NACA-0012翼型上层流分离泡的破裂和再形成循环：流场动力学
Bursting and reformation cycle of the laminar separation bubble over a NACA-0012 aerofoil: The dynamics of the flow-field
摘要：
采用动态模态分解（DMD）和本征正交分解（POD）对NACA-0012翼型在雷诺数为$5\times 10^4$、马赫数为$0.4$和不同攻角下失速起始时的流场进行了详细的流动解剖研究。DMD和POD识别出三种不同的主要流动模式：1）低频下的全局振荡流动模式（LFO模式1）; 2）低频下翼型吸力面上的局部振荡流动模式（LFO模式2）;以及3）高频下沿翼型尾流的局部振荡流动模式（HFO模式）。LFO模式1的特点是绕翼型的全局涡动流、沿翼型弦的涡动压力以及产生和维持三涡组的过程。LFO模式2的特点是TCV上游涡（UV）的扩张和平流。HFO模态起源于翼型前缘，沿翼型尾流方向以振荡模态为特征。HFO模态存在于所有研究的攻角，并导致流场的全局振荡。翼型周围的整体流动振荡与前缘附近分离剪切层的层流部分相互作用，并触发产生和维持TCV的无粘绝对不稳定性。当TCV的UV扩展时，它向下游平流并为HFO模式提供能量。LFO模式1、LFO模式2和HFO模式相互加强，直到失速开始时LFO模式1和2超过HFO模式。在$9.7^{\circ} \leq α\leq 10.0^{\circ}$攻角下，低频流动振荡（LFO）现象充分发展。在较大迎角时，HFO模态超过LFO模态2，翼型经历完全失速。
英文摘要：
Detailed flow dissection using Dynamic Mode Decomposition (DMD) and Proper Orthogonal Decomposition (POD) is carried out to investigate the dynamics of the flow-field around a NACA-0012 aerofoil at a Reynolds number of $5\times 10^4$, Mach number of $0.4$, and at various angles of attack around the onset of stall. Three distinct dominant flow modes are identified by the DMD and the POD: 1) a globally oscillating flow mode at a low-frequency (LFO mode 1); 2) a locally oscillating flow mode on the suction surface of the aerofoil at a low-frequency (LFO mode 2); and 3) a locally oscillating flow mode along the wake of the aerofoil at a high-frequency (HFO mode). The LFO mode 1 features the globally oscillating-flow around the aerofoil, the oscillating-pressure along the aerofoil chord, and the process that creates and sustains the triad of vortices. The LFO mode 2 features the expansion and advection of the upstream vortex (UV) of the TCV. The HFO mode originates at the aerofoil leading-edge and features the oscillating mode along the aerofoil wake. The HFO mode exists at all of the investigated angles of attack and causes a global oscillation in the flow-field. The global flow oscillation around the aerofoil interacts with the laminar portion of the separated shear layer in the vicinity of the leading-edge and triggers an inviscid absolute instability that creates and sustains the TCV. When the UV of the TCV expands, it advects downstream and energies the HFO mode. The LFO mode 1, the LFO mode 2, and the HFO mode mutually strengthen each other until the LFO mode 1 and 2 overtake the HFO mode at the onset of stall. At the angles of attack $9.7^{\circ} \leq α\leq 10.0^{\circ}$ the low-frequency flow oscillation (LFO) phenomenon is fully developed. At higher angles of attack, the HFO mode overtakes the LFO mode 2, and the aerofoil undergoes a full stall.
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