弯曲渠道中的振荡流和二次流

本文以弯道理想不可压缩流动的流线和等势线为曲线坐标系给出了弯道内不可压缩粘性振荡流动的一阶解以及边界层流型下的二阶解,以直角弯道为例对弯道內的粘性振荡流动以及二次流的影响进行了讨论。     

弯道水流数值模拟研究

采用边界拟合坐标技术,生成边界处正交的曲线网格,在曲线坐标系中进行弯道水流数值计算。借鉴现有关于弯道水流流速分布的研究成果,对曲线坐标系平面二维浅水方程作了修正,修正后的方程组能够计入弯道环流引起的横向的动量交换,即考虑了二次流对流线弯曲的复杂水力特性的影响。通过连续弯道实例计算,验证了该模型的可靠性和实用性。     

“蜻蜓点水，紫燕穿波”－－短道速滑运动员安全转弯的力学分析

通过对短道速滑运动员在弯道滑行时的受力分析，得出了运动员安全转弯的力学条件，解释了运动员在弯道滑行时，为什么要采取深蹲、向弯道内侧倾斜、用左手触摸冰面等滑跑姿势，并对机动车辆的平地翻车现象进行了讨论.     

90°弯道明渠横向环流的大涡模拟研究

采用气液两相流大涡模型模拟90°弯道明渠水流的水力特性。使用有限体积法对控制方程进行离散;使用压力隐式算子分裂PISO(Pressure-Implicit with Splitting of Operators)算法求解速度与压力的耦合;采用了VOF(Volume of Fluid)法模拟自由水面。通过计算得到弯道处流速、压强系数、横向环流强度等水力参数的分布规律。分别比较弯道纵向流速与横向流速的模拟值与实验值,二者吻合较好。通过分析弯道环流的流态特征得出:因受重力作用与水流在流经弯道时发生的离心现象的影响,凸岸附近较凹岸存在较大的二次流强度的结果;凸岸与凹岸压强系数均沿程增大,当达到弯顶时,水面横比降达到最大值;弯道横向环流强度随弯道角度增大而逐渐增大,其最大值出现在明渠底部。     

曲线坐标系下平面二维浅水模型的修正与应用

借鉴有关弯道水流流速分布的研究成果,计入弯道环流引起的横向的动量交换,对曲线坐标系平面二维浅水方程做了修正;采用边界处正交的曲线网格生成技术,处理复杂的计算区域边界。采用修正后的模型对90°弯道水流进行了数值模拟,并与原模型的计算结果及实测资料进行了比较,结果表明该模型能够有效地模拟流线弯曲的复杂水流的水力特性。     

带滩槽地形的连续弯道中纵向流速横向分布解析

本文基于沿水深积分的动量方程,假定二次流项和弯道附加应力项沿横断面呈线性分布,提出了预测弯道垂线平均纵向流速的解析计算方法,进一步提出了河槽区和河滩区垂线平均纵向流速沿断面分布的求解模式,并将其应用于带滩槽地形的反向连续弯道水槽中. 根据实测数据率定计算参数,该模式可计算不同出口水深条件下断面垂线平均纵向流速分布,计算结果与实测数据吻合良好.分析了线性分布假设中参数随水深变化的取值规律和沿横断面分布特点,并对参数进行了敏感性分析,分析表明线性假设中一次项系数分区位置对流速峰值的大小和位置影响较大,常数项根据地形横比降变化进行分区取值,流速计算值对常数项在水平段和斜坡段分区位置较为敏感,并根据参数的敏感度提出了参数沿水槽的均值作为参考值.讨论了动量方程中二次流项和弯道附加应力项沿弯道的横向分布规律,进一步认识线性假设的适用范围,结果表明线性假设在本文试验水槽中适用于弯道沿程.研究成果有助于认识带滩槽地形的连续弯道纵向流速分布特征及其形成机制.      

