振荡剪切流与一种新的二维自维持过程
Oscillatory shear flows and a new 2D self-sustaining process
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中文总结 AI 辅助
本研究在振荡均匀剪切流中发现新的二维自维持过程，揭示其由一维片层与二维涨落构成的结构及线性不稳定机制，给出高雷诺数下的标度律，为振荡壁面流动研究提供新视角。
中文摘要 AI 辅助
本文采用非线性开尔文模模型，在空间均匀的振荡剪切流中发现了一种新的自维持过程（SSP）。该自维持过程由高能量的展向不变一维“片层”和强度较弱的展向相关二维涨落组成。它瞬时为二维结构，在流动-横切剪切平面内具有随时间变化的不变方向，且具备曲张对称性。片层随时间振荡且呈线性不稳定，会产生涨落，涨落随后发生非线性自相互作用以强化片层。研究表明，片层的线性不稳定性源于升力机制与“前推”效应的共同作用，后者由片层的“流向”剪切产生。当雷诺数$Re\rightarrow\infty$时，渐近分析与数值模拟均显示自维持过程的下支满足涨落速度$|\boldsymbol{u}'|\sim Re^{-1/2}$的标度关系。本文还讨论了该结果对振荡壁面约束流动的启示。
英文摘要
A new self-sustaining process (SSP) is identified in an oscillating spatially-homogeneous shear flow using a nonlinear Kelvin mode model. This SSP consists of energetic spanwise-invariant 1D `sheets' and weaker spanwise-dependent 2D fluctuations. It is instantaneously 2D, with a time-dependent invariant direction in the flow-cross-shear plane, and has a varicose symmetry. The sheets oscillate in time and are linearly unstable generating fluctuations which then nonlinearly self-interact to reinforce the sheets. The linear instability of the sheet is shown to be due to a lift-up mechanism acting in tandem with `push-forward', which arises due to the `streamwise' shear of the sheets. As the Reynolds number $Re\rightarrow\infty$, both asymptotics and numerics show SSP lower branches with fluctuation velocity $|\boldsymbol{u}'|\sim Re^{-1/2}$. Implications are discussed for oscillatory wall-bounded flows.
