On the influence of trailing-edge serrations on open-rotor tonal noise

Abstract

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.

Optimization of Trailing-Edge Serrations to Reduce Open-Rotor Tonal Interaction Noise

“…Tonal interaction noise of the reference and the serrated configuration noise cancellation gets stronger. The analytically calculated spectra and the trend of increasing PWL reduction with higher frequencies agree with the results that Weckmüller et al [12] found with unsteady RANS and FW-H.…”

“…Figure 5 shows a contour plot of the optimization area. In the optimization the engine orders up to 49 were considered for the tonal noise calculation for better comparability with the study done by Weckmüller et al [12]. In the optimization results a trend for the number and depth of the serrations can be identified.…”

“…In 2012 Weckmüller et al [12] investigated the influence of trailing-edge serrations on tonal interaction noise of the contra-rotating open-rotor AI-PX7 designed by AIRBUS for JTI Clean Sky (see Fig. 1).…”

Performance prediction and progress towards multi-disciplinary design of contra-rotating open rotors

“…The third objective related to the accurate prediction of tonal noise is achieved by coupling URANS solutions of the CFD solver TRACE to a far-field pressure extrapolation tool C3D_FWH (27,28,29) . A special formulation of the Ffowcs Williams and Hawkings method was developed for permeable surface in which the surface integration is replaced by a line integration thanks to an azimuthal decomposition of the flow-field solution (30) .…”

“…In order to decrease the computation time and increase the accuracy, the phase-shift method was applied. The application of URANS to CROR is discussed by Schnell et al (10,20) and Weckmüller and Guérin (27,28) . The mesh strategy developed for CROR is presented in Fig.…”