Preliminary Study on Time-Spectral and Adjoint-Based Design Optimization of Helicopter Rotors

We propose in this paper an adjoint-based design optimization methodology that is particularly efficient for flow simulation of helicopter rotors in combination with a TimeSpectral (TS) method. The TS method is a fast and efficient algorithm to simulate the unsteady periodic flows with the narrow frequency spectrum which are often encountered in rotorcraft, turbomachinery, and fixed-wing flutter analysis. It represents the time discretization of the flow solver by Fourier-based modes/bases in which periodic steady-state is assumed throughout the computation. The steady-state assumption reduces the computational cost significantly, which makes it possible to use the efficient adjoint method applicable to the rotor design problem. The adjoint solution method is widely accepted as an inexpensive way to obtain the sensitivity information of flow solutions to a large number of design parameters. Integrated with gradient-based optimization technique, the adjoint method has been an essential module in aerodynamic/aero-structural shape optimizations. As a preliminary study before we progress towards a more complete and practical desgin optimization of helicopter rotor, this study focuses on the accuracy and validity of our current design optmization tools. First of all, for the validation of the aerodynamic tools, a flight 8534 test condition of the UH-60A configuration is simulated with time-spectral computation, and the accuracy and efficieny ofthe time-spectral method is demonstrated in comparison with the time accurate results and experimental data. A loose coupling of the time-spectral method and the structural analysis via UMARC is also demonstrated to prove the capability of the time-spectral method to simulate the helicopter rotor problem in a comprehensive way. Finally, a simple design application problem of the hover flight condition is considered and an analysis with single blade coupled with free wake model is employed. A baseline blade shape is modified to an optimal shape to minimize torque while the thrust is maintained or enhanced.