The effect of a low-viscosity near-wall film on bypass transition in boundary layers
The effect of a low-viscosity near-wall film on bypass transition in boundary layers
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DOI：
10.1017/jfm.2015.214
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发表时间：
2015-05
影响因子：
3.7
通讯作者：
Seo Yoon Jung;T. Zaki
中科院分区：
文献类型：
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作者：
Seo Yoon Jung;T. Zaki
使用直接数值模拟 (DNS) 检查二流体边界层中的旁路转变。考虑粘性较小的壁膜，并在两种不同的粘度比和自由流湍流强度下评估对过渡位置的影响。粘性较小的壁膜吸收了来自外部流的平均剪切力，削弱了升力机制，并改变了边界层内的扰动场。这些效应都有利于延迟旁路转换的开始。然而，两流体界面上的粘度和平均剪切不连续性引入了一种在边界层中产生壁法向涡量的新机制，因此可以促进向湍流的转变。采用条件平均统计和条纹跟踪技术来检查壁膜对旁路过渡过程的影响。研究表明，尽管在两流体界面处产生了额外的扰动，但外部流体中条纹的较弱放大可以延迟湍流的分解。壁膜延迟转变的功效在中等水平的自由流湍流强度下得到证明，但随着湍流强度的增加而降低。
Bypass transition in a two-fluid boundary layer is examined using direct numerical simulations (DNSs). A less-viscous wall film is considered and the impact on transition location is evaluated at two different viscosity ratios and free-stream turbulence intensities. The less-viscous wall film absorbs the mean shear from the outer stream, weakens the lift-up mechanism, and alters the disturbance field inside the boundary layer. These effects all favour a delay in the onset of bypass transition. However, the viscosity and mean-shear discontinuities across the two-fluid interface introduce a new mechanism for the generation of wall-normal vorticity in the boundary layer, and can therefore promote transition to turbulence. Conditionally averaged statistics and streak tracking techniques are adopted in order to examine the impact of the wall film on the bypass transition process. It is shown that the weaker amplification of the streaks in the outer fluid can delay breakdown to turbulence, despite the additional disturbance generation at the two-fluid interface. The efficacy of the wall film in delaying transition is demonstrated at moderate level of free-stream turbulence intensity, but is reduced as the turbulence intensity is increased.