方背Ahmed模型非定常尾迹的实验研究

方背Ahmed模型是一种简化的商用车类车体模型，气流在尾部发生分离形成回流区，使背部产生负压继而带来较大的气动阻力。利用风洞实验对1/4缩比的方背Ahmed模型的非定常尾迹进行了精细测量和统计分析，实验雷诺数为9.2×104。背部压力、粒子图像测速（PIV）和热线测量结果表明方背Ahmed模型的非定常尾迹呈现出3种流动特征的相互耦合:左右涡结构不对称分布的双稳态特征、水平以及垂直方向的涡脱落和回流区的周期性抽吸。其中双稳态现象在非定常尾迹中占主导作用，表现出2种稳定状态（水平不对称）的交替出现（转换概率P_转换=0.149），且每种稳定状态可维持较长的时间尺度（平均维持时间约为6 s），其频谱特性满足-2次幂律分布；水平和垂直方向剪切层振荡引起的涡脱落频率分别为Sr_H=0.13和0.17；回流区周期性抽吸的频率为Sr_H=0.07。3种流动结构共存并相互作用，从而使方背Ahmed模型的非定常尾迹呈现复杂的三维湍流特性。

Experimental investigation of the unsteady wake of the square-back Ahmed model

The square-back Ahmed model is a simplified commercial vehicle body model. The flow separates at the tail to form a recirculation region, leading to the negative pressure on the back and large aerodynamic drag. The unsteady wake of the 1/4-scale square back Ahmed model were finely measured and statistically analyzed via the wind tunnel test. The experimental Reynolds number was 9.2×104. The results of the back pressure measurement, particle image velocimetry (PIV) and hot-wire measurement show that the unsteady wake pattern of the square back Ahmed model basically consists of three flow characteristics: the bi-stable behavior (random asymmetry between the left and right vortex structures), vortex shedding in the horizontal and vertical direction, and pumping of the recirculation region periodically. Among them, the bi-stable phenomenon plays a dominant role in the wake. It shows the alternating occurrence of two stable states (the transition probability is 0.149). Each stable state can maintain a long time scale (the average duration is about 6 s), and its spectral characteristics satisfy the -2 power law distribution; the corresponding frequency of the periodic pumping oscillation is Sr_H=0.07; and the vortex shedding frequencies corresponding to the oscillations in the horizontal direction and the vertical direction are Sr_H=0.13 and Sr_H=0.17, respectively. The interaction among these structures exists, and thus the unsteady wake of the square back Ahmed model presents complex three-dimensional features.