Vortex mode bifurcation and lift force of a plunging airfoil at low reynolds numbers
Vortex mode bifurcation and lift force of a plunging airfoil at low reynolds numbers
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DOI：
10.2514/6.2010-390
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发表时间：
2010-01
期刊：
影响因子：
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通讯作者：
D. Cleaver;Zhijin Wang;I. Gursul
中科院分区：
文献类型：
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作者：
D. Cleaver;Zhijin Wang;I. Gursul
在雷诺数为10，000时，在失速前、失速和失速后攻角下，对NACA 0012翼型的倾翻进行了力和支持粒子图像测速测量。失速前和失速迎角在较大振幅时的升力系数表现出由初始条件确定的分支的突然分叉。随着频率的逐渐增加，观察到非常高的正升力系数，这被称为模式A。在同一频率下，翼型脉冲启动时，观察到负升力系数，这被称为模式B。模式A流场与俯冲运动底部附近的后缘涡配对相关，导致向上偏转的射流，以及由此产生的强烈上表面前缘涡。模式B流场与俯冲运动顶部附近的后缘涡配对有关，导致向下偏转的射流，以及由此产生的弱上表面前缘涡。由于后缘涡流强度不足，在小振幅下没有观察到分叉，也没有在较大迎角下观察到分叉，因为后缘涡流强度更大的不对称性，这导致了向下偏转模式B的自然偏好B尾流。
Force and supporting particle image velocimetry measurements were performed on a plunging NACA 0012 airfoil at a Reynolds number of 10,000, at pre-stall, stall, and post-stall angles of attack. The lift coefficient for pre-stall and stall angles of attack at larger amplitudes showed abrupt bifurcations with the branch determined by initial conditions. With the frequency gradually increasing, very high positive lift coefficients were observed, this was termed mode A. At the same frequency with the airfoil impulsively started, negative lift coefficients were observed, this was termed mode B. The mode A flow field is associated with trailing-edge vortex pairing near the bottom of the plunging motion causing an upwards deflected jet, and a resultant strong upper surface leading-edge vortex. The mode B flow field is associated with trailing-edge vortex pairing near the top of the plunging motion causing a downwards deflected jet, and a resultant weak upper surface leading-edge vortex. The bifurcation was not observed for small amplitudes due to insufficient trailing-edge vortex strength, nor at larger angles of attack due to greater asymmetry in the strength of the trailing-edge vortices, which creates a natural preference for a downward deflected mode B wake.