Design of a Low-Noise Open Rotor Using an Implicit Harmonic Balance Method

Abstract

A coaxial contra-rotating open rotor belongs to the next generation of aero-engines. It has an efficiency that is about 30% higher than that of a conventional turbojet engine. However, because of the high noise level, the open rotor has not been introduced into the commercial aviation market. Although there have been numerous efforts to reduce its noise level, the need to accurately predict the unsteady and complex flow field around the open rotor makes it difficult to apply the conventional design methodologies. In this paper, we introduce a state-of-the-art design methodology for solving the unsteady flow field problem of the low-noise open rotor design. A harmonic balance method that is an order of magnitude more efficient than the conventional time accurate CFD method is used to predict the aerodynamic performance of the open rotor. With the accurate formulation of the governing equations through the harmonic balance method, a design method that uses a surrogate model is employed to find optimum configuration that minimizes the noise level and total power at a constant thrust level. A noise prediction is made using the Farassat formula, derived from the Ffowcs-Williams-Hawking’s equation. To efficiently search for the optimum configuration, the design optimization is divided into the rotor topology design level and blade planform design level. In a previous study, we investigated the optimum rotor topology parameters such as the blade radii, rotor spacing, and pitch angle of the aft rotor. In this paper, an investigation is conducted to determine the optimum planform variables such as the twist angle, chord length for several design sections, and tip shape control parameters for the aft rotor. A genetic algorithm is used as a multi-objective optimization algorithm in combination with the Kriging surrogate model. Through the planform design for the aft rotor, the noise level and power consumption of the optimum rotor are reduced by 0.6 dB and 6.8% respectively.