Transient effects of orthogonal pipe oscillations on laminar developing incompressible flow

This paper presents a numerical study of the transient developing laminar flow of a Newtonian incompressible fluid in a straight horizontal pipe oscillating around the vertical diameter at its entrance. The flow field is influenced by the tangential and Coriolis forces, which depend on the through‐flow Reynolds number, the oscillation Reynolds number and the angular amplitude of the pipe oscillation. The impulsive start of the latter generates a transient pulsating flow, whose duration increases with axial distance. In any cross‐section, this flow consists of a pair of symmetrical counter‐rotating vortices, which are alternatively clockwise and anti‐clockwise. The circumferentially averaged friction factor and the axial pressure gradient fluctuate with time and are always larger than the corresponding values for a stationary pipe. On the other hand, local axial velocities and local wall shear stress can be smaller than the corresponding stationary pipe values during some part of the pipe oscillation. The fluctuation amplitude of these local variables increases with axial distance and can be as high as 50% of the corresponding stationary pipe value, even at short distances from the pipe entrance. Eventually, the flow field reaches a periodic regime that depends only on the axial position. The results show that the transient flow field depends on the pipe oscillation pattern (initial position and/or direction of initial movement). Copyright © 2000 John Wiley & Sons, Ltd.     

分数导数粘弹性模型描述的土中管桩竖向振动的频域解

在分数导数粘弹性本构模型的基础上综合考虑桩周土和桩芯土的平衡方程和几何方程建立了桩周土和桩芯土的竖向运动的控制方程．在频率域内利用分离变量法和分数导数的性质求解了桩周土和桩芯土竖向振动控制方程．考虑管桩与桩周土、管桩与桩芯土的边界连续性条件以及三角函数的正交性得到了分数导数粘弹性模型描述的土中管桩的竖向振动,通过数值分析研究了管桩和土体模型参数和几何参数对管桩的桩顶复刚度的影响规律．结果显示：桩芯土本构模型的分数导数的阶数对管桩竖向振动的影响较桩周土本构模型的阶数要小,且与频率有一定关系;桩芯土与桩周土的模型参数比τ1 和τ2 对等效阻尼的影响较对刚度因子的影响要大;管桩桩周和桩芯的直径比d 越小,管桩复刚度的实部和虚部就越大;土体力学性能对管桩竖向振动的影响要比管桩桩身力学性能的影响小．     

水平悬链线中点集中力作用下的非线性分析及计算

首先,对均布力作用下悬链线理论进行了深入分析研究,推导出求解水平张力单参数和双参数形式的隐含超越方程,在算法上验证了两种方程的等价性。对于悬链线固有的广义倾角、曲率半径、约束反力和水平张力的嵌入关系等给出了具有新意的表达公式。然后,建立了水平悬链线在对称集中力作用下的平衡方程,对于不同垂向集中力作用下水平张力的变化情况和最大垂直距离差所在水平位置随集中力的变化等进行了非线性计算。本文在理论构建和分析方面具有创新性,在理论探索和工程应用上具有重要意义。     

水平井体积压裂缝网表征及流动耦合模型

针对岩石脆性系数高且发育天然裂缝的储层,提出表征水平井体积压裂形成裂缝网络的三种基本模式,并将渗流过程划分为油藏流动和缝网内部流动.在此基础上,利用势叠加原理导出油藏流动控制方程,利用有限差分方法建立缝网内部有限导流等式;其次,采用星三角变换法处理人工缝与天然缝的交汇流动;最后,耦合两部分流动矩阵方程得到水平井体积压裂缝网渗流数学模型.该模型表明:当水平井改造段长度一定时,压裂段数与段内分簇数是决定产能的最主要因素,其次是人工裂缝半长和人工缝导流能力,而天然裂缝密度和导流能力对产量影响较小.实例应用表明,实际产油量与模型计算值一致,误差较小.     

