Study of Secondary Instability of a Streaky Boundary Layer Under Spanwise-Localized Free-Stream Vortical Disturbances
Study of Secondary Instability of a Streaky Boundary Layer Under Spanwise-Localized Free-Stream Vortical Disturbances
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展向局部自由流涡旋扰动下条纹边界层的二次不稳定性研究
DOI：
10.4208/aamm.2018.s12
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发表时间：
2019
影响因子：
1.4
通讯作者：
Jisheng Luo
中科院分区：
文献类型：
--
作者：
Yongming Zhang;Xiangzhen Chen;Jisheng Luo
利用双全局不稳定性分析方法研究了展向局部自由流涡扰动（FSVD）作用下的条纹边界层二次不稳定性。在不同的流向位置和在整个时间段内的不同时刻进行的不稳定性分析，并考虑在时空（xt）平面的结果，这样的不稳定模式被发现在四个不稳定区。通过模增长积累的比较，我们发现二次不稳定性对旁路跃迁的贡献比T-S波大得多，中心强条纹在二次不稳定性中起着关键作用。中心低速条纹处的弯曲型外模是主要的不稳定模，这与实验观察相一致。中心高速条纹处的内模也有很高的增长率，但其增长不能持续积累，导致其在二次不稳定性中的作用不大。本文发现的所有不稳定模态都出现在基流梯度方向的Fjortoft拐点处，这意味着这些模态具有相同的物理性质，即，它们由剪切流的弯曲不稳定性引起。
Secondary instability of a streaky boundary layer under spanwise-localized free-stream vortical disturbances (FSVD) is investigated using BiGlobal instability analysis. The instability analysis is performed at different streamwise locations and at different instants in a whole time period and the results in space-time (xt) plane are taken into consideration, such that unstable modes are found in four unstable zones. According to the comparison of mode growth accumulation, we find that the contribution of secondary instability to bypass transition is much more important than T-S waves and that the center strong streak plays the key role in secondary instability. The sinuous type outer mode at center low-speed streak is the dominant unstable mode and this agrees with experimental observation. The inner mode at center high-speed streak also has high growth rate, but its growth cannot keep accumulating, resulting in.its unimportant role in secondary instability. All unstable modes found in this paper appear at Fjortoft inflection points of the basic flow in gradient direction, implying that the modes have the same physical nature, i.e., they are caused by inflectional instability of the shear flow.