Noise Generation by Airfoils and Rotors with Porous and Serrated Trailing Edges

Abstract

Trailing-edge (TE) noise is an important noise source for airfoil applications that operate near populated areas. This thesis aims to develop novel (porous, serrated, and porous-serrated) geometries for the TE noise control of airfoils/rotors and investigate their noise generation mechanisms.

First, the acoustic absorption of ten additively manufactured porous specimens is characterised to facilitate the design of porous TE geometries. The aeroacoustic performance and near-wake characteristics of eleven novel TE designs are then measured at various velocities in UNSW Anechoic Wind Tunnel. Their noise attenuation performance on laminar-transitional boundary layer TE (LBL-TE) and turbulent boundary layer TE (TBL-TE) noise are evaluated. Fluctuating velocity results indicate that the proposed designs influence LBL-TE noise generation by altering the flow characteristics around the TE. A TBL-TE noise intensity factor is proposed to relate near-wake flow statistics to TBL-TE noise generation, showing good consistency with the measured TBL-TE noise level. A high-frequency-broadband noise increase is observed for all porous TE designs.

Moreover, the aeroacoustic performance of three sets of rotor blades with integrated novel TEs is evaluated at various pitch angles and RPMs on UNSW Rotor rig. Compared with serrated blades, porous blades show better low-frequency noise attenuation. At frequencies where the porous structures have good acoustic absorption, porous blades can effectively control TE noise at all operating conditions, indicating the acoustic absorption may contribute to TE noise attenuation by altering the acoustic scattering efficiency.

In addition, Large-Eddy Simulations (LES) are performed on a porous and a reference airfoil. Ffowcs-Williams and Hawkings (FWH) acoustic analogy results of porous airfoil capture the high-frequency excessive noise and agree well with single microphone measurements. Flow simulation results reveal that the TBL-TE noise reduction for porous TE is mainly due to an attenuation of convection velocity and spanwise correlation, and the excessive noise is originated from the interaction of the permeated turbulent flow and pore geometries.

Finally, a wind turbine noise prediction model based on a noise scaling function is proposed. It accurately predicts the noise spectra and overall noise levels of a full-scale wind turbine using the aerodynamic and acoustic data of lab-scale airfoil models.