时间非对称性对二维扑翼气动性能的影响

Effect of Time Asymmetry on Aerodynamic Performance of Two-Dimensional Airfoil

摘要: 为了研究时间非对称性对扑翼气动性能的影响，通过求解N-S方程和使用重叠网格技术研究了下行程时间与整个运动时间的比$k=0.3~0.7$ 的二维翼型NACA0014的升阻系数。分析了在时间非对称的条件下，扑动和俯仰运动之间的相位差对升阻系数的影响。结果表明，在相位差$\varphi =\frac{\pi }{2}$ 时，下行程时间短的扑翼在一个周期内的平均升力系数更小。适当延长下行程时间可以提高平均升力系数，下行程时间过长会降低平均升力系数。在相位差$\varphi =\frac{3\pi }{2}$ 时，相较于对称运动的扑翼，平均升力系数只有在$k=0.3$ 时才显著增加，机翼在其他非对称运动下的平均升力系数小于对称运动下的平均升力系数。非对称运动的扑翼的平均推力系数均大于对称运动的。翼型周围的压力场结果表明，下行程时翼型下表面的压力均大于上表面的压力，上行程时下表面的压力均小于上表面的压力。

Abstract: In order to study the effect of motion asymmetry of flapping wing on aerodynamic performance, this paper establishes a model of plunging motion and pitching motion that includes asymmetry. The lift coefficients and drag coefficients of the two-dimensional airfoil NACA0014 at $k=0.3$ to $k=0.7$ are investigated by solving the N-S equations and overset grid technique. In addition, based on the existing studies, the effect of the phase difference between the plunging and pitching motions on the lift and drag coefficients is also analyzed when the asymmetric motion is considered. The results show that at phase difference $\varphi =\frac{\pi }{2}$ , the asymmetric motion with a short downstroke time decreases the average lift coefficient in one cycle. Appropriate prolongation of downstroke time can increase the mean lift coefficient, and too long downstroke time will decrease the mean lift coefficient. At the phase difference $\varphi =\frac{3\pi }{2}$ , the mean lift coefficient can be significantly increased only at $k=0.3$ , and the mean lift coefficient of the wing under other asymmetric motions is smaller than that of the symmetric motion. In addition, asymmetric motion can increase the mean thrust. The results of the pressure contour around the airfoil shows that the pressure of lower surface of the airfoil is all greater than the upper surface during the downstroke motion, and the pressure of lower surface is all less than the upper surface pressure in the upstroke motion. This verifies that the downstroke of the flapping wing generates positive lift, and the upstrokes generate negative lift.