Search Results (1,276)

Deep coalbed methane (CBM) represents a strategic successor field for unconventional oil and gas exploration and development in China, and fluid mobility is a key parameter determining CBM recovery rates. Existing NMR-based studies on coal rock pore structure and fluid mobility suffer from three deficiencies: a lack of coal rock classification based on T₂ spectral morphology, failure to incorporate fractal characteristics into pore classification, and insufficient understanding of fluid mobilization mechanisms in different pore types during gas-driven recovery. This study investigates deep coal rocks of the Taiyuan Formation in the Linxing Block, eastern Ordos Basin, using low-field nuclear magnetic resonance (LF-NMR), saturation gas displacement experiments, and fractal theory. Deep coal rocks were classified into three types based on T₂ spectral peak morphology under saturated conditions: Type I (central main peak), Type II (left-shifted main peak), and Type III (balanced bimodal peak). A fractal-based method was established to subdivide fluid-filled pores into four types: P1-1, P1-2, P1-3, and P2. Through multiple nitrogen displacement experiments, the fluid mobilization characteristics of each pore type at different displacement stages were quantitatively characterized. A fluid mobility index was proposed to comprehensively evaluate the overall fluid mobility of coal rocks. The results indicate that Type I coal rocks exhibit the highest fluid mobility (54.83% after three displacement cycles), with P1-3 pores as the primary mobile fluid reservoir, whereas Type II and Type III coal rocks show lower mobility (27.70% and 32.89%, respectively), with P1-2 pores as the dominant contributors. Pore structure complexity exhibits a significant nonlinear evolutionary relationship with fluid mobility. The fluid mobility index demonstrates a strong positive correlation with the degree of mobile fluid, validating its effectiveness in characterizing fluid mobility in deep coal rock reservoirs. These findings provide a theoretical foundation for sweet spot identification and development optimization in deep coal gas reservoirs. Full article

Microelectromechanical systems (MEMS) have a widespread use in several applications such as signal filtering, time referencing, and sensing. This paper explores the nonlinear dynamic behavior of a MEMS resonator using a reduced-order modeling approach. The study focuses on how multi-modal formulation and detuning affect the system’s response under primary resonance. Using the method of multiple scales, amplitude–phase response equations are derived, and time-domain simulations are generated with the Runge–Kutta method. Two mode combinations are examined: the first mode combined with the second mode and the first mode with the third mode for multi-modal influence evaluation. Results indicate that the first mode provides the dominant behavior to MEMS response, while the second and third modes exhibit minimal participation despite the nonlinearities retained in the presented multi-modal model. Additionally, a detuning study reveals that the geometric and forcing nonlinear effects are stronger near resonance and diminish as the system moves away from it. The analysis suggests that the significant features of the response can be captured in the case of primary resonance using only the first mode, which offers an effective modeling approach. From a design perspective, finding that the first mode alone is sufficient means that the essential dynamic behavior of the MEMS resonator can be predicted and controlled by focusing on its first mode of vibration. In practical terms, this greatly allows engineers to optimize geometry, driving voltage, or control parameters to target the first mode natural frequency without accounting for higher modes, which reduces computational cost and design complexity. Full article

