论文标题
在随机底部地形上的波边界层中的接收性和过渡
Receptivity and transition in a wave boundary layer over random bottom topography
论文作者
论文摘要
进行直接数值模拟，以研究在随机有组织的波浪样底部地形上发展的孤立波边界层中的接受性和过渡机制。边界层流从底部显示对宽带扰动的选择性响应，并发展出流向流向的条纹。当条纹达到较高的幅度时，它们通过调节小规模的波动和压力场间接扩大流向流的涡流。这些较强的涡流反过来又更有效地搅动边界层，并通过提升机构进一步加强条纹。这种非线性反馈循环增加了边界层对粗糙度水平的敏感性，并在共享相同雷诺数的情况下产生巨大的变化，而粗糙度高度不同。根据条纹的幅度，观察到三种不同的局部崩溃场景：（i）条纹较弱的地区的二维波浪不稳定性； （ii）具有中等振幅条纹的区域内的内部剪切层不稳定性； （iii）在高振幅条纹高度升高的区域中，迅速增长的外剪切层不稳定性。内部不稳定性在所有过渡路径中的生长速率最慢，这证实了先前关于中等振幅条纹稳定作用的预测（Önder＆Liu，J。Fluid。Mech。，第896、2020卷，2020年，A20）。
Direct numerical simulations are conducted to study the receptivity and transition mechanisms in a solitary wave boundary layer developing over randomly organized wave-like bottom topography. The boundary layer flow shows a selective response to broadband perturbations from the bottom, and develops streamwise-elongated streaks. When the streaks reach high amplitudes, they indirectly amplify streamwise-elongated vortices through modulating small-scale fluctuations and pressure fields. These stronger vortices in turn stir the boundary layer more effectively and further intensify streaks via the lift-up mechanism. This nonlinear feedback loop increases the sensitivity of the boundary layer to the roughness level and yields dramatic variations among cases sharing the same Reynolds number with differing roughness height. Three different local breakdown scenarios are observed depending on the amplitude of the streaks: (i) two-dimensional wave instabilities in the regions with weak streaks; (ii) inner shear-layer instabilities in the regions with moderate-amplitude streaks; (iii) rapidly growing outer shear-layer instabilities in the regions with highly elevated high-amplitude streaks. Inner instabilities have the slowest growth rate among all transition paths, which confirms the previous predictions on the stabilising role of moderate-amplitude streaks (Önder & Liu, J. Fluid. Mech., vol. 896, 2020, A20).
