Acta Mechanica Sinica最新文献

Researchers have achieved remarkable control over material properties by designing novel architectures, particularly for tuning Poisson’s ratio. Despite abundant existing approaches, significant design space remains unexplored. This work presents two metamaterial designs exhibiting directionally dependent zero Poisson’s ratio, i.e., one-way zero Poisson’s ratio. In other loading directions, these metamaterials display positive or negative Poisson’s ratio. This selectivity stems from a mode switching mechanism between “unbuckling” and “buckling” of well-designed members within the metamaterials. Theoretical analysis reveals the conditions governing this mode switch, numerical simulation and experiments confirm the one-way Poisson’s effect. Furthermore, the high stiffness contrast within these buckling-prone members yields a pronounced asymmetry in equivalent moduli of the metamaterials under tension and compression, breaking the inherent symmetry of the elastic matrix of conventional materials. This asymmetry is then exploited to design metamaterial beams with asymmetric bending stiffness. Our findings and the design strategy presented here pave the way for developing advanced metamaterials with previously unattainable and unexpected Poisson’s ratios.

A robust generalized continuum model called the wavelength-dependent strain gradient continuum model (WDSGM) has been proposed to predict dispersion properties of two-dimensional (2D) periodic lattice metamaterials. The key idea lies in replacing the classical Taylor expansion of displacement fields with a wavelength-dependent one, naturally leading to new equations of motion and therefore a significantly improved capability of predicting dispersion characteristics. For different 2D lattices, dispersion results derived from the proposed WDSGM are verified by comparing with those obtained from the discrete model and the existing strain gradient continuum model (SGM) in the irreducible Brillouin zone. Based on the proposed model, the effects of SG orders have been investigated. Results suggest that considering the wavelength-dependent Taylor expansion and increasing the SG order are beneficial to improving the predictive performance of continuum models. The proposed model is free of any instability issue which is challenging for many existing SG methods. Under given parameters, the proposed WDSGM with eighth-order truncation is enough to predict the dispersion relation of three lattices, i.e., the square, triangular and hexagonal lattices throughout the irreducible Brillouin zone.

This study presents the design, verification, and calibration of a spherical inertial sensor particle engineered to achieve kinematic equivalence with a solid sphere. Utilizing micro-electro-mechanical systems inertial measurement unit technology, this 40 mm particle is capable of measuring triaxial acceleration up to ±16g (g = 9.81 m/s²) and triaxial angular velocity up to ±2000°/s, with a high sampling rate of 1000 Hz sustained over one hour. The sensor particle features a dual-layered spherical structure designed to ensure equivalence in shape, density, center of mass, moment of inertia, and elastic modulus compared to a solid sphere. The performance of the sphere is calibrated and verified with a series of physical experiments. The experiment of the sphere freely sinking in still water confirmed the accuracy of the data measured by the sensor particle and its equivalence to a solid aluminum sphere. This study provides a more representative tool for measuring particle motion information in homogeneous dense granular experiments.

The autonomous navigation capabilities of the lunar roving vehicles (LRVs) rely on optical sensors. However, lunar dust emitted by the wheels diffuses around the rover and causing surface adsorption, threatening the performance of the optical detection system. This study delves into the distribution of dust emissions caused by rovers and their effects on the light transmission of optical sensors’lenses. A multiscale hierarchical discrete element method (MSH-DEM) incorporating ground experiments was adopted to analyze the dust emissions from LRVs, providing an assessment of dust deposition on sensor lenses under different driving conditions. Results show that the adsorption probability is close to 100% for particle sizes less than 10 µm or collision velocities less than 1 m/s. After the LRV traveled forward at a speed of 10 km/h for 1 h, the light transmission of the sun sensor decreases by 5.29%, and that of the star sensor decreases by 1.88%. The two stereo cameras are minimally affected by the dust deposition. Left-steering conditions will increase the dust deposition on the stereo cameras and star sensors located on the right side of the LRV. Uphill conditions have a mitigating effect on lunar dust deposition, while downhill increases the dust deposition on the star sensor. These findings are crucial for assessing the potential impact of lunar dust on optical sensors and the accuracy of autonomous navigation.

