Abstract: In order to investigate the interaction mechanism between boundary layer separation and secondary flows in a diffusing S-duct，numerical simulations were performed for the internal flow field of the diffusing S-duct at different inlet conditions. The results show that boundary layer separation and secondary flows are closely related to each other: the large area of low-energy fluid caused by boundary layer separation significantly increases the strength and size of secondary flows，in turn，secondary flows can change the distribution of the separation region. The development of secondary flows is dependent on the competition between streamwise vortices，which are induced from the two opposite bends of the S-duct. For the case with long straight section，boundary layer is more prone to separation，and a‘lifted vortex’pattern is observed at the S-duct outlet. The total pressure recovery is 0.978，the total pressure distortion coefficient and swirl coefficient are 0.55 and 0.035，respectively，suggesting that the aerodynamic performance is considerably worse than the case without long straight section.

Key words: Diffusing S-duct；Inlet boundary layer；Boundary layer separation；Secondary flow；Total pressure recovery；Total pressure distortion

摘要： 为了研究S形扩压器边界层分离和二次流之间相互作用的机制，采用数值模拟方法，对不同进口边界层条件下的S形扩压器内流场进行了计算。结果表明:边界层分离和二次流之间存在着密切关系，边界层分离带来的大面积低速区显著地加大了二次流的强度和尺度，二次流又反过来会改变分离区的分布。S形扩压器二次流的发展可认为是两个弯道产生的流向涡相互对抗的过程，对于进口带有长等直段的方案，边界层更易发生分离，扩压器出口二次流最终发展成为“上抬涡”模式。此时总压恢复系数等于0.978，而总压畸变指数和旋流系数分别高达0.55和0.035，其气动性能明显劣于无长等直段的方案。

关键词: S形扩压器；进口边界层；边界层分离；二次流；总压恢复；总压畸变