On the Influence of Trailing-Edge Serrations on Open-Rotor Tonal Noise

Tonal noise of contra-rotating open rotors can be divided into self and interaction noise. The latter can be subdivided into potential field interaction noise, noise due to the interaction of the front-rotor wakes with the aft-rotor blades and tip vortex interaction noise. The excited tonal sound field is closely related to the properties of the impinging wakes provided that the tip vortex interaction is not the dominant source of interaction noise. Designing the trailing edge of the front-rotor blades with serrations will enhance the mixing in the wakes which should have a beneficial effect on the interaction tones. This process is investigated with 3D RANS calculations of 3 serration designs. Through assessment of the wake properties one configuration is down selected for evaluation of the far-field sound levels. These are computed using a 3D URANS approach coupled to an integral method that implements the FW-H acoustic analogy. The effect observed with the serrations on the far-field directivity is twofold: at low frequencies the sound energy is redistributed in the polar arc; at higher frequencies a systematic reduction of the sound emission is observed.