Abstract: In order to investigate the mechanics of the formation of the secondary flows induced by the isentropic sidewall compression in the supersonic flat-plate boundary layers，a simplified flow model was proposed. Based on the small perturbation approach and numerical study，the nonuniformity of Mach number distribution of the inflow was investigated to illustrate its importance for the formation of the secondary flows. The results show that，the secondary flows induced by the sidewall compression are the characteristics of the mean flow-field and independent of the instantaneous interaction between the disturbances and the vortexes. Due to the different characteristics of propagation of compression waves in the main stream and the boundary layer，the flows are compressed non-uniformly by the sidewall，resulting in the cross-movement of the fluid in the boundary layer compared to that in the mainstream. The distinction of direction between density gradient and pressure gradient or barocliny is one of the important factors leading to the formation and development of the streamwise vortex.

Key words: Secondary-flow；Streamwise vortex；Boundary layer；Small perturbation approach；Inlet

摘要： 为了揭示等熵侧压诱导的超声速平板边界层二次流的形成机理，基于建立的简化流场模型，结合小扰动分析和数值计算，研究了来流边界层马赫数不均匀分布因素对二次流形成的贡献。研究表明，侧压诱导的二次流是流场的平均行为，和瞬时的扰动与涡的相互作用无关。压缩波在主流和边界层中的传播特征不同，导致在侧壁压缩作用下主流和边界层流体压缩不均匀，从而产生了边界层流体相对于主流的横向运动；密度梯度和压力梯度矢量方向不一致，即流动的斜压性是流向涡形成和发展的一个重要因素。

关键词: 二次流；流向涡；边界层；小扰动分析；进气道