Study of low-frequency oscillation in the wake of a two-dimensional generic vehicle profile

Abstract

Flow around road vehicles is characterized by a massive wake,which is related to aerodynamic drag, unsteady loading, wind noise et al. Therefore, good understanding of the wake dynamics is important. In the present work,a low-frequency oscillation in the wake of a two-dimensional generic vehicle profile is investigated at a reasonably high Reynolds number Re=2.3 × 104 by direct numerical simulation (DNS). Although flow around vehicles is three-dimensional, some flow features are quasi-two-dimensional,such as separations over the head and over a straight back,for which a two-dimensional profile can be used. The flow solution is validated against the existing experimental data, and the calculation presents capacity for capturing dominant flow structures. Results show that a low-frequency oscillation is observed in the amplitude of the lift fluctuation. Other governing properties are also found varying,corresponding to the lift signal at the low frequency. The evolution of the wake during the high- and low-amplitude periods is examined, illustrating the greater lift fluctuation corresponds to the alternative vortex shedding,while the weaker fluctuation corresponds to the simultaneous vortex shedding. The velocity at the upper and the lower shear layers and the surface pressure acting on the trailing edges are comprehensively investigated, indicating the attenuated lift fluctuation is because the surface pressure acting on the upper and the lower sides cancel out,as vortices are shed AB18simultaneously. The mechanism of the exitance of two shedding modes is discussed based on the instabilities of the shear layer and the wake,respectively. 汽车尾部的大尺度流动分离是汽车外流场的显著特征之一，其与气动阻力、结构振动和震荡密切相关。应用直接数值模拟（DNS）方法研究在雷诺数 Re = 2.3 × 104 下某二维类车体尾迹中的低频震荡。该类车体常被用于汽车空气动力学学术研究，因为其轮廓可涵盖到货车主要几何特征，且其尾迹与货车的准二维尾迹具有共性。计算结果对照现有的实验结果进行验证，二者之间的差异经过评估确认了该计算的正确性。流场结果显示低频震荡与尾迹中两种模态之 间 的 相 互 转 换 有 关 。 这 两 种 模 态 分 别 由 Kelvin-Helmholtz 不稳定性及尾迹的不稳定性主导。前种模态下，尾部分离的上、下剪切层在几乎相同的时刻产生涡脱落；而在后种模态下，上、下剪切层交替地产生涡脱落形成卡门涡。根据不稳定性理论讨论了模态转换的机理，并阐述了因低频震荡而产生的气动力变化。