Trajectory similarity computation is a key functionality in many applications, such as abnormal trajectory detection and traffic-mode classification. However, existing methods either incompletely exploit trajectory features or overlook the effects of noisy and critical points, compromising the accuracy of trajectory similarity computation. This paper presents SimCFT, a novel trajectory representation learning framework for accurate trajectory similarity computation. SimCFT consists of two parallel channels. In the dynamic status channel, point-level features (e.g., position, speed, movement distance, movement angle, timestamp) are extracted and fed into a Dynamic Gated Recurrent Unit (DGRU) with an attention mechanism to capture temporal dependencies and dynamic variations. In the spatial context channel, a weighted grid graph is built from historical trajectory data, and Node2Vec is employed to learn the grid embeddings. A Transformer encoder with a noise recognition mechanism (TransNR) is followed to identify and de-emphasize noisy trajectory points. The two channels are fused via a learnable coefficient to produce the final trajectory embeddings. The model is then optimized with the unsupervised InfoNCE contrastive loss, which pulls similar trajectories together while pushing dissimilar ones apart. Experiments on three real-world datasets show that SimCFT significantly outperforms six state-of-the-art methods in similarity accuracy. The source code is available at https://github.com/xianghz123/SIMCFT.git.

Fe-N-C catalysts, especially FeN4 moieties, hold great promise for boosting the oxygen reduction reaction (ORR). However, they still suffer from sluggish reaction kinetics due to the unsatisfactory adsorption energy of intermediates resulting from the symmetrical distribution of electrons in FeN4 active sites. Strategies in single-atom catalysis have primarily focused on tailoring the local coordination of metal centers. Here, we advance this approach by demonstrating that the intrinsic properties of the support can be harnessed as active, cooperative components. We report a symmetry-broken O-Fe-N3 moiety anchored on a hierarchical porous carbon sphere with a negatively charged surface. This "smart" carrier establishes an interfacial electric field that facilitates O2 activation and electrostatically repels harmful intermediates (e.g., H2O2), concurrently enhancing activity and stability. Theoretical calculations also reveal that the resultant structure can trigger energy level splitting of Fe 3d orbitals, thereby regulating the hybridization of the central Fe 3d orbital. Combined with the optimized electronic structure of the Fe site, the catalyst achieves an exceptional half-wave potential of 0.90 V vs. RHE and remarkable durability. A flexible Zn-air battery using this catalyst delivers a high-power density of 249 mW cm-2. This work establishes a new "active site-smart carrier" synergistic design principle, opening a distinct avenue for next-generation electrocatalysts.

Abstract This study investigates the transport mechanisms of Na + and OH − ions and their hydrated clusters in mineral oil (MO) and vegetable oil (VO) using molecular dynamics simulations, complemented by experimental validation. Key findings reveal that distinct ion transport behaviors are governed by oil polarity and hydration effects. In non-polar MO, despite weak electrostatic contributions, the formation of hydrated clusters significantly reduces ion mobility. This reduction is due to an increased hydrodynamic radius and enhanced van der Waals interactions between the cluster and the oil medium. OH − and its hydrated clusters exhibit lower mobility than their Na + counterparts, which is attributed to stronger polarization effects and a larger effective radius. Conversely, in weakly polar VO containing ester groups, strong initial ion-dipole interactions suppress the mobility of bare ions. However, the formation of hydration shells shields the ionic charge, weakens Coulombic interactions with the polar oil matrix, and consequently leads to a slight increase in mobility with cluster size. This results in an opposing trend for hydrated cluster size dependence in the two oils: mobility decreases in MO but increases in VO. Analyses of radial distribution functions and interaction energies confirm that hydration increases the ion–oil distance and reduces electrostatic binding, leading to a more pronounced charge-shielding effect in VO than in MO. The simulations align with experimental observations of higher overall ionic mobility in MO than in VO but highlight discrepancies in absolute values. These differences arise from the simplified ion model (Na + /OH − only) used in simulations, compared to the diverse ionic species present in real oils. Crucially, incorporating hydrated cluster transport significantly improves the agreement between simulation and experiment. These insights advance the fundamental understanding of ion dynamics in dielectric liquids, emphasizing the critical role of hydration, and offer practical guidance for optimizing oil-based systems in applications such as power transformers and electrokinetic devices.

