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首页> 外文会议>Vehicle aerodynamics, 2012. >Spectral Density and Time Scales of Velocity Fluctuations in the Wake of a Simplified Car Model
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Spectral Density and Time Scales of Velocity Fluctuations in the Wake of a Simplified Car Model

机译：简化汽车模型尾迹中速度波动的频谱密度和时标

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摘要

Although CFD is considered a well-established tool in the development of car aerodynamics it is obvious that there is still a strong demand on verification and validation of schemes and models involved in the numerical codes. This is reflected by the constantly high number of publications dealing with the matter. One of the test cases most often adopted for the purpose is the generic car model proposed by Ahmed et al. [1]. This is because of its geometrical simplicity and the realistic complex flow field which is reproducibly generated in its wake and makes it a selective test case. Another reason is the availability of comprehensive experimental data sets, amongst others the one we reported in an earlier paper [2]. The velocity data covers the complete flow field around the model but focuses on the wake region. Besides mean values it provides turbulence quantities such as Reynolds stresses and other higher order statistical moments measured using laser-Doppler anemometry (LDA). After reviewing the results of numerous CFD approaches and comparing those to the experimental data it turns out that most deficiencies arise from the failing prediction of detachment and reattachment of the flow on the rear body at a slant angle of 25°. A closer analysis reveals a severe underestimation of the fluctuating quantities in the wake and most notably in the shear layers between the outer flow and the recirculation region. This fact in turn can be directly associated with the difficulties in predicting reattachment. Because of these findings further LDA measurements with high spectral resolution were performed at selected locations. One major aim of the study was to identify the time scales of the fluctuations and in this way to come to suggestions what measures are advisable to reduce the uncertainties both in CFD and experimental results.
机译：尽管CFD被认为是汽车空气动力学发展中公认的工具，但是很明显，对数字代码中涉及的方案和模型的验证和确认仍然存在强烈的需求。有关此事的出版物数量不断增加反映了这一点。最常用的测试用例之一是Ahmed等人提出的通用汽车模型。 [1]。这是因为它的几何简单性和现实的复杂流场，该流场在其尾流中可重复生成，并使其成为选择性测试用例。另一个原因是可获得全面的实验数据集，其中之一是我们在较早的论文中报道的数据[2]。速度数据涵盖了模型周围的完整流场，但侧重于尾流区域。除平均值外，它还提供湍流量，例如雷诺应力和使用激光多普勒风速计（LDA）测量的其他高阶统计矩。在回顾了众多CFD方法的结果并将这些结果与实验数据进行比较之后，我们发现大多数缺陷是由于对后车身上的气流以25°的倾斜角分离和重新附着的预测失败而引起的。仔细分析发现，尾流中，尤其是外流和再循环区域之间的剪切层中的波动量被严重低估了。反过来，这一事实可能与预测重新连接的困难直接相关。由于这些发现，在选定的位置进行了具有高光谱分辨率的进一步LDA测量。这项研究的主要目的是确定波动的时间尺度，并以此方式提出建议采取哪些措施来减少CFD和实验结果的不确定性。

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