Models of high velocity impacts into dust-covered ice: Application to Martian northern lowlands

The detection of fresh impact craters with bright floors and ejecta (arising from fresh clean water ice) in the northern lowlands of Mars (Byrne et al., 2009b, Science 325, 1674), together with observations of polygonal structures and evidence from the Phoenix probe, suggests that there are substantial water ice deposits just below the surface over large areas. Specifically in cases of the larger craters observed, the impacts themselves may have raised the temperature and the pressure of the water ice deposits locally to values which allow phase changes. In this paper, we use smoothed particle hydrodynamics to model hyper-velocity impacts. We estimate peak shock pressures in a solid water ice target covered by a layer of loose material, modeled by pre-damaged dunite. In addition, we account for the possibility of a thin layer of sub-surface water ice by using a three-layer model where the ice is surrounded by dunite. We find that the peak shock pressures reached in the simulated events are high enough to produce several 100-1000 kg of liquid water depending upon the impact parameters and the exact shock pressure needed for the phase change. A difficulty remains however in determining whether liquid is generated or whether a type of fluidized ice is produced (or indeed some combination of the two). We also note that the process can become rather complex as the number of layers increases because of reflections of the shock at sub-surface boundaries—a process which should lead to increased fluidization.     

Simulation of the Askervein flow. Part 2: Large-eddy simulations

Large-eddy simulations of the neutrally stratified flow over the Askervein Hill were performed, to improve the knowledge of the flow obtained from field measurements and numerical simulations with Reynolds averaged Navier-Stokes (RANS) methods. A Lagrangian dynamic subgrid model was used but, to avoid the underdissipative character near the ground, it was merged with a damped Smagorinsky model. Simulations of a flat boundary-layer flow with this subgrid model showed that the turbulent vertical motions and shear stress were better resolved using grids with a stream to spanwise aspect ratio Δx / Δy = 2 than with an aspect ratio Δx / Δy = 1. Regarding the flow over the Askervein Hill, it was found that large-eddy simulations provide an acceptable solution for the mean-velocity field and better predictions of the turbulent kinetic energy in the upstream side of the hill than the model. However, as with the model, grid convergence was not achieved in the lee side and the size of the zone with reversed flow increased with the grid refinement. Nevertheless, the existence of the intermittent separation predicted with unsteady RANS in part one of this work seems unquestionable, due to the deceleration of the flow. In our opinion, a better modelling of the decelerating boundary layer in the lee side is required to improve the results obtained using equilibrium assumptions and achieve grid convergence.     

Multi-resolution adaptive modeling of groundwater flow and transport problems

Many groundwater flow and transport problems, especially those with sharp fronts, narrow transition zones, layers and fingers, require extensive computational resources. In this paper, we present a novel multi-resolution adaptive Fup approach to solve the above mentioned problems. Our numerical procedure is the Adaptive Fup Collocation Method (AFCM), based on Fup basis functions and designed through a method of lines (MOL). Fup basis functions are localized and infinitely differentiable functions with compact support and are related to more standard choices such as splines or wavelets. This method enables the adaptive multi-resolution approach to solve problems with different spatial and temporal scales with a desired level of accuracy using the entire family of Fup basis functions. In addition, the utilized collocation algorithm enables the mesh free approach with consistent velocity approximation and flux continuity due to properties of the Fup basis functions. The introduced numerical procedure was tested and verified by a few characteristic groundwater flow and transport problems, the Buckley–Leverett multiphase flow problem, the 1-D vertical density driven problem and the standard 2-D seawater intrusion benchmark–Henry problem. The results demonstrate that the method is robust and efficient particularly when describing sharp fronts and narrow transition zones changing in space and time.     

基于细观力学的混凝土劈拉破坏率相关特性研究

提出了一种高效的混凝土细观力学预处理方法,其中包括基于背景网格的骨料投放与细观三组分的识别;建立了颗粒离散元的混凝土细观力学模型。利用该模型对不同应变率下混凝土的劈拉试验进行了数值仿真,仿真结果显示出的混凝土动力强度提高的规律以及不同应变率下混凝土的破坏模式与试验结果吻合较好。研究表明,随着应变率的提高,混凝土破坏时产生更多弥散状分布的裂纹,同时有些裂纹穿过强度较高的骨料,导致混凝土在高应变率破坏过程中消耗更多的能量,是混凝土的动力强度提高的重要原因。此外,在高应变率下,力链产生更多的分叉,空间分布的不均匀性和随机性增加,因此,在高应变率下混凝土动力强度表现出更大的离散性。     