During the winding process of a coil winding machine, excessive tension can cause wire deformation, over-stretching, or breakage, while insufficient tension may lead to slackness, accumulation, and wrinkling. The magnitude of winding tension directly affects product quality and operational performance. This paper addresses the challenges of inadequate constant-tension control accuracy and excessive fluctuations in the unwind system of winding machines under disturbances. By integrating specific operational scenarios, a fuzzy PID control strategy suitable for actual production environments is designed. Based on an established coupling model relating unwind tension to roll diameter, unwind speed, and moment of inertia, conventional PID and fuzzy PID control simulation models are developed in the MATLAB/Simulink platform. These models evaluate both control strategies under noise disturbances and abrupt tension changes. A systematic comparative analysis examines the dynamic response characteristics, steady-state accuracy, and anti-interference capabilities. Results demonstrate that the fuzzy PID control, integrated with actual winding machine conditions, effectively suppresses tension fluctuations induced by nonlinear disturbances, reducing adjustment time by 3 s compared to conventional PID control. This indicates that the production-condition-integrated fuzzy PID control exhibits smaller overshoot, enhanced robustness, and superior dynamic response and better meets precision requirements for wire winding tension control.

Power transformers serve as crucial hub equipment in modern power systems, hence their reliability and security have been the major concern to the power industry. The vibration of the cores of the transformers due to magnetostriction can result in severe security issues, and is one of the dominant origins of the noise during the operation of the transformers. In this manuscript, we take a 10 kV transformer as an example to establish a magneto-solid-acoustic coupling finite element model of the transformers, taking into account the magnetostrictive properties of the cores. The reliability and accuracy of the model is then demonstrated via A posteriori error estimation. The vibration noise of the reference transformer is then investigated based on the verified model.

The SF6/N-2 gas mixture is a promising alternative to reduce SF6 usage in the power industry, with surface streamers being the dominant discharge form. This study develops an electrical-optical system based on the Pockels effect to measure the spatiotemporal morphology of surface streamers in pure N-2 and SF6/N-2 gas mixtures. In pure N-2, surface streamers are filamentary with bifurcations. In contrast, in SF6/N-2 gas mixtures, the morphology of surface streamers initially appears filamentary and transforms into clusters under alternating current applied voltage. The formation of clustered channels may be explained in terms of the lateral drift and diffusion of the net space charge. Two fluid models analyze the experimental results. The fluid simulation reveals a sheath between the surface streamer and the dielectric surface. The propagating surface streamer floats above the dielectric, with a net space charge layer lying at its bottom, sustaining its propagation through the net-charge-induced field. Due to the presence of the sheath, the surface streamer's morphology and properties are similar to those of volume streamers. This study enhances the understanding of surface streamer properties in insulating gases.

Using Lagrangian coherent structures (LCSs), the mass transport process of large-scale coherent structures in a buoyant jet diffusion flame is studied numerically to gain a key understanding of combustion instability. First, the reacting flow fields of the jet diffusion flame are numerically simulated for two cases, a stable case without considering the buoyancy effect and an unstable case that includes the buoyancy effect corresponding to combustion instability. In particular, large-scale Kelvin-Helmholtz instability (KHI) vortex structures outside the flame surface are captured in the unstable case. Then, the ridges in finite-time Lyapunov exponent (FTLE) fields that sketch the distinct regions in the reacting flow field are extracted as LCSs. The mass transport process is studied by comparing the distribution of LCSs with reaction fields. The flame surface is found to coincide with the attracting LCSs, which separate the fuel and air on opposite sides of the flame surface. In contrast, the reaction products, including species and reaction heat, are confined in the boundaries consisting of repelling LCSs on the inner and outer sides of the flame surface. Finally, the evolution of LCSs is tracked to analyze the generation of KHI vortices and species transport in the jet diffusion flame. With the aid of LCSs, it is found that there exist some saddle-type flow features separating the distinct flow structures into several regions, and the jet fuel and airflow enter into the separated regions from different open boundaries. Importantly, the attracting LCSs in the separated regions then bulge outwards, forming KHI vortices as the attracting LCSs further stretch and fold. Furthermore, the vortices continue moving upwards while species flow from repelling LCSs to attracting LCSs, leading to mixing. These results show that attracting and repelling LCSs can act as the flame surface and the mass and energy transport boundary of the reaction products, respectively. In summary, the work presented can provide a new method in combustion controlling to estimate the location of the flame surface and the transport boundaries of species from the velocity field, by using LCSs.

