Acta Mechanica Sinica最新文献

Physics-informed neural networks (PINNs) have recently emerged as a powerful tool to solve differential equations for nonlinear mechanics. However, PINNs struggle with singular perturbation problems due to their locally abrupt behavior and singularities. The Poincaré-Lighthill-Kuo (PLK) method has been efficiently used to address these problems by applying perturbation expansions to both dependent and independent variables. This paper proposes a combination of the PLK method and PINNs, termed PLK-PINNs. The PLK-PINNs employ a parametric expression through two neural networks: one representing the mapping from parametric variables to independent variables, and the other approximating the solution of dependent variables with respect to parametric variables. Moreover, an auxiliary loss term is proposed to constrain the Jacobian determinant of the mapping within a constant sign interval to ensure the bijectivity of the mapping. The effectiveness of the proposed method is demonstrated through tests on typical singularity-shift and secular-term problems, with conventional PINNs in comparison.

High-performance natural fiber composites have attracted global research interest due to their renewable nature, inherent biodegradability, excellent specific properties, and wide availability. The mechanical performance of single plant fiber-reinforced composites is relatively low, which severely limits their application in engineering. In this paper, the mechanical properties and failure mechanisms of ramie/basalt fiber-reinforced composites are studied. In order to do that, first, two biodegradable natural fibers, ramie fiber and basalt fiber, were selected as the reinforcing phase. Bisphenol A-type epoxy vinyl ester resin (HS-4430RT) was selected as the matrix. Four different configurations ([Ramie₄]_S, [Basalt₄]_S, [Ramie₂/Basalt₂ ]_S, and [Basalt₂/Ramie₂]_S) of composite laminates were fabricated using the vacuum-assisted resin transfer molding process. Then, quasi-static tensile tests and acoustic emission detection were conducted to investigate the mechanical performance of its tension specimens. Finally, the mechanical performance of the four different configurations of composite laminates under various impact energies was studied through a low-velocity impact test, and the impact response and energy absorption characteristics were evaluated. The research findings will provide valuable guidance for developing environmentally friendly ramie-basalt fiber-reinforced composites that meet mechanical performance requirements.

Enhancing the nonconservative force exerted on the ferromagnetic material is a key strategy for improving magnetoelastic coupling in multiferroic composites. Inspired by the mechanism of pulsed lasers injecting phonons directly into ferromagnetic materials, we propose a method to introduce and amplify phonons by constructing a “phonon channel”. Analyzing the magnon excitation spectrum reveals that magnetoelastic coupling is significantly enhanced with an increasing amplification coefficient A_p of nonconservative force in the phonon channel. When the external magnetic field is parallel to the wave vector of the spin wave, the amplitude of magnons in the anti-crossing region exhibits left-right symmetry with respect to the crossing point. In contrast, when the magnetic field is perpendicular, the symmetry becomes up-down. Introducing a coupling layer into the multiferroic composites provides a viable approach to realizing the phonon channel. Taking a multiferroic composite structure consisting of polyvinylidene fluoride and yttrium iron garnet as an example, we compare the electrical and acoustic properties of multiferroic composites with and without a coupling layer in the microwave frequency band. The enhancement of electrical and acoustic properties confirms the effectiveness of the phonon channel in multiferroic composites.

Direct numerical simulations of turbulent boundary layers over two types of discontinuous converging and diverging riblets are performed to validate their drag reduction performances and investigate their impacts on turbulence statistics and coherent structures. To suppress the total drag increases in the diverging region of traditional converging and diverging riblets (T-riblets), we design new converging and diverging riblets (N-riblets) with the heights gradually decreasing from the convergence line to the diverging line. Results show that both riblet configurations reduce the skin-friction drag, but the pressure drag is increased. The N-riblets are able to relieve the net drag increase from 7.42% to 0.93%, suggesting their potential in reducing the total drag. Both the discontinuous converging and diverging riblets cause the generation of large-scale secondary flows that originate from the ribbed walls and persist in the downstream wakes over the smooth walls. Their impacts on the time-averaged flow fields and Reynolds stresses are shown to be different between the T- and N-riblets. Moreover, two-point correlations of streamwise velocity fluctuations are calculated to show their modifications on coherent structures, providing more insights to explain the different impacts of T- and N-riblets on the fluid dynamics over the ribbed walls and in the downstream wakes.

Shock wave/boundary layer interaction (SWBLI) and shock/shock interaction (SSI) have drawn considerable focus due to their potential to cause severe aerodynamic heating. Research (Ai et al., 2024) demonstrated that within a high-enthalpy freestream environment, the 25°/55° double-cone flow exhibits critical coupling between SWBLI and SSI when considering high-temperature effects. This study extends the investigation into the effects of corner bluntness on double-cone flow characteristics, employing numerical simulations and the shock polar method. The results highlight a critical fillet radius beyond which the separation length decreases inversely with increasing bluntness, and below which it stabilizes at a larger value. As bluntness decreases, four distinct flow states are identified: weak interaction state, transition state, strong interaction state, and separation state. The separation state features a complex interaction of Type VI SSI, Type IV SSI, and SWBLI. Modest bluntness counterintuitively amplifies thermal loads, while substantial bluntness reduces the peak heat flux by 54.4%. Thermodynamic non-equilibrium downstream of the bow shock remains insensitive to bluntness, yet air dissociation and heat transfer correlate strongly with flow state evolution. Parameter evolution is further analyzed along wall-normal profiles at load extrema and streamlines through the triple point across bluntness variations. These insights hold significant implications for the industrial design and manufacturing of hypersonic vehicles.

