椭圆前缘平板壁面冷却下的旁路转捩
Bypass transition under wall cooling over a flat plate with an elliptical leading edge
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中文总结 AI 辅助
本研究通过壁面解析大涡模拟，发现壁面冷却不改变平板边界层旁路转捩机制与起始位置，但产生热条带并使速度条带向壁面靠近，且高速条带在转捩前演化中起关键作用。
中文摘要 AI 辅助
本研究采用壁面解析大涡模拟方法，研究了受自由来流湍流影响的平板边界层中的旁路转捩。考虑了两种热配置：绝热壁面和均匀冷却壁面。平板几何包含椭圆前缘，并设计为可直接进行实验复现，为未来的数值与实验比较提供了框架。解析的前缘使得能够直接考察旁路转捩的早期阶段。两种模拟均重现了经典的感受性、涡倾斜、抬升、条带放大、二次不稳定性和湍斑增长序列。壁面法向输运项被识别为条带形成的前兆，而剪切遮蔽被量化并关联到自由来流扰动对边界层的频率依赖性穿透。壁面冷却并不改变旁路转捩机制，也不改变转捩起始位置。相反，它在速度条带之外产生了热条带，并使速度条带略微向壁面靠近，这与最优扰动预测一致。在两种热条件下，速度条带保持相似的振幅、增长率和展向间距。条件分析揭示了高速与低速条带之间的显著不对称性。尽管击穿系统性地发生在低速条带内，但最强的转捩前演化发生在高速条带群体中，该群体在间歇性开始之前经历了显著放大并逐渐向壁面位移。这些观察表明，高速条带可能积极促进条带场的重组，导致二次不稳定性和击穿。
英文摘要
This study uses wall-resolved large-eddy simulations to investigate bypass transition in a flat-plate boundary layer subjected to free-stream turbulence. Two thermal configurations are considered: an adiabatic wall and a uniformly cooled wall. The plate geometry includes an elliptic leading edge and is designed for direct experimental reproduction, providing a framework for future numerical-experimental comparisons. The resolved leading edge allows direct examination of the early stages of bypass transition. Both simulations recover the classical sequence of receptivity, vortex tilting, lift-up, streak amplification, secondary instability and turbulent-spot growth. The wall-normal transport term is identified as a precursor of streak formation, while shear sheltering is quantified and linked to the frequency-dependent penetration of free-stream disturbances into the boundary layer. Wall cooling does not modify the bypass-transition mechanisms nor the onset location of transition. Instead, it generates thermal streaks alongside the velocity streaks and shifts the latter slightly closer to the wall, consistent with optimal-perturbation predictions. The velocity streaks retain similar amplitudes, growth rates and spanwise spacings in both thermal conditions. A conditional analysis reveals a pronounced asymmetry between high- and low-velocity streaks. Although breakdown is systematically observed within low-velocity streaks, the strongest pre-transitional evolution occurs within the high-velocity streak population, which undergoes significant amplification and a progressive displacement towards the wall before the onset of intermittency. These observations suggest that high-velocity streaks may actively contribute to the reorganisation of the streak field leading to secondary instability and breakdown.
