Investigation of Three-Dimensional Dynamic Stall Effects on Rotor Blades

现代直升机在高速向前或机动飞行中的飞行包线主要受到后退旋翼桨叶上的动态失速的限制。如果机翼或转子叶片的俯仰角暂时超过其最大迎角，就会出现这种复杂的、高度不稳定的三维现象，在静态情况下，在该迎角下气流会分离。在动态失速期间，在前缘处发展出强烈的旋涡，这些旋涡对流到下游并引起升力和低头俯仰力矩的大幅度增加。这些载荷波动和振动远远超过其静态最大值，因此可靠的预测和对这一现象的深刻理解是非常重要的。在先前的项目中，在DLR Göttingen对俯仰有限翼和两叶片模型转子的动态失速进行了实验研究，并在IAG进行了相应的CFD模拟。根据所获得的结果，现在将在一个非刚性四叶旋翼上研究这种现象，该旋翼具有一个实际的、双后掠的叶尖几何形状，分别在悬停和爬升飞行构型下工作。同样，实验是在德国航天中心哥廷根的转子测试设施中进行的，该设施在前一个项目中使用。现在，它的重点是调查的影响，新的叶尖和相互作用的现象与转子叶片的结构动力特性。为了考虑模拟中结构动力学的影响--这在前一个项目中很重要--必须通过流体-结构耦合来扩展数值处理链。在IAG，流动求解器FLOWer和结构求解器CAMRADII（应用于直升机模拟）或SIMPACK（迄今为止应用于风力涡轮机模拟）之间的耦合已得到很好的建立。此外，混合RANS/LES方法，它产生了显着更好的结果，在完全分离流的情况下，但引入了新的数值困难，是要进一步改进的模拟方面。此外，层流-湍流过渡也需要考虑。实验和模拟之间的密切合作可以验证数值方法，获得关于模型所需复杂性的知识，并得出“最佳实践”指南。为了提高结果到真实的直升机构型的可移植性，还将进行前飞构型的模拟，以便深入了解其它动态失速机理，如由桨叶-涡流相互作用引起的失速。

The flight envelope of modern helicopters in high-speed forward or maneuvering flight is primarily limited by dynamic stall on the retreating rotor blade. This complex, highly unsteady and three-dimensional phenomenon occurs, if a wing or a rotor blade is temporarily pitched beyond its maximum angle of attack, at which flow would separate in the static case. During dynamic stall, strong vortices evolve at the leading edge, which convect downstream and induce a massive increase in lift and nose-down pitching moment. These load fluctuations and vibrations exceed their static maximum values by far, thus the reliable prediction and a profound understanding of the phenomenon is of great importance.During the predecessor project, dynamic stall on a pitching finite wing and a two-bladed model rotor was experimentally investigated at DLR Göttingen and corresponding CFD simulations were carried out at IAG. Based on the findings thus obtained, the phenomenon is now to be investigated on a non-rigid four-bladed rotor with a realistic, double-swept bladetip geometry operating in a hover respectively climb-flight configuration. Again, the experiments are conducted at DLR Göttingen in the rotor test facility, which was used during the predecessor project. Now, it is focused on the investigation of the influence of the novel blade tip and the interaction of the phenomenon with the structural-dynamic properties of the rotor blade. To account for the effects of the structure dynamics in the simulations – which was found to be important during the predecessor project – the numerical process chain must be extended by a fluid-structure coupling. At IAG, coupling between the flow solver FLOWer and the structural solvers CAMRADII, applied to helicopter simulations, or SIMPACK, hitherto applied to wind turbine simulations, is well established. Furthermore, the hybrid RANS/LES approach, which yields significantly better results in case of completely separated flow but introduces new numerical difficulties, is to be further improved on the simulation side. Moreover, laminar-turbulent flow transition is to be considered, too.The close cooperation between experiment and simulation allows to validate numerical methods, to gain knowledge regarding the required complexity of the model and to derive “best practice” guidelines. To improve the transferability of the results to real helicopter configurations, there will also be simulations of a forward flight configuration, to gain insight into other dynamic stall mechanisms, like being caused by blade-vortex interaction.