定床弯道内水沙两相运动的数值模拟

在适体同位网格中采用非正交曲线坐标系下的三维k-ε-kp固液两相双流体湍流模型研究弯道内水流和悬浮泥沙运动,主要计算了试验室S型水槽内清水流动的三维流场、120°弯道内水沙两相流动中底沙与底流的运动轨迹以及S型水槽内水沙两相流动的两相流场和泥沙浓度场. 对于S型水槽内清水流动,数值结果与试验结果吻合良好. 120°弯道内水沙两相流动中固液两相的运动轨迹在弯道直线段基本重合,在弯道内泥沙轨迹逐步偏离水体轨迹,其偏离程度随泥沙粒径增大而增大. 从S型水槽内水沙两相流动计算结果中发现泥沙纵向流速在壁面附近比水流纵向速度大,在远离壁面区域比水流纵向速度小;弯道内泥沙横向流速比水流横向流速小;垂向流速在直线段和泥沙沉速相当,在弯道内受螺旋水流影响而变化;两相流速差别随泥沙粒径增大而变大;泥沙浓度呈现下浓上稀的分布,在弯道内横向断面上呈现凸岸大凹岸小的分布,泥沙浓度随泥沙粒径增大而减小.      

基于CFD的河流弯道二次流的三维数值模拟

为了系统地研究河流弯道在二次流影响下水沙运动的特点,本文基于CFD(Computational Fluid Dynamics)数值模拟和物理模型实验的方法,对河流弯道二次流进行了模拟和测量;应用雷诺时均法模型,结合Eulerian两相流模型对河流弯道二次流及其泥沙冲淤规律进行了数值模拟;以SIMPLE方法对压力场和速度场进行了耦合计算,并对辽宁省阜新市细河某段支流河道进行了物理模型实验,数值模拟的结果与实验结果基本一致,表明了本文方法的正确性。在此基础上,对90°弯曲的梯形断面河道的二次流结构和冲淤变化进行了分析,为防洪和水利设施的建设提供理论依据。     

Carrousel氧化沟固-液两相水力特性数值模拟

为研究氧化沟内的固-液两相流,本文采用数值计算的方法,以典型的四沟道Carrousel氧化沟为研究对象,通过将污泥沉降速率模型与两相湍流混合物模型耦合,建立了氧化沟固-液两相湍流混合物模型,从而实现了垂向上固-液两相的分离。模拟结果显示:氧化沟内的污泥浓度变化趋势总体呈现出与水流流速变化趋势相反的分布规律,即越接近沟底,污泥浓度越大,流速越小;越接近水面,污泥浓度越小,流速越大。此外,在大弯道处,形成弯道环流,造成了外弯道的冲刷和内弯道的淤积,大弯道内壁处的污泥浓度整体高于弯道外壁处。本研究结果对实际工程中防止或减少沟内污泥沉积,提高氧化沟污水处理效率有一定的指导意义。     

F1赛车弯道技术的力学分析

 以上海国际赛车场的赛道设计为例，通过对F1赛车弯道技术的力学分析，说明了 F1赛车手为什么要在比赛中选择奇特的行车路线和超车技术，以适应弯道速度控制和直道 高速行驶对车手和赛车的严格要求, 揭示了F1比赛既要速度更要技术和经验的魅力所在．     

90°弯管内气固两相湍流流动的数值研究

利用两相湍流KET模型对90°弯管内气固两相湍流流动进行了数值模拟,得到了弯管内两相流动的一些规律,并提出用颗粒动理学压力来定性表征弯管内磨损严重部位,为管道抗磨损设计提供了一定的理论依据。     

Numerical analysis of developing turbulent flow in a 180° bend tube by an algebraic Reynolds stress model