Hydrogen-bond organization plays a central role in determining the structure and properties of water from molecular to macroscopic scales. In this study, we propose a geometry-based descriptor for small hydrogen-bonded water clusters, (H₂O)_n, with n = 2–6. The central element of the proposed geometric framework is the dimensionless geometric index, S_n = d/l, where d is the center-to-molecule distance in a cluster configuration and l is the nearest-neighbor O···O distance associated with hydrogen-bonded water molecules. The geometric descriptor is not intended to replace quantum-chemical calculations or to provide a direct measurement of hydrogen-bond energy, lifetime, or number. Instead, it provides a compact geometric framework for describing the structural organization of small hydrogen-bonded water clusters. The obtained geometric trend is compared with selected Nuclear Magnetic Resonance (NMR), Møller–Plesset perturbation theory (MP2), and radial distribution function data as complementary qualitative and semi-quantitative references. The proposed geometric index S_n = ${\displaystyle \frac{d}{l}}$ was further compared with MP2 quantum-chemical O···O distances for (H₂O)_n clusters, n = 2–6, using the oxygen atoms as structural nodes of the hydrogen-bonded motifs. This comparison showed that the exponential increase in S_n is consistent with the characteristic O···O donor–acceptor length scale of approximately 2.8 Å, linking the geometric framework with calculated molecular geometries. Over the limited interval n = 2–6, the geometric index Sn increases monotonically and nonlinearly with cluster size. The quantum-chemical reference data previously reported in our study, comprising GIAO-DFT-calculated ¹H chemical shifts obtained for MP2-optimized water-cluster geometries, show a rapid nonlinear increase from the dimer to the pentamer, followed by the onset of saturation in the pentamer–hexamer range. The semi-empirical stabilization parameter evaluated in the present study indicates increasing relative stabilization, with a reduced incremental change around n ≈ 4–5. The qualitative consistency of these size-dependent trends supports the use of Sn as a compact geometric descriptor of hydrogen-bond organization in small water clusters, without interpreting it as a direct quantitative measure or mechanistic framework of hydrogen-bond cooperativity. Importantly, liquid water is not treated as a system of closed cyclic clusters; cyclic motifs are used only as frameworked geometric reference configurations for small hydrogen-bonded aggregates. The geometric trend is qualitatively compared with selected quantum-chemical, spectroscopic, and radial distribution function data and should be regarded as an empirical geometric approximation over the limited interval n = 2–6. These findings indicate that geometric, spectroscopic, and quantum-chemical descriptors reflect related, but not identical, aspects of hydrogen-bond organization. The proposed approach links cluster geometry, O···O intermolecular distances, and hydrogen-bond connectivity in a simplified geometric description. Full article

Liquid-lubricated spherical spiral-groove hydrodynamic bearings (SSGB) sustain composite loads under high-speed conditions. This study establishes a numerical framework for SSGB lubrication and proposes a physics-guided hybrid surrogate model to accelerate rotor dynamic analysis. The spherical Reynolds equation is solved via the finite-difference method to determine pressure distributions and dynamic coefficients. Results indicate SSGB exhibits quasi-isotropic behavior, with load capacity and stiffness increasing linearly with rotational speed. Direct stiffness rises with groove depth but varies non-monotonically with groove width ratio, whereas damping generally declines as these geometric parameters increase. The developed surrogate model achieves high prediction accuracy ($R^2=0.9969$). Embedding this model into rotor equations reveals typical soft-spring nonlinearities: deeper grooves increase critical speed and resonance amplitude, while excessive groove width ratios trigger amplitude jumps and system instability. This work provides an efficient approach for predicting the dynamic response of complex rotor-bearing systems. Full article

We study a Caputo fractional three-point boundary value problem of order $\alpha \in (c-1,c]$ and establish three interrelated contributions, all resting on a single refined pointwise ${\mathbf{L}}^2$ estimate for the associated Green function $\mathbf{Gr}(t,s)$. First, we derive a tighter upper bound for ${\sup }_{0<t<1}{\int }_0^1{\mathbf{Gr}}^2(t,s)ds$ by retaining a sign-definite negative mixed term that the classical ${\mathbf{L}}^1$-based analysis of Shivanian discards. The resulting admissible Lipschitz constant ${\kappa }_B$ is explicit in $\alpha$, a, b, c and exceeds Shivanian’s constant ${\kappa }_S$ under an explicit algebraic condition; a closed-form refinement ${\kappa }_B^{\ast }\ge {\kappa }_B$ follows by maximising the pointwise bound in closed form. On Shivanian’s benchmark, the admissible constant rises from $14.646$ to $23.220$ and then to $32.165$. Second, the same contraction constant yields an explicit Ulam–Hyers stability theorem for this problem. While a stability estimate already follows from the classical ${\mathbf{L}}^1$ condition, the refined constant both enlarges the range of admissible Lipschitz constants for which stability is certified and yields a strictly smaller stability constant. Third, we establish quantitative continuous-dependence bounds with respect to the nonlinearity h and the boundary parameter a, and characterize the deterioration of the contraction-based boundary-parameter estimate as the problem approaches resonance. For a fixed Lipschitz constant, this estimate becomes singular as the perturbed contraction factor approaches one and ceases to apply once the contraction condition fails. A more accurate analysis of the Green function thus simultaneously sharpens solvability conditions, stability estimates, and sensitivity bounds for nonlinear Caputo fractional boundary value problems. Full article

