二维类车体尾迹中的低频震荡

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 simultaneously. The mechanism of the exitance of two shedding modes is discussed based on the instabilities of the shear layer and the wake, respectively.