基坑稳定性的塑性极限分析上限法研究

基坑稳定性验算是基坑支护设计中的一项重要内容,将极限分析上限理论、有限元离散思想和数学规划方法结合起来研究基坑的稳定性。首先将基坑土体用三角形有限单元离散,然后根据上限定理构建同时满足公共边速度不连续条件、单元塑性流动约束条件和单元速度边界条件的机动许可速度场,并由内、外功率相等条件建立目标函数,构建基坑稳定性分析的上限法数学规划模型,通过对模型的优化求解,得到基坑整体稳定性分析的极限荷载(或安全系数)上限解及相应的破坏机构,最后分析了土体抗剪参数、支护结构嵌固深度及基坑开挖深度等因素对基坑整体稳定性的影响规律。     

Fully computable bounds for a staggered discontinuous Galerkin method for the Stokes equations

We first propose a guaranteed upper bound for an arbitrary order staggered discontinuous Galerkin (staggered DG) method for the Stokes equations with the use of the global inf–sup constant. Equilibrated stress reconstruction and velocity reconstruction are the main ingredients in the construction of the error estimator. Next, to improve the error estimation and to overcome the difficulties caused by the calculation of the global inf–sup constant, a refined error control relying on local inf–sup constants is also developed. Some minimization techniques and an explicit method are then established to facilitate the construction of the refined error control. Finally, some benchmark examples are tested to compare the performances of the proposed error estimators.     

基于Qt的移动机器人上位机软件设计与实现

为了实现移动机器人运行过程状态数据的可视化,以面向对象的C++编程语言,设计了一款基于跨平台的Qt图形库框架的上位机监测软件。该软件对接收到的状态数据进行解码、分离并绘制移动机器人实时运行轨迹和状态信息波形曲线等功能。首先概述了自行设计搭建的移动机器人硬件平台的结构和原理以及软件的实现流程;然后简要介绍了上位机软件的开发环境,并阐述了软件的各项功能以及其设计与实现过程;最后以移动机器人路径跟踪运动控制作为实验对象,验证了所设计上位机软件的有效性。     

Graphene-Based Advanced Membrane Applications in Organic Solvent Nanofiltration

Owing to the increasing need to mitigate excessive organic solvent waste, the efficient separation and recovery of organic solvents have received major research attention in recent years. The membrane-based organic solvent nanofiltration (OSN) process has demonstrated its feasibility in addressing this problem with low energy costs, compared to conventional separation techniques, such as adsorption, liquid–liquid extraction, and solvent evaporation. Recently, membranes made of 2D graphene-based materials have shown great promise because they attain high solvent flux and solute rejection using easy processing methods. Thus, this paper focuses on state-of-the-art studies of graphene-based membranes used in OSN processes, which include syntheses, characterizations, performance evaluations, membrane fouling, and simulation studies, in combination with the development of the “upper-bound” line to indicate the performance of graphene-based membranes. In this paper, critical challenges involved in the development of graphene-based membranes are also focused on and discussed to map out the future directions of these membranes in industrial OSN processes. In addition to OSN, this paper pertains to a broader audience in other separation processes, particularly in the fields of gas separation and water treatment.     

A hybrid formation path planning based on A* and multi-target improved artificial potential field algorithm in the 2D random environments

Traditional artificial potential field algorithm for multi-robot formation is easy to fall into local minima and the path planning efficiency is low. To this end, we propose a new method of a hybrid formation path planning based on A* and multi-target improved artificial potential field algorithm (A*-MTIAPF) that provides the optimal collision free path and improves the efficiency for multi-robot formation path planning. The A*-MTIAPF algorithm integrates global path planning and local path planning. The novelties include combining A* with the improved artificial potential field algorithm and dividing multiple virtual sub-target points on the global optimal path of A* planning. Firstly, A* algorithm is used to complete the global path planning. Secondly, the improved artificial potential field algorithm which takes multiple sub-target points divided by the global optimal path as virtual target points is used to complete local path planning by switching target points. In addition, we propose a double priority judgment control algorithm (DPJC) to solve the collision problem among multiple robots by setting double priority to determine the movement order of each robot. Then, a new experimental method is designed by using the randomly generated 2D maps to verify the effectiveness of the proposed method. The results show that our method has advantages that it solves the local minimum problem, improves the efficiency of formation path planning and avoids collision among multiple robots over existing methods.     

