Investigation of Three-Dimensional Dynamic Stall Effects on Rotor Blades

• Applicants: Professor Dr.-Ing. Ewald Krämer; Dr. Holger Mai
• Subject Area: Fluid Mechanics
• Term: from 2014 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 250992397

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.

• DFG Programme: Research Grants