软弱结构面表面形态与充填度的力学特性研究

将压力梯度法与逆瞬态法结合,提出了适合长输管道的泄漏点定位方法. 该方法将泄漏 点前后的摩阻系数以及泄漏流量系数看作优化控制变量、水击方程的数值计算结果与测量值 之差的绝对值作为控制目标,利用遗传算法优选泄漏点前后的摩阻系数,与压力梯度法结合 不断迭代求解泄漏点的位置. 通过输水管道的泄漏实验得到该方法漏点定位的平均误差为 4%左右.     

基于瞬变流和遗传算法的管道泄漏辨识

输水系统管道的泄漏影响瞬变水击波波形的畸变和衰减特性,对泄漏的定位和定量是泄漏辨识研究的重要环节.考虑水力瞬变中非恒定摩阻的影响,将泄漏参数反映在建立的管道瞬变流时域数学模型中,给出离散网格和求解方法,分析不同泄漏参数对瞬变水击波的影响,用遗传算法结合瞬变流时域模型的特征线法最小化管道测点压力实测和计算响应值的均方误差来辨识泄漏参数,其中实测信号首先经过预滤波和小波去噪处理.通过模型试验对瞬变流数学模型和泄漏反问题分析模型作了率定和验证,结果表明此法能有效的辨识出小泄漏参数,并且检测信号主动,阀门扰动可操作性强.     

瞬变流摩阻计算及摩阻对水力瞬变的影响

摩阻损失的精确计算对于长输管道的水力瞬变分析是很重要的一个环节、过去为减少计算工作量，采用固定摩阻系数和流量一阶近似法计算瞬交流摩阻，精度有限．本跟踪液流流态变化，采用变摩阻系数和流量二阶插值法计算瞬交流摩阻，提高了摩阻计算精度．同时，从理论上分析了管道摩阻对水力瞬变的影响，澄清了一些模糊认识．     

黏弹性管道水柱分离弥合水锤有限体积法模型

基于有限体积法二阶Godunov求解格式对黏弹性输水管道中水柱分离弥合现象进行建模和模拟研究. 在传统的弹性管道模型基础上考虑管道黏弹性效应的影响. 在瞬变流控制方程中引入管道黏弹性项和动态摩阻项, 采用有限体积法进行求解, 考虑压力修正系数来模拟自由气体对计算单元的影响, 同时为避免数值模拟结果产生虚假震荡引入斜率限制器MINMOD函数; 通过虚拟单元法进行边界构建, 实现了计算区域的统一计算. 将所建模型计算结果与已有模型结果、试验结果进行对比, 并对影响模型的各参数进行敏感性分析. 结果表明, 本文模型能够准确模拟出纯水锤、水柱分离弥合水锤两种情况下的瞬态压力变化, 均能与试验数据高度吻合; 与传统的特征线方法相比, 当库朗数C_r小于1时, 有限体积法二阶Godunov格式计算结果更准确、稳定; 在压力波动的衰减过程中, 黏弹性效应相比于管道摩阻起主导作用; 与弹性管道模型相比, 考虑管道黏弹性效应后可显著提高模拟结果的准确性, 尤其是压力波峰值的相对误差明显降低.      

含泄漏及弯头三维输水管道内水锤波传播特性研究

建立了含泄漏及弯头的三维几何模型,基于仿真软件,采用k-ε模型模拟了水锤波在管道的传播规律,分析了泄漏、弯头及流体粘度变化对输水管道内水锤波传播特性的影响。利用计算结果解释了非恒定摩阻模型影响水锤波波速这一现象。研究表明:(1)管道存在泄漏时,泄漏孔的存在并不改变水锤波的传播速度,弯头和粘性效应影响水锤波的传播速度;粘性越大,水锤波衰减幅度越大;泄漏位置越接近管道末端,水锤波衰减越快。(2)摩擦阻力的存在造成管道内水锤波呈现持续的线充效应,管道内壁的粗糙程度和管道的长度决定了线充效应的增压幅度,长度越大,线性充填效应越明显。     

游泳衣料的表面摩阻测量

本文介绍用二维激光流速仪对游泳衣料的局部表面摩阻作对比性实验的一组测量结果.由于游泳衣表面为具有高度柔弹性的粗糙壁面,所以表面摩阻系数的值由离壁面稍远的对数律层的速度分布求得.实验结果表明,不同衣料的表面摩阻的差别可达90%左右,国产的单向弹性奥纶游泳衣的表面摩阻系数最小.由此可知,这种具有纵向纹理的衣料对于减少水对运动员的阻力最为有利.     