A numerical analysis has been performed for three‐dimensional developing turbulent flow in a 180° bend tube with straight inlet and outlet section used by an algebraic Reynolds stress model. To our knowledge, numerical investigations, which show the detailed comparison between calculated results and experimental data including distributions of Reynolds stresses, are few and far between. From this point of view, an algebraic Reynolds stress model in conjunction with boundary‐fitted co‐ordinate system is applied to a 180° bend tube in order to predict the anisotropic turbulent structure precisely. Calculated results are compared with the experimental data including distributions of Reynolds stresses. As a result of this analysis, it has been found that the calculated results show a comparatively good agreement with the experimental data of the time‐averaged velocity and the secondary vectors in both the bent tube and straight outlet sections. For example, the location of the maximum streamwise velocity, which appears near the top or bottom wall in the bent tube, is predicted correctly by the present method. As for the comparison of Reynolds stresses, the present method has been found to simulate many characteristic features of streamwise normal stress and shear stresses in the bent tube qualitatively and has a tendency to under‐predict its value quantitatively. Judging from the comparison between the calculated and the experimental results, the algebraic Reynolds stress model is applicable to the developing turbulent flow in a bent tube that is known as a flow with a strong convective effect. Copyright © 2005 John Wiley & Sons, Ltd.     

连续弯道水流紊动特性试验研究

采用电磁流速仪, 在接近天然河道形态的连续弯道中, 对沿程断面上各点的脉动流速进行量测.根据测得的数据分析得到了紊动能谱、紊动强 度、紊动切应力、紊动能、紊动参数等的全断面分布.研究结果表明水流紊动主要集中在10 Hz以下,不同环流形式与各种紊动量之间会互相影响,边壁处纵向紊动强烈、横向和垂向紊动较弱.     

The numerical simulation of turbulent flows of Newtonian and a sort of non-Newtonian fluid in 180-degree square sectioned bend

Using k-εmodel of turbulence and measured wall functions.turbulent flows ofNewtonian(pure water)and a sort of non-Newtonian fluid(dilute,drag-reduction solutionof polymer in a180-degree curved bend were simulated numerically.The calculated resultsagreed well with the measured velocity profiles.On the basis of calculation andmeasurement,appropriateness of turbulence model to complicated flow in which the large-scale vortex exists was analyzed and discussed.     

Laser-Doppler measurements of laminar and turbulent flow in a pipe bend

Laser-Doppler measurements are reported for laminar and turbulent flow through a 90° bend of circular cross-section with mean radius of curvature equal to 2.8 times the diameter. The measurements were made in cross-stream planes 0.58 diameters upstream of the bend inlet plane, in 30, 60 and 75° planes in the bend and in planes one and six diameters downstream of the exit plane. Three sets of data were obtained: for laminar flow at Reynolds numbers of 500 and 1093 and for turbulent flow at the maximum obtainable Reynolds number of 43 000. The results show the development of strong pressure-driven secondary flows in the form of a pair of counter-rotating vortices in the streamwise direction. The strength and character of the secondary flows were found to depend on the thickness and nature of the inlet boundary layers, inlet conditions which could not be varied independently of Reynolds number. The quantitative anemometer measurements are supported by flow visualization studies. Refractive index matching at the fluid-wall interface was not used; the measurements consist, therefore, of streamwise components of mean and fluctuating velocities only, supplemented by wall pressure measurements for the turbulent flow. The displacement of the laser measurement volume due to refraction is allowed for in simple geometrical calculations. The results are intenden for use as benchmark data for calibrating flow calculation methods.     

Turbulent pipe flow downstream a 90° pipe bend with and without superimposed swirl

In the present work, the turbulent flow downstream a 90° pipe bend is investigated by means of stereoscopic particle image velocimetry. In particular, the three dimensional flow field at the exit of the curved pipe is documented for non-swirling and swirling flow conditions, with the latter being generated through a unique axially rotating pipe flow facility. The non-swirling flow was examined through snapshot proper orthogonal decomposition (POD) with the aim to reveal the unsteady behaviour of the Dean vortices under turbulent flow conditions, the so-called “swirl-switching” phenomenon. In respect to the swirling turbulent pipe flow, covering a wide range of swirl strengths, POD has been employed to study the effect of varying strength of swirl on the Dean vortices as well as the interplay of swirling motion and Dean cells. Furthermore, the visualised large scale structures in turbulent swirling flows through the bend are found to incline and tear up with increasing swirl intensity. The present time-resolved, three component, experimental velocity field data will provide a unique and useful database for future studies; in particular for the CFD community.     