The effect of pulsating jet on the unsteady film cooling performance was studied by experimental and numerical simulation. The FLIR thermal infrared thermal camera was used the measure the adiabatic temperature of the surface. The large eddy simulation was conducted for analyzing the adiabatic film cooling effectiveness at four different jet non-dimensionalized pulsated frequency of St = 0, 0.1, 0.2, and 0.3. The analysis of the dynamic model decomposition of both the velocity and temperature fields obtained by numerical calculation was performed to obtain the coupling relationship between the flow and heat transfer. Results show that the cooling effectiveness of steady film cooling (St = 0) decreases with the increase of blowing ratio. At low blowing ratio (M = 0.65), the cooling efficiency of the pulsating jet is significantly lower than the steady-state jet. At the blowing ratio of 1.0 and 1.5, the cooling efficiency of the low frequency pulsating jet (St = 0.1) is a little higher than the steady-state jet, indicating that the low frequency pulsation under the high blowing ratio can improve the coverage of the cooling air. At high pulsating frequency (St = 0.3), the cooling effectiveness is obviously declined.

The transport of atmospheric pollutants under global wind is analyzed from viewpoint of dynamical system, with the Lagrangian coherent structures (LCSs) extracted from the wind field. First, the distribution of global atmospheric pollutants is presented using the data of inhalable particulate matters and dust aerosols. Then, the transports of pollutants are analyzed using hyperbolic LCSs extracted from the global wind field, with a detailed analysis of the North Atlantic region. Finally, the formation and evolution of vortices in the process of pollutants transport are analyzed further by Lagrangian-averaged vorticity deviation (LAVD) method. It can be concluded that the transport of pollutants is closely related to the hyperbolic LCSs in the wind field. Importantly, these structures as transport barriers can act as the transport channels of particulate matters, and one conceptual design to control atmospheric pollutants efficiently could be provided tentatively, based on studies presented. (C) 2022 L&H Scientific Publishing, LLC. All rights reserved.

The evolution of flow structures during dynamic stall of a two-dimensional pitching National Advisory Committee for Aeronautics 0012 airfoil is studied using the variational Lagrangian coherent structures (LCSs), and the mass transport and vorticity transport are precisely analyzed using LCSs and lobe dynamics for further understanding the nature of flow phenomena in dynamic stall. First, the variational LCS algorithm is improved to be efficiently used in the accurate extraction of flow structures. Then, both the hyperbolic LCSs and elliptic LCSs are computed numerically in the whole process of dynamic stall to analyze the evolution of flow structures in detail. Further, a high-accuracy LCS-advection method is used in the advection of LCSs to quantitatively analyze the mass transport and vorticity transport in the evolution of LCSs utilizing lobe dynamics based on nonlinear dynamics. Finally, the evolution and motion of primary leading edge vortex (LEV) and trailing edge vortex (TEV) identified by elliptic LCSs are analyzed in depth. The results obtained can provide a deeper insight into the nature of flow phenomena in dynamic stall from the viewpoint of nonlinear dynamics. Specifically, the nature of evolution of primary LEV and the TEV and the reasons for the changes of lift coefficients are clarified from the viewpoint of fluid transport. To explain it briefly, the variational LCSs and lobe dynamics are powerful tools to quantitatively analyze the evolution of flow structures and fluid transport.

The bifurcation analysis of nonlinear combustion instability in a Rijke burner is studied numerically. First, a saturated flame model and a third-order saturated flame model are proposed, and the mode truncation effects are investigated by studying their influences on system eigenvalues. Then, bifurcation analysis on the flame location, average flame intensity, damping, and time delay in the flame model is conducted with the new flame models. Results show that the burner driven by the saturated model loses stability via supercritical Hopf bifurcations, whereas the system driven by the third-order saturated model shows the characteristic of subcritical Hopf bifurcations, accompanied by bistable regimes emerging in the bifurcation diagrams. Finally, the effects of velocity perturbations are analyzed further, and the results show that a given system may oscillates with different frequencies if the disturbance is imposed to different Galerkin modes. This study reveals that the process of the combustion system’s transition to instability depends strongly on the nonlinear term in the flame model.