Nacre exhibits exceptional mechanical properties, which are attributed to its brick-mortar microstructure with an integration of stiff mineral platelets and soft organic interfaces. The rapidly developing 3D printing technique has been used to make nacre-inspired composites with similar brick-mortar structure. It is known that the strain hardening phenomenon plays an important role in the high strength and toughness of natural nacre. However, the role of strain hardening on the mechanical properties of biomimetic nacreous composites still lacks theoretical evaluation and experimental confirmation. Based on a mesomechanical theoretical model, we derive the stress-strain response and macroscopic strength of the brick-mortar structure under uniaxial tension. The brick-mortar structure shows three typical failure modes, according to the occurrence of strain hardening and platelet fracture. Furthermore, we investigate how the occurrence of strain hardening depends on its geometry and constituent properties. It is found that increasing the aspect ratio of the platelets promotes strain hardening, while increasing the stiffness of the soft phase leads to the disappearance of strain hardening. Furthermore, we utilize bi-material 3D printing technology to prepare biomimetic nacre samples and conduct uniaxial tensile mechanical tests. We observe the occurrence of strain hardening with the increase in the length of the platelets, resulting in a significant increase in the strength and fracture strain of artificial nacre. Our result highlights the significant role of strain hardening in regulating the mechanical properties of nacre-like composite materials.

In space-based gravitational wave detection missions and other space experiments utilizing the ultra-static and ultra-stable spacecraft, the drag-free control system (DFCS) plays a key role in maintaining the free-falling motion of the test masses (TMs). However, high-precision ground-based verification of DFCS faces a great challenge in suppressing and evaluating multiple disturbances and noises accurately. To this end, this article first proposes an active disturbance rejection controller for TM with a two-stage torsion pendulum. The dual-loop scheme with robust control approach is introduced for this underactuated pendulum system, which significantly reduces the impact of seismic noise on TM. Subsequently, on the basis of suspended TM and the controlled Stewart platform, the closed-loop dynamics of a ground-based DFCS validation system is conducted. The emulating capability of this system for in-orbit flight dynamics is analyzed. Furthermore, complex propagation mechanism models of multiple disturbances and noises are built, and residual acceleration and tracking performance are precisely evaluated.?Finally, numerical simulations are performed to validate the theoretical work. The presented work provides a design and analysis methodology for ground-based verification of DFCS.

A heat transfer system is proposed to enhance the thermal performance of a microchannel by positioning a rigid cylinder upstream of the staggered flexible beams as passive vortex generators. By using a fluid-structure-thermal coupling solver, the effects of the number and spacing of flexible beams on specific physical quantities, such as the local Nusselt number and Colburn factor, were investigated. The results indicate that the periodic pressure variations induced by the wake of the cylinder cause vibrations of the flexible beams, which generate stronger vortices and enhance flow mixing. The staggered flexible beams make vortices closer to the wall, leading to more significant disturbances to the thermal boundary layer and improving convective heat transfer. Further analysis shows that a sufficient number of flexible beams are necessary to effectively perturb the far-field thermal boundary layer due to the dissipation of wake vortices, although this also increases the pressure drop. The spacing of the flexible beams affects the strength of the local vortices, and appropriate spacing increases the intensity of the vortices. By considering both heat transfer performance and energy consumption, the optimal configuration of passive vortex generators is identified in this study.

