Investigation and Prediction of Transitional Airfoil Self-Noise

Abstract

Direct numerical simulations (DNS) have been conducted of the flow over NACA-0006 and NACA-0012 airfoils. The airfoil flows have been found to exhibit pronounced transitional phenomena and multiple sources of noise have been observed. Data transformed to the frequency domain, and third-octave averaged, reveal that for low frequencies, the contribution of trailing edge noise radiation is significant while for the high frequencies the radiated noise appears to be due only to the flow events in the transition/reattachment region on the suction side. Cross-correlations of acoustic and hydrodynamic quantities, combined with ray-acoustic theory, are shown to identify the main source locations. Even though the cross-correlation maps capture the trailing edge contribution, the highest correlation with the farfield observer locations is found in the transition/reattachment region, implying that this is the main source region for the transitional cases investigated here. Surface pressure peaks associated with the transitional behavior lead to decreased accuracy when predicting self-noise using classical trailing edge theory based on surface pressure difference. Using data from the DNS the application of ramping functions to Amiet's surface pressure jump function are evaluated. It is shown that the right choice of ramping function can considerably improve predictions of the scattered pressure field and the total surface pressure difference.