On the slit trailing edges for aerofoil self-noise reduction

This paper presents an experimental and analytical investigation into the use of trailing edge slits for the reduction of aerofoil trailing edge noise. The noise reduction mechanism is shown to be fundamentally different from conventional trailing edge serrations, relying on destructive interference from highly compact and coherent sources generated at either ends of the slit. This novel approach is the first to exploit the coherence intrinsic to the boundary layer turbulence. Furthermore, the study demonstrates that trailing edge slits not only achieve superior noise reductions compared with sawtooth serrations of the same amplitude at certain conditions, but also offer frequency-tuning capability for noise reduction. Noise reduction is driven by the destructive interference between acoustic sources at the root and tip of the slit, which radiate with a phase difference determined by the difference in times taken for the boundary layer flow to convect between the root and tip. Maximum noise reductions occur at frequencies where the phase difference between these sources is $180^\circ$ . The paper also presents a detailed parametric study into the variation in noise reductions due to the slit length, slit wavelength and slit root width. Additionally, a simple two-source analytic model is proposed to explain the observed results. Wind tunnel measurements of the unsteady flow field around the trailing edge slits are also presented, providing insights into the underlying flow physics.