转捩翼型流动中的间歇性涡合并与极端阻力
Intermittent Vortex Merging and Extreme Drag in Transitional Airfoil Flow
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中文总结 AI 辅助
该研究通过NACA0012翼型的二维数值模拟，揭示转捩翼型流动中极端阻力源于成簇涡释放与合并，提出通过secondary涡量动力学控制涡释放时机可缓解极端气动载荷。
中文摘要 AI 辅助
偏离标称开尔文-亥姆霍兹涡脱的间歇性现象会在转捩翼型流动中产生罕见且显著的阻力突增。我们通过对NACA0012翼型的二维直接数值模拟研究这些事件，模拟条件为：攻角5°，来流马赫数0.4，基于弦长的雷诺数分别为5×10⁴和5×10⁵。在较低雷诺数下，事件解析分析显示，单个主涡通过壁面产生的反向 secondary 涡量的喷发从分离剪切层释放；每次喷发会中断发育中的主涡与其供能剪切层的连接，将涡释放到下游。标称涡脱时，到达后缘区域的涡对应一次此类释放，且保持足够孤立，可通过后缘而无强烈集体相互作用。极端事件则由成簇的涡释放产生：短时间内发生多次 secondary 涡量喷发，形成初始流向间距小的紧凑主涡群；差分对流进一步减小其间距，促进后缘附近的强烈相互作用，这些涡的组合压力足迹产生局部吸力峰，导致阻力急剧上升，相互作用形式包括长时间变形、丝化及快速核心合并。在Re=5×10⁵时也观察到类似的紧凑涡结构和后缘附近相互作用，表明下游事件路径在涡更小的情况下依然存在。这些发现提示，通过 secondary 涡量动力学控制涡释放时机，或可成为破坏成簇释放、缓解极端气动载荷的途径。
英文摘要
Intermittent departures from nominal Kelvin--Helmholtz shedding can produce rare and pronounced drag excursions in transitional airfoil flow. We examine these events using two-dimensional direct numerical simulations of flow over a NACA0012 airfoil at an angle of attack of $5^\circ$, a freestream Mach number of $0.4$, and chord-based Reynolds numbers of $5\times10^4$ and $5\times10^5$. At the lower Reynolds number, event-resolved analysis shows that individual primary vortices are released from the separated shear layer through the eruption of wall-generated, opposite-signed secondary vorticity. Each eruption interrupts the connection between a developing primary vortex and its feeding shear layer, releasing the vortex downstream. During nominal shedding, the vortex reaching the trailing-edge region is associated with a single such release and remains sufficiently isolated to pass the trailing edge without strong collective interaction. Extreme events instead arise through clustered vortex release, in which several secondary-vorticity eruptions occur within a short interval and produce a compact group of primary vortices with small initial streamwise spacing. Differential convection further reduces their spacing and promotes strong near-trailing-edge interactions, where the combined pressure footprint of these vortices produces a localized suction peak and a sharp increase in drag. These interactions range from prolonged deformation and filamentation to rapid core coalescence. Similar compact vortex organization and near-trailing-edge interactions are recovered at $Re=5\times10^5$, indicating that the downstream event pathway persists despite the smaller vortical scales.These findings suggest that controlling vortex-release timing through secondary-vorticity dynamics may provide a route to disrupt clustered release and mitigate extreme aerodynamic loading.
