利用 resolvent 分析层流分离泡内扰动的跨频放大
Cross-frequency amplification of perturbations in a laminar separation bubble using resolvent analysis
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中文总结 AI 辅助
本文采用 LES、SPOD 及 resolvent 分析，揭示层流分离泡内驻留条纹的能量源于斜向 KH 波与驻留条纹的跨频放大耦合，为相关流动机制提供了新解释。
中文摘要 AI 辅助
本文对平板上逆压梯度诱导的层流分离泡（LSB）进行了大涡模拟（LES），其基于来流位移厚度的雷诺数为 410，自由流马赫数为 0.25。该泡的平均峰值反向流占比达 21.4%，在无任何外部激励的情况下，通过局部绝对不稳定性区域维持自激涡脱。对 LES 数据应用谱正交分解（SPOD），在 LSB 内识别出三种主导相干结构：涡脱频率下分离剪切层内的二维及斜向开尔文-亥姆霍兹（KH）波，以及再附附近近零频率下的驻留展向周期条纹。对平均流的经典 resolvent 分析显示，KH 波在一定展向波数范围内存在强对流放大，但在低频条纹形成区域仅预测到弱放大，该区域主导增益小几个数量级且无主导一阶机制存在。这一与 SPOD 能量的差异表明，条纹并非由同频线性放大维持，而是由经典框架视为无法解释输入的本征激励提供能量。对时间周期基流的谐波 resolvent 分析揭示了潜在机制：基流非定常性将涡脱频率下的斜向 KH 波与驻留条纹通过跨频放大耦合，产生的增益远大于直接同频放大的增益，该跨频途径为解释再附附近观测到的驻留条纹如何获得能量提供了可能。
英文摘要
A large-eddy simulation (LES) of a laminar separation bubble (LSB) induced by an adverse pressure gradient over a flat plate is performed at an inflow displacement-thickness-based Reynolds number of 410 and a free-stream Mach number of 0.25. With a mean peak reverse flow of 21.4%, the bubble sustains self-excited vortex shedding through a local region of absolute instability, in the absence of any external forcing. Spectral proper orthogonal decomposition (SPOD) applied to the LES data identifies three dominant coherent structures within the LSB: two-dimensional and oblique Kelvin--Helmholtz (KH) waves in the separated shear layer at the vortex-shedding frequency, and stationary spanwise-periodic streaks near reattachment at near-zero frequency. Classical resolvent analysis of the mean flow identifies strong convective amplification of the KH waves over a range of spanwise wavenumbers, but predicts only weak amplification in the low-frequency, streak-forming region, where the leading gain is orders of magnitude smaller and no dominant rank-one mechanism is present. This discrepancy with the SPOD energy indicates that the streaks are not sustained by same-frequency linear amplification, but are instead energized by the intrinsic forcing, which the classical framework treats as an unexplained input. Harmonic resolvent analysis of the time-periodic base flow reveals the underlying mechanism: the base-flow unsteadiness couples the oblique KH wave at the shedding frequency to the stationary streak through cross-frequency amplification, yielding a gain far larger than that of the direct same-frequency amplification. This cross-frequency route provides a likely explanation for how the stationary streaks observed near reattachment are energized.
