当自由流湍流度较大时边界层会发生bypass转捩，这也是工程中常见的一种转捩。目前对展向局部自由流湍流下边界层bypass转捩的二次失稳的内在机理，认识还不充分。本项目拟从我们过去对边界层内条纹演化的研究结果出发，对带条纹的边界层进行二次失稳分析，通过考察其三个方面的特性，以求对bypass转捩二次失稳的本质有更全面、深刻的理解。第一，拟通过比较二次失稳的增长率和层流中T-S波的增长率，确认二次失稳对bypass转捩的贡献是大还是小。第二，拟通过考察二次失稳特征模态的特征函数分布，确认起主导作用的条纹是一个还是多个。第三，拟通过考察二次失稳特征模态的相速度和特征函数分布，确认起主导作用的模态是内模态还是外模态。

Bypass transition takes place in the boundary layer at a high level of free-stream turbulent intensity, and it is also the usual type of transition in engineering problems. It is yet to fully developed that our understanding of the internal mechanism of secondary instability in boundary layers for spanwise localized free-stream turbulence. The proposed project is based on the results we got in the research of streaks development in the boundary layer, and will do secondary instability analysis on the streaky boundary layer. By investigating its three aspects, we can have a more comprehensive and profound understanding of the nature of secondary instability for bypass transition. Firstly, by comparing the growth rate of secondary instability and that of T-S waves in laminar flow, we can confirm whether the contribution of secondary instability to bypass transition is dominant or not. Secondly, by studying the eigenfunction shape of secondary instability eigenmode, we can confirm whether one or more streaks plays a key role for secondary instability. Thirdly, by studying the phase speed and eigenfunction shape of secondary instability eigenmode, we can confirm whether inner or outer mode is the dominant one for secondary instability.