Turbulence-driven secondary flows in a curved pipe

If the torque exerted on a fluid element and the source of streamwise vorticity generation are analyzed, a turbulence-driven secondary flow is found to be possible in a curved pipe. Based on this analysis, it is found that the secondary flow is primarily induced by high anisotropy of the cross-stream turbulent normal stresses near the outer bend (furthest from the center of curvature of the bend). This secondary flow appears as a counterrotating vortex pair embedded in a Dean-type secondary motion. Recent hot-wire measurements provide some evidence for the existence of this vortex pair. To verify the formation and extent of this turbulence-driven vortex pair further, a near-wall Reynolds-stress model is used to carry out a detailed numerical investigation of a curved-pipe flow. The computation is performed specifically for a U-bend with a full developed turbulent flow at the bend entrance and a long straight pipe attached to the exit. Numerical results reveal that there are three vortex pairs in a curved pipe. The primary one is the Dean-type vortex pair. Another pair exists near the pipe core and is a consequence of local pressure imbalance. A third pair is found near the outer bend and is the turbulence-driven secondary flow. It starts to appear around 60° from the bend entrance, grows to a maximum strength at the bend exit, and disappears altogether at about seven pipe diameters downstream of the bend. On the other hand, calculations of developing laminar curved-pipe flows covering a range of pipe-to-bend curvature ratios, Reynolds number, and different inlet conditions fail to give rise to a third cell near the outer bend. Therefore, experimental and numerical evidence together lend support to the formation of a pair of turbulence-driven secondary cells in curved-pipe flows.Research supported by the Office of Naval Research under Grant No. N0014-81-K-0428 and by the David Taylor Research Center, Annapolis, Maryland, under Contract No. N00167-86-K0075.     

Numerical modeling of liquid-waste infiltration from storage facilities into surrounding groundwater and surface-water bodies

This study focuses on the infiltration of saturated brine from liquid-waste storage facilities into the surrounding groundwater and surface-water bodies. The storage facilities are situated at the Verkhnekamsk Potassium and Magnesium Salt Deposit (Perm krai, Russian Federation); they store highly mineralized brine of potassium, sodium, and magnesium chlorides. An analytical solution of a one-dimensional equation of contaminant transport by a homogeneous steady-state groundwater flow is used to estimate the time it takes the contaminant to travel from the storage facilities to the nearest surface water body, as well as to evaluate the formation time of a stationary concentration profile, with contaminant adsorption in the porous matrix either neglected or taken into account. The contaminant concentration at the point of brine entry into the surface water body is calculated. The ANSYS Fluent software package is used for direct 3D simulation of brine infiltration into the surrounding medium. The simulation results revealed different stages of contaminant propagation in the porous medium. The contaminant was found to spread over a wide area with an almost uniform high brine concentration close to the saturation value. The contaminant reaches the nearest riverbed approximately 20 days after the start of infiltration. The estimates of the time required for the contaminant front to reach the surface water body obtained by three-dimensional simulation agree with the analytical estimates derived from a one-dimensional model. The proposed system of physical models adequately describes the hydrodynamic processes accompanying the operation of large storage facilities and can be used to predict contaminant-front propagation in the groundwater near storage facilities.     

Particulate Dispersion in a Turbulent Serpentine Channel

Particulate dispersion in an S-shaped duct, with periodicity between inlet and exit, is studied by direct numerical simulation. Stokes numbers range from 0.125 to 6.0. In a straight, turbulent channel flow, eddies are responsible for particulate impact. Turbophoresis causes a mean drift toward the wall. In a curved channel, particle inertia can be the dominant cause of impact. Above the lowest Stokes number, particles form into a plume that leaves the inner bend and flows toward the outer wall. Turbulence then disperses the plume. Heavier particles cross the bend and reflect from the outer wall, forming a high concentration layer near the surface. The heaviest particles reflect again from the wall and are dispersed across the duct by turbulence. An empirical formula is used to analyze the propensity for particle impacts to erode the wall. The region of maximum erosion is not the region of maximum number of impacts, nor is it where the impact velocity is highest: the impact angle determines where erosion is largest.