Techniques for improving computational speed in numerical simulation of casting thermal stress based on finite difference method

Finite difference method (FDM) was applied to simulate thermal stress recently, which normally needs a long computational time and big computer storage. This study presents two techniques for improving computational speed in numerical simulation of casting thermal stress based on FDM, one for handling of nonconstant material properties and the other for dealing with the various coefficients in discretization equations. The use of the two techniques has been discussed and an application in wave-guide casting is given. The results show that the computational speed is almost tripled and the computer storage needed is reduced nearly half compared with those of the original method without the new technologies. The stress results for the casting domain obtained by both methods that set the temperature steps to 0.1 ℃ and 10 ℃, respectively are nearly the same and in good agreement with actual casting situation. It can be concluded that both handling the material properties as an assumption of stepwise profile and eliminating the repeated calculation are reliable and effective to improve computational speed, and applicable in heat transfer and fluid flow simulation.

Numerical Investigation by the Finite Difference Method of the Laser Hardening Process Applied to AISI-4340

This paper presents a numerical and experimental analysis study of the temperature distribution in a cylindrical specimen heat treated by laser and quenched in ambient temperature. The cylinder studied is made of AISI-4340 steel and has a diameter of 14.5-mm and a length of 50-mm. The temperature distribution is discretized by using a three-dimensional numerical finite difference method. The temperature gradient of the transformation of the microstructure is generated by a laser source Nd-YAG 3.0-kW manipulated using a robotic arm programmed to control the movements of the laser source in space and in time. The experimental measurement of surface temperature and air temperature in the vicinity of the specimen allows us to determine the values of the absorption coefficient and the coefficient of heat transfer by convection, which are essential data for a precise numerical prediction of the case depth. Despite an unsteady dynamic regime at the level of convective and radiation heat losses, the analysis of the averaged results of the temperature sensors shows a consistency with the results of microhardness measurements. The feasibility and effectiveness of the proposed approach lead to an accurate and reliable mathematical model able to predict the temperature distribution in a cylindrical workpiece heat treated by laser.

高铁开通对城市能源利用效率的影响机制及空间溢出效应研究

将高铁开通视为一项准自然实验,基于2006—2019年城市面板数据,运用多期双重差分法实证检验高铁开通对城市能源利用效率的影响,并进一步分析其影响机制及空间溢出效应。研究发现:(1)高铁开通对城市的能源利用效率具有显著的提升作用。(2)高铁开通通过提高铁路货运效率、替代公路客运、优化产业结构及促进技术创新,提升了城市的能源利用效率。(3)高铁开通对邻近城市的能源利用效率存在正向空间溢出效应,并存在800公里的空间距离衰减边界。(4)高铁开通对城市能源利用效率的提升作用在不同区域、等级、规模、资源禀赋的城市存在显著的异质性,其中对中西部城市、小城市、普通城市、资源型城市的能源利用效率提升作用更为显著;此外,高铁开通对衰退型的资源型城市能源利用效率的促进效果最大,对成熟型的资源型城市的影响次之,但对成长型和可再生型的资源型城市不存在显著性影响。研究结论为理解高铁在能源利用效率方面的作用提供了新视角和新证据,也为实现“节能减排”和“交通强国”提供了政策启示。

城市高铁建设如何促进企业数字化转型?——来自中国上市公司的证据

随着新一轮科技革命和产业变革的持续推进,数字经济已经成为国民经济的核心增长极之一。基于2000—2019年中国上市公司和地级及以上城市面板数据样本,采用双重差分法考察城市高铁建设对企业数字化转型的影响及其作用机制,研究结果表明:首先,城市高铁建设对企业数字化转型具有明显的推动效应,其辐射范围约为20千米;其次,这种积极影响主要是通过降低数字化市场准入门槛、提升数字化技术创新水平和调整数字化管理战略而实现的;再次,城市高铁建设对企业数字化转型的推动效应在网络基础设施建设完善和市场化水平较高的地区以及融资约束宽松的企业样本中表现得更加明显。研究结果有助于揭示高速铁路的微观经济效应,也可为引导企业进行数字化转型提供重要的学理支持和政策参考。

超声波风速传感器风场误差的数值模拟

为了研究矿用超声波时差法风速传感器测量结果受风场误差的影响,采用计算流体力学方法对不同结构类型的传感器在不同风向、风速下的流场和声道速度分布进行分析。研究结果表明:对射式结构的测量风速受风向角的影响明显大于反射式;对射式结构在45°风向角下经修正后的相对误差最小,且优于反射式;3种反射式结构中,换能器凹陷安装对测量风速的影响最小,凸起安装其次,半凸安装影响最大;换能器凹陷安装受风向角的影响最小,凸起安装和半凸安装影响相当;3种安装方式下,半凸安装在45°风向角下经修正后的测量误差最小;超声波风速传感器选择合适的结构布局,可以提升测量准确度。