Deformation of thin straight pipes under concentrated forces or prescribed edge displacements

The purpose of this paper is to present an efficient analytic method for obtaining the deformation of thin straight pipes, subjected to prescribed edge displacements or concentrated loads.The approach uses the mixed formulation where unknown functions are combined with trigonometric terms. A variational procedure is used to obtain the system of ordinary differential equations. For the applied load a Fourier approach is used to represent the load as an analytical function. For the prescribed displacement, three solutions for the ovalization are evaluated and a method based on energy contribution of each term is used to obtain their superposition.In contrast to finite element method the proposed method is efficient and can be applied to other boundary condition problems leading to continuous displacement and stress fields with a low number of unknowns. Comparisons with experimental and finite element procedures show good agreement that enhances the merits of the analytical solutions proposed.The value of this method is based on solving the differential equations rather than using commercial codes. So far, the solution of prescribed edge displacements has been limited to one term. This paper discusses how to add further terms using the mixed formulation, thus, presenting a novel procedure.     

Optimal transient growth in turbulent pipe flow

The optimal transient growth process of perturbations driven by the pressure gradient is studied in a turbulent pipe flow. A new computational method is proposed, based on the projection operators which project the governing equations onto the subspace spanned by the radial vorticity and radial velocity. The method is validated by comparing with the previous studies. Two peaks of the maximum transient growth amplification curve are found at different Reynolds numbers ranging from 20 000 to 250 000. The optimal flow structures are obtained and compared with the experiments and DNS results. The location of the outer peak is at the azimuthal wave number n=1, while the location of the inner peak is varying with the Reynolds number. It is observed that the velocity streaks in the buffer layer with a spacing of 100δ_v are the most amplified flow structures. Finally, we consider the optimal transient growth time and its dependence on the azimuthal wave length. It shows a self-similar behavior for perturbations of different scales in the optimal transient growth process.     

Wave forces on a submerged horizontal plate - Part I: Theory and modelling

This paper is concerned with the propagation of nonlinear gravity waves over a thin horizontal plate submerged in water of shallow depth. An unsteady solution of the problem is obtained by use of the theory of directed fluid-sheets for the two-dimensional motion of an incompressible and inviscid fluid. Particular attention is paid to the calculation of the nonlinear wave-induced vertical and horizontal forces and overturning moment by solving the Level I Green–Naghdi equations. The theoretical formulation of the problem is given in this paper (Part I), while the results due to solitary and cnoidal waves, and comparisons with the available experimental data are given in a companion paper under the same title (Part II).     

Ambient noise near the sea-route

Ambient noise data measured in an experiment conducted in shallow water near a sea-route were analyzed. It was observed that, at low frequency, the horizontal correlation has an obvious difference from that predicted by the classical ambient noise model. The theoretical analyses show that this phenomenon is caused by wind noise together with the discrete shipping noise nearby. An ambient noise model was proposed to include the effects caused by both the noise sources. Data measured at different times verify that the proposed model can be used to forecast the ambient noise field in shallow water near the sea-route. Supported by the National Natural Science Foundation of China (Grant No. 10734100), the Knowledge Innovation Program of the Chinese Academy of Sciences (Grant No. KZCX1-YW-12-2), the National Basic Research Program of China (Grant No. 2007CB209603), and the Open Foundation of China National Petroleum Corporation Key Laboratory (Grant No. GPKL0804)     

热管吸液芯的研究进展

文中对热管内吸液芯的研究进展进行了阐述,着重从对热管传热性能起主要影响作用的比表面积、孔隙率、渗透率、有效毛细半径、密度、有效导热系数等结构参数方面进行了分析,最后介绍了多孔泡沫金属这一新型的热管吸液芯,并指出了今后的研究方向.     

竖直环形通道内液氮流动沸腾的MUSIG模型分析 第二部分:径向气泡直径和界面面积浓度的预测

由于考虑了气泡的破裂和聚合,同两流体模型相比,MUSIG模型(多尺寸组模型)能更准确地描述流场内气泡直径。采用MUSIG模型详细分析了不同壁面热流量,液体入口速度,过冷度以及不同管道高度时通道内气泡相界面面积、当地气泡直径、空泡系数等参数沿径向的分布。分析结果表明,MUSIG模型可用来预测泡弹状流型转变区的流动参数,也即该模型拓展了两流体模型的使用范围。