Coupling Rotor Dynamics with a Parallel Airflow Simulation

The airflow around a helicopter and especially around the main rotor is very complex, non-uniform and highly non-linear. The wake of a helicopter’s main rotor interacts very strongly with the rotor itself, giving rise to a number of problems that can only be addressed if the geometry and intensity of the wake is known with a high degree of accuracy. The best compromise between computational cost and accuracy are vortex-lattice formulations known as Freewake codes. The Institute of Flight Systems of the German Aerospace Center (DLR) in cooperation with DLR’s Simulation and Software Technology operates a modern Freewake implementation which has been optimized for computations on a workstation using GPGPU accelerators. The code is coupled via file IO to a comprehensive rotor simulation tool called S4. Freewake receives blade motion and circulation data from S4 for a trimmed state, then calculates a number of revolutions using this steady-state and returns induced inflow velocities. This conventional weak coupling scheme for Freewake codes is very robust and converges after only a few iterations. The flip-side is that only trimmed steady-state solutions can be computed. While these cases cover most of the flight time of a helicopter, there are phases that cannot be covered e.g. take-off and landing, maneuvering flight, flight in turbulent atmosphere and autorotation which is an emergency maneuver helicopters use in case of engine failure. Here the Rotor is decoupled from the engine and is only driven by the slipstream. During this maneuver the rotor speed varies significantly making a weak coupling scheme all but useless. This talk focuses on the integration of the Freewake code base into S4 and the development of different coupling schemes. Another focus are recent developments of the Freewake pertaining to the integration of fuselage-rotor-interference as an additional induced velocity and its transport through the wake as well as further optimizations of the code and a nested MPI/OpenMP parallelization scheme