Investigation of Three-Dimensional Dynamic Stall Effects on Rotor Blades

现代直升机在高速前向或机动飞行时，其飞行包线主要受到旋翼后向叶片动态失速的限制。这种复杂的、高度不稳定的三维现象发生在机翼或旋翼叶片暂时倾斜超过最大迎角的情况下，在静态情况下，气流会在最大迎角处分离。在动态失速过程中，前缘形成强涡，并向下游对流，导致升力和俯仰力矩大幅增加。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种现象的深刻理解具有重要意义。在前一个项目中，在DLR Göttingen对俯仰有限翼和双叶片模型旋翼的动态失速进行了实验研究，并在IAG进行了相应的CFD模拟。基于所获得的发现，现在将在一个非刚性四叶旋翼上研究这种现象，该旋翼具有真实的双掠叶尖几何形状，在悬停分别进行爬升-飞行配置。同样，实验是在DLR Göttingen的转子测试设施中进行的，这是在前一个项目中使用的。目前，研究的重点是新型叶顶的影响以及这种现象与动叶结构动力特性的相互作用。为了在模拟中考虑结构动力学的影响，必须通过流固耦合来扩展数值过程链，这在之前的项目中是很重要的。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）之间的耦合已经很好地建立起来。此外，混合RANS/LES方法在完全分离流的情况下得到了更好的结果，但也带来了新的数值困难，在模拟方面有待进一步改进。此外，还考虑了层流-湍流的过渡。实验和模拟之间的密切合作可以验证数值方法，获得有关模型所需复杂性的知识，并得出“最佳实践”指导方针。为了提高结果在真实直升机配置中的可转移性，还将进行前向飞行配置的模拟，以深入了解其他动态失速机制，例如由叶片-涡相互作用引起的失速机制。

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.