Leading edge blowing: A targeted approach to reduce aerofoil noise

Abstract

This thesis investigates steady leading-edge blowing as an active flow-control strategy for reducing aerofoil self-noise from a NACA 65(12)-10 aerofoil. The work considers three noise mechanisms: trailing-edge tonal noise, leading-edge broadband turbulence-interaction noise, and separation-stall noise. Far-field acoustic measurements are combined with hot-wire anemometry, surface-pressure measurements, and aerodynamic force measurements so that the acoustic response can be interpreted alongside the underlying flow physics. The first part of the study examines trailing-edge tonal noise. The results show that leading-edge blowing can strongly suppress the discrete tonal radiation associated with a suction-side laminar separation bubble and the aeroacoustic feedback loop it supports. Under the most effec-tive conditions, tonal levels are reduced by up to 20–25 dB. The main effect is not a large shift in tonal frequency, but a weakening of the organised instability and of the flow-acoustic coupling that sustains the tone. The second part addresses leading-edge broadband noise under both isotropic and anisotropic turbulent inflow conditions. Here, the effectiveness of the control is found to depend strongly on the incoming turbulence, particularly its length scale, anisotropy, and spatial coherence. When the actuation is suitably matched to the inflow, measurable broadband reductions are achieved. These reductions are linked to a weakening of spanwise coherence and to a shortening of the effective interaction length at the leading edge, showing that the control acts by modifying the structure of the incoming disturbances before they are scattered into sound. The final part of the thesis considers separation-stall noise at high incidence. In this regime, leading-edge blowing delays the onset of stall-related separation by up to about 3.7◦ and reduces the associated low-frequency acoustic radiation by up to 5 dB. The measurements show that the blowing modifies the separated-flow topology and weakens the large-scale unsteady structures that dominate the noise generation process. Overall, the thesis shows that steady leading-edge blowing is more than a case-specific method of reducing noise. It acts as a targeted control strategy that can influence several aerofoil self-noise mechanisms through changes to the near-field flow structure. The central finding is that noise reduction is achieved by weakening the coherence and organisation of the source regions that radiate most efficiently. This provides a basis for the future design of quieter aerofoils and blade sections using tunable leading-edge actuation in applications such as aircraft, turbomachinery, propellers, and wind turbines.