【作者】 丁林；

【导师】 张力； Michael M.Bernitsas；

【作者基本信息】 重庆大学 ， 动力工程及工程热物理， 2013， 博士

【摘要】 钝体绕流时的旋涡脱落是自然界中最普遍的一种物理现象，包含着许多复杂的流动机理。旋涡脱落在钝体表面形成周期性脉动作用力，当钝体固定方式为弹性支撑或允许发生形变时，脉动力将引发钝体产生周期性振动，钝体振动又进一步影响尾流旋涡脱落形态，这种流体与结构物之间相互作用的问题即为流致振动(Flow-induced motion, FIM)。在许多与流体有关的机械与工程中，流致振动是一个涉及安全性的重大问题。涡致振动水生清洁能源（VIVACE）系统将流致振动与能源利用相结合，使这种潜在的破坏性现象在清洁能源领域得到有效利用。为了提高VIVACE系统的能量密度，本文采用被动湍流控制（PTC）和多柱体结构改善系统能量收集性能。另外，随着近年来海洋工程、风工程、航天航空和核工程的飞速发展，多柱体结构流致振动问题受到广泛关注，已成为流体力学理论研究和应用领域的重要课题之一，尚存在大量的问题需要进一步深入研究。因此，对于多柱体流致振动的研究具有非常重要的学术意义与应用价值。本文针对被动湍流控制下的多圆柱流致振动问题进行数值分析和实验研究，详细分析了多圆柱流致振动特性和尾流旋涡形态，探讨了各种因素对多柱体旋涡脱落的影响，揭示了多柱体流致振动产生机理，探索改善多柱体流致振动和提高涡致振动水生清洁能源系统能量密度的方法。采用Spalart-Allmaras湍流模型，求解非稳态雷诺平均纳维-斯托克斯方程组（RANS）获得多圆柱流致振动的数值解，并在美国密歇根大学海洋可再生能源实验室（MRELab）完成相应实验测试，得到了以下结论。首先，针对PTC单圆柱流致振动系统展开数值计算和实验研究，获得不同来流速度下圆柱振幅、频率和近尾迹流场特征，建立了描述柱体流致振动能量转换装置发电功率的数学模型，分析了最优能量收集速度范围。当雷诺数（Re）在3×10~4≤Re≤1.3×10~5范围内时，PTC单圆柱流致振动RANS数值结果与实验测试极为吻合，能够清晰观察到涡致振动（VIV）初始分支、VIV上部分支和驰振。柱体每个振动周期产生的脱体旋涡数量随着Re数的上升而增加。PTC单圆柱VIVACE系统在整个测试Re数范围内都能有效进行能量收集，模拟结果表明，在VIV上支和驰振起始区域具有最优能量收集效率。其次，采用GGI（General Grid Interface）界面结合拓扑网格变形技术，首次对串行排列的弹性支撑PTC双圆柱结构涡致振动和驰振进行数值计算，并在MRELab完成了实验测试。研究表明PTC双圆柱的流致振动振幅和频率响应数值模拟与实验结果趋势一致，两个圆柱均清晰观察到VIV初支和上支、VIV-驰振过渡分支及驰振。当Re=30000时，上游圆柱尾流旋涡形态为典型的2S模式，下游圆柱受到上游圆柱脱体旋涡的影响，运动几乎受到抑制；当Re=59229时，上游圆柱尾涡形态在2P和2P+2S之间切换，下游圆柱尾涡形态为2P模式。在PTC双圆柱发生驰振时，数值模拟获得高达3.5D的振幅（D为圆柱直径），实验测试中的圆柱撞击水槽安全停止阀，达到实验水槽所允许的振幅极限。然后，针对串行排列的三个PTC圆柱，研究了各个圆柱流致振动规律，首次通过数值模拟可视化分析了涡致振动过渡到驰振的原因及驰振产生机理。数值模拟和实验研究均表明，在光滑圆柱表面采取PTC措施，能有效刺激边界层分离和旋涡脱落，扩大柱体流致振动同步范围，使高Re数下数值结果与实验测试相吻合。在圆柱振动进入VIV上部分支以后，随着来流速度增加，线性粘滞阻尼模型能有效模拟高Re数PTC圆柱流致振动。圆柱流致振动各个分支间的转移往往伴随尾流旋涡形态的改变，在PTC圆柱由VIV过渡到驰振时，流致振动产生机理也随之发生变化。PTC圆柱驰振的产生是由升力不稳定性引起的，在脱体旋涡形成过程中，圆柱上下侧的分离剪切层被拉伸到几乎垂直于来流方向的位置，造成圆柱上下侧出现较大压差，从而使圆柱受到的升力急剧增大产生驰振。最后，通过数值模拟和实验分析对串行排列的PTC四圆柱的流致振动特性开展了研究。研究发现，在串行排列的四圆柱系统中引入PTC，是实现数值结果与实验测试相互匹配的关键因素。在3×10~4≤Re≤1.05×10~5范围内，四个圆柱均观察到了VIV初支、VIV上支、VIV-驰振过渡分支和驰振。PTC圆柱振动由VIV上支向驰振过渡时，实验发现水槽自由液面会造成圆柱振幅突然降低，使柱体驰振出现振幅波动。在Re=50000时，来自上游第1个圆柱的脱体旋涡使第2个圆柱受到的流体作用力变得不稳定，极大削弱圆柱横向升力，从而导致第2个圆柱出现低振幅和低频率振动现象。由于上游PTC圆柱产生的脱体旋涡对下游圆柱运动产生明显影响，使各个圆柱振动频率相互关联，第2/3/4圆柱的振动频率随流速的变化趋势相似。上游PTC圆柱每个振动周期旋涡产生数量往往多于下游柱体，尾流旋涡形态也更为复杂。更多还原