Conventional low-frequency antennas face a trade-off between miniaturization and radiation efficiency due to wavelength limitations. Although magnetoelectric (ME) antennas can overcome the electrical size constraint, existing designs lack structural tunability and performance enhancement. This paper proposes a Bézier curve-based (BCB) ME antenna that features a variable cross-section, introducing a shape tuning factor for precise geometric configuration. Using the lumped-mass method, the functional relationship between resonant frequency and the shape tuning factor is derived, establishing the theoretical basis for frequency tuning. A nonlinear multi-field coupled numerical simulation model is established for performance prediction. The BCB structure modifies internal stress distribution, enabling spatial reconstruction of magnetization modulation. The proposed design is validated by comparing the analytical model with our simulation results and literature-reported experimental data. Results show that the BCB design reduces resonant frequency and enhances converse ME (CME) coupling and far-field radiation without increasing material volume. Under clamped and free boundary conditions, the minimum resonant frequencies reach 7.2 kHz and 11.1 kHz, respectively, with CME coupling improved by 124% and 140%. When the shape tuning factor proposed in this work is set to 1/2, the proposed design degenerates into a traditional antenna with uniform cross-sections, which verifies the consistency of the established model. Full article

Nonlinear absorption (NLA) in silver nanoparticles (Ag NPs) is strongly affected by plasmonic field enhancement. However, its wavelength- and energy-dependent evolution on the red side of the surface plasmon resonance (SPR) band remains insufficiently understood. Here, we systematically investigate the NLA properties of Ag NPs using open-aperture (OA) Z-scan measurements combined with femtosecond transient absorption (TA) spectroscopy. The results show that Ag NPs exhibit saturable absorption (SA) under low excitation energies, whereas higher excitation energies induce a mixed SA/reverse saturable absorption (RSA) response. A pronounced wavelength dependence is also observed as the excitation approaches the SPR band, where nonlinear absorption gradually evolves with spectral detuning. TA measurements revealed ultrafast carrier dynamics characterized by a fast electron–phonon relaxation process (~3.3 ps) followed by a slower lattice cooling process (~46 ps). These results demonstrate that the nonlinear response is governed by the interplay between plasmonic resonance and ultrafast carrier relaxation, providing insights for the design of plasmonic nanomaterials for ultrafast nonlinear photonic applications. Full article

Frozen sand molds exhibit broad application prospects in aerospace, large-scale complex castings, and high-end equipment manufacturing owing to their high-strength particle-bonding structure and excellent low-temperature stability. However, their brittle–plastic characteristics make them susceptible to collapse, spalling, and load fluctuations during conventional milling, resulting in nonlinear and unstable milling force behavior. To address this issue, a longitudinal–torsional resonant ultrasonic-assisted milling method was proposed, and an instantaneous milling force model incorporating the effective cutting time was established based on the elemental cutting theory and the oblique cutting force model. Through a series of milling experiments, the milling force coefficients at different spindle speeds were calibrated using the average milling force coefficient method. The identified milling force coefficient models exhibited high fitting accuracy, with coefficients of determination ($R^2$) exceeding 0.9. The developed model was then employed to investigate the effects of various machining conditions on the milling forces of frozen sand molds. The relative error between the predicted and experimentally measured average milling forces was calculated to evaluate the prediction accuracy. The results show that the relative errors between the predicted and experimental milling forces in the X-, Y-, and Z-directions were 9.76%, 8.43%, and 8.45%, respectively, all below 10%, demonstrating the reliability and accuracy of the proposed model. Cutting depth and cutting width were identified as the dominant factors affecting the milling force, whereas the ultrasonic-assisted milling process effectively reduced the milling force, with the most pronounced load-reduction effect observed for conventionally prepared frozen sand molds. This study provides a theoretical basis and practical guidance for process optimization and parameter selection for the efficient and low-load machining of frozen sand molds. Full article

