The present study investigates experimentally on the development of Tollmiem-Schlicting (T-S) instabilities and its associated aerofoil noise using a highly instrumented NACA 0012 profile with four angles of attack (α = 0◦, 2◦, 4◦ and 6◦) at moderate Reynolds numbers, ranging from 1.8×105 to 4.6×105 based on the chord length. The measured far-field noise spectra captured clearly both the broadband and multiple discrete tonal noises centred around a dominant peak, consistent with the existing literatures. Examinations on the near-field pressure and wake velocity spectra suggests that the dominant tones and its harmonics propagate over the entire flow field, extending both upstream to the leading-edge as well as downstream beyond the near-wake region, which agrees with the characteristics of vortex noise as proposed by Paterson et al. On the other hand, the dominant tones scale very well with f = 9.4 nU 0.8 across the entire range of flow velocities at lower angles of attack, corroborating with the feedback loop mechanisms derived by Tam. Evidences from the near- and far-field noise measurements prompt further investigations into the correlations between near-field pressure fluctuations and wake velocity. The results suggest not only the existence of periodic large-scale structures, consistent with the characteristics of hydrodynamic instability, but also a phase reversal in the near-wake region close to the aerofoil trailing-edge. The present results demonstrate that the aerofoil noise stemming from T-S instabilities could be attributed to a combination of the vortex disturbances and near-wake feedback loop and their dynamic and complex interactions.
|Title of host publication||25th AIAA/CEAS Aeroacoustics Conference|
|Place of Publication||Delft, Netherland|
|Number of pages||13|
|Publication status||E-pub ahead of print - 18 May 2019|