Wake-aerofoil interaction noise control with trailing-edge serrations

Abstract

The present study experimentally investigates the effective reduction of wake-aerofoil interaction noise in a tandem aerofoil configuration using trailing-edge sawtooth serrations. The far-field noise results clearly show that at the ‘head-on’ wake impingement, the use of sawtooth serrations on the trailing-edge of the front aerofoil can lead to substantial reduction of the wake-aerofoil interaction noise of approximately 10 dB without any noticeable increase at other frequencies and changes to directivity pattern. Subsequently, detailed near-field measurements were carried out to investigate further the flow dynamics associated with the reduction of the interaction noise. The flow field and unsteady loading results suggest that the wake-aerofoil interaction primarily takes place within the first 30% of the chord close to the leading-edge area, while towards the trailing-edge, the flow is influenced more heavily by the boundary layer development. The velocity and wall pressure fluctuation spectra show significant decrease of the energy frequency content of the flow structures over the wake-aerofoil interaction frequency range after the wake impingement, due to the modification of the wake turbulence by trailing-edge serrations. This leads to notable reduction in the unsteady aerodynamic loading, particularly in the lift direction. Further coupled pressure–velocity analyses reveal that the sawtooth serrations modify the energy frequency content associated with the dominant turbulence eddies by spreading it over a wider frequency range. Near-field to far-field pressure coherence also reveals the loss of coherence, and thus abatement of the radiated far-field noise. The experimental results are relevant to practical applications in rotor–stator and outlet guide vanes.