Streak instabilities in boundary layers beneath free-stream turbulence
Streak instabilities in boundary layers beneath free-stream turbulence
复制标题
DOI：
10.1017/jfm.2013.677
复制
发表时间：
2014-02
影响因子：
3.7
通讯作者：
M. Hack;T. Zaki
中科院分区：
文献类型：
--
作者：
M. Hack;T. Zaki
用直接稳定性分析的方法研究了边界层条纹的二次不稳定性。基本流量是在旁路过渡的直接模拟中计算的。自由流扰动的随机性导致边界层内条纹频谱的形成，个别条纹的局部不稳定性的放大引发了湍流的破裂。建立了稳定性分析对直接数值模拟中观察到的不稳定性进行预测的能力。此外，分析表明，可以识别特定的条纹，这些条纹分解为下游更远的湍流。两种特殊的条纹结构通常会导致这些局部不稳定性的增长：向边界层边缘抬升的低速条纹，以及边界层内高速和低速条纹之间的局部重叠。基本模式可以归因于Vaughan&Zaki提出的内模和外模的一般分类(流体力学杂志，681卷，2011年，第116-153页)。统计分析表明，Blasius边界层有利于外部不稳定的放大。不利的压力梯度促使击穿通过内模转化为湍流。
Abstract The secondary instability of boundary layer streaks is investigated by means of direct stability analysis. The base flow is computed in direct simulations of bypass transition. The random nature of the free-stream perturbations causes the formation of a spectrum of streaks inside the boundary layer, with breakdown to turbulence initiated by the amplification of localized instabilities of individual streaks. The capability of the instability analysis to predict the instabilities which are observed in the direct numerical simulation is established. Furthermore, the analysis is shown to identify the particular streaks that break down to turbulence farther downstream. Two particular configurations of streaks regularly induce the growth of these localized instabilities: low-speed streaks that are lifted towards the edge of the boundary layer, and the local overlap between high-speed and low-speed streaks inside the boundary layer. It is established that the underlying modes can be ascribed to the general classification of inner and outer modes which was introduced by Vaughan & Zaki (J. Fluid Mech., vol. 681, 2011, pp. 116–153). Statistical evaluations show that Blasius boundary layers favour the amplification of outer instabilities. Adverse pressure gradient promotes breakdown to turbulence via the inner mode.