Robust Formation Maneuvers Through Sliding Mode for Multi-agent Systems With Uncertainties

This paper develops a robust control method for formation maneuvers of a multi-agent system. The multi-agent system is leader-follower-based, where the graph theory is utilized to describe the information exchange among the agents. The control method is exercised via sliding mode methodology where each agent is subjected to uncertainties. The technique of nonlinear disturbance observer is adopted in order to overcome the adverse effects of the uncertainties. Assuming that the uncertainties have an unknown bound, the formation stability conditions are investigated according to a given communication topology. In the sense of Lyapunov, not only the formation maneuvers of the multi-agent system have guaranteed stability, but the desired formations of the agents are also realized. Compared with other two control approaches, i.e., the basic sliding mode approach and the fuzzy sliding mode approach, some numerical results are presented to illustrate the effectiveness, performance and validity of the robust control method for formation maneuvers in the presence of uncertainties.      

Variable Structure Feedback Construction Algorithm for controlling measurement-imploded leader–follower ground robots

This paper presents a systematic approach for decreasing the error growth of a formation system consisted of a leader and a number of followers which are open-loop in position measurements (position, linear velocity and relative distance to the leader). Using conventional algorithms to control such a system may lead to an unstable network with increasing consensus error. A 3-phase algorithm is proposed to address open-loop systems of the mentioned form. The algorithm is specifically beneficial to the case where linear velocity and position sensors get damaged. The main idea of the proposed algorithm is to use fundamental relations between feedbacks and construct some of them using the others which in case of car-like ground vehicles, angular sensors’ feedbacks are used to estimate linear feedbacks. The error and input mathematical relations are formulated using kinematic of car-like robots. The stability of the proposed method is proved using a quadratic CLF, and the numerical simulation results are presented to assess the effectiveness of the algorithm compared to a conventional open-loop form. The algorithm is also applied on a pair of ground car-like robots. The experimental results verify the conclusions based on simulation. Various simulation results show that the proposed algorithm has improved the divergence rate of follower’s path from leader’s, from 0.3 $\frac{m}{sec}$ for the unit of velocity’s uncertainty to about zero.     

Process,Policy, and Implementation of Pool‐Wide Drawdowns on the Upper Mississippi River: A Promising Approach for Ecological Restoration of Large Impounded Rivers

The Upper Mississippi River (UMR) has been developed and subsequently managed for commercial navigation by the U.S. Army Corps of Engineers (USACE). The navigation pools created by a series of lock and dams initially provided a complex of aquatic habitats that supported a variety of fish and wildlife. However, biological productivity declined as the pools aged. The River Resources Forum, an advisory body to the St. Paul District of the USACE, established a multiagency Water Level Management Task Force (WLMTF) to evaluate the potential of water level management to improve ecological function and restore the distribution and abundance of fish and wildlife habitat. The WLMTF identified several water level management options and concluded that summer growing season drawdowns at the pool scale offered the greatest potential to provide habitat benefits over a large area. Here we summarize the process followed to plan and implement pool‐wide drawdowns on the UMR, including involvement of stakeholders in decision making, addressing requirements to modify reservoir operating plans, development and evaluation of drawdown alternatives, pool selection, establishment of a monitoring plan, interagency coordination, and a public information campaign. Three pool‐wide drawdowns were implemented within the St. Paul District and deemed successful in providing ecological benefits without adversely affecting commercial navigation and recreational use of the pools. Insights are provided based on more than 17 years of experience in planning and implementing drawdowns on the UMR. Copyright © 2015 John Wiley & Sons, Ltd.     

Some factors affecting delayed ettringite formation in heat-cured mortars

Although more than 10 years of studies on delayed ettringite formation (DEF) have led to consensus in numerous areas of past disagreements, some questions remain experimental work is needed to complete the knowledge of this pathology. Following this objective, this paper studies the influence of pre-existing microcracking, wetting/drying cycles and the type of sulfated addition on DEF in steam cured mortars. The mortar specimens were prepared using an Ordinary Portland Cement and two types of sulfate were added to the mixtures: calcium sulfate (CaSO₄) or sodium sulfate (Na₂SO₄). The results confirm the well-known effect of temperature: no expansion was observed in any of the mixtures cured at room temperature. Moreover, no expansion was observed after 800 days for the reference mortar or for the mortar containing calcium sulfate but all the specimens of heat-cured mortars containing sodium sulfate expanded markedly after about 50 days whatever the supplementary treatments applied (thermal shrinkage or wetting/drying cycles). These results show the significant role played by alkalis in the occurrence of delayed ettringite. The supplementary treatments intended to cause prelimiray microcracking of the specimens did not promote expansion but contributed to a slight acceleration of the reaction. The ultimate values of expansion were similar to those obtained with sound mortars.