Atomic Force Microscopy (AFM) uses oscillating cantilever-shaped microprobes to measure nanometer-scaled sample topography. Spatial resolution of AM-AFM is determined by the dynamics of the cantilever-sample system including thermomechanical noise of the cantilever. Conventional AFM systems drive the cantilever harmonically in resonance, where resolution can only be improved by changing system and process parameters. We take a different approach keeping cantilever, sample, and control method (AM-AFM) unchanged. Instead, we operate the AFM cantilever in nonlinear parametric resonance. The corresponding excitation scheme is produced via electronic feedback. Because of the artificial source of the nonlinear parametric excitation, we implement arbitrary system behavior and prove the theoretical findings experimentally. We analyze the influence of excitation parameters on dynamic system behavior including an explanation of the nonlinear limitation mechanism of the cantilever amplitude in parametric instability and amplitude reduction mechanism during approach of the cantilever to the sample. We find a mechanism which could reduce tip damage. The dependence of the cantilever’s thermomechanical noise on parametric excitation is investigated. Our analysis yields the resolution of parametric resonance AFM and enables systematic selection of parametric excitation parameters to improve resolution of existing AFM systems. We provide evidence that responsivity is enhanced by 30%, thermomechanical noise by a factor of 4.5, and resolution by a factor of $5.7$ for identical systems when parametric resonance AFM is used. Full article

Although electric vehicles have been widely adopted in recent years, insufficient charging infrastructure in remote areas may still compromise the continuity of emergency operations involving field medical electric vehicles (EVs). Motivated by the need for temporary DC energy support from a donor vehicle to a field medical EV, this paper investigates an isolated DC–DC energy-sharing converter based on a series-resonant dual-active-bridge (SRDAB) topology and its associated control method. First, the SRDAB converter is employed to satisfy the requirements of low-voltage input, galvanic isolation, voltage step-up, and DC power transfer. Based on the fundamental harmonic approximation (FHA), a steady-state power relationship and a control-oriented dynamic model are derived to characterize the coupling between the phase-shift angle, transferred power, and output voltage. Subsequently, a fractional-order model predictive control strategy tuned offline using the grey wolf optimizer (GWO-FOMPC) is developed to address donor-side input-voltage variations, recipient-side equivalent-load disturbances, and the nonlinear power-transfer characteristics of the SRDAB converter. In this strategy, a fractional-order proportional–integral outer loop generates the reference transferred power, while a fractional-order predictive inner loop analytically determines the phase-shift command online. Finally, simulations and converter-level experiments validate the dynamic regulation performance of the proposed control strategy under emulated energy-sharing conditions for field medical EVs. Full article

This paper presents the design, fabrication, and experimental validation of a 3.45 GHz RF-to-DC voltage-doubler rectifier intended for sub-6 GHz wireless power transfer and RF energy harvesting applications. The proposed rectifier uses low-barrier SMS7630 Schottky diodes, a distributed microstrip impedance-matching network, and an FR4 substrate optimized for low RF input power operation. The design was developed using nonlinear Harmonic Balance simulations in Advanced Design System (ADS) and experimentally characterized through reflection-coefficient measurements, output DC voltage measurements, and RF input power sweeps. A quantitative comparison between simulated and measured results demonstrates good agreement in the impedance-matching characteristics, with a measured resonance frequency of 3.438 GHz and a minimum reflection coefficient of −25.22 dB. The fabricated prototype achieved a maximum measured output DC voltage of 1.58 V and a peak RF-to-DC conversion efficiency of 25.9% at an RF input power of +2.9 dBm. The experimental results validate the practical implementation of the proposed rectifier topology and demonstrate its feasibility as a proof-of-concept RF energy harvesting building block for ultra-low-power IoT and wireless sensing applications. The presented implementation also provides a solid foundation for future optimization using low-loss microwave substrates, improved matching networks, and complete rectenna integration. Full article

While input shaping is an effective control technique for reducing residual oscillation in systems with continuous actuation, many systems can only be actuated at discrete time steps. Thus, to apply this control technique to discrete time systems the input shapers must be discretized. This process can reduce the effectiveness of the shapers. Furthermore, nonlinearities in frequency behavior, such as those observed in linear resonant actuators (LRAs), can further diminish the effectiveness of input shapers. This paper examines the effectiveness of previously proposed discretization approaches and seeks to develop an improved approach for creating input shapers for linear resonant actuators (LRAs). LRAs are often utilized in consumer electronics to generate haptic signals. This application presents the dual challenge of minimizing residual peak accelerations while maintaining large transient peak accelerations. Both challenges are addressed herein. Full article

