Harmonic forcing of a laminar bluff body wake with rear pitching flaps
Harmonic forcing of a laminar bluff body wake with rear pitching flaps
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DOI：
10.1017/jfm.2022.520
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发表时间：
2022-07
影响因子：
3.7
通讯作者：
A. E. Giannenas;S. Laizet;Georgios Rigas
中科院分区：
文献类型：
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作者：
A. E. Giannenas;S. Laizet;Georgios Rigas
摘要对二维钝体尾流在两个后俯仰襟翼的谐波作用下的响应进行了数值研究。尾迹是由一个基于高度的雷诺数Re=100的矩形产生的，其特征是层流涡脱落。研究了相内“蛇形”和相外“鼓掌”的两种强制策略。研究了钝体展弦比（$AR=1,2,4$）、扑动频率、扑动幅值、扑动长度和雷诺数等因素对颤振速度的影响。对于蛇形运动，当尾迹被迫接近旋涡脱落频率时，观察到强烈的均方根阻力基本共振。对于拍击运动，当施加的力接近旋涡脱落频率的两倍时，观察到微弱的次谐波共振，导致升力均方根增加，而阻力均方根不受影响。这两种共振都加剧了旋涡脱落，并且在蛇形运动中观察到伴随的平均阻力增加。迫使远离共振区，这两种运动通过尾流对称和推进机制导致相当大的阻力减少。由于扑翼运动，每个振荡周期形成两个涡旋偶极子，削弱了涡旋的自然脱落，被认为是主要的对称机制。提出了一个单一的标度参数，以压缩两种运动在大范围的扑翼频率、幅值和扑翼长度范围内的平均阻力减少。最后，对强制策略的性能评估表明，鼓掌比蛇形更有效。
Abstract A numerical study on the response of a two-dimensional bluff body wake subjected to harmonic forcing imposed by two rear pitching flaps is performed. The wake is generated by a rectangle at a height-based Reynolds number $Re=100$, characterised by laminar vortex shedding. Two forcing strategies are examined corresponding to in-phase ‘snaking’ and out-of-phase ‘clapping.’ The effects of the bluff body aspect ratio ($AR=1,2,4$), flapping frequency, flapping amplitude, flap length and Reynolds number are investigated. For the snaking motion, a strong fundamental resonance of the root mean square (r.m.s.) drag is observed when the wake is forced near the vortex shedding frequency. For the clapping motion, a weak subharmonic resonance is observed when the forcing is applied near twice the vortex shedding frequency resulting in an increase of the lift r.m.s. whereas the drag r.m.s. remains unaffected. Both resonances intensify the vortex shedding and a concomitant mean drag increase is observed for the snaking motion. Forcing away from the resonant regimes, both motions result in considerable drag reduction through wake symmetrisation and propulsion mechanisms. The formation of two vortex dipoles per oscillation period due to the flapping motion, which weaken the natural vortex shedding, has been identified as the main symmetrisation mechanism. A single scaling parameter is proposed to collapse the mean drag reduction of the forced flow for both motions over a wide range of flapping frequencies, amplitudes and flap lengths. Finally, the assessment of the performance of the forcing strategies has revealed that clapping is more effective than snaking.