基于PSO-PP的围岩稳定性评价

围岩的稳定性评价是一个复杂的不确定系统问题。结合投影寻踪方法、粒子群算法和逻辑斯谛曲线函数,建立了围岩稳定性评价的粒子群优化投影寻踪(projection pursuit based on particle swarm optimization,PSO-PP)模型。该模型一方面采用粒子群算法优化投影指标函数及逻辑斯谛曲线函数参数,确保了模型的准确性;另一方面利用逻辑斯谛曲线函数建立投影值与经验等级之间的非线性关系。模型的测试结果显示了良好的精度,实例分析结果与实际状态完全一致,表明该模型在围岩稳定性评价中的可行性和有效性。     

缝洞型油藏大尺度可视化水驱油物理模拟实验及机理

缝洞型碳酸盐岩油藏由于其缝洞组合关系的复杂性,使得储层流体的渗流规律有别于常规油藏。根据某油田奥陶系缝洞型碳酸盐岩的实际情况设计制作了大尺度可视化物理模型,并按照现场实际设计了二注三采的实验井网及注采方案,以应用物理模拟实验来研究缝洞型碳酸盐岩油藏的开发特征及油井生产规律。结合实验结果,利用J.Cruz-Hernández建立的数学模型对生产数据进行拟合,以研究缝洞型碳酸盐岩油藏中流体的渗流机理,J.Cruz-Hernández数学模型中的空间系数和流动速度对采收率影响较大,提出模型的改进应考虑重力分异作用、溶洞比例等影响因素。     

城市雨洪模拟技术研究进展

从对水文过程描述的角度对城市雨洪模拟技术进行了回顾,认为管网汇流阶段算法较为成熟,而产流和坡面汇流阶段还有待深入研究.从模型构建思想的角度回顾其发展历程,阐明了经验性模型和分布式概念性模型的局限性以及分布式物理模型的良好发展前景.通过总结国内外几个具代表性城市雨洪模型的特点,从特定模型的角度进一步分析城市雨洪模型的现状,并指出中国与国外模型研究的差距.对与城市雨洪模拟精度密切相关的基础数据收集与管理技术进行了讨论,并提出了城市雨洪模拟技术的发展方向.     

基于Godunov格式的溃坝水流数学模型

为了更好地把握溃坝洪水风险,减小因溃坝洪水而造成的人员生命和财产损失,建立了基于Godunov格式的一维、二维溃坝水流耦合数学模型。一维溃坝水流模型采用HLL格式的有限体积法求解,二维溃坝水流模型采用基于非结构网格的Roe格式离散求解,在一维、二维模型的链接处采用重叠计算区域的方法实现一维模型和二维模型之间的水力要素信息交换。经弯道溃坝算例和断面突变溃坝算例验证,该耦合模型具有良好的可靠性和适用性,验证后的耦合模型为大尺度的溃坝水流数值模拟打下了基础。     

河流生态系统结构功能整体性概念模型

在完善与整合现有河流生态系统结构功能概念及模型的基础上,提出河流生态系统结构功能整体性概念模型。水文情势、水力条件和地貌景观格局是对河流生态系统结构与功能具有关键影响的3大生境要素,结构功能模型的核心是建立以3大生境要素为构架的生命支持系统与河流生命系统之间的相互作用和相互制约关系,同时考虑由于人类活动引起生境要素变化对于河流生态系统的影响。河流生态系统结构功能整体性概念模型由以下4种模型组成:河流四维连续体模型、水文情势-河流生态过程耦合模型、水力条件-生物生活史特征适宜模型以及地貌景观空间异质性-生物群落多样性关联模型,这4种模型的一体化整合,基本概括了河流生态系统结构功能的整体特征。     

河南省泌阳县贾楼乡陡岸村泥石流灾害及其防治

通过对当地地形地貌、暴雨特征、流域面积、地层岩性、地质构造、水文地质条件、泥石流物源的调查与分析研究,针对该泥石流具有危险度高、流量大、流速缓的特点,结合受灾对象及其分布区域与整体布局和局部治理的关系,按照防治结合、因地制宜、突出重点的原则设置了拦挡滞洪、导流防护、漫水路相结合的综合防治工程措施。拦挡工程为有效发挥工程泻洪、拦渣、调节、固床、稳坡和控制固体物质补给量,预防沟道下切及沟壑发展,减少泥石流对下游村庄危害,起到了积极作用;通过导流堤的建筑限制泥石流的流速、流向,将泥石流顺畅地通过受灾对象比较集中的区域,排入下游非危害区;漫水路为当地群众出行提供方便,又是应急通道。工程实施后综合效益明显。