大直径盾构并行施工对邻近高铁桩板结构路基的影响程度兮区研究

[目的]大直径盾构在长距离、小净距并行邻近高铁施工时,对既有结构及周边环境的变形要求极为严格,需要根据影响程度分区,对可能受到较大影响的区域进行防护加固。[方法]依托上海市轨道交通市域线机场联络线盾构并行沪杭高铁路基段工程,通过FLAC3D有限差分软件建立模型,分析盾构并行施工对高铁桩板结构路基的变形影响,明确不同施工条件下隧道与既有高铁的安全距离,得到盾构并行施工的影响程度分区。[结果及结论]增大隧道埋深、高铁桩板结构桩长和隧道与路基的水平距离均能减小盾构并行施工引起的路基水平位移;增大隧道埋深会增加路基竖向位移,但受地质因素影响,出现增大隧道埋深,路基沉降减小的情况;得到不同施工条件下满足变形控制指标的安全距离及盾构并行既有高铁桩板结构路基的影响程度分区。

不同处理方式对胆道疾病患者外科手术后胃肠功能恢复的影响研究

外科手术后胃肠功能是否可以快速恢复关系到患者预后效果。为此探讨了不同处理方式给胆道疾病患者外科手术后胃肠功能恢复带来的影响。将80例胆道疾病患者分成试验1组和试验2组,常规术后处理是试验1组采取的处理方式,早期营养支持是试验2组采取的处理方式,观察患者术后胃肠功能恢复和预后效果。结果显示,从胃肠功能恢复时间方面来看,试验2组排便时间、进食时间、肠鸣音恢复时间和肛门排气时间均短于试验1组;从营养指标和免疫功能方面来看,试验2组患者各指标改善效果均优于试验1组,且并发症发生率低于试验1组,生活质量高于试验1组。研究发现,对于接受外科手术治疗的胆道疾病患者,需要采取正确的术后处理方式,早期进行营养支持,以加快胃肠功能恢复,改善预后。

Transportation Infrastructure and Productivity Growth:Effects of Railway Speed-Up on Firm's TFP in China

Improvement of transportation infrastructure quality will lead to more sufficient market competition and promote the flow of resources with greater efficiency. This paper considers China's railway speed-up in 2007 as a quasi-natural experiment on China's transportation infrastructure quality improvement. With the initial operation of electric multiple units(EMUs) as the basis of grouping, this research examines the effect of railway speed-up on corporate total factor productivity(TFP) growth by the differencein-differences(DID) method. Overally, the results reveal positive effects both on firms' technological change and efficiency improvement, which lead to the increase of TFP. Based on subsamples divided by different regions and types of enterprises, further analysis indicates that the productivity of exporter, non-state and coastal firms has been mostly affected by the railway speed-up. These conclusions are verified by a placebo test. Besides, firms within "one-hour economic circle" have been shown more sensitive to the effect of railway speed increase.

Research on Effect of Oil Film Inertia on Piston Skirt by Iteration Method

The influence of oil film inertia on piston skirt lubrication in a high speedengine is investigated by an iteration method that alternately solves the Navier-Stocks equationsand the Reynolds equa-tion by finite element method and difference method. The Reynolds lubricationequation including oil film inertia is developed, in which the inertia coefficient is introduced toinvestigate the effect of oil film inertia. The iteration procedure and finite formulation ofsolving the new Reynolds lubrication equation are given to analyze the effect of oil film on pistonskirt in this kind of engine. The calculation results show that the oil film inertia has someeffects on the friction force, pressure force and load capacity of oil film and its effect isobvious for the last. The Reynolds lubrication equation proposed can be also used to analyze thelubrication performance of the piston skirt in low or medium speed engine and some other lubricationproblems generally excluding oil film inertia with the inertia coefficient being set at zero.

Comparison of Fixed Point Methods and Krylov Subspace Methods Solving Convection-Diffusion Equations

The paper first introduces two-dimensional convection-diffusion equation with boundary value condition, later uses the finite difference method to discretize the equation and analyzes positive definite, diagonally dominant and symmetric properties of the discretization matrix. Finally, the paper uses fixed point methods and Krylov subspace methods to solve the linear system and compare the convergence speed of these two methods.

Study on the subgrade deformation under high-speed train loading and water–soil interaction

It is important to study the subgrade characteristics of high-speed railways in consideration of the water–soil coupling dynamic problem,especially when high-speed trains operate in rainy regions.This study develops a nonlinear water–soil interaction dynamic model of slab track coupling with subgrade under high-speed train loading based on vehicle–track coupling dynamics.By using this model,the basic dynamic characteristics,including water–soil interaction and without water induced by the high-speed train loading,are studied.The main factors-the permeability coefficien and the porosity-influencin the subgrade deformation are investigated.The developed model can characterize the soil dynamic behaviour more realistically,especially when considering the influenc of water-rich soil.

