【作者】 宋昕；

【导师】 谷正气；

【作者基本信息】 湖南大学 ， 车辆工程， 2012， 博士

【摘要】 随着我国高等级公路通车里程数的快速增长,汽车实际行驶的平均速度有了明显的提高。同时能源危机、油价上涨引发的汽车轻量化又使得车身重量显著降低。这些原因使得汽车空气动力学性能尤其是气动升力对汽车性能的影响越来越大。伴随着生活水平的不断提高,消费者对汽车的安全性、舒适性等性能日益重视,这就对汽车的品质提出了更高的要求。气动升力作为评价汽车空气动力学性能的重要指标之一,不仅直接影响到汽车的安全性,同时也间接影响到汽车的燃油经济性。高速公路上经常出现的汽车发飘,汽车抬头现象更是进一步引起了相关人员对气动升力的关注到目前为止,汽车阻力系数的试验测定方法以及数值计算方法已经比较成熟,试验结果与计算结果吻合较好,但是对于升力系数,试验结果与计算结果经常存在较大误差。相对于阻力系数,升力系数的试验测定结果更易受模型安装、风洞结构等因素的影响,这就造成了升力系数的测量结果重复性差,且同一模型在不同风洞的测量结果经常会有较大差别。汽车的升力主要源于其上下表面的压差。由于汽车离地面很近,且底盘结构凹凸不平,底部流场相对复杂,计算结果易受网格、湍流模型等的影响,与试验结果的一致性较差。除此之外,与阻力越小越好不同,升力系数的优化目标则需根据实际情况具体分析,如赛车通常需要较大的负升力来帮助它实现高速转弯,而对于重型卡车则需要减小负升力,以降低轮胎与地面之间的摩擦阻力,从而实现低油耗。综上所述展开气动升力的试验和仿真研究不仅具有重大的理论意义,同时也能更好地指导工程开发。鉴于在气动升力的研究中存在上述问题,本文旨在通过大量的试验和仿真找出一套能获得较高精度气动升力的方法。在此基础上结合多体动力学,分析气动升力对整车直线行驶能力的影响。与此同时找出对气动升力较敏感的造型参数,最后提出一系列有效的造型优化方案改善整车的直线行驶能力。本文的主要研究内容如下：1.通过在湖南大学HD-2风洞中展开大量的试验研究,讨论了模型安装方式、地面附面层厚度对升力测量结果的影响。在此基础上对9种不同车型进行了风洞试验,讨论了汽车阻力系数与升力系数之间的关系。然后针对MIRA阶梯背模型研究了气动升力随离地间隙、俯仰角、侧偏角的变化规律,并结合测压试验、PIV试验分析了部分原因。2.通过大量仿真探讨了如何提高数值计算的精度与效率,主要做了以下几方面的工作：①对比了不同网格、不同壁面函数、不同湍流模型、不同压力耦合格式、不同压力离散格式对气动升力计算精度与速度的影响；②探讨了速度梯度自适应和Y+自适应法对改善网格合理性,提高计算精度的影响；③以Ahmed模型为研究对象,通过试验设计的方法对Realizable λ-ε湍流模型中的经验系数进行了优化。3.通过大量的仿真计算以及某MPV车型油泥模型的风洞试验,分析了气动升力系数对整体以及局部造型参数的敏感性。4.通过构建某轿车的动力学模型,探讨了气动升力对该轿车直线行驶能力、直线加速能力以及直线制动能力的影响5分析了工程实例中的某轿车的空气动力学性能,并讨论了在考虑气动力的情况下该轿车的直线行驶能力。在此基础上,根据气动升力的有关优化原则,对该轿车的造型进行了优化,提高了该轿车的直线行驶能力。更多还原

【Abstract】 With the rapid growth of high-grade highway traffic mileage, the average speed of automotive is greatly improved. The body weight is significantly reduced for the tendency of making light automotive which is caused by energy crisis and rising oil prices. For these reasons, the automotive aerodynamic characteristics, especially aerodynamic lift, have increasing influence on the automotive performance. With the continuous improvement of living standards, consumers put more emphasis on vehicle safety, comfort and so on, thus higher requirements for the quality of the automotive is put forward. As one of the most important evaluating indicator of aerodynamic performance, aerodynamic lift not only directly affects the safety of the automotive, but also indirectly affect the automotive’s fuel economy. The phenomenon that the driver feels the automotive shimmy dangerously at high speed arouses the concern of the relevant researcher on aerodynamic lift.So far, the experimental and computational methods about drag coefficient have been developed maturely, and experimental results agree well with computational results. As to the lift coefficient, experimental results are always different from computational results. Relative to the drag coefficient, the lift coefficient is more vulnerable to the uncertainties of the model installation, wind tunnel structure and so on, thus the repeatability of the lift coefficient is bad, and the lift coefficients are quite different for the same model which is measured in different wind tunnels. The lift of automotive is mainly from the pressure difference between its upper surface and underbody. For the uneven surface of chassis, the flow field around the bottom is complex when the automotive is driving close to the ground, thus the computational results are vulnerable to the influence of the grid, turbulence model and so on, and are often different from the experimental results. In addition, different from drag coefficient which is the smaller the better, the lift coefficient optimization objectives need to be analized specificly according to the actual situation, for a racing car a large amount of negative lift is required to help it cornering at high-speed, while for a heavy duty truck,.the negative lift is want to be reduced to decrease the frictional risistance between the tires and the floor, so as to achieve low fuel consumption. In summary, start the experimental and computational study on aerodynamic lift, and find out a reasonable way to evaluate the aerodynamic lift characteristics of different automotive not only of great theoretical significance, but also to conduct the project development.According to the problems mentioned above, the objective of this paper is to find out a method which can obtain high accuracy aerodynamic lift coefficient by a large number of experiments and simulations. On this basis, the influence of aerodynamic lift on vehicle straight-line driving ability is analyzed by combining with the multi-body dynamics; meanwhile the styling parameters which are sensitive to aerodynamic lift are identified. Finally, series schemes which are useful to improve vehicle straight-line driving ability are put forward. The main research contents are as follows:1. The influence of model installation and the thickness of the boundary layer on the results of lift measurement are discussed by conducting a large amount of experiments in HD-2wind tunnel of Hunan University. Based on this, the relationship between drag coefficient and lift coefficient is discussed by conducting wind tunnel tests with9different vehicle models. Finally, the laws that the lift coefficients change with the ground clearances, pitch angles, yaw angles are also studied by wind tunnel tests, its mechanism was analyzed by combining with surface pressure tests and PIV tests.2. How to improve the numerical accuracy and efficiency is discussed by simulations, the works were done as follows:①Comparing the results of different grids, different wall functions, different turbulence models, different pressure-coupled schemes and different pressure discretization schemes;②Discussing the influence of velocity gradient adaptation and Y+adaptation on improving the grids and the results accuracy;③Based on Ahmed model, the empirical parameters in the Realizable k-ε model were optimized by DOE.3. Based on a large number of simulations on model and the wind tunnel tests about some MPV, the sensitivity of the aerodynamic lift coefficient to the styling parameters was analyzed.4. Created the dynamic model of some automotive, and discussed the influence of aerodynamic lift on the straight-line driving ability of this automotive.5The aerodynamic characteristics of some automotive which is from an engineering project is analized, and its straight-line driving ability under aerodynamic force is also discussed. Based on this, Styling of this automotive is optimized according to relative optimization rules, and its straight-line driving ability is improved.更多还原