ONERA, The French Aerospace Lab, Aerodynamics Aeroelasticity Acoustics Department (DAAA), 8 rue des Vertugadins, 92190 Meudon, France.
Université de Toulouse, INPT, UPS-Institut de Mécanique des Fluides de Toulouse (IMFT), Allée Camille Soula, 31400 Toulouse, France.
Phys Rev E. 2017 Jun;95(6-1):063111. doi: 10.1103/PhysRevE.95.063111. Epub 2017 Jun 20.
The two-dimensional and time-periodic wake flows produced by a pitching foil are investigated numerically for a fixed flapping amplitude. As the flapping frequency is increased, three regimes are identified in the time-marching nonlinear simulations. The first regime is characterized by nondeviated wake flows with zero time-averaged lift. In the second regime, the wake flow is slightly deviated from the streamwise direction and the time-averaged lift is slightly positive or negative. The third regime is characterized by larger deviations of the wake, associated with larger values of both the time-averaged lift and the thrust. The transition from the first to the second regime is examined by performing a Floquet stability analysis of the nondeviated wake. A specific method is introduced to compute the time-periodic, nondeviated wake when it is unstable. It is found that one synchronous antisymmetric mode becomes unstable at the critical frequency where deviation occurs. Investigation of its instantaneous and time-averaged characteristics show that it acts as a displacement mode translating the nondeviated wake away from the streamwise direction. Finally, it is demonstrated that the transition from the second to the third regime is linked to nonlinear effects that amplify both antisymmetric and symmetric perturbations around the foil.
二维周期性扑翼水翼的尾迹流的研究 为了固定的扑翼幅度进行数值模拟。随着扑翼频率的增加，在时间推进非线性模拟中识别出三个区域。第一区域的特征是非偏离尾迹流，升力的时间平均值为零。在第二区域中，尾迹流稍微偏离流向，升力的时间平均值为正或负。第三区域的特征是尾迹的更大偏离，伴随着更大的时间平均值升力和推力。通过对非偏离尾迹进行 Floquet 稳定性分析，研究了从第一区到第二区的过渡。引入了一种特定的方法来计算不稳定时的周期性非偏离尾迹。发现一个同步反对称模式在发生偏离的临界频率处变得不稳定。对其瞬时和时间平均值特征的研究表明，它作为一种位移模式，将非偏离的尾迹从流向方向移开。最后，证明了从第二区到第三区的过渡与非线性效应有关，这些效应放大了翼片周围的反对称和对称扰动。
