Simulation of mixed-mode fracture using SPH particles with an embedded fracture process zone

This paper focuses on the modelling of mixed-mode fracture using the conventional smoothed particle hydrodynamics (SPH) method and a mixed-mode cohesive fracture law embedded in the particles. The combination of conventional SPH and a mixed-mode cohesive model allows capturing fracture and separation under various loading conditions efficiently. The key advantage of this framework is its capability to represent complex fracture geometries by a set of cracked SPH particles, each of which can possess its own mixed-mode cohesive fracture with arbitrary orientations. Therefore, this can naturally capture complex fracture patterns without any predefined fracture topologies. Because a characteristic length scale related to the size of the fracture process zone is incorporated in the constitutive formulation, the proposed approach is independent from the spatial discretisation of the computational domain (or mesh independent). Furthermore, the anisotropic fracture responses of materials can be naturally captured thanks to the orientation of the fracture process zone embedded at the particle level. The performance of the proposed approach demonstrates its potentials in modelling mixed-mode fracture of rocks and similar quasi-brittle materials.     

A continuum‐discrete model using Darcy's law: formulation and verification

This paper presents a numerical scheme for fluid‐particle coupled discrete element method (DEM), which is based on poro‐elasticity. The motion of the particles is resolved by means of DEM. While within the proposition of Darcian regime, the fluid is assumed as a continuum phase on a Eulerian mesh, and the continuity equation on the fluid mesh for a compressible fluid is solved using the FEM. Analytical solutions of traditional soil mechanics examples, such as the isotropic compression and one‐dimensional upward seepage flow, were used to validate the proposed algorithm quantitatively. The numerical results showed very good agreement with the analytical solutions, which show the correctness of this algorithm. Sensitivity studies on the effect of some influential factors of the coupling scheme such as pore fluid bulk modulus, volumetric strain calculation, and fluid mesh size were performed to display the accuracy, efficiency, and robustness of the numerical algorithm. It is revealed that the pore fluid bulk modulus is a critical parameter that can affect the accuracy of the results. Because of the iterative coupling scheme of these algorithms, high value of fluid bulk modulus can result in instability and consequently reduction in the maximum possible time‐step. Furthermore, the increase of the fluid mesh size reduces the accuracy of the calculated pore pressure. This study enhances our current understanding of the capacity of fluid‐particle coupled DEM to simulate the mechanical behavior of saturated granular materials. Copyright © 2014 John Wiley & Sons, Ltd.     

云贵高原中部倒春寒特征及其极端类型大气环境场分析

利用1981—2021年云贵高原中部45个气象观测站逐日平均气温资料,以及NCEP2.5°×2.5°再分析日资料，采用倒春寒指数计算、分级以及合成分析等方法，分析云贵高原中部倒春寒天气年际、年代际变化特征，并确定特重级倒春寒年和无倒春寒年为倒春寒极端类型年，对比分析其环流特征，以期找出其预报指标。结果表明：（1）1981—2021年云贵高原中部区域年度倒春寒标准化指数总体呈下降趋势,且在1998年前后具有一个十分明显的突变（α=0.01）。特重级和重级倒春寒年基本发生在20世纪80年代和90年代，进入21世纪后，以轻级倒春寒年和无倒春寒年为主。（2）在特重级倒春寒年，500 hPa极涡偏强，亚洲中高纬呈现出西高东低的异常环流型，有利于东亚地区环流径向度加大，同时50°N以南温度基本呈负距平分布，表明冷空气活动异常活跃，从而造成地面云贵准静止锋偏强。与此同时，700 hPa上，云贵高原受异常气旋式环流东侧的偏东气流控制，与华北异常反气旋式环流底部的偏东气流在云贵高原上空辐合，将南海和东海的水汽向云贵高原上空输送，从而有利于低温和降水天气的形成。在无倒春寒年，上述形势相反。（3）在特重级倒春寒年，前期夏季、秋季、冬季和同期春季中东太平洋海温均表现为东部型厄尔尼诺；而在无倒春寒年，均表现为中部型厄尔尼诺。     

Projections of Annual Mean Air Temperature and Precipitation over the Globe and in China During the 21st Century bythe BCC Climate System Model BCC_CSM1.0

Evaluating the projection capability of climate models is an important task in climate model development and climate change studies. The projection capability of the Beijing Climate Center (BCC) Climate System Model BCC_CSM1.0 is analyzed in this study. We focus on evaluating the projected annual mean air temperature and precipitation during the 21st century under three emission scenarios (Special Report on Emission Scenarios (SRES) B1, A1B, and A2) of the BCC_CSM1.0 model, along with comparisons with 22 CMIP3 (Coupled Model Intercomparison Project Phase 3) climate models. Air temperature averaged both globally and within China is projected to increase continuously throughout the 21st century, while precipitation increases intermittently under each of the three emission scenarios, with some specific temporal and spatial characteristics. The changes in globally-averaged and China-averaged air temperature and precipitation simulated by the BCC_CSM1.0 model are within the range of CMIP3 model results. On average, the changes of precipitation and temperature are more pronounced over China than over the globe, which is also in agreement with the CMIP3 models. The projection capability of the BCC_CSM1.0 model is comparable to that of other climate system models. Furthermore, the results reveal that the climate change response to greenhouse gas emissions is stronger over China than in the global mean, which implies that China may be particularly sensitive to climate change in the 21st century.     

Broadband strong motion simulation in layered half-space using stochastic Green’s function technique

The stochastic Green’s function method, which simulates one component of the far-field S-waves from an extended fault plane at high frequencies (Kamae et al., J Struct Constr Eng Trans AIJ, 430:1–9, 1991), is extended to simulate the three components of the full waveform in layered half-spaces for broadband frequency range. The method firstly computes ground motions from small earthquakes, which correspond to the ruptures of sub-faults on a fault plane of a large earthquake, and secondly constructs the strong motions of the large earthquake by superposing the small ground motions using the empirical Green’s function technique (e.g., Irikura, Proc 7th Japan Earthq Eng Symp, 151–156, 1986). The broadband stochastic omega-square model is proposed as the moment rate functions of the small earthquakes, in which random and zero phases are used at higher and lower frequencies, respectively. The zero phases are introduced to simulate a smooth ramp function of the moment function with the duration of 1/fc s (fc: the corner frequency) and to reproduce coherent strong motions at low frequencies (i.e., the directivity pulse). As for the radiation coefficients, the theoretical values of double couple sources for lower frequencies and the theoretical isotropic values for the P-, SV-, and SH-waves (Onishi and Horike, J Struct Constr Eng Trans AIJ, 586:37–44, 2004) for high frequencies are used. The proposed method uses the theoretical Green’s functions of layered half-spaces instead of the far-field S-waves, which reproduce the complete waves including the direct and reflected P- and S-waves and surface waves at broadband frequencies. Finally, the proposed method is applied to the 1994 Northridge earthquake, and results show excellent agreement with the observation records at broadband frequencies. At the same time, the method still needs improvements especially because it underestimates the high-frequency vertical components in the near fault range. Nonetheless, the method will be useful for modeling high frequency contributions in the hybrid methods, which use stochastic and deterministic methods for high and low frequencies, respectively (e.g., the stochastic Green’s function method + finite difference methods; Kamae et al., Bull Seism Soc Am, 88:357–367, 1998; Pitarka et al., Bull Seism Soc Am 90:566–586, 2000), because it reproduces the full waveforms in layered media including not only random characteristics at higher frequencies but also theoretical and deterministic coherencies at lower frequencies.     

不同地质成因岩石地应力分布规律的统计分析

通过收集我国大陆地区大量的实测地应力资料,筛选出了受局部影响小的实测数据,建立了岩浆岩、沉积岩和变质岩地应力随埋深分布的散点图,并根据各自的分布规律进行了相应的回归分析。结果发现,三大类岩石的地应力分布规律存在差异,其主要与岩石地质成因相关。三大类岩石的垂直应力总体上随埋深呈线性增大,量值接近各自上覆岩层自重;最大水平主应力随埋深均呈线性增大,相同埋深条件下,火成岩量值最大,变质岩次之,沉积岩最小;而沉积岩浅部最小水平应力大于岩浆岩;三大类岩石的水平差应力总体上随埋深也呈线性关系,相同埋深条件下,火成岩较大,而沉积岩较小;浅部岩浆岩平均水平主应力与垂直应力之比最大,而沉积岩较小。     

城市化:相变与自组织临界性

城市化的本质是一种自组织临界相变过程 ,在时间和空间上分别联系着自组织临界性(SOC)和空间复杂性。城市化过程作为SOC有 3个方面的标志 :时间方向的 1/ f涨落、空间方面的分形结构和等级方式的Zipf定律 ,其共性是无标度性。分形中心地景观和位序 -规模分布分别是城市化自组织临界过程在空间和等级两个领域的实证判据。在城市化的临界相变过程中 ,城市体系通常演化为无标度网络 ,而位序 -规模法则乃是空间领域的无标度性在等级领域的结构性“投影”。基于SOC思想研究城市化过程有助于理解自组织城市网络的复杂空间动力学 ,从而为发展城市演化的模拟实验方法提供理论思路     

松辽平原黑土有机碳含量时空分异规律

松辽平原典型黑土分布区土壤有机碳的空间分异规律及不同开垦年限土壤有机碳含量变化的研究结果表明:随着纬度的增加,黑土总有机碳平均含量由11.43g/kg增加至20.83g/kg,有机碳组分含量和HA/FA比值均表现出增加的趋势。随着土壤剖面加深,黑土总有机碳及其组分含量均呈下降的趋势,土壤中的HA-C逐步被FA-C和HU-C所替代。土壤开垦前50年,黑土总有机碳及其组分含量下降迅速,以后下降趋于缓慢,至开垦130年后,有机碳含量水平基本处于相对稳定状态,变化不明显。     

The Nature and Predictability of the East Asian Extreme Cold Events of 2020/21

Three extreme cold events invaded China during the early winter period between December 2020 to mid-January 2021 and caused drastic temperature drops,setting new low-temperature records at many stations during 6?8 January 2021.These cold events occurred under background conditions of low Arctic sea ice extent and a La Ni?a event.This is somewhat expected since the coupled effect of large Arctic sea ice loss in autumn and sea surface temperature cooling in the tropical Pacific usually favors cold event occurrences in Eurasia.Further diagnosis reveals that the first cold event is related to the southward movement of the polar vortex and the second one is related to a continent-wide ridge,while both the southward polar vortex and the Asian blocking are crucial for the third event.Here,we evaluate the forecast skill for these three events utilizing the operational forecasts from the ECMWF model.We find that the third event had the highest predictability since it achieves the best skill in forecasting the East Asian cooling among the three events.Therefore,the predictability of these cold events,as well as their relationships with the atmospheric initial conditions,Arctic sea ice,and La Ni?a deserve further investigation.