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Dynamic stall on rotor blades

A rotor airfoil model installed in the Transonic Wind tunnel Göttingen (TWG)

Measured static and dynamic pitching moment curves of the rotor blade OA209

Numerical simulation with the DLR-TAU code

Under fast forward flight or manoeuvre, the retreating blade of a helicopter can temporarily stall (Dynamic stall), leading to a rapid change in pitching moment. Dynamic stall is an inherently unsteady phenomenon which is significantly affected by the interaction of the laminar/turbulent boundary layer transition, flow separation, vortex growth and propagation, and reattachment of the flow. During dynamic stall, large peaks in lift, pitching moment and drag appear, and these cause an undesirable increase in the mean drag. The flight envelope of a helicopter is limited to avoid the torsional impulse of dynamic stall, which can cause structural damage to the rotor by overloading the pitch links, and structural vibrations which reduce the comfort of passengers or damage the cabin.

In the department of helicopters activities performed in the DLR/ONERA project SIMCOS (Advanced Simulation and Control of Dynamic Stall) include extensive wind tunnel experiments and unsteady numerical simulations on dynamically-pitching rotor-blades to investigate and control dynamic stall. A combination of experiments in small-scale wind tunnels and full-scale experiments in large scale wind tunnels including the Transonic Wind Tunnel Göttingen (DNW-TWG) is being used to investigate the parameters influencing the formation and progression of dynamic stall on an airfoil in order to better predict the stall on a helicopter rotor blade.

The investigations use an array of measurement techniques including unsteady pressure, force and acceleration measurements, as well as variants of particle image velocimetry with a high temporal and spatial resolution of the flow field. Based on the experimental data, extensive two and three-dimensional unsteady flow simulation are performed with the DLR-TAU code to improve the numerical prediction of dynamic stall. The newest boundary layer transition and turbulence models are being tested in order that, in the near future, the dynamic stall performance will be a part of the quantitative numerical analysis of new rotor blade designs.

The research on dynamic stall is performed in cooperation with national and international partners. This includes a long-term and close cooperation with the French Aerospace Lab (ONERA), and cooperations with the NASA, US-Army and Konkuk University Seoul.
The research is also performed in close cooperation with the German and European helicopter industry.

Literature

• K. Richter, A. Le Pape, T. Knopp, M. Costes, V. Gleize, A.D. Gardner, "Improved Two-Dimensional Dynamic Stall Prediction with Structured and Hybrid Numerical Methods". In: American Helicopter Society 65th Annual Forum Proceedings, 2009
• Mulleners, Karen and Raffel, Markus (2010) The Onset of Dynamic Stall: a Time-Resolved Approach. 36th European Rotorcraft Forum, 07.-09. Sept. 2010, Paris, France
• Kindler, Kolja and Mulleners, Karen and Schneider, Oliver and Raffel, Markus (2010) Dynamic Stall on a Fully Equipped Helicopter Model. 36th European Rotorcraft Forum , 07.-09. Sept. 2010, Paris, France.