On a three-dimensional investigation of airfoil turbulence-impingement noise and its reduction by leading-edge tubercles

The present work addresses the turbulence-impingement noise of an airfoil and its reduction by a wavy cut of the leading edge, referred to as serrations or tubercles. It is primarily aimed at completing existing experimental databases for both straight-edge and serrated airfoils, by measurements also performed off the mid-span plane on a portion of sphere, in an open-jet anechoic wind tunnel. The three-dimensional investigation is a required condition for further applications in rotating-blade noise modeling. The turbulence is generated by a grid placed upstream of the nozzle contraction. An