【作者】 杨博；

【导师】 傅立敏；

【作者基本信息】 吉林大学 ， 车辆工程， 2004， 硕士

【摘要】 轿车外流场中的尾流场和车轮流场十分重要。尾流场特性与车辆的气动阻力和气动升力密切相关，而基本形体周围的流动特性由于旋转车轮的存在变得非常复杂。对于轿车车轮流场，先前的研究中常常进行了过多的简化，所以有必要开展比较详细的在车轮旋转影响下的轿车外流场研究。当今世界的全尺寸风洞中只有少数几座带有移动地板系统，目前进行全尺寸轿车车轮旋转风洞试验仍然比较困难，而且旋转车轮的表面压力也难以获得。CFD技术可以在这个领域发挥有效的作用，因此我们应用CFD技术比较详细地分析了某国产轿车的尾流场和车轮流场，以便为风洞试验研究提供依据、积累数据。网格生成是CFD技术的一个关键内容。网格生成方法多种多样，如何在特定的研究和设计阶段选择网格生成的方法和形式是一个重要的问题。前人的研究中虽然都提及了网格方案，但是都没有对多种方案进行比较系统地对比分析，因此有必要针对轿车外流场进行详细的网格技术的探讨。目前的CFD软件一般都是采用迭代法求解离散方程。防止迭代过程中求解发散非常重要。影响解的收敛性的主要因素除了湍流模型、数值离散的格式和相应的数值控制方法外就是网格质量。对于轿车外流场这种实际问题，一般要采用比较成熟的湍流模型，数值离散格式和相应的数值控制方法，因此，提高CFD软件收敛性和精度的最不确定因素就是网格生成技术。本文比较系统地论述了网格技术基础，包括结构化网格、非结构化网格、混合网格的生成方法和特点，以及自适应性网格、笛卡尔网格的特点。为轿车外流场模拟选用的网格方案提供依据。针对某国产轿车，对比分析了五种网格方案，给出网格生成的一般步骤。选取了三种网格方案，应用大型CFD软件STAR-CD中选用成熟的高雷诺数的湍流模型和SIMPLE算法来求解此轿车的外流场，对比不同网格方案的计算结果。文中重点分析此车型的尾流场和车轮流场，分别应用了车轮固定和车轮<WP=67>旋转两种边界条件，对比了两种条件下的计算结果。在车轮旋转条件下，比较前轮后方流场和后轮后方流场的特点，比较此轿车阻力系数与升力系数计算值与风洞试验值，分析车轮流场对于汽车设计的意义，为深入研究轿车外流场的特性积累数据。CFD技术不仅在概念车阶段发挥了重要作用，而且在对已经批量生产车型的改进中也是必要的设计工具。我们尝试去改进此国产轿车的空气动力特性，论述了在后轮口处加装盖板的改进方案，分析这种方案的可行性，计算并且对比加装盖板后的后轮后方流场和尾流场的特性，对加装盖板后的燃油经济性进行初步的预测。本文得出以下主要研究结论：1. 关于该国产轿车的空气动力特性研究，四面体、三棱柱和六面体相结合的混合网格策略是比较有效的。2. 最终没有被选中的网格方案不是无效的，而是不适合在这个特定的使用范围。四面体和三棱柱结合的方案计算结果出现了很大的误差，而完全六面体的方案计算结果是发散的，主要原因是车轮附近的网格质量低。3. 本文采用的混合方案和传统混合方案有所不同，在研究对象的边界附近使用了四面体与三棱柱，在外侧空间使用六面体，这样就比较充分地发挥了三种单元的优点，也是本文的创新点。4. 在本文的研究情况中，车轮转动条件下计算的CD和CL值都要比车轮固定条件下的系数小，这与前人的试验研究结果相吻合，对于旋转车轮周围的流动特性还需要进一步的研究。此轿车的CD计算值偏大，CL计算值与试验值吻合较好。5. 关于车轮流场，压力峰值都在轮腔上方靠近外侧的边缘附近，因此若将轮腔外缘考虑成平面曲线是不够精确的，这个结论是对前人研究的补充。6. 加装后轮口盖板后的空气动力性能的确有可以提高的趋势和潜力，CD值下降了约4.2%，CL值下降了约45.3%。通过盖板方案的计算结果、可行性分析和经济性预测，认为此设计方案是合理的。7. 加装盖板后，后轮后方的流场发生了较大的变化，没有出现正压区，<WP=68>尾流场的结构没有出现大的变化，只是涡流的强度稍低。8. CFD可以帮助研发人员更好地理解流动的状态，以便为进一步的风洞试验研究积累相关数据。应用CFD进行流动特性的研究和各种空气动力数据的变化趋势的预测，对于汽车的空气动力学设计具有十分重要的意义。人们对于汽车设计的要求越来越高，设计周期越来越短，CFD技术在汽车设计中发挥了重要的作用。本文对轿车外流场CFD研究中多种网格技术进行了分析，提出新的混合网格方案，指出不同的网格策略会对计算结果造成相当大的影响。根据不同研究阶段和不同的研究条件，推荐了不同的网格方案，这为轿车外流场的CFD研究的前处理提供了重要依据。关于车轮流场总结了前人的研究，通过对此轿车车轮流场的详细分析，论证了轮腔外缘形状会直接影响车轮区域的流动状态，这补充了前人的研究结论，使车轮流场的研究取得了进展。通过论证在已批量生产车型基础上进行CFD改进设计的可行性，提出改善空气动力特性的有效方案，为汽车空气动力学设计积累数据，提供依据，并且为进一步的CFD研究和风洞试验研究做了铺垫，对工程实际具有一定的参考和指导意义。更多还原