In this study, a set of deep-water electric explosion experimental systems were designed and built to explore the dynamics of Cu/Al wire electric explosion bubbles and the characteristics of shock wave and bubble pulsation pressure under different water depth conditions. By conducting multiple sets of experiments in deep water conditions of 100–2000 m, combined with the third-order monotonic upstream-centered scheme for conservation laws scheme and the finite volume numerical method of the approximate Riemann solver Harten-Lax-van Leer contact, the dynamics of bubbles and the propagation process of explosion pressure in the electrical explosion of wires under deep water conditions were accurately simulated. Numerical verification shows that the results are highly consistent with those of the comparative experiments in terms of peak shock wave overpressure, duration, etc., with an error of less than 6.4%. The results of the deep water electric explosion experiments indicate that, at a fixed explosion distance, the peak shock wave overpressure of Cu/Al wires does not change significantly with water depth, and the decrease range is 2%–14%. The positive pressure duration of the shock waves for both metal wires gradually decreases with increasing water depth. Regarding bubble parameters, as the water depth increases, the maximum bubble radius and the first pulsation periods of both types of metal wires decrease, and the pulsation pressure drops by up to 37%. The hydrostatic pressure restricts bubble expansion and accelerates its contraction process so that the bubble energy reaches a peak of approximately 2100 J after 1000 m and no longer increases. Overall, these research results provide valuable data support and technical references for gaining insight into the bubble dynamics in the deep sea.

An experimental study is conducted to investigate the spatiotemporal evolution of the turbulent boundary layer (TBL) over a flat plate towed in a large tank. The friction Reynolds number (Re_τ) covers the moderate-Re_τ range of 1200 ≤ Re_τ ≤ 3100. While time-resolved stereoscopic particle image velocimetry (TR-SPIV) is used to measure the streamwise-wall-normal plane, time-resolved planar PIV (TR-PIV) is employed to simultaneously measure the wall-parallel plane. Large-scale coherent structures such as large-scale motions (LSMs), very large-scale motions (VLSMs) and “footprints” of hairpin vortices in the two orthogonal planes are captured by the simultaneous TR-SPIV and TR-PIV measurements, enabling investigations of their spatiotemporal dynamics and interactions from a three-dimensional perspective. The spatial characteristics of the LSMs and VLSMs, which are elongated and inclined in the streamwise direction and meandering in the spanwise direction, are observed. Owing to the discrepancy in the convection velocities of different coherent structures, low- and high-speed LSMs (or VLSMs) interact and stretch vigorously, generating strong shear and leading to merging or splitting. Furthermore, conditionally averaged flow fields based on events of strong shear reveal that low-speed coherent structures move away from the wall to the edge of the TBL, whereas high-speed coherent structures entrain towards the wall. Finally, two-point correlations of different flow variables are made to examine the spatial coherence of these large-scale coherent structures. The outer-scaled length and width as well as the angle of inclination of the correlation contours remain unchanged within the Reynolds number range considered in this work; however, the length and width scales vary with the wall-normal height.

To accurately predict the three-dimensional flow characteristics of the flow field inside a waterjet propulsion pump, data assimilation (DA) method based on unsteady ensemble Kalman filter (EnKF) is used for the reconstruction of the flow field of a pump at different flow rates Q/Q_opt = 0.85, 1, 1.15, where Q_opt is optimal flow rate at the design point. As a compensation to the spatial limitation of planar particle image velocimetry (PIV) measurements, dynamic delayed detached-eddy simulation (DDES) results validated by the PIV data is used to provide the observational data at the optimized probe locations. In DA procedure, the shear stress transport (SST) model constants are optimized by the EnKF approach. The model constants are subsequently rescaled and fitted to form a variation with the flow rate, which is extended to the prediction of the flow field with other flow rates in the vicinity of the design condition. The results show that the SST model with recalibrated constants has improved the prediction of the internal flow field in the waterjet propulsion pump, especially the separation flow in the diffuser section. The modified model constants mainly reduce the eddy viscosity and significantly improve the fluctuation characteristics in the flow field. This study provides a reference for the fast and accurate prediction of the flow field information in the waterjet propulsion pump.