In this work, we perform large-eddy simulations of the high-Reynolds-number turbulent flows over large two-dimensional transverse bars, aiming to establish the potential relationship between the mixing-layer-like shear flows in the roughness layer and the drag on the turbulent flow above. We systematically change the cavity width while keeping the height of the bars constant, and change the height of the bars with fixed cavity width for a parametric study. The results show that the mixing-layer-like shear flows emerge from the rear edge of the bars due to flow separation and develop along the crest of the cavities. As the cavity width increases, the inflectional mean velocity profile is gradually smoothed, which is related to the increasing roughness function. Meanwhile, the roughness function remains almost unchanged when increasing the height of the bars with fixed cavity width, due to similar momentum transport in the mixing-layer-like shear flows. Analysis based on the Navier-Stokes equations shows that most of the roughness drag is contributed by the Reynolds shear stress in these shear layers. The enhanced Reynolds shear stress leads to increased drag as the cavity width increases, while changing the roughness height has little impact on the mixing layer, resulting in little change in the roughness function. We also found that cavities with a width-to-height ratio equal to 1 have strong vertical motions inside the cavities but smaller drag than expected.

The paper designed a petal-shaped carve beam structure (PCBS) inspired by the petal features, and its band gap properties and elastic wave propagation behavior are explored. The influence of the resonance block radius R and beam width t on the energy band structure of the PCBS is investigated separately. The research reveals that increase of the resonant block radius R leads to a wider the second band gap and higher fractional bandwidth (FB). Increase of the beam width t leads to a higher band gap frequency. The FB reaches up to 125% after parameter optimization. Subsequently, the band gap’s formation mechanism is explored using vibration mode, and it was found that the resonant block’s local resonance is the reason for the development of the wide-frequency band gap. Finally, the properties of elastic wave transmission within the PCBS are investigated. The consequence demonstrates the PCBS has remarkable band gap properties with wide frequency band gap occurs around 2972 and 16996 Hz with widths of 4887 and 5171 Hz, respectively, accompanied by an FB reaching 90.24%. The peak attenuation within the band gap reaches 350 dB. This highlights its ability for broad-frequency wave control and offers a theoretical reference for novel vibration reduction design.

The numerical manifold method, extensively utilized in numerical computations, faces significant challenges in generating complex manifold elements, particularly for three-dimensional applications. To overcome this challenge, the meshfree numerical manifold method is developed by integrating the moving least-squares method into the numerical manifold method, effectively bypassing the need for meshing complex geometric objects. However, the implementation of the moving least-squares method introduces computational efficiency issues. To mitigate these, parallel computing methods have been incorporated, resulting in a tenfold increase in the speed of assembling the stiffness matrix with central processing unit parallelism, and a twentyfold increase with graphics processing unit parallelism. The static mechanical system equations for the meshfree numerical manifold method are derived using the Galerkin method. The method’s effectiveness and accuracy are then validated through a series of numerical experiments. The experiments demonstrated that the meshfree numerical manifold method achieves a high precision with minimal nodes and integration points. Additionally, positioning nodes outside the domain significantly improves computational accuracy at the boundaries.

The traditional method of performance degradation prediction and maintenance of rolling bearings only considers a single sensor signal, which makes it difficult to automatically partition degradation stages and prone to over-detection. A new method of performance degradation evaluation and maintenance of rolling bearings based on data-level fusion, adaptive health state partitioning, and state maintenance is proposed. Firstly, considering the degradation and impact in the process of bearing deterioration, the multi-sensor signals are dynamically weighted to achieve data-level fusion. Secondly, a bearing health index was established based on fast spectral correlation, Wasserstein distance, and linear rectification techniques. On this basis, by combining the Bayesian information criterion and the elbow rule, the precise division of rolling bearing health state is realized through hidden Markov model regression. Then, random forest was used to classify and predict the data to verify the validity of the proposed data fusion method and health indicator. Finally, condition-based maintenance strategy based on the fourth moment, stress-strength interference model, and Gamma process is proposed to avoid excessive detection and reduce maintenance costs. Through accelerated degradation experiments and field validation tests on the rolling bearing test data set of Xi’an Jiaotong University and FEMTO (PRONOSTIA), the accuracy and superiority of the proposed method in the prediction and maintenance of bearing health state are verified.

In nature, some undulating propulsion organisms with broad pectoral fin utilize ground effect to swim near walls such as the seabed. Inspired by these organisms, the immersed boundary method was adopted to carry out a three-dimensional numerical simulation of a self-propelled wave plate in ground effect. We had taken into account the three-dimensional flow characteristics of the wavy plate from aspects such as the initial height from the ground, the undulating parameters, and the geometric features of the body. It is found that the undulating rules of travelling wave plate in ground effect need to be controlled in order to obtain better motion performance in ground effect, and the wavy plate can enhance the thrust force rather than the lift force compared with flapping propulsion. The characteristics of the wake vortex structure of the wavy plate change with the undulating amplitude. The optimal fluctuation amplitude enables the plate to achieve a relatively good propulsion speed. Compared with other amplitudes, the swimming efficiency can be increased by 66% at the optimal fluctuation amplitude. Increasing the undulating frequency does not alter the structure of the wake vortices, but it results in a uniform enhancement of the wake vorticity intensity of the plate. When the frequency increases from 1.8 to 2.6, the cruising speed increases by 48%, and the swimming efficiency improves by 30%. For the three-dimensional shape, different shape characteristics also have an impact on it. The plate with larger aspect ratio has a higher speed and a higher swimming efficiency under the same undulating parameters.