基于水力瞬变与扩展卡尔曼滤波的管道流体监测与泄漏定位

引入一种长输管道流体监测与泄漏定位的新方法,将管道流动的瞬变流模型转化为状态空间模型的描述,以管线沿程流量、压强水头为状态变量,管道进口流量和出口压力视做非线性动态系统的控制输入,出口流量和进口压力观测序列构成系统的测量向量。基于小信号原理首先线性化处理非线性模型,然后用扩展的卡尔曼滤波器结合传统的双曲方程特征线解法估计泄漏尺寸与位置,并实时模拟出管道流体的压力流量过程及其沿管道的分布。试验和仿真算例表明此法模拟的管道流动状态能较快收敛到稳定状态,并且泄漏尺寸估计与定位的结果与给定值比较吻合。因此引入扩展的卡尔曼滤波能够提高瞬变流模拟管道非定常流动的准确性和跟踪能力。     

小问题(42)

133.一轮半径为R,重量为P,在其铅垂直径的上端B点处作用水平力Q。轮与水平面间的滚动摩阻系数为δ。问水平力Q使轮子只滚动而不滑动吋,轮与 ...     

小问题

133.一轮半径为R,重量为P,在其铅垂直径的上端B点处作用水平力Q。轮与水平面间的滚动摩阻系数为δ。问水平力Q使轮子只滚动而不滑动吋,轮与     

摩擦与滚阻对被动行走器步态影响的研究

本文研究了圆弧足被动行走器支撑足与地面间的摩擦系数和滚阻系数对被动行走器步态的影响.首先分别利用扩展的赫兹接触力模型和LuGre摩擦模型描述了支撑足与地面接触点处的法向支撑力和切向摩擦力,并考虑了行走过程中支撑足所受的滚动摩阻;其次利用第二类Lagrange方程推导出了该系统的动力学方程,并通过与已有成果的对比确定了合适的LuGre摩擦模型参数;最后仿真分析了摩擦系数和滚阻系数对被动行走器步态的影响.研究发现:摩擦系数的改变虽然对被动行走器行走的平均速度、步幅,以及支撑足接触点处的最大法向接触力的影响较小,但摩擦系数的减小会改变其行走步态类型,如发生倍周期分岔甚至混沌现象;然而,滚阻系数的改变会对行走器行走的平均速度、步幅,以及支撑足接触点处的最大法向接触力的影响较大,尚未发现滚阻系数的改变会引起其行走步态的变化.     

Wake layer in a thermal turbulent boundary layer with pressure gradient

The open equations of thermal turbulent boundary layer subjected to pressure gradient have been analysed by method of matched asymptotic expansions at large Reynolds number. The flow is divided into outer wake layer and inner wall layer. The asymptotic expansions are matched by Millikan-Kolmogorov hypothesis. The temperature profile in overlap region yields composite law which reduce to log. law for moderate pressure gradient and inverse half power law for strong adverse pressure gradient. In case of a shallow thermal wake, the matching result of outer wake layer reduces to composite temperature defect law, which is more general than the classical log. law. The comparison of data for thermal boundary layer with strong adverse pressure gradient is also considered. Received on 26 May 1998     

Effect of quadratic pressure gradient term on a one-dimensional moving boundary problem based on modified Darcy’s law

A relatively high formation pressure gradient can exist in seepage flow in low-permeable porous media with a threshold pressure gradient, and a significant error may then be caused in the model computation by neglecting the quadratic pressure gradient term in the governing equations. Based on these concerns, in consideration of the quadratic pressure gradient term, a basic moving boundary model is constructed for a one-dimensional seepage flow problem with a threshold pressure gradient. Owing to a strong nonlinearity and the existing moving boundary in the mathematical model, a corresponding numerical solution method is presented. First, a spatial coordinate transformation method is adopted in order to transform the system of partial differential equations with moving boundary conditions into a closed system with fixed boundary conditions; then the solution can be stably numerically obtained by a fully implicit finite-difference method. The validity of the numerical method is verified by a published exact analytical solution. Furthermore, to compare with Darcy’s flow problem, the exact analytical solution for the case of Darcy’s flow considering the quadratic pressure gradient term is also derived by an inverse Laplace transform. A comparison of these model solutions leads to the conclusion that such moving boundary problems must incorporate the quadratic pressure gradient term in their governing equations; the sensitive effects of the quadratic pressure gradient term tend to diminish, with the dimensionless threshold pressure gradient increasing for the one-dimensional problem.     

