simplified eigenvalue problem, from which the resonance frequencies are obtained. The analytic formulae proposed in this study theoretically explain the already known geometric tendencies of the resonance frequencies and reveal new geometric factors influencing resonance frequencies, which were previously unknown. This approach is expected to facilitate obtaining several important resonance frequencies of the SparkJet actuator more promptly and accurately and to provide a deeper understanding of the nature of the complex oscillation phenomena inside the actuator.

High-Order Models for Gas Transport in Multiscale Coal Rock.

The coal reservoirs exhibit great heterogeneity and strong anisotropy in multiscale pore/fracture structures. Developing highly accurate multiscale models for real-time prediction of microgaseous flow in complicated porous media with pronounced contrast in transport coefficients is crucial but not yet available, which is time-consuming, expensive, and even computationally impossible. In this study, a multiscale approximate solution of the gas flow and pressure field is derived in this paper for predicting coalbed methane (CBM) transport in macro-microscopic two-scale porous media of typical coal rocks in Guizhou Province, China, and the detailed finite element algorithm is established. The multiscale approximate solutions are constructed from the first- and second-order auxiliary cell functions and low- or high-order derivatives of the homogenized pressure field, which can accurately approximate the solution of the original problem to a certain extent. The numerical accuracy and validity of the multiscale approximate solutions under various working conditions are analyzed in detail by comparing and verifying the results with the direct numerical simulation results of fine-mesh high-precision finite elements. The results show that the homogenized approximate solution can only roughly capture the characteristics of the macroscopic pressure evolution, the first-order approximate solution can partially reproduce the macro-microscopic pressure oscillation phenomenon, and the second-order approximate solution can accurately predict the pressure profile and its evolution law with time in porous media with macro-micro configuration under various complex conditions. The second- and high-order two-scale approximate solutions constructed in this paper exhibit high numerical accuracy and excellent computational efficiency. We also develop multiscale approximate solutions for predicting microgaseous flow at a low reservoir pressure where the slippage effects are very pronounced. Potential applications of our high-order models to the coal reservoirs with a hierarchical matrix and fractures are discussed. The developed multiscale models exhibit excellent applicability to investigating the crossflow effects and coupling mechanisms between the matrix and cleats or fractures with multiple configurations in the CBM and shale gas reservoirs, which is crucial for the effective implementation of hydraulic fracturing technology. This study provides a fundamental understanding of gas transport in multiscale matrix-fracture networks, which helps to improve the optimization design of hydraulic fracturing in tight reservoirs.

The study of forced oscillations in the non-linear system of an individual traction drive

BACKGROUND: The processes taking place in the ‘traction electric drive – wheel – road’ system during acceleration and braking cause increased dynamic loads on drive components, which may lead to a breakdown. Therefore, it is important to control the drive in a way to minimize and to suppress the given processes. To make it possible, the control system is to be equipped with a resistance torque observer at the electric motor shaft. In addition, in any system, there is a heighten interest in study of arise of resonances, which come with abrupt increase of oscillations amplitudes. Therefore, the features of oscillation phenomena in the given non-linear system are to be studied. AIM: Identification of the peculiarities of oscillatory processes, resonance phenomena in the systems of electromechanical drive of vehicles, which are nonlinear technical systems. METHODS: The study of features of the oscillating processes and the study of capabilities of arise of resonant phenomena were conducted using analysis of the differential equations system describing the operation of the non-linear system. RESULTS: The features of the oscillating phenomena in non-linear systems of interaction between an elastic wheel and road as well as capabilities of arise of resonant phenomena were considered. It is defined that arise of the resonant phenomena in the considered systems is not possible due to breakdown of them. The behavior of modes of interaction between an elastic wheel and road during intensive acceleration and braking was analyzed. The abrupt shock behavior of change rate of wheel torque and current consumed by a drive as well as features of lowering of them when using the self-oscillating phenomena suppression were found. CONCLUSION: The practical value of the study lies in ability of using the proposed conclusions at development of units of a traction electric drive and at synthesis of vehicle motion control systems.

Hybrid adaptive moment estimation based performance measure approach for complex reliability-based design optimization

The complexity of engineering problems often poses challenges, especially in reliability-based design optimization, where the nonlinearity of the performance function is not known in advance. The substantial costs and oscillating results significantly impede the engineering application for highly nonlinear problems. To address this issue, a hybrid adaptive moment estimation (HAME) method is proposed in this work. Given that the performance measure approach is heavily influenced by the gradient of performance function, this method employs the statistical moment information of the gradient, thereby accelerating the search for the minimum performance target point. By combining gradient mean and modified chaos control as new search directions, oscillation phenomena are resolved. To filter the aggregated gradient calculations, the gradient variance and state variable are introduced to approximate the gradients for weakly nonlinear performance functions or in later iterations by the dynamic identification of the nonlinearity of the performance function. The proposed method eliminates the limitation of taking the upper bound for the chaos control factor and addresses the balance issue between efficiency and robustness. The advantages of the proposed method are demonstrated through inverse reliability analysis problems, several highly nonlinear classic RBDO examples, and engineering applications of the crank rod mechanism of an engine.

Numerical study on flow instability in parallel helical tubes under ocean conditions

The superimposed coupling of flow oscillations generated by transient external forces under ocean conditions and flow oscillations in parallel helical tubes during two-phase flow instability may lead to more complex flow oscillation phenomena in parallel helical tubes. However, there is still a lack of understanding regarding the effect of ocean conditions on the two-phase flow instability within parallel helical tubes. The two-phase flow instability in parallel helical tubes under ocean conditions is numerically investigated in the present study. The effects of ocean conditions on the flow behavior and instability threshold in parallel helical tubes are studied with mass flux 200 kg/m2s and inlet subcooling within 5–70 °C. The results show that the mass flow rate decreases as the inclination angle increases. A larger rolling amplitude or shorter period increases the flow and temperature oscillation amplitude. Two characteristic frequencies are found in the natural circulation flow instability under the rolling motion. As the rolling amplitude increases, the compound flow oscillation amplitude is larger and more chaotic with a certain phase shift. The effect of stationary inclining on the flow instability threshold is rather significant than rolling effect. The results of the present study can serve as a significant reference for the operation and design of the helical-coiled once-through steam generator under ocean conditions.