Institute Of Mechanics,Chinese Academy of Sciences
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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
Flow boiling heat transfer in enhanced tubes with herringbone and dimple structure
No full textThis paper experimentally investigates the flow boiling heat transfer characteristics of smooth tubes (ST) and advanced new enhanced tubes (HT and EHT) made of aluminum, using R32 refrigerant. The effects of the tubes on heat transfer coefficients (HTC) and frictional pressure drop during flow boiling were explored. The study examined the flow boiling heat transfer performance of the enhanced tubes and ST tubes under varying conditions of mass flux (100 similar to 350 kg/m(2)s), saturation temperatures (279 similar to 288 K), and refrigerant inlet vapor quality (0.1 similar to 0.9), and analyzed the impact of the enhancement structures on flow boiling heat transfer. The results are that the HTC increases with rising saturation temperature, mass flux, and average vapor quality, with the HT tube exhibiting the best HTC, followed by the EHT tube. The result is that the frictional pressure drop increases with mass flux and average vapor quality and decreases with saturation temperature. Among them, the EHT tube exhibits the highest frictional pressure drop, while the pressure drop of the HT tube is closer to that of the ST tube. Additionally, four flow boiling heat transfer correlations were predicted the HTC of the ST tube, and the correlation with the best prediction results was selected to predict the HTC for the HT and EHT tubes. The revised correlation can verify over 95 % of the data within a +/- 15 % range, establishing a new HTC correlation for enhanced tubes
An asymmetric 2D braiding strategy for balancing hoop and axial strength in SiC/SiC composite nuclear fuel cladding
No full textTo address the challenge of inverted hoop and axial strength in SiC/SiC composite cladding for nuclear reactors, this study introduces an innovative asymmetric two-dimensional (2D) braided design. A multi-scale model was constructed to predict the mechanical properties of the claddings with different braiding structures. Leveraging a chemical vapor infiltration/chemical vapor deposition (CVI/CVD) process, gradient braided specimens with braiding angles of 30 degrees/45 degrees and 50 degrees/42 degrees were fabricated to systematically reveal the regulatory mechanism of the braiding angle on mechanical properties. Results indicate that the inner braid angle has a more significant impact on mechanical properties, while the outer braid design can compensate for the mechanical property deficiencies caused by the inner braid angle, thereby overcoming the inherent conflict between hoop and axial strengths in traditional symmetric designs. Raman spectroscopy revealed a residual compressive stress of-2.07 GPa in the large-angle braiding structure (50 degrees), contributing to improved hoop strength via prestress strengthening. Parameter weighting analysis indicated that the inner-layer braiding angle primarily dictates hoop strength, while axial strength is co-regulated by both braiding angle and porosity. This research provides a theoretical foundation for multi-objective optimization of nuclear fuel cladding performance
An asymmetric 2D braiding strategy for balancing hoop and axial strength in SiC/SiC composite nuclear fuel cladding
No full textTo address the challenge of inverted hoop and axial strength in SiC/SiC composite cladding for nuclear reactors, this study introduces an innovative asymmetric two-dimensional (2D) braided design. A multi-scale model was constructed to predict the mechanical properties of the claddings with different braiding structures. Leveraging a chemical vapor infiltration/chemical vapor deposition (CVI/CVD) process, gradient braided specimens with braiding angles of 30 degrees/45 degrees and 50 degrees/42 degrees were fabricated to systematically reveal the regulatory mechanism of the braiding angle on mechanical properties. Results indicate that the inner braid angle has a more significant impact on mechanical properties, while the outer braid design can compensate for the mechanical property deficiencies caused by the inner braid angle, thereby overcoming the inherent conflict between hoop and axial strengths in traditional symmetric designs. Raman spectroscopy revealed a residual compressive stress of-2.07 GPa in the large-angle braiding structure (50 degrees), contributing to improved hoop strength via prestress strengthening. Parameter weighting analysis indicated that the inner-layer braiding angle primarily dictates hoop strength, while axial strength is co-regulated by both braiding angle and porosity. This research provides a theoretical foundation for multi-objective optimization of nuclear fuel cladding performance
Energy dissipation mechanism of high-entropy alloys /CFRP/Al laminates under hypervelocity impact
No full textFiber metal laminates (FMLs) have garnered significant attention due to their high specific stiffness and strength, making them ideal candidates for spacecraft meteoroid/debris shielding in deep-space missions. In this study, an advanced energetic FMLs structure was designed and subjected to hypervelocity impact testing at 3.2 km/s and 5 km/s. The results indicate that, despite experiencing similar shock pressures and shock wave durations upon impact, the energetic FMLs structures with equivalent areal density exhibited superior ballistic performance compared to traditional metal or FMLs structures. These energetic FMLs structures have the ability to split the projectile into smaller parts and scatter the projectile shards over a larger space. Further analysis revealed that the shock heating effect of the energetic FMLs bumper accelerates the oxidation reactions of both projectile and bumper fragment elements due to its unique local plastic deformation characteristics and energy release mechanisms, thereby significantly enhancing the protective capability
