Abstract The use of rear cavities at the base of a square-back Ahmed body has been experimentally evaluated as a passive control device under cross-wind conditions with yaw angles β ≤ 10 ° , by means of pressure, force and velocity measurements. A comparative study has been performed at a Reynolds number R e = 10 5 , considering the reference square-back body (i.e. the body without any passive control device), and the same body implementing both straight and curved cavities as add-on devices. It is shown that the performance of a straight cavity, which is widely acknowledged as a robust drag reduction device for car models, is hindered under moderate cross-wind conditions, and does not constitute an efficient control strategy, especially when compared with a curved cavity. In particular, when the freestream is aligned with the body, the curved cavity provides a stronger attenuation of the fluctuating nature and the bi-stable dynamics of the wake (characteristic of the wake behind a square-back Ahmed body) than the straight one. Besides, the reduced size of the near wake, which is provoked by flow re-orientation and the reduced span between the rear edges of the curved cavity, leads to an important base pressure recovery, that translates into relative reductions of the drag of 9.1 % in comparison with the reference case (i.e. 2.6 % , with respect to the straight cavity). The results are considerably improved under cross-wind conditions, since the increase with the yaw angle of the force is particularly intense for the body with the straight cavity and attenuated for the model with the curved cavity. Thus, the relative reduction of the drag coefficient with respect to the reference body becomes negligible for the straight device at a yaw angle of 10°, while it still represents approximately a 10 % for the curved cavity. Furthermore, flow visualizations show that the wake is deflected as the incident flow is increasingly yawed, leading to the formation of a single leeward vortex core that approaches progressively the body, decreasing the base pressure. This phenomenon is minored when a curved cavity is implemented, increasing the low pressure induced at the body base.

中文翻译：

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侧风条件下具有不同后腔的三维钝体减阻

摘要 在偏航角 β ≤ 10 ° 的侧风条件下，通过压力、力和速度测量，在方背 Ahmed 车身底部使用后腔作为被动控制装置进行了实验评估。比较研究已经在雷诺数 Re = 10 5 下进行，考虑到参考方背车身（即没有任何被动控制装置的车身），以及将直线和弯曲腔体作为附加装置的同一车身。结果表明，被广泛认为是汽车模型的强大减阻装置的直腔的性能在中等侧风条件下受到阻碍，并且不构成有效的控制策略，尤其是与弯曲腔相比时. 特别是，当自由流与身体对齐时，弯曲腔比直腔提供了更强的波动衰减和尾流的双稳态动力学（方背艾哈迈德体后面的尾流特征）。此外，由流动重新定向和弯曲空腔后边缘之间的跨度减小引起的近尾流尺寸减小，导致重要的基础压力恢复，这意味着阻力相对减少了 9.1%与参考案例相比（即 2.6 %，相对于直型腔）。结果在侧风条件下得到了显着改善，因为力随偏航角的增加对于具有直腔的主体特别强烈，而对于具有弯曲腔的模型则减弱。因此，对于偏航角为 10° 的直线装置，阻力系数相对于参考体的相对减少可以忽略不计，而对于弯曲腔，它仍然代表大约 10%。此外，流动可视化显示，随着入射流越来越偏航，尾流被偏转，导致形成单个背风涡核，逐渐接近身体，降低基础压力。当采用弯曲的腔体时，这种现象会减少，从而增加在主体底部引起的低压。导致形成一个背风涡核，逐渐接近身体，降低基础压力。当采用弯曲的腔体时，这种现象会减少，从而增加在主体底部引起的低压。导致形成一个背风涡核，逐渐接近身体，降低基础压力。当采用弯曲的腔体时，这种现象会减少，从而增加在主体底部引起的低压。