Frequency-Targetable Aerofoil Self-Noise Reduction

This paper presents an experimental study to investigate a novel, yet simple trailing edge treatment to enable aerofoil self-noise reduction with frequency-tuning capability based on the acoustical interference mechanism. Using a structurally-rigid slit trailing edge where the edges become parallel to the mean flow direction, it can facilitate a phase-cancellation of the acoustical pressure waves between two scattering sources located at the slit root and tip. By forcing the condition of the acoustical destructive interference to establish the Strouhal number as 0.5, 1.5 and so on, one could fine-tune the peak frequency for the maximum noise reduction when the distance between the root and tip of the slit trailing edge, and the convection velocity of the turbulent eddies over the slit surfaces, are known. Similarly, at Strouhal number of 1.0, 2.0 and so on, the slit trailing edge can also trigger a constructive acoustical interference mechanism whereby it can radiate more efficiently than that of a baseline, straight trailing edge. Therefore, for a given slit trailing edge configuration, the radiated spectrum will feature an oscillatory characteristics reflecting to the noise reduction and noise increase across the entire frequency. A slit trailing edge with small slit amplitude is found to be quieter than a serrated trailing edge with the same amplitude. The slit trailing edge therefore represents an ideal passive control device for the reduction of aerofoil turbulent broadband self-noise because it is quieter, which remains so at high inflow velocity. Most importantly, a slit trailing edge has a frequency-tuning capability that is not found in a serrated trailing edge.