Airfoil trailing edge noise reduction by the introduction of sawtooth and slitted trailing edge geometries

This paper compares the measurements of the trailing edge self noise reduction obtained using sawtooth and slit serrations on a NACA651210 airfoil. This work is relevant to reducing the noise from aircraft engines, aircraft wings and wind turbines. A detailed experimental study conducted in the ISVR's open-jet wind tunnel reveals noise reduc- and the boundary layer has been tripped so as to become turbulent. Measurements of the static pressure coefficient distribution along the chord of the airfoil are also reported. This is to allow the effects on lift to be assessed. Noise measurements for the sawtooth serrations are compared to the theory derived by Howe. Howe's theory is extended to include a series of slits and compared to experiments. It is shown theoretically that for a sawtooth profile high levels of noise reduction can be achieved, either when the serration wavelength γ is smaller than the boundary layer thickness 8 or when the root-to-tip distance h is larger than δ. It is shown theoretically that the slit serrations are not an effective noise reduction treatment since the noise reduction asymptotes to zero at high frequencies. Experimental measurements of the noise reduction obtained using trailing edge sawtooth and slits are shown to be significantly less than that predicted. The noise is shown to increase at frequencies above some critical frequency, which is shown to depend only on f ₀δ U _c ≈ 1 and independent of serration geometry.