The evolution of vortex structure and mass transport process during dynamic stall of the two-dimensional pitching NACA0012 airfoil are studied in detail, using Lagrangian coherent structures (LCSs), and the nature of dynamic stall is given from viewpoint of nonlinear dynamics. First, the numerical method with SST k−ω turbulence model is used to simulate the flow field around the pitching oscillation airfoil. Then, the Lagrangian-averaged vorticity deviation (LAVD) method is used to analyze the formation and evolution of vortex structure during dynamic stall. Further, the circulation development process of the primary leading edge vortex (LEV) is studied, with comparison between LAVD method with Q criterion. Finally, the dynamic behaviors, such as the mass transport and evolution of flow structure during the process of dynamic stall, are analyzed in depth by using LCSs, and the fluctuation of lift coefficient and the dynamic stall are investigated from the viewpoint of nonlinear dynamics. The results show that the evolution of vortex structure and mass transport are closely related to the fluctuation of lift coefficient during the dynamic stall, and the low-pressure region formed by vortices and the complex mass transport and mixing process on the suction surface of the airfoil are the main reasons for the high lift during the dynamic stall process. In particular, the primary LEV, which can be accurately identified and tracked by LAVD method, plays a very important role in lift enhancement, and the primary LEV and the secondary LEV form and grow in turn by the intermittent feeding process of shear layer. Additionally, the fluid particles in front of the leading edge of the airfoil enter the LEV through a special mass transport channel, which is textured by LCSs and opened and closed intermittently. More importantly, the saddle point of LCSs is an important component of vortex boundary, and its dynamic behaviors represent the behaviors of vortex. The first separation of saddle point from the shear layer at leading edge indicate the separation of primary LEV from the suction surface of the airfoil, and the occurrence of dynamic stall. The second separation of saddle point from the shear layer at leading edge indicate that primary LEV begins to leave the upper region of the airfoil, corresponding to a sharp drop in lift coefficient. Generally, compared to the traditional visualization techniques, the Lagrangian analysis based on LCSs and LAVD can provide a deeper insight into the dynamics of the vortex in flow field around pitching oscillation airfoil and the dynamic mechanism of dynamic stall.

In engineering, the combustion chamber with a backward step is very popular, and it is a kind of flame stabilizer. In this type of combustion chamber, there will be shedding vortices at the step due to the instability of the flow field. The shedding vortices will carry reactants to move downstream and burn, resulting in unstable heat release and then pressure and velocity fluctuations of the sound field, thereby, finally, forming a combustion-vortex-acoustic interaction process. If a positive feedback loop is formed between the unstable heat release and the pressure fluctuation of sound field, combustion instability will occur, and it is also referred to as thermoacoustic oscillation due to vortex shedding. Combustion instability frequently occurs in many practical systems or equipment, and its induced significant pressure oscillations have a serious influence on the normal operation of the equipment. Recently, the combustion instability has been extensively studied experimentally, but the theoretical investigation on its nature is still rare. Since combustion instability is a complicated nonlinear phenomenon, it is necessary to study its nature from the viewpoint of nonlinear dynamics. Based on the one-dimensional simplified model of thermoacoustic instability involving vortex shedding proposed by Matveev and Culick, the typical nonlinear phenomenon in thermoacoustic oscillation induced by vortex shedding is studied. The study focuses on the initial value sensitivity of the system, the influence of key parameters on thermoacoustic oscillation, and the phenomenon of vortex-acoustic lock-on. Firstly, the Galerkin method is used to approximate the governing equation, and the partial differential equations are reduced to a set of ordinary differential equations. Then, the first ten modes are selected, and the pressure and velocity fluctuations of sound field under different system parameters are obtained by MATLAB program. Finally, the thermoacoustic instability of the system under different initial disturbances, the influences of different steady flow velocity on the thermoacoustic oscillation of the system, and the phenomenon of vortex-acoustic lock-on in thermoacoustic oscillation are studied in detail. The results show that the system of thermoacoustic oscillation involving vortex shedding is extremely sensitive to initial values, and there are a rich variety of nonlinear phenomena. With steady flow velocity increasing, the amplitude of pressure fluctuation augments generally. However, the similar structures are found in several intervals of steady flow velocity, and the amplitude first decreases and then increases. In particular, it is verified that the system oscillates periodically by integer (f(p)/f(s)) multiple of the vortex impinging frequency (f(s)), that is, the vortex-acoustic frequency locking with the number of revolutions f(p)/f(s), which is found in experiment and can be regarded as an important characteristic of periodic thermoacoustic oscillation.

In this paper, an extension of advancing front technique (AFT) on new target surface (NTS) after virtual topology operations has been achieved in order to suitably represent CAD model and enhance the accuracy of numerical analyses. The extension of AFT has been accomplished through two stages. In the first stage, through analysis of the connection features of the intersecting lines with geometric continuity, a new notion of NTS is proposed. Based on the new notion, the topology of NTS is adjusted to generate meshes and meet the requirements of numerical analyses. In the second stage, AFT is extended accordingly in order to realize the topology of NTS. Specifically, based on the concise description of the front configuration of AFT, all of the subordinate aspects of AFT, including meshing order, generation of elements and nodes, and validity checking, are optimized. With the extension of AFT in this fashion, the target surfaces of AFT have been extended from the parametric surfaces to the composite surfaces. Finally, two examples are calculated with the extended AFT. The results of the calculation have verified that the topology of the composite surfaces is suitable and that the extended AFT is accurate and efficient.