基于数值模拟分析CBN刀具车削参数对SAE-5120钢残余应力的影响

零部件表面的残余应力对其屈服极限、疲劳寿命有很大影响,残余拉应力与压应力对零部件的疲劳影响也存在差异。本试验主要研究CBN刀具切削SAE-5120钢时的表面残余应力,利用有限元数值分析方法设计不同切削速度、进给量和背吃刀量等切削参数,分析CBN刀具切削SAE-5120钢的表层应力状态以及切削用量对残余应力的影响,优化产生最小残余应力的切削参数,并结合极差法分析切削速度、进给量和背吃刀量对残余应力影响的显著度。有限元结果表明:采用CBN刀具切削SAE-5120时,工件表层会产生残余拉应力;切削速度对残余应力的影响最显著,其次是进给量;切削速度为200m/min和进给量为0.2mm/r时,工件表层残余应力最小。

基于移动平台的三维风速风向动态测量系统

由于风速风向对帆船等移动平台的影响,需要实现对风速风向的动态测量,因此设计了一款基于时差法的正四面体非正交结构超声波阵列的动态测量系统。该系统通过测量超声波在顺风和逆风下传播的速度不同,结合测量装置的结构解析,得到风速风向和俯仰角,利用磁力计补偿方位信息数据和中值滤波算法,解决了在移动平台上实时动态采集风速风向和俯仰角的问题。经实际测试,系统风速测量误差在±1 m/s以内,风向测量偏差不超过0.8°,俯仰角测量偏差不超过1°,达到预期设计要求。

高速线材穿水冷却过程的传热分析及数值模拟

差分计算模型和水冷换热模型是影响高速线材温度计算准确性的两个主要因素。提出了一种新的离散化网格划分方式,并据此推导出具有表面温度节点的一维有限差分模型。采用Fluent软件模拟得出水冷管内冷却水的流速场,结合开冷温度对水冷温降的影响规律,构建了高速线材穿水冷却过程的非线性换热模型,并用现场实测数据验证了模型的精度,误差为±10%。结合差分模型和水冷换热模型研究了预冷段水冷参数对线材表面温降和心表温差的影响。结果表明:该模型可应用于线材水冷参数优化和在线水冷温控等。

基于Lankarani赫兹接触模型的弓网动力学仿真研究

该文通过有限元方法建立简单链型悬挂接触网模型并对其进行模态分析,通过使用Lankarani赫兹接触模型,运用显式中心差分法对弓网耦合动力学方程进行求解,基于上述理论创建弓网动力学仿真平台,采用模块化的思想进行弓网动力学的仿真分析。在此基础上,采用EN50318标准对弓网仿真平台的正确性进行验证。最后,通过对考虑迟滞阻尼效应的Lankarani赫兹接触模型进行动力学仿真,分析Lankarani赫兹接触模型中重要参数对仿真结果的影响。

巷道掘进空顶距确定的差分方法及其应用

提出了一种空顶距确定方法。该方法首先建立顶板稳定分析力学模型,然后采用差分法计算得到顶板应力分布规律与空顶距的关系,根据顶板应力与其抗压强度和抗拉强度关系判定顶板稳定性,在此基础上确定合理掘进空顶距。结合张双楼煤矿9802工作面回采巷道生产地质条件,应用该方法确定了巷道合理空顶距为6 m。在6 m大循环进尺掘进条件下采取2 m小循环内临时支护、高预紧力锚杆支护技术,现场应用效果表明,该方法有效控制了巷道围岩变形、提高了掘进速度。

冬小麦冠层温度及其影响因素探析

采用涡度相关法测定冬小麦农田潜热通量和显热通量 ,研究农田热量平衡各分量及风速对冠层温度和冠层 大气温差的影响结果表明 ,净辐射通量是影响冠层温度高低的主要能量因子 ,二者呈极显著线性相关关系 (α =0 .0 1) ,潜热通量、显热通量和土壤热通量对冠层温度的影响与天气状况有关 ,风速变化对冠层温度的影响作用不显著 ,而与冠层

高铁开通对中国城市房价的影响研究

本文基于2005—2013年中国地级市非平衡面板数据,利用连续时间双重差分方法(DID)实证研究了高铁开通对沿线地级市房价的影响。研究发现:高铁开通整体上会显著推高地级市的房价;高铁开通对房价的影响具有区域差异性,对中西部地区房价的影响大于东部地区;地级市修建的高铁站等级越高、高铁站数量越多,高铁对该地区房价的影响越大。进一步研究还发现,加速人口流动和资本流动是高铁开通推动房价上涨的重要机制。

一种矿井通风中精确测定风速的方法

针对超声波传感器接收超声波脉冲信号时由于存在起振过程,导致脉冲到达接收传感器的时间难以精确测定的问题,提出一种矿井通风中精确测定风速的方法,即通过拟合接收传感器接收到的正弦信号曲线及其包络线,计算出超声波脉冲信号到达传感器的时刻。对模拟采样数据的计算及分析结果表明,该方法能够达到较高的测时精度,从而能够保证矿井巷道测风精度。

速度差法超声波流量计硬件延时误差分析及对策

基于速度差法超声波流量计的测量原理,分析了硬件延时对测量精度造成的误差。提出可采用二次回波法和声速法来提高测量精度,利用数学模型分析2种方法的可行性,最后通过实验验证这2种方法,实验结果表明二次回波法和声速法对提高了超声波流量计测量精度的可行性和有效性。