压力管道爆裂压力模型比较研究和改进的剩余强度评价方法

在役管道剩余强度评价使用传统评价方法B31G和APIRP-579时,由于存在一些不确定因素使评价结果偏保守,但它能确保管道安全运行.同时,现在国内外研究机构又制定了一些新的管道爆裂压力预测模型.一些科研机构通过预测管道爆裂压力来对比这些评价方法.但在他们的对比中,使用B31G时并没有对安全因子修正,不能准确的预测管道的爆裂压力,所以在可靠性评价和剩余强度评价中使用B31G不能达到好的效果.本文使用安全因子对这些模型进行修正,预测管道的爆破压力极限,并与常用极限情况模型评价公式进行对比.结果表明,在极限情况时,双剪条件模型是一个较为准确的爆破压力极限预测模型,同时,它适合于结构可靠性评价.     

栓状流密相气力输送

首先介绍了气力输送的实验设备.评述了水平栓流气力输送的压力降计算方法,用3种不同的方法计算了压力降并与实验数据进行比较.此外评述了用特征线方法进行水平管的数值模拟,倾斜管的压力降计算和长距离的栓流气力输送.最后展望了该领域的发展方向.      

Improvement of a pressure gradient method and its application to an unsteady flow problem

The pressure gradient method using velocity components and components of a pressure gradient as dependent variables has been modified to solve incompressible Newtonian fluid flow problems numerically. Applying this modified method to unsteady-state development of flow in a circular cavity shows that, at least for the case of a low Reynolds number flow, relative errors produced by the proposed method are smaller for most time intervals than those produced by the primitive velocity-pressure variable method and by the standard pressure gradient method. Also it is found that the modified and standard pressure gradient methods can be applied to the unsteady circular cavity flow at a moderate Reynolds number of at least up to 200.     

Inverse laminar boundary-layer problems with assigned shear. The mechul function revisited

An inverse method is presented which accurately determines the pressure distribution for assigned wall shear in a two-dimensional, laminar, incompressible boundary layer. The method reformulates the mechul function scheme of Cebeci and Keller to produce a stable solution in the marching direction and to increase accuracy in the normal direction. In the reformulation a modified pressure gradient parameter variation in the normal direction is used in conjunction with three-point backward differences for streamwise derivatives and fourth-order accurate splines for normal derivatives. The resulting spline-finite difference equations are solved by Newton-Raphson iteration together with partial pivoting. Numerical solutions are presented for self-similar and non self-similar flows and compared with published results.     

A finite element method for transient compressible flow in pipelines

A finite element method is developed to solve the partial differential equations describing the unsteady flow of gas in pipelines. Excellent agreement is obtained between simulated results and experimental data from a fullscale gas pipeline. The method is used to describe very transient flow (blowout), and to determine the performance of leak detection systems, and proves to be very stable and reliable.     

含NaCl超细水雾对不同阻塞率管道内爆炸的抑制

针对长距离输气管道频发的爆炸问题,在自行搭建的水平透明管道平台中开展了含NaCl超细水雾对不同阻塞率管道爆炸特性的影响研究。通过对瓦斯爆炸压力、火焰传播速度等特征参数进行分析,探究含NaCl超细水雾与不同阻塞率泄压口对爆炸特性的影响规律。结果表明:仅在不同阻塞率（0、0.2、0.4和0.6）的泄压口作用下的管道瓦斯爆炸,爆炸超压随着管道阻塞率的增大而增强,阻塞率与火焰锋面传播至管道末端时间呈非线性关系,在阻塞率为0.2时火焰平均速度最快;两者共同作用下,雾通量为8.4 mL、质量分数为8%的NaCl超细水雾阻火抑爆效果最佳,最大爆炸压力下降幅度可达59.2%;含NaCl超细水雾直接作用于爆炸火焰从而抑制爆炸传播。