【Abstract】 The wake survey and the flow field around wheels are really important in the whole external flow field of sedans. Characteristics of wakes have great effects on the aerodynamic drag and lift of sedans. It is noticeable that the flow around the basic body shape is complicated by the presence of the rotating wheels. With regard to the flow around a sedan’s wheels, there are too many simplifications in the past studies. Thus, it is necessary to carry out studies about the external flow around a sedan in detail with the rotating wheels condition. At present, it is still difficult to make full-scale sedan experiments in wind tunnels because full-scale rolling roads are rare in the full-scale wind tunnels of the world and surface pressures on rotating wheels are also difficult to obtain. Therefore, we analyzed the wake and flow around the wheels of a homemade sedan in detail with CFD technology in order that we could provide gist and data for wind tunnel experiments.Mesh generation is a critical content in CFD Technology. There are so many kinds of methods of mesh generation that it is important to decide how to select reasonable methods in certain studies. Mesh generation methods were mentioned in most of past research; however, there were not any contrasts among several different methods. Hence, it is significant to make some detailed discussions about the mesh generation technology.Today, the iteration technique dominates most of CFD software. And it is really important to prevent the divergence during the iterating process. Besides turbulence models, discretization schemes and algorithms, the quality of meshes affected the convergence of the computation directly. Toward the actual problems, such as the external flow field around a sedan, it is recommended to apply mature turbulence models, discretization schemes and algorithms. Consequently, the technique of mesh generation is the most uncertain factor in improving the convergence ability and precision of CFD software. This paper illustrated systematically the basics of mesh generation technique, including the methods of generating structured meshes, unstructured meshes, hybrid meshes and their features respectively and mentioned the characteristics of adaptive meshes and cartesian meshes. Recommended methods of selecting mesh strategies were provided for the research of external flow field around sedans. <WP=70>To a certain homemade sedan, five mesh strategies were compared and the general steps of creating meshes were put forward. Then, the mature turbulence model of k-εHigh Reynolds Number and SIMPLE Solution Algorithms were applied to compute the whole external flow field of the sedan using three mesh strategies. The computational results were also compared.The wake survey and the flow field around wheels are emphasized in this paper. The computation results are compared using stationary and rotating wheels condition. Characteristics of flow fields behind wheels were compared between the front wheel and the rear one. Computational results of CD and CL were compared with wind tunnel experiments. The significance of the flow field around wheels was discussed in the automobile design area in order that data for further research of the external flow field around a sedan could be accumulated.CFD is playing an important role not only in the concept car stage, but also in the aerodynamic improvement of vehicles that had been produced. We attempted to ameliorate the aerodynamic performance of this homemade sedan and illustrated the plan of adding a cover on the rear wheel arch. Feasibilities of this plan were discussed. Characteristics of the flow field around the rear wheel and the wake were compared with the counterparts of the prototype. Then, the fuel savings of the plan were estimated roughly.Main conclusions included:1. With regard to research about aerodynamic characteristics of this homemade sedan, the hybrid meshes strategy of tetrahedral cells, prismatic cells and hexahedral cells is effective and efficient.2. Strategies tha更多还原