Numerical simulation of fast granular flow facing obstacles on steep terrains

The interaction between fast shallow granular flow and obstacles on steep terrain is an important aspect of granular mechanics and defending against geological hazards. In this study, we used a depth-averaged model for granular flow facing obstacles on steep terrains in a bed-fitted coordinate system where the obstacle system is treated as a local bed deviation term. A second-order Riemann-free scheme is extended to compute the depth-averaged model with a wetting–drying technique, which is verified by several granular flow cases, such as aluminum bar collapse and granular flow runout on a steep slope. Numerical simulations were performed for the case of granular flow facing a (i) single hemispherical obstacle and (ii) system of three hemispherical obstacles to produce a dynamical process and deposit profile, and show good agreement with experimental results. Granular flow facing a single obstacle on a concave plane produces a detached shock wave that moves upstream and a tailing rapid transition zone that moves down, which will merge to form a new shock for deposition. Granular flows facing a three-hemisphere obstacle system produce a tailing rapid transition zone that moves downstream and a downstream wavy shock that results from the interaction of three bow shocks in front of each obstacle. The downstream wavy shock moves upstream and merges with the upstream transition zone to form a new curved shock, which later relaxes to a deposit owing to bed friction. These findings provide some supplemental understandings of flow structures of fast granular flow facing obstacles.     

Constructing sparse Davenport–Schinzel sequences

For any sequence $u$, the extremal function $Ex(u,j,n)$ is the maximum possible length of a $j$-sparse sequence with $n$ distinct letters that avoids $u$. We prove that if $u$ is an alternating sequence $abab\dots$ of length $s$, then $Ex(u,j,n)=\Theta \left(s{n}^2\right)$ for all $j\ge 2$ and $s\ge n$, answering a question of Wellman and Pettie (2018) and extending the result of Roselle and Stanton that $Ex(u,2,n)=\Theta \left(s{n}^2\right)$ for any alternation $u$ of length $s\ge n$ (Roselle and Stanton, 1971).Wellman and Pettie also asked how large must $s\left(n\right)$ be for there to exist $n$-block $DS(n,s\left(n\right))$ sequences of length $\Omega \left(n^{2-o\left(1\right)}\right)$. We answer this question by showing that the maximum possible length of an $n$-block $DS(n,s\left(n\right))$ sequence is $\Omega \left(n^{2-o\left(1\right)}\right)$ if and only if $s\left(n\right)=\Omega \left(n^{1-o\left(1\right)}\right)$. We also show related results for extremal functions of forbidden 0–1 matrices with any constant number of rows and extremal functions of forbidden sequences with any constant number of distinct letters.     

A transient solution method for the finite element incompressible Navier-Stokes equations

A numerical procedure for solving the time-dependent, incompressible Navier-Stokes equations is presented. The present method is based on a set of finite element equations of the primitive variable formulation, and a direct time integration method which has unique features in its formulation as well as in its evaluation of the contribution of external functions. Particular processes regarding the continuity conditions and the boundary conditions lead to a set of non-linear recurrence equations which represent evolution of the velocities and the pressures under the incompressibility constraint. An iteration process as to the non-linear convective terms is performed until the convergence is achieved in every integration step. Excessively artificial techniques are not introduced into the present solution procedure. Numerical examples with vortex shedding behind a rectangular cylinder are presented to illustrate the features of the proposed method. The calculated results are compared with experimental data and visualized flow fields in literature.     

Co-ordinated movements to avoid collisions of a twin-robot system

A method of modelling collision avoidance between separate coordinated arms in the process of changing hands is presented. The shortest distance between two forearms is chosen as the criterion for kinematic modelling. Trajectories of various geometries are classified with respect to different collision situations. Parameters to be adjusted to avoid collisions are also discussed. To illustrate the modelling technique, two redundant anthropomorphic robots are considered to cooperate in rotating a wheel.     

Collision‐free poisson motion planning in ultra high‐dimensional molecular conformation spaces

The function of protein, RNA, and DNA is modulated by fast, dynamic exchanges between three‐dimensional conformations. Conformational sampling of biomolecules with exact and nullspace inverse kinematics, using rotatable bonds as revolute joints and noncovalent interactions as holonomic constraints, can accurately characterize these native ensembles. However, sampling biomolecules remains challenging owing to their ultra‐high dimensional configuration spaces, and the requirement to avoid (self‐) collisions, which results in low acceptance rates. Here, we present two novel mechanisms to overcome these limitations. First, we introduce temporary constraints between near‐colliding links. The resulting constraint varieties instantaneously redirect the search for collision‐free conformations, and couple motions between distant parts of the linkage. Second, we adapt a randomized Poisson‐disk motion planner, which prevents local oversampling and widens the search, to ultra‐high dimensions. Tests on several model systems show that the sampling acceptance rate can increase from 16% to 70%, and that the conformational coverage in loop modeling measured as average closeness to existing loop conformations doubled. Correlated protein motions identified with our algorithm agree with those from MD simulations. © 2018 Wiley Periodicals, Inc.     

OTDR测试误差及规避方法探讨

目前,光缆作为城市轨道交通系统之中的关键传输介质,其本身的安全性会直接影响轨道交通的运营安全,这也就突出了光缆测试的重要性。而在光缆工程施工及线路维护工作开展过程中,OTDR(光时域反射仪)作为必不可缺的一个测试仪器,能将故障点位置迅速发现并确定,同时可对障碍性质及类别准确判定,从而提供可靠的数据,从而在分析光纤特性参数之中加以使用。基于此,在分析OTDR工作原理、测试方式及主要参数的基础上,剖析了测试中可能出现的误差,并提出了几点规避方法,以供参考。     

基于STM32的无线充电小车综合实验设计

综合实验旨在设计一款基于STM32的无线充电小车,实验采用两轮电机驱动智能小车,使车身质量减轻,同时减少小车的电能消耗。选用STM32F103ZET6单片机作为核心板完成对小车的整体控制,在循迹避障方面通过红外传感器来采集路面信号,信号经过分析处理后,使用L298N电机驱动模块来驱动小车运动。在供电部分采用可充电锂电池作为小车的电源供应,并配套无线充电模块用于对电池的充电。通过综合实验的训练达到拓展学生的创新思维的目的。     

基于Arduino单片机的智能机器人小车的研究

本文基于Arduino嵌入式系统技术,设计了一种具有自主避障功能,同时能够进行循迹控制的四轮驱动的巡检机器人小车。     

Insta-echoes: Selective exposure and selective avoidance on Instagram

Many studies have examined the degree to which social media facilitate echo chambers that keep users from diverse political perspectives (Bakshy, Messing, & Adamic, 2015; Parmelee & Bichard, 2012; Zhu, Skoric, & Shen, 2017). The present study expands the debate regarding selective exposure and selective avoidance to political Instagram use. A survey of politically active Instagram users measured how often users expose themselves to opinion-reinforcing political leaders and the extent to which users avoid opinion-challenging leaders. The findings indicate a high level of selective avoidance behavior on Instagram, especially by users who are conservative, Republican, very ideological, and female.