Sensitivity of aerofoil self noise reductions to serration flap angles

Abstract

The serration amplitude and serration wavelength are traditionally regarded as the primary geometrical variables that can affect the noise performance of an add-on, flat plate type serrated trailing edge. This experimental study investigates another serration geometrical variable, namely the serration flap angle that could potentially affect the self-noise reduction of an aerofoil. The experiment was carried out at Brunel aeroacoustic facility, on a NACA65(12)–10 aerofoil. The serrated flat plates were manufactured to form in several flap angles: ±15°,±10°,±5° and ±0° as the reference. Preliminary investigation on the effect of serration amplitude, without the flap angle, confirms with other findings that the largest level of broadband noise reduction is achieved when the amplitude of the serrated flat plate is large. It is also worth reporting that broadband noise can already be reduced even by attaching a large chord length of unserrated, straight flat plate. When the serrated flat plate contains a flap angle, it is generally observed that a flap-up position (positive flap angle) is more favourable for broadband noise reduction, while the opposite is true for the flap-down position (negative flap angle). The best flap-up position is when the positive flap angle is small, at around +5°. Unfortunately, a small flap-down position, i.e. -5° is the worst performer amongst the test cases (lowest level of broadband noise reduction at low frequency, and highest noise increase at high frequency). Therefore, even a small misalignment of the trailing edge serration due to the manufacturing defect could potentially degrade (or enhance) the overall aerofoil self-noise reduction because the serration is found to be sensitive to small flap angles.