Stable and uniform pesticide application is essential for precision crop protection, input-use efficiency, and the reduction of off-target losses in large-scale farming systems. Large boom sprayers are important agricultural machines for high-efficiency crop protection, but their wide and flexible booms are highly sensitive to terrain-induced excitation, chassis motion, liquid sloshing, and hydraulic nonlinearities. These disturbances can cause roll, yaw, vertical oscillation, and boom-end height variation, thereby reducing spray uniformity, increasing drift risk, and threatening operational safety. Hybrid active–passive boom suspension systems have therefore become a key enabling technology for modern precision spraying. This review summarizes research progress in the structural design, dynamic modeling, sensing, and control of boom suspension systems for large-scale agricultural sprayers. Mainstream commercial machines commonly use double-pendulum active–passive suspension architectures, in which passive components attenuate high-frequency vibration and active subsystems improve low-frequency terrain-following performance. Recent studies have advanced electro-hydraulic actuation, disturbance compensation, adaptive control, multi-sensor fusion, and field evaluation methods; however, a unified framework for coupling boom dynamics, spray quality, sensing accuracy, and whole-machine operation remains incomplete. Key challenges include rigid–flexible–hydraulic coupling, underactuated control, uncertain parameters, external disturbances, and posture estimation errors caused by boom elastic deformation and sensor noise. Future research should emphasize rigid–flexible–fluid-coupled modeling, adaptive output-feedback control with disturbance and resonance suppression, terrain-preview and multi-source perception, and coordinated chassis–boom-spray control. These developments can support more stable, efficient, and environmentally responsible spraying operations in modern precision agriculture. Full article

This paper investigates vibration suppression of finite-length cylindrical shells using linear locally resonant absorbers and nonlinear energy sinks (NESs). A theoretical model is first established for a finite-length locally resonant cylindrical shell equipped with periodically distributed linear cantilever-beam absorbers based on Flügge’s thin shell theory. By combining the modal superposition method and the harmonic balance method, analytical expressions for the vibration responses and average velocity levels of the cylindrical shell are derived. The theoretical model is validated by finite element simulations, and the locally resonant bandgap characteristics are further analyzed. Parametric studies are conducted to clarify the effects of absorber length, thickness, width, and distribution density on the bandgap position, bandgap width, and vibration attenuation performance. On this basis, nonlinear energy sinks are introduced as an additional broadband vibration suppression strategy and investigated through experiments and COMSOL transient simulations. The experimental results show that the NESs effectively suppress resonance peaks near the bandgap edges and improve broadband vibration attenuation. Furthermore, transient simulations under different excitation amplitudes demonstrate the excitation-amplitude-dependent response characteristics of the NESs, indicating enhanced nonlinear energy transfer under higher excitation levels. The results provide theoretical guidance for the design of finite-length locally resonant cylindrical shells and further verify the broadband vibration suppression capability of nonlinear energy sinks through experimental and numerical investigations. Full article

The aim of this study is to determine the reliability level (the probability of failure-free operation) of a masonry building with load-bearing walls based on the conducted experimental investigations. The objective of the study is to compare the obtained reliability and failure risk values with the corresponding values calculated using the results of the structural certification. In 2017–2018, and subsequently in 2023–2024, a comprehensive structural certification of the multi-apartment residential building stock was carried out for the first time in the city of Almaty. A total of 1609 multi-story masonry buildings with heights of two to four stories were identified. Based on the certification results, quantitative estimates of the prior and posterior probabilities of failure and reliability for masonry buildings were obtained for the first time. The recurrence of earthquakes was taken into account. The novelty of the study lies in the experimental investigation of a three-story masonry building of series 308. The dynamic excitation was generated by an inertial vibration machine installed on the floor slab. As the inertial load increased, the resonant vibration period changed by a factor of three. This indicates that the building underwent significantly nonlinear deformation. The structure sustained substantial damage. Using statistical simulation methods based on the experimental data, the prior probabilities of failure for masonry buildings were calculated. In this case, the seismic action was modeled as a non-stationary random process with the deterministic envelope proposed by F. F. Aptikaev. Probabilistic estimates of the reliability of masonry buildings were obtained from the certification results both with and without taking into account the recurrence of earthquakes. The obtained estimates of reliability and failure probability can be used to develop practical recommendations aimed at reducing risk and expected losses in the event of possible earthquakes. It is recommended that masonry buildings with load-bearing brick walls either be structurally strengthened or be demolished. Full article