【Abstract】 The vortices shed from a bluff body is a kind of widespread physical phenomena,which involves many complex flow mechanisms. Vortex shedding results in fluctuatingforces acting on the bluff body, which can cause vibrations when the bluff body iselastically mounted or allowed deformation. This vibration further changes the nature ofthe vortex formation. The interaction between the fluid and structure is called FlowInduced Motion (FIM). FIM is typically treated as destructive phenomenon because ofthe fatigue damage may be potentially introduced. With entirely opposite objective ofprevious efforts, which are mainly focused on reducing FIM effects, VIVACE (VortexInduced Vibration for Aquatic Clean Energy) converter is designed to generate powerby utilizing this potentially disastrous phenomenon. With the rapid development ofocean engineering, wind engineering, aerospace, and nuclear engineering, FIM ofmultiple cylinders has attracted a wide spread attention and becomes one of theimportant subjects in the theory and engineering applications of the hydrodynamics. Butthe research on this topic is still in the stage of exploration and needs to be intensivelystudied. Research on FIM of multiple cylinders has great significance in theory andapplication.In this thesis, the FIM of multiple cylinders with Passive Turbulence Control (PTC)are simulated using2-Dimensional Unsteady Reynolds-Averaged Navier-Stokesequations with the Spalart-Allmaras one equation turbulence model. Numerical resultsare compared with experiments conducted in the Marine Renewable Energy Lab(MRELab) of the University of Michigan in the USA. The effects of the parameters ofPTC-cylinders on the vortex shedding, wake vortex patterns, characteristics of bodymotion, hydro-oscillation force, and energy conversion are investigated in the presentstudy. The mechanisms of FIM, such as VIV and galloping, are explored as well. Theenergy conversion efficiency of VIVACE converter is estimated.Firstly, the flow and body kinematics of the transverse motion of a spring-mountedcircular cylinder are investigated. The simulated Reynolds number range for whichexperiments were conducted in the MRELab is30000<Re<130000, which falls in thehigh-lift TrSL3regime, where TrSL indicates Transition in Shear Layer. PTC in theform of selectively distributed surface roughness is used to alter the cylinder FIM. Theamplitude-ratio curve in the simulation results clearly shows three different branches, including the VIV initial and upper branch, and a galloping branch. The numericalbranches are similar to those observed experimentally. The number of the vortices shedfrom the PTC-cylinder in one oscillating cycle increases with flow velocity. Power isharnessed in the entire test ranges of Reynolds number. The optimal energy conversionefficiency of VIVACE is obtained in the starting area of VIV upper branch andgalloping in the simulation results.Secondly, the FIM of two rigid circular cylinders, on end linear-springs, in tandemare numerically studied and verified by experimental data. The key point in thesimulations is the utilization of Topological Mesh Changes combined with General GridInterface. PTC is being used to enhance FIM of cylinders in the VIVACE Converter toincrease its efficiency and power density in harnessing marine hydrokinetic energy. Theamplitude-ratio and frequency results in CFD are in excellent agreement withexperimental data showing the initial and upper branches in VIV, transition from VIV togalloping, and galloping. Vortex structures are studied using high-resolution imagingfrom the CFD results showing typical2S structure in the initial branch and both2P+2Sand2P in the upper branch of VIV. In the galloping branch, amplitudes of3.5diametersare reached before the channel stops are hit.Furthermore, a series of simulations are performed for validating the ability of thecode to predict hydrodynamic loads and response of three spring-mountedPTC-cylinders undergoing flow induced motion using2-D URANS. Validation isperformed by comparing simulation results to the experiments. The results show that thecylinder FIM is enhanced with the application of PTC, the flow over the cylindersurface is altering in a way that generates higher lift which is shown better synchronizedwith the motion. The classical linear viscous damping model used in the simulationsmatches well with the physical damping model because the velocity of oscillations isnot near zero when the PTC-cylinder is in the VIV upper branch or galloping. Forcylinder in FIM, the transition between branches is accompanied by vortex patternchange. In galloping, the driving mechanism is not based on the alternating vortices buton the lift instability caused by negative damping due to the lift force induced by theasymmetry of the geometry of the circular cylinder due to the turbulence stimulation.Finally, the flow induced motions of four cylinders with PTC in tandem are studiedby numerical simulation and experimental analysis. The study results show that thereason for the successful numerical prediction of the experimental results lies in theapplication of the turbulence stimulation in the form of the PTC. The VIV initial branch and upper branches, transition from VIV to galloping, and galloping are observed for allcylinders. The four-cylinder system extracts so much hydrokinetic energy from the fluidflow that a significant drop in free surface level occurs between the first and fourthcylinders. As the velocity increases, so does the response amplitude, making thecylinders reach closer to the free surface and resulting in the strong fluctuations exhibit.For Re=50000, the upstream vortices shedding from the1stcylinder directly and closelyinteract with the downstream cylinders. Especially for the2ndPTC-cylinder, the liftforce is weakened which results in low amplitude and low frequency vibration. Thevariation trends of oscillation frequency for the2/3/4cylinders are similar. The numberof vortex shed from the upstream cylinder is always more than that from thedownstream one, which leads to a more complex vortex pattern for the upstreamcylinder.更多还原