The lobe dynamics and mass transport between separation bubble and main flow in flow over airfoil are studied in detail, using Lagrangian coherent structures (LCSs), in order to understand the nature of evolution of the separation bubble. For this problem, the transient flow over NACA0012 airfoil with low Reynolds number is simulated numerically by characteristic based split (CBS) scheme, in combination with dual time stepping. Then, LCSs and lobe dynamics are introduced and developed to investigate the mass transport between separation bubble and main flow, from viewpoint of nonlinear dynamics. The results show that stable manifolds and unstable manifolds could be tangled with each other as time evolution, and the lobes are formed periodically to induce mass transport between main flow and separation bubble, with dynamic behaviors. Moreover, the evolution of the separation bubble depends essentially on the mass transport which is induced by lobes, ensuing energy and momentum transfers. As the results, it can be drawn that the dynamics of flow separation could be studied using LCSs and lobe dynamics, and could be controlled feasibly if an appropriate control is applied to the upstream boundary layer with high momentum.

Nonlinear waves produced in an incompressible boundary layer driven by a travelling wave are investigated, with damping considered as well. As one of the typical nonlinear waves, the spike-like wave is governed by the driven-damped Benjamin-Ono equation. The wave field enters a completely irregular state beyond a critical time, increasing the amplitude of the driving wave continuously. On the other hand, the number of spikes of solitary waves increases through multiplication of the wave pattern. The wave energy grows in a sequence of sharp steps, and hysteresis loops are found in the system. The wave energy jumps to different levels with multiplication of the wave, which is described by winding number bifurcation of phase trajectories. Also, the phenomenon of multiplication and hysteresis steps is found when varying the speed of driving wave as well. Moreover, the nature of the change of wave pattern and its energy is the stability loss of the wave caused by saddle-node bifurcation.

An experimental program has been designed in order to study pollutants dispersion at a complex site with a focus on stable conditions, which are still challenging for numerical modelling. This experimental program is being conducted at the SIRTA site in a southern suburb of Paris and consists in measuring, in near field, the turbulence and the pollutants dispersion. The aim of this program is to characterize the fine structure of turbulence and associated dispersion through high temporal and spatial resolution measurements. Then, these measurements allow to validate and improve the performance of CFD simulation for turbulence and dispersion in a heterogeneous field. The instrumental set up includes 12 ultrasonic anemometers measuring continuously wind velocity and temperature at 10 Hz, and 6 photo-ionization detectors (PIDs) measuring gas concentration at 50 Hz during tracer tests. Intensive observations periods (IOPs) with gas releases have been performed since March 2012.First of all, a detailed study of flow on the site is made, because it must be characterised and properly simulated before attempting to simulate the pollutants dispersion. This study is based on two years of continuous measurements and on measurements performed during IOPs. Turbulence strong anisotropy in the surface layer is characterized by calculating variances, integral length scales and power spectra of the three wind velocity components. Propagation of turbulent structures between sensors has been characterized with velocity correlations. Energy spectra show several slopes in different frequency regions. Also, data analyses show impact of terrain heterogeneity on the measurements. The forest to the north of experimental field modifies wind velocity and direction for a large northerly sector. It induces a strong directional wind shear and a wind deceleration below the forest height. Numerical simulations are carried out using the CFD code Code_Saturne in RANS mode with a standard k-e closure adapted for atmospheric flows and a canopy model for the forest. These simulations are shown to reproduce correctly the characteristics of the mean flow on the measurements site, especially the impact of the forest for different wind directions, in both neutral and stable stratification. Simulations results also show the directional wind shear and the turbulent kinetic energy increase induced by the forest. A sensitivity study has been made for various values of forest density and shows that the typical features of canopy flow become more pronounced as canopy density increases. Pollutants dispersion study are made for several IOPs. Concentration data analysis shows a consistency with previous measurements made in a near-source region where the plume scale is smaller than the large-scale turbulence eddies. Concentration fluctuations are characterized through concentration time series, histogram and statistical analysis. The internal subrange can be observed in the concentration spectra. Next, pollutants dispersion are modelled by transport equations for concentration and its variance. The mean concentrations show a good agreement with measurements in values for all the IOPs studied, except that the position of the concentration peak depends on the accuracy of simulated wind rotation below the forest height. The concentration fluctuations obtained from simulations seem to be affected significantly by the initial condition and the modelling of the dissipation term. A sensitivity study to the parameterisation is then presented