Abstract
Within the DLR project VicToria an aerodynamic and aero-acoustic optimization of helicopter rotor blades is performed. During the optimization, three independent flight conditions are considered: hover, cruise and descent flight. The first two flight conditions drive the power requirements of the helicopter rotor, while the descent flight is the loudest flight condition for current helicopter generations. To drive down the required power and the emitted noise, a multi-objective design approach coupled with surrogate models is utilized to find a Pareto optimal set of rotors. This approach allows to identify the trade-offs to be made when laying emphasis on either goal function. The underlying CFD simulations utilize fourth-order accurate spatial schemes to capture the vortex dominated flow of helicopter rotor blades. The paper presents the validation of the setups, the optimization results and the off-design analysis of a chosen set of blades from the Pareto front. The conclusion is that the utilization of the Pareto front approach is necessary to find good rotor designs, while the utilization of high-order methods allows for efficient CFD setups.
Highlights
Helicopters play a unique role among all aircraft due to their universal and versatile mission applicability
For the Chimera interpolation, an eighthorder Lagrangian interpolation [27] is utilized on the finest L1 grid, while for the coarser grids (L2 and level 3 grid (L3)) this is reduced to second-order linear interpolation as not enough overlap is given anymore
From the investigation of these three flight conditions, it is decided to go with the level 2 grid (L2) grid of each case as it provides a good trade-off between accuracy and speed
Summary
Helicopters play a unique role among all aircraft due to their universal and versatile mission applicability. Fabiano and Mavriplis utilize 2.3 million grid points and 95 design variables, while Wang et al optimized 92 design variables on a mesh with seven million grid points These examples are among the first to have an adjoint formulation for the unsteady flight conditions of the helicopter. In the hover flight condition, they have the adjoint solution available, but in forward flight, they utilize surrogate models to efficiently obtain the gradients Their approach is based on the Nash game, which allows finding a continuous Pareto front using a gradient optimizer. Smith et al [13] validate their CFD tools using the data generated by the HART II wind tunnel campaign Their conclusion is that to correctly model the vortex dominated flow of helicopter rotor blades, either many grid points are necessary or higher order methods in space, or even both. An off-design analysis of a selected set of rotor blades is discussed, before the paper is closed with a summary and outlook for future work
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