Thermodynamic-kinetic relationship in Pd-based metallic glasses
No full textEstablishing a direct correlation between thermodynamic and kinetic behaviors in metallic glasses is of paramount importance, yet it remains an unresolved challenge in the field. Here, we conduct a comprehensive investigation on Pd-based metallic glasses, integrating dynamic mechanical analysis and differential scanning calorimetry to probe the interplay between mechanical relaxation and thermodynamic properties. Our results demonstrate that the temperature-dependent evolution of excess entropy remarkably parallels the kinetic spectrum, providing compelling evidence for a strong thermodynamic-kinetic relationship. Notably, we quantitatively explore the relationship between stress relaxation kinetics and excess entropy. This work provides new insights into the intrinsic coupling between thermodynamic disorder and mechanical relaxation behaviors in metallic glasses, offering a novel framework for understanding glass transition dynamics
Data-driven modeling of wind farm wake flow based on multi-scale feature recognition
No full textAccurate and efficient predictions of wind flow developments with wake effects accounted are crucial for wind farm layouts and power forecasting. Existing methods can be broadly classified as physical measurement, numerical simulations, physics-based modeling, and data-driven modeling. The first two is of high cost in terms of time and resources, the third suffers from low accuracy due to limited physics modeled, while the last one takes advantage of the large amount of high-quality data available and has become increasingly popular. This study proposes a rapid data-driven modeling method for wind farm wake flow, inspired by video frame interpolation and based on the principle of similarity, which utilizes a multi-scale feature recognition technique. The method transforms wind farm field data into images and predicts wake flow by identifying, matching, and interpolating features from a limited set of wake flow images using the Scale-Invariant Feature Transform (SIFT) and Dynamic Time Warping (DTW) approaches. To demonstrate the effectiveness of the proposed method, six representative cases were evaluated, encompassing mini wind farms with varying turbine spacings, different turbine sizes, combinations of spacing and size variations, different numbers of turbines, and various degrees of wind direction misalignment. A Mean Absolute Percentage Error (MAPE) ranging from 0.68% to 2.28% is achieved. Due to its ability to flexibly compute both 2D and 3D wake flow fields, the proposed method offers unique computational efficiency advantages over Large Eddy Simulation (LES) and Meteodyn WT in scenarios where two-dimensional wake flow fields are sufficient to meet industrial requirements. Therefore, this method can be employed for the extension of the wake flow database serving wind farm design, power prediction, etc., as an alternative to measurements, numerical simulation, and physics-based modeling, balancing efficiency and accuracy
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
Dynamics of molten droplet/pool fusion in additive manufacturing
No full textThis mini-review first clarifies a very basic process of the fusion-solidification between molten droplet/pool in additive manufacturing (AM) from its technical principle and various experimental observations on the material defects, such as porosity, separation and splashing, erosion and surface concave, and balling effects. Then, the dynamical similarities between the droplet/pool coalescence at normal temperature and the molten droplet/pool fusion at high temperature are illustrated to reveal the fusion-solidification-induced mechanism of material defects, where the droplet/pool fusion phenomena in the early stage have significant influences on the solidification of molten pool in the late stage for the AM process. Finally, some thoughts on the modeling of the fusion-solidification process between molten droplet/pool are stated by referring to the droplet-pool dynamics, which is benefit to the Eulerian-Lagrangian simulation on the prediction of AM process, so as to change the traditional trial-and-error methodology and improve the manufacturing technique
Physical and MPM modelling of sand column collapse with different moisture and density conditions
No full textThe sand column collapse test is a simple but useful experiment for investigating the dynamic behaviour of granular flow, which is an important topic in engineering geology and the validation of numerical models. Previous studies have not adequately considered the influence of soil moisture and density conditions. In this study, a series of sand column collapse tests were conducted, considering five water contents ranging from 0 to 10 % and two relative densities of 40 % and 58 %. Particle Image Velocimetry (PIV) was utilised to post-process the experimental results. A hydro-mechanical coupled Material Point Method (MPM), improved by incorporating a non-linear strain hardening/softening law, was employed to back-analyse the physical model tests. The measured and computed results show that as water content increases, the degree of collapse and post-collapse runout distance initially decrease, consistent with changes in Bishop's stress, affected by suction and interparticle water meniscus. As relative density increases, both the degree of collapse and the post-collapse runout distance decrease due to the greater shear strength and Bishop's stress. The MPM simulations closely matched experimental results, confirming the model's accuracy in simulating large deformations in both dry and unsaturated soils