俯仰振荡翼型推阻力转变滞后机制数值研究

扑翼产生的反卡门涡街被认为是一种推力型尾迹,但已有研究指出,随着斯特劳哈尔数(St)增大,低雷诺数下俯仰振荡翼型的净推力产生明显滞后于反卡门涡街的出现。为探究该现象背后的物理机制,对NACA0012翼型在雷诺数1000条件下作简谐俯仰运动的流场进行了数值模拟。采用翼型表面积分方法和基于有限控制体的气动力估计方法分别研究了翼面分布力特性和尾迹流场特性变化对阻力-推力转变临界点的影响。翼面分布力积分结果表明,当翼型振荡参数进入对应反卡门涡街尾迹形态区域时,翼面压力分布产生的推力分量无法克服剪切层的黏滞阻力,是造成俯仰翼型推力产生滞后于反卡门涡街出现的主要原因。对尾迹流场及相应的推阻力特性变化的分析表明,尽管反卡门涡街会产生“喷流效应”,但在St较小时,其产生的动量诱导推力无法克服反卡门涡自身的涡致阻力和尾迹流场压力损失引入的附加阻力,因此即使存在反卡门涡街也不能产生净推力,进而从流场分析的角度进一步解释了这一滞后现象的发生机制。

Numerical investigation on lag mechanism of drag-to-thrust transition for a pitching airfoil

The reverse Bénard-von Kármán(RBvK)vortex street produced by flapping wings is considered to be a thrust-generating wake,but studies have shown that as the Strouhal number increases,the generation of a net thrust for a pitching airfoil at low Reynolds numbers lags significantly behind the appearance of the RBvK wake.To reveal the physical mechanism behind this phenomenon,the flow field of the NACA0012 airfoil undergoing a simple harmonic pitching motion at a Reynolds number of 1000 is numerically studied.Both a surface integration method and a finite control volume based force estimation method are used to investigate the effects of the surface stress distribution and the wake flow variation on the drag-to-thrust transition,respectively.The results indicate that the lag phenomenon exists for all pitching amplitudes considered,and the degree of lag which quantified as the difference between the Strouhal number of the drag-to-thrust transition and that of the neutral wake,decreases as the pitching amplitude gets larger.The distributed force characteristics suggest that when the airfoil oscillation parameters enter into the wake pattern regime corresponding to the RBvK vortex street,the thrust component due to the pressure distribution on the airfoil surface cannot overcome the viscous drag of the shear layer,which is responsible for the lag between the thrust generation and the formation of the RBvK wake.The aerodynamic decomposition based on the flow field indicates that the thrust characteristic is dominated by three terms:the vortex thrust,the momentum induced thrust,and the pressure induced thrust.The variations of the wake flow and the corresponding thrust characteristics show that,although the RBvK wake can produce a"jet effect",it cannot generate enough momentum induced thrust to overcome both the vortex drag of the RBvK vortex itself and the additional drag due to the pressure loss in the wake,when the Strouhal number is small.Therefore,a net thrust cannot be generated even though there exists a RBvK vortex street.The mechanism of the lag phenomenon is further explained from the perspective of the flow field analysis.