Institute Of Mechanics,Chinese Academy of Sciences
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Exceptional tensile properties induced by interlayer-compatible deformation in a gradient ultra-nanograined Cu
No full textIn this study, a gradient ultra-nanograined (GUNG) Cu was prepared by surface rolling and shearing processing at liquid nitrogen temperature. Microstructural analysis reveals a significant presence of ultrananograins (similar to 5-20 nm) within the topmost surface layer (SL), transitioning to coarser grains beneath, culminating in a gradient structure over 600 mu m deep. The GUNG Cu exhibits an exceptional strength-ductility synergy, achieving yield strengths of 250-330 MPa and uniform elongations of 17 %-30 %. The deformation mechanisms of GUNG Cu are elucidated through in-situ electron backscatter diffraction and microscopic digital image correlation, highlighting the interlayer-compatible deformation of GUNG Cu under tensile loading. It is noteworthy that the topmost ultra-nanograined SL (within depths of 0-2 mu m) in GUNG Cu maintains high mechanical stability with minimal change in grain size during tensile plastic deformation, whereas the subsurface layer (at a depth of similar to 15 mu m) displays a deformation-driven grain coarsening behavior, facilitating deformation compatibility across individual layers. The enhanced strength-ductility synergy exhibited in GUNG Cu can be attributed to the interplay between interlayer compatible deformation and hetero-deformation induced (HDI) hardening, in which softer and harder layers interact with each other, thus promoting the strain hardening throughout the GUNG structure. The present findings provide a more profound understanding of deformation compatibility and HDI hardening mechanisms in gradient structures, demonstrating how tailored microstructural heterogeneity can potentially circumvent the traditional strength-ductility trade-off in nanostructured materials. (c) 2025 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology
Semi-analytical modeling of coating-crack-defect interactions using a combined distributed dislocation technique and numerical equivalent inclusion method
No full textCoatings play a critical role in controlling stress concentrations, mitigating crack-defect interactions, and enhancing the durability of tribological components under frictional loading. Understanding the interactions among cracks, coatings, and defects at the microscale is therefore critical for elucidating the underlying mechanisms and ensuring the design and reliability of coated materials in friction-related applications. This studyudy investigates the effect of coating on the interaction between a multi-branched crack and arbitrarily shaped inhomogeneities or voids. The governing equations for coatings, inhomogeneities, voids and cracks are fully coupled into a unified model. Furthermore, the stress solutions for the crack with multiple branches at any angle and length are innovatively derived in the half plane with the help of the Distributed Dislocation Technique (DDT). Based on the numerical equivalent inclusion method (NEIM) and Fast Fourier Transform (FFT) algorithms, a semi-analytical scheme with a multi-stage iterative procedure is presented to obtain the final stress solutions and the stress intensity factors (SIFs). Benchmark examples compared with finite element method (FEM) results validate the numerical implementation. The proposed semi-analytical method overcomes limitations related to crack branching, inhomogeneity shapes, and mesh complexity, offering enhanced flexibility and computational efficiency
Multi-scale analyses of flow separation around rectangular prisms in uniform flow
No full textTurbulent flows around infinitely spanned rectangular prisms in uniform flow with streamwise aspect ratios, AR = 1, 2, and 3 were studied using time-resolved particle image velocimetry (TR-PIV) at a Reynolds number of 16,200 based on free-stream velocity and prism height. These aspect ratios span regimes that transition from direct shear layer shedding in the wake (AR1) to intermittent reattachment (AR2) and mean reattachment on the prism surface (AR3). The mean flow topology, Reynolds shear stress, and turbulent transport were analyzed. Spatiotemporal characteristics were investigated using two-point correlation, integral time scales, reverse flow areas, and proper orthogonal decomposition (POD) of the vorticity field. The results reveal non-monotonic variations of statistical and structural characteristics with aspect ratio. The case of AR2 possesses the largest recirculation region as well as the largest spatial and temporal scales of coherent structures. The wake exhibits quasi-periodic fluctuations concentrated in a single frequency for AR1, and AR3 but dual frequencies for AR2. The POD of the vorticity effectively decomposed a wide range of scales. Depending on the aspect ratio, spectra of the POD coefficients revealed concentrated spectral energy at the dominant vortex shedding frequency, its harmonics and at Kelvin-Helmholtz instability frequencies associated with small-scale vortices near the leading edge
Multi-scale analyses of flow separation around rectangular prisms in uniform flow
No full textTurbulent flows around infinitely spanned rectangular prisms in uniform flow with streamwise aspect ratios, AR = 1, 2, and 3 were studied using time-resolved particle image velocimetry (TR-PIV) at a Reynolds number of 16,200 based on free-stream velocity and prism height. These aspect ratios span regimes that transition from direct shear layer shedding in the wake (AR1) to intermittent reattachment (AR2) and mean reattachment on the prism surface (AR3). The mean flow topology, Reynolds shear stress, and turbulent transport were analyzed. Spatiotemporal characteristics were investigated using two-point correlation, integral time scales, reverse flow areas, and proper orthogonal decomposition (POD) of the vorticity field. The results reveal non-monotonic variations of statistical and structural characteristics with aspect ratio. The case of AR2 possesses the largest recirculation region as well as the largest spatial and temporal scales of coherent structures. The wake exhibits quasi-periodic fluctuations concentrated in a single frequency for AR1, and AR3 but dual frequencies for AR2. The POD of the vorticity effectively decomposed a wide range of scales. Depending on the aspect ratio, spectra of the POD coefficients revealed concentrated spectral energy at the dominant vortex shedding frequency, its harmonics and at Kelvin-Helmholtz instability frequencies associated with small-scale vortices near the leading edge
Droplet sliding on an inclined substrate with chemical defects
No full textWe numerically study the dynamics of a droplet sliding on an inclined and inhomogeneous substrate under gravity force using three-dimensional diffuse interface method. On the path of the sliding droplet, there are two transversely located defects with the shape of circle, and their wettability is different from the substrate. Our aim is to identify the regimes of the droplet motion and figure out the critical conditions between the different regimes. By varying the distance between the two defects, the inclination angle of the substrate, and the size of the defects, three regimes are found according to the status of the droplet: capture, breakup and release. By analyzing geometrical relationships, the shape of the contact line and the force balance of the droplet at rest, we derive critical conditions between each two regimes. The theoretical predictions of critical conditions agree well with our numerical simulations, which provide a comprehensive understanding of the underlying mechanisms of a droplet sliding on a chemical defect substrate
A novel flexible membrane boundary method based on color function in DEM simulations for triaxial tests of granular materials
No full textThis technical note presents a novel flexible membrane algorithm for DEM-based triaxial test simulations. Using color functions to calculate the normal vectors of membrane particles, the algorithm can model mechanical response of flexible membrane under large deformations and curvatures. It also uses imaginary particles to improve calculation accuracy of normal vector nearby high curvature area. After validation based on lab test result, the algorithm effectively reproduces the stress-strain behavior of specimens under triaxial compression, offering a more precise and reliable tool for studying the mechanical response of granular materials
Wind farm fluid mechanics for high-penetration wind energy
No full textAdvancements in aerodynamics during the early 20th century laid the foundation for modern wind energy. The increasing penetration of wind power presents novel challenges in fluid mechanics, which stem from an incomplete understanding of the dynamics of wind turbine wakes and their interactions with the atmospheric flow. This article provides a comprehensive review of the current understanding of the mechanisms of wind turbine wakes and wake-atmosphere interactions. It summarizes existing models for wind turbine wakes and explores control strategies for mitigating wake losses and tracking power reference signals. Finally, it delves into research trends in the field and summarizes the review
Direct numerical simulation of Rayleigh-Bénard convection based on physics-informed neural networks with transfer learning
No full textRayleigh-B & eacute;nard (RB) convection, characterized by a fluid layer with bottom heating and top cooling, serves as a fundamental model system in fluid dynamics research, serves as an essential paradigm for studying thermally driven flows, offering fundamental understanding of heat transfer, fluid mixing, and turbulent transition processes that occur widely in nature and industrial systems. This study introduces the application of Physics-Informed Neural Networks (PINNs) augmented with transfer learning techniques. Using transfer learning, our aim is to take advantage of the knowledge gained from training PINNs on a Ra condition to improve predictions for other Ra values. Preliminary results show that transfer learning-enhanced PINNs successfully capture the convective regime while avoiding convergence to steady-state solutions, enabling efficient prediction across varying Rayleigh (Ra) numbers without requiring full retraining. Furthermore, different ways of transferring models are also proposed to explore the feasibility of knowledge transfer across different natural convection configurations, including cases with varying inclination angles and Prandtl (Pr) numbers. The effective incorporation of transfer learning into PINNs have demonstrated promising capabilities for RB convection modeling, suggesting several key areas for future investigation. Further advanced transfer strategies suited to particular physical systems and conditions can be investigated as PINNs develop
Numerical investigation of mixed-phase turbulence in flow past a partially merged plate
No full textLarge-eddy simulation (LES) is conducted to study the statistical properties of mixed-phase turbulence induced by the breaking of bow waves in flow past a partially submerged plate. The simulation is performed using a finite difference method, with the air-water interface captured by a coupled level-set and volume-of-fluid method. Four cases are conducted to investigate the effects of Froude number on turbulent statistics, including the mean velocity, turbulence kinetic energy, and turbulence mass flux (TMF), which is an additional unclosed term in the Reynolds-averaged momentum equation. The TMF, especially its vertical component, shows a complex behaviour with respect to the Froude number. This property of the TMF imposes high demands on the robustness of the closure model of TMF. The present LES data is further used to examine a closure model of the TMF production term, which shows a high correlation with the data obtained from LES
Permeability anisotropy and non-Darcy effect of gas shale: A case study of S1l<sub>1-1</sub><SUP>1</SUP> sublayer of Longmaxi Formation in Sichuan Basin, China
No full textPermeability anisotropy and non-Darcy effect are critical factors in determing the production capacity of gas shale. Through combined testing using the cubic-block and crushed-particle sample methods, we systematically characterized the percolation characteristics of 27 samples from the main production layer (S1l(1-1)(1) sublayer) of the Longmaxi Formation. Results show the quartz matrix support system dominates percolation perpendicular to bedding, with permeability (K-V approximate to 1-500 nD) showing high consistency with crushed sample results (K-C), controlled by porosity-quartz content co-evolution. However, clay directional alignment significantly enhances bedding-parallel percolation advantage (K-P/K-V = 10(1)-10(3)), with every 5 % increase in clay content elevating K-P by approximately one order of magnitude. Based on Knudsen number (Kn), we demonstrate that Darcy flow and slip flow are the main flow forms while pure Knudsen flow is rare in shale gas development. Finally, an integrated evaluation framework of permeability to optimize shale gas exploration is developed by incorporating with actual well performance. From the perspective of permeability, reservoirs with Kn < 0.2 and K-P/K-V < 100 exhibit better exploration and development potential