基于不规则钙质砂颗粒发展的拖曳力系数模型及其在细观流固耦合数值模拟中应用

钙质砂广泛分布于我国南海海域,作为岛礁填筑材料,其渗透性很大程度上决定着填筑后土体的固结和沉降。拖曳力系数是表达液体对土体颗粒表面力的参数,也是表征颗粒状土体渗透能力的一个重要指标,目前国内外对具有极不规则形状钙质砂拖曳力系数的研究十分有限。在前期大量钙质砂颗粒沉降试验基础上,建立了能够考虑形状系数的颗粒与液体相互作用拖曳力系数理论公式;在已建立的计算流体动力学-离散元法(CFD-DEM)流固耦合理论框架下,将新发展的拖曳力系数公式嵌入到流固耦合程序中,模拟钙质砂颗粒在液体中沉降过程。通过将模拟结果与室内沉降试验对比,验证了新建立的拖曳力模型及程序实现的准确性。研究结果表明,较CFD-DEM程序中已有的未考虑颗粒形状的拖曳力系数半经验模型,新的拖曳力系数模型能够更准确地模拟不规则颗粒在液体中沉降过程;同时,在数值建模中无需采用异形颗粒即可以反映颗粒形状对拖曳力的影响,这将大大降低数值模拟的计算量,提高计算效率。研究可进一步应用于钙质砂水中堆填后固结沉降以及冲刷等实际工程问题分析中。     

基于不规则钙质砂颗粒发展的拖曳力系数模型及CFD-DEM流固耦合数值方法

钙质砂广泛分布于我国南海海域,作为岛礁填筑材料,其渗透性很大程度上决定着填筑后土体的固结和沉降。拖曳力系数是表达液体对土体颗粒表面力的参数,也是表征颗粒状土体渗透能力的一个重要指标,目前国内外对具有极不规则形状钙质砂拖曳力系数的研究十分有限。在前期大量钙质砂颗粒沉降试验基础上,建立了能够考虑形状系数的颗粒与液体相互作用拖曳力系数理论公式;在已建立的CFD-DEM流固耦合理论框架下,将新发展的拖曳力系数公式嵌入到流固耦合程序中,模拟钙质砂颗粒在液体中沉降过程。通过将模拟结果与室内沉降试验对比,验证了新建立的拖曳力模型及程序实现的准确性。研究结果表明,较CFD-DEM程序中已有的未考虑颗粒形状的拖曳力系数半经验模型,新的拖曳力系数模型能够更准确地模拟不规则颗粒在液体中沉降过程;同时,在数值建模中无需采用异形颗粒即可以反映颗粒形状对拖曳力的影响,这将大大降低数值模拟的计算量,提高计算效率。研究可进一步应用于钙质砂水中堆填后固结沉降以及冲刷等实际工程问题分析中。     

基于不规则钙质砂颗粒发展的拖曳力系数模型及CFD-DEM流固耦合数值方法

钙质砂广泛分布于我国南海海域,作为岛礁填筑材料,其渗透性很大程度上决定着填筑后土体的固结和沉降。拖曳力系数是表达液体对土体颗粒表面力的参数,也是表征颗粒状土体渗透能力的一个重要指标,目前国内外对具有极不规则形状钙质砂拖曳力系数的研究十分有限。在前期大量钙质砂颗粒沉降试验基础上,建立了能够考虑形状系数的颗粒与液体相互作用拖曳力系数理论公式;在已建立的CFD-DEM流固耦合理论框架下,将新发展的拖曳力系数公式嵌入到流固耦合程序中,模拟钙质砂颗粒在液体中沉降过程。通过将模拟结果与室内沉降试验对比,验证了新建立的拖曳力模型及程序实现的准确性。研究结果表明,较CFD-DEM程序中已有的未考虑颗粒形状的拖曳力系数半经验模型,新的拖曳力系数模型能够更准确地模拟不规则颗粒在液体中沉降过程;同时,在数值建模中无需采用异形颗粒即可以反映颗粒形状对拖曳力的影响,这将大大降低数值模拟的计算量,提高计算效率。研究可进一步应用于钙质砂水中堆填后固结沉降以及冲刷等实际工程问题分析中。     

基于不规则钙质砂颗粒发展的拖曳力系数模型及CFD-DEM流固耦合数值方法

钙质砂广泛分布于我国南海海域,作为岛礁填筑材料,其渗透性很大程度上决定着填筑后土体的固结和沉降。拖曳力系数是表达液体对土体颗粒表面力的参数,也是表征颗粒状土体渗透能力的一个重要指标,目前国内外对具有极不规则形状钙质砂拖曳力系数的研究十分有限。在前期大量钙质砂颗粒沉降试验基础上,建立了能够考虑形状系数的颗粒与液体相互作用拖曳力系数理论公式;在已建立的CFD-DEM流固耦合理论框架下,将新发展的拖曳力系数公式嵌入到流固耦合程序中,模拟钙质砂颗粒在液体中沉降过程。通过将模拟结果与室内沉降试验对比,验证了新建立的拖曳力模型及程序实现的准确性。研究结果表明,较CFD-DEM程序中已有的未考虑颗粒形状的拖曳力系数半经验模型,新的拖曳力系数模型能够更准确地模拟不规则颗粒在液体中沉降过程;同时,在数值建模中无需采用异形颗粒即可以反映颗粒形状对拖曳力的影响,这将大大降低数值模拟的计算量,提高计算效率。研究可进一步应用于钙质砂水中堆填后固结沉降以及冲刷等实际工程问题分析中。     

考虑粒间滚动阻力的CFD-DEM流-固耦合数值模拟方法

在离散元(DEM)和计算流体动力学(CFD)程序基础上,运用流-固相互作用力方程建立三维CFD-DEM细观耦合数值模拟模型。CFD-DEM的控制方程包括流体-颗粒相互作用力方程、CFD的流体和DEM的颗粒运动方程。其中,考虑到土颗粒的形状效应,引入颗粒滚动阻力机制,介绍了适用性较强的滚动阻力模型。在开源耦合程序CFDEM的框架下,定制CFD开源程序Open FOAM与DEM开源程序LIGGGHTS之间的双向耦合,实现基于颗粒细观的流-固耦合计算。通过砂堆和砂土渗流的模拟分别验证了所嵌入的滚动阻力模型的有效性及耦合模型的可行性。模拟结果表明,圆柱土体中向上渗流流速与水力梯度关系与经典的Ergun理论解接近;耦合模型中所引入的颗粒间滚动阻力模型能够很好地反映颗粒形状对砂堆形成休止角和土体堆积孔隙率的影响。     

层流区Shields曲线分布规律试验

为探明层流条件下Shields曲线的分布规律,运用电荷耦合组件(Charge Coupled Device,CCD)成像技术结合激光扫描及计算机图像处理技术,在水和甘油混合液中对无黏性均匀玻璃颗粒的起动拖曳力进行测量,并根据层流绕流理论建立了无黏性均匀颗粒的滚动起动模型,提出层流条件下无黏性均匀颗粒的量纲一起动拖曳力公式.试验观测发现在层流范围内希尔兹数呈规则的带状分布,流体作用引起颗粒床表面粗化并导致床面颗粒突起减小,使颗粒起动拖曳力增大1倍以上.结果表明颗粒床表面结构性状直接影响着颗粒起动拖曳力的大小,在层流区Shields曲线具有带状分布特性.     

考虑钙质砂细观颗粒形状影响的液体拖曳力系数试验

钙质砂作为南海岛礁填筑常用的岩土材料,其渗透性很大程度上决定着填筑后土体的固结和沉降。拖曳力系数是表达流体对土体颗粒表面力的参数,也是表征颗粒状土体渗透能力的一个重要参数,目前国内外对钙质砂拖曳力系数的研究十分有限。首先引入一个修正的三维参数 对钙质砂这种天然非规则颗粒材料的形状进行定量描述,然后开展一系列单个钙质砂颗粒在液体中沉降试验,利用高速相机记录颗粒沉降过程,结合图像处理技术获得颗粒沉降平衡速度Ut,进而计算出拖曳力系数CD和雷诺数Re,最后拟合出包含CD、Re及 三个参数的钙质砂拖曳力系数半经验模型。结果发现,在相同雷诺数条件下钙质砂的形状系数 越大,拖曳力系数越小。通过与其他研究结果对比发现,其表面微孔隙越发育,拖曳力系数越小的规律。该模型能够考虑不规则颗粒形状对拖曳力系数的影响,从而提高对土体渗透性预测的精度,对南海岛礁填筑工程中钙质砂固结和沉降的计算也具有重要意义。     

南极威德尔海冬季冰脊形拖曳力

利用2006年南极威德尔海冬季科学考察期间由机载激光高度计测得的海冰上表面形态数据,以拖曳分割理论为基础研究了冰脊形拖曳力及其对冰-气总拖曳力的贡献以及中性条件下对应10 m高度处风速的冰-气拖曳系数C_dn(10)。结果显示,在密集冰区,冰脊形拖曳力及其对总拖曳力的贡献随冰脊强度(脊高/脊距)的增大呈递增趋势,而随粗糙长度的增大而减小,对应于威德尔海冬季典型冰脊强度和粗糙长度,占总拖曳力的35%,说明了冰脊形拖曳力在冰-气界面动量交换中的重要作用。C_dn(10)随冰脊强度的增大而增大,但冰脊强度较小时,C_dn(10)随粗糙长度增大而增大;而对较大的冰脊强度,C_dn(10)则随粗糙长度增大而减小。     

等结构矿物的光性异向指数

在研究透明矿物光学异向性时,人们总是用三个主折射率 N_g、N_m、N_p 和重折射率N_g-N_p 来表示。本文作者在一系列的论文中使用了折射度指示体和光性异向指数的新概念。用光性异向指数(Optical anisotropy index 即 OAl 值)研究矿物光学异向性与晶体结构关系时,远远比折射率和重折射率的概念更有意义得多。     

水流拖曳力对斜坡浅层土稳定性的影响分析

为研究强降雨条件下地表径流和裂隙水流的拖曳力作用对斜坡稳定性的影响,建立斜坡地表径流和地下水渗流耦合分析模型。采用Navier–Stokes方程描述地表径流和裂隙水流,Brinkman-extended Darcy方程描述土体和岩石中的渗流,并根据不同介质交界面处流速相等及剪应力连续的边界条件,推求各个介质中的流速分布。应用Newton内摩擦定律求得水流对斜坡产生的拖曳力,进而将拖曳力嵌入刚体极限平衡理论对斜坡稳定性进行分析。典型斜坡实例计算结果表明,当未考虑拖曳力效应时,斜坡的安全系数为1.164,考虑水流对斜坡土体产生拖曳力效应时,斜坡的安全系数为1.089,斜坡安全系数降低了6.44%。这表明拖曳力作用对斜坡的稳定性存在不利影响,斜坡土体在近乎临界稳定状态下拖曳力将对斜坡失稳起到决定性作用。最后分别讨论了考虑拖曳力和不考虑拖曳力情况下斜坡的安全系数与径流水深、斜坡倾角、土层厚度之间的关系。     

Undrained lateral load capacity of piles in clay using artificial neural network

This paper describes the application of the artificial neural network model to predict the lateral load capacity of piles in clay. Three criteria were selected to compare the ANN model with the available empirical models: the best fit line for predicted lateral load capacity (Q_p) and measured lateral load capacity (Q_m), the mean and standard deviation of the ratio Q_p/Q_m and the cumulative probability for Q_p/Q_m. Different sensitivity analysis to identify the most important input parameters is discussed. A neural interpretation diagram is presented showing the effects of input parameters. A model equation is presented based on neural network parameters.     

散粒粗糙床面的当量糙率尺度特征

为揭示散粒粗糙床面的当量糙率尺度特征,采用4种散粒粗糙床面开展了8组粒子图像测速法(PIV)水槽明渠紊流试验,探讨了理论床面位置、当量糙率尺度和阻力系数的沿程分布规律,分析了当量糙率尺度与阻力系数的内在关系。结果表明:(1)散粒粗糙床面明渠水流特性沿程可分为颗粒区和粒间区,各区的理论床面位置参数ξ、相对当量糙率尺度k_s0和阻力系数f具有显著差异,颗粒区0. 65≤ξ≤0. 85,k_s0波动明显,f较大;粒间区ξ=1,k_s0变化相对平缓,f较小。(2) k_s0沿程平均值与颗粒直径d成正比,与排列间距Lx及水流雷诺数Re成反比,在d、Lx及Re相同时,梅花形排列的k_s0大于矩形排列。(3) f随k_s0呈递增变化,变化特性与相对光滑度h/d密切相关。研究成果可为散粒粗糙床面沿程阻力系数确定提供科学参考。     

Regional time-predictable modeling in the Hindukush–Pamir–Himalayas region

The seismic characteristic of Hindukush–Pamir–Himalaya (HPH) and its vicinity is very peculiar and has experienced many widely distributed large earthquakes. Recent work on the time-dependent seismicity in the Hindukush–Pamir–Himalayas is mainly based on the so-called “regional time-predictable model”, which is expressed by the relation log T=cM_p+a, where T is the inter-event time between two successive main shocks of a region and M_p is the magnitude of the preceded main shock. Parameter a is a function of the magnitude of the minimum earthquake considered and of the tectonic loading and c is positive (0.3) constant. In 90% of the cases with sufficient data, parameter c was found to be positive, which strongly supports the validity of the model. In the present study, a different approach, which assumes no prior regionalization of the area, is attempted to check the validity of the model. Nine seismic sources were defined within the considered region and the inter-event time of strong shallow main shock were determined and used for each source in an attempt at long-term prediction, which show the clustering and occurrence of at least three earthquakes of magnitude 5.5≤M_s≤7.5 giving two repeat times, satisfying the necessary and sufficient conditions of time-predictable model (TP model). Further, using the global applicability of the regional time- and magnitude-predictable model, the following relations have been obtained: log T_t=0.19 M_min+0.52M_p+0.29 log m₀−10.63 and M_f=1.31M_min−0.60M_p−0.72 log m₀+21.01, where T_t is the inter-event time, measured in years; M_min the surface wave magnitude of the smallest main shock considered; M_p the magnitude of preceding main shock; M_f the magnitude of the following main shock; and m₀ the moment rate in each source per year.

These relations may be used for seismic hazard assessment in the region. Based on these relations and taking into account the time of occurrence and the magnitude of the last main shock in each seismogenic source, time-dependent conditional probabilities for the occurrence of the next large (M_s≥5.5) shallow main shocks during the next 20 years as well as the magnitudes of the expected main shocks are determined.     

The influence of the void fraction on the particle migration: A coupled computational fluid dynamics–discrete element method study about drag force correlations

Granular soils subjected to flow through their soil skeleton can show a behaviour in which fine particles migrate through the pore space between coarser particles. This process is called internal instability or suffusion. This contribution deals with the numerical analysis of the migration of fine particles in a soil column subjected to fluid flow with unresolved coupled computational fluid dynamics–discrete element method (CFD–DEM) with special regards to the used drag force correlation. The contribution investigates the influence of the Schiller–Naumann model and its extension with a voidage term on the migration behaviour of fine particles. The voidage term is further varied with a parameter, which controls the impact of the change of the void fraction on the drag force. It could be observed that the Schiller–Naumann model does not yield in a suffusive behaviour while the extended models show significant particle migration. Thereby, increasing the impact of the void fraction on the drag force results in stronger particle migration. These results reveal the need for good validation techniques. They indicate how the drag force correlation can be adapted to depict the correct particle migration behaviour.     

On the implicit immersed boundary method in coupled discrete element and lattice Boltzmann method

The coupled discrete element method and lattice Boltzmann method (DEMLBM) has increasingly drawn attention of researchers in geomechanics due to its mesoscopic nature since 2000. Immersed boundary method (IBM) and immersed moving boundary (IMB) are two popular schemes for coupling fluid particle in DEMLBM. This work aims at coupling DEM and LBM using the latest IBM algorithm and investigating its accuracy, computational efficiency, and applicability. Two benchmark tests, interstitial fluid flow in an ideal packing and single particle sedimentation in viscous fluid, are carried out to demonstrate the accuracy of IBM through semi-empirical Ergun equation, finite element method (FEM), and IMB. Then, simulations of particle migration with relatively large velocity in Poiseuille flow are utilized to address limitations of IBM in DEMLBM modeling. In addition, advantages and deficiencies of IBM are discussed and compared with IMB. It is found that the accuracy of IBM can be only guaranteed when sufficient boundary points are used and it is not suitable for geomechanical problems involving large fluid or particle velocity.     

Refinement of thrust faulting models for the Central New Madrid seismic zone

We present the results of the joint relocation of events recorded during 1989–1992 by the PANDA network in the central New Madrid seismic zone. The near-surface material in the study area is a gently-dipping layer of poorly consolidated sediments with low P-wave velocity and high V_p/V_s (estimated values: 1.8 km s⁻¹ and 3). The sediments are underlain by high-velocity Paleozoic rocks. Under the network the difference in sediment thickness is only 0.6 km, but because of the low velocities the location of the events using layered models is affected by errors. Application of the joint hypocentral determination (JHD) technique to a subset of 580 events shows that the single-event locations may be in error by as much as 1 km in depth, depending on where the events are located. Analysis of synthetic data generated for a realistic 3-D velocity model supports the JHD results. The analysis of synthetic data also suggests that a V_p/V_s≤ 2.3 is more appropriate for the post-Paleozoic Mississippi embayment sediments. Based on the JHD locations we present a new interpretation of the seismicity, with two en-echelon SW-dipping thrust faults connected by a west-dipping thrust fault. These faults appear associated with the Reelfoot scarp and its northern extension, the Kentucky bend scarp.     

考虑颗粒破碎的钙质砂修正邓肯-张E-B模型

在常规应力水平下颗粒发生破碎是钙质砂有别于其他砂土的重要性质之一,且由于颗粒破碎的存在,使用传统的本构模型无法很好地模拟钙质砂的力学行为。因此本文以最为普及的本构模型之一--邓肯-张E-B模型为基础,对其进行颗粒破碎方面的修正以得到一个能用于钙质砂的本构模型。具体方法为:首先本文采用Hardin提出的相对破碎Br这一指标来度量颗粒破碎的大小。之后研究分析得出了颗粒破碎对邓肯-张模型参数(内摩擦角φ、割线模量E50及体积模量B)的影响规律。然后通过颗粒破碎与输入能量之间的关系将各状态下无法直接确定的相对破碎Br与可确定的应力-应变状态联系起来。最终得到了一个考虑颗粒破碎的钙质砂修正邓肯-张E-B模型。为验证模型的准确性及适用性,本文还使用该模型对4种不同粒径范围且试验围压不同的钙质砂的三轴排水行为进行了模拟。结果表明拟合效果较好,模型能适用于各种不同粒径范围的钙质砂,并且在颗粒破碎较大的情况下明显优于传统邓肯-张模型。     

碎(块)石路堤孔隙空气对流运动的Darcy定律适用性

针对青藏高原多年冻土区路基工程中采用的碎石路堤地温调控技术,分析了碎石层孔隙空气对流运动符合Darcy定律的适用范围及其可能偏离Darcy定律的非线性效应,并给出了考虑非线性效应的Darcy定律修正方程.同时,讨论了适合于路堤碎石层的空气渗透系数及其表征空气流动非线性效应的Ergun常数的估算公式.     

An analysis of drag force on wind of shrubs simulated in a wind tunnel

Shrubs that exert drag force on the wind significantly affect the air–surface interaction in arid and semiarid regions, and some coastal regions where aeolian processes are active. An understanding of the drag force on wind of shrubs provides important information on the dynamics of shrubs in reducing wind erosion, and their geomorphological significances. Thus, we analyzed the drag force and drag coefficient for model shrubs using wind measurements obtained by particle image velocimetry in a scaled wind tunnel simulation. The drag force was found to be a function of wind velocity and shrub density. Both drag force and drag coefficient revealed a critical shrub density of around 0.08. The vertical drag force changed direction when the shrub density became greater or less than the critical density. The drag coefficient increased rapidly with shrub density when the density was below 0.08, but it increased very gently when the density was increased beyond the critical value. The results have an important theoretical significance for our understanding of the dynamics of vegetated dunes and the windbreak mechanism of shrubs.