Zhanfeng Qi

This study numerically investigates the propulsive performance of a semi-active flapping foil near a free surface at Re=105 using the volume of fluid and overset grid methods. Results indicate that the free-surface effect (FSE) is strongly modulated by the reduced frequency (f*), with an empirical transition point identified at f*=0.44. At low reduced frequency, the free surface amplifies the pressure differential across the foil and intensifies wake vortices, significantly enhancing thrust. Conversely, at high reduced frequency, proximity to the free surface suppresses leading-edge vortex shedding and destabilizes the wake, leading to momentum blockage and reduced thrust. Analysis reveals that the transition in FSE influence is fundamentally driven by a shift in the passive pitching mode, triggered by the variation in reduced frequency. Furthermore, a novel dynamic equilibrium between the semi-active foil and the fluid is identified, characterized by periodic fluctuations within the hydrodynamic cycle. These fluctuations, driven by fluid–structure interaction and constrained by the restoring force, eventually converge into a stable limit cycle. This study offers new trend-wise insights into bio-inspired propulsion mechanisms under free-surface constraints within the present two-dimensional numerical framework.

Global navigation satellite system (GNSS) is indispensable for consumer electronics (e.g., smartphones) but faces severe spoofing threats that manipulate positioning/timing. However, traditional signal quality monitoring (SQM) methods rely on fixed multi-correlator fusion, which lacks adaptability to dynamic spoofing, leading to unstable detection. To address these limitations, we first define the spoofing signal-to-noise ratio (SSNR) to quantitatively evaluate theoretical detection probability of SQM metrics, providing a benchmark for metric selection. We then prove the suboptimality of linear fusion using Cauchy-Schwarz inequality, which cannot reach the theoretical SSNR upper bound, further confirming the need for nonlinear solutions. Building on this theoretical insight, we propose a dedicated spoofing convolution network (SpoofConvNet) tailored for consumer electronics. Adopting a three-stage framework (multi-correlator spatiotemporal feature input, end-to-end convolutional neural network processing, and spoofing state output), SpoofConvNet directly targets the flaws of traditional methods. Its shared encoder integrates spatiotemporal feature extraction and noise suppression, while 2-dimensional convolutions simultaneously capture spatial correlator anomalies and temporal frame trends. Its lightweight architecture aligns with the hardware constraints of consumer devices. Experiments validate SpoofConvNet outperforms both traditional SQM methods and mainstream neural networks, providing a practical anti-spoofing solution for GNSS-enabled consumer electronics.

Spoofing attacks pose a severe threat to global navigation satellite system (GNSS) integrity in vehicular navigation, highlighting the need for effective detection methods. Current detection approaches often rely on fixed signal quality monitoring (SQM) metrics, which do not adapt well to varying spoofing conditions, leading to reduced detection accuracy. To address this challenge, an adaptive spoofing detection framework based on voting ensemble learning (VEL) is proposed. First, a correlator output model under spoofing is derived, quantitatively revealing the influence of phase difference (between spoofing and authentic signals) on in-phase or quadrature-phase channel energy distribution and delay difference on correlation distortion, filling the theoretical gap in energy transfer mechanisms. Second, windowed SQM metrics are designed, enhancing sensitivity to stealthy spoofing. Third, a VEL framework integrating decision tree, random forest, and k-nearest neighbor has been developed, dynamically selecting optimal windowed metrics through real-time feature spoofing parameter correlation, while remaining compatible with in-vehicle microcontrollers. Experiments on the Texas spoofing test battery dataset show that the proposed VEL method achieves 90%+ detection rates, outperforming fixed metrics and OR-gate combinations. Field tests on autonomous vehicles confirm its robustness in actual environments. This framework provides a flexible and practical solution for safeguarding GNSS services in vehicular navigation.

The conventional heaving flapping-foil propulsion system, which couples heaving and pitching motions, shows considerable application potential in the field of underwater propulsion owing to its performance advantages. To further advance underwater propulsion technology, this study proposes a fully active rotating flapping foil propulsion device with compound rotating–pitching motion. A three-dimensional flapping foil of a National Advisory Committee for Aeronautics (NACA) section 0012, subjected to coupled rotating and pitching motions, is simulated using the finite volume method, the sliding mesh technique, and user-defined functions. The research analyzes the effects of maximum angular displacement, pitching amplitude, and nominal rotational speed on its propulsive performance. Results indicate that increasing the maximum angular displacement can effectively enhance the thrust generated by the rotating flapping foil. However, with the increase in pitching amplitude, the mean thrust coefficient of the rotating flapping foil first increases and then decreases, suggesting that an optimal pitching amplitude exists for the rotating flapping foil propulsion system. Further analysis was conducted on the effect of nominal rotational speed on the propulsive performance of the rotating flapping foil. It was found that the nominal rotational speed exhibits a positive correlation with the generated thrust: the higher the nominal rotational speed, the greater the thrust output of the rotating flapping foil. Furthermore, compared with conventional heaving flapping foils, the rotating flapping foil achieves a 22.5% increase in thrust under identical operational conditions. The full-active rotating flapping foil proposed in this study exhibits excellent hydrodynamic performance, paving a new way to develop underwater propulsion technology.

We numerically investigate the propulsion performance of a flapping foil operating near a free surface, focusing on the effects of submergence depth, Strouhal number, pitching amplitude, and surface features. A two-dimensional flapping foil of a National Advisory Committee for Aeronautics (NACA) section 0015, subjected to coupled pitching and heaving motions, is simulated using the Volume of Fluid (VOF) method and overset grid techniques at a Reynolds number of 1 × 105. Results show that submergence depth critically affects the hydrodynamic performance of the flapping foil by modulating the ground effect. At small submergence depths, interactions between the free surface and flapping foil induce energy dissipation and reduce thrust generation, particularly on the upper surface of the foil during its ascending phase. However, as submergence depth increases beyond a threshold, i.e., more than five times the flapping chord length, the ground effect diminishes rapidly. Further, the Strouhal number exhibits a positive correlation with the thrust generation of the flapping foil, enhancing propulsion through intensified dipole structures and jet-like wake effects. In contrast, pitching amplitude shows a negative correlation, as excessive amplitudes decrease propulsion performance. Moreover, the comparative analysis of free and rigid surfaces highlighted that the free surface introduces energy dissipation and slightly degrades propulsion performance at small submergence depths.

Wave glider with webbed wings (WGWW) is an innovative unmanned surface vehicle, powered by wave energy and solar energy. Its energy supply conundrum has been thoroughly resolved by “exploiting local resources” and can achieve long-term and large-scale ocean data acquisition for its outstanding endurance. Flexible webbed wings (FWWs) are crucial constituents of WGWW, transforming wave energy into driving force through fluid–structure interaction (FSI) under their hyper-elastic deformation. It is of great significance to analyze the influence of FWW material parameters on their driving force to improve the wave energy conversion efficiency of WGWW. In this paper, the numerical simulation model of the fluid–structure interaction of FWWs is first established, based on the bidirectional FSI theory. Moreover, the overset method and dynamic mesh technology are introduced to solve the problems of large deformation and easy divergence in the thin plate structure of FWWs. The optimization analysis of the FSI dynamic performance of FWWs under different material parameters are accomplished, including the different materials, surface hardness, and shape size. Finally, a principle prototype of WGWW equipped with inertial navigation sensor is built and a wave tank experiment is carried out. The elastic deformation and driving force generation of FWWs are analyzed, and the feasibility of WGWW driven by FWW is verified. The relevant results provide scientific basis and reference for the material selection and optimal design of FWWs and have important significance for improving the dynamic performance of WGWW.

A wave glider with webbed wings (WGWWs) is a new type of unmanned surface robot that combines wave energy and solar energy as its energy supply, driven by flexible webbed wings (FWWs). Wave gliders with webbed wings are already playing an important role in marine science research. Flexible webbed wings are significant components of wave gliders with webbed wings that achieve the absorption and conversion of wave energy through bidirectional fluid–structure coupling with water flow. To address the issues of large deformations and nonconvergence under the strong coupling action of flexible webbed wings, a dynamic model of flexible webbed wings is first established on the basis of an analysis of the motion principle of a wave glider with webbed wings. The Mooney–Rivlin model was subsequently applied to describe the stress–strain relationship of a rubber hyperelastic material for flexible webbed wings. By adopting the overset method and dynamic mesh technology, employing the system coupling interface based on the data interaction platform of the ANSYS Workbench, as well as mechanical and fluid solvers, the transient dynamic characteristics of fluid–structure coupling of flexible webbed wings under different working conditions are obtained. Finally, by setting different sea conditions, the influences of wave height and period on the dynamic characteristics of flexible webbed wings are analyzed. The results indicate that the greater the wave height and the smaller the wave period are, the greater the power output of the flexible webbed wings is. Additionally, the influence of the wave height ratio period on the dynamic characteristics of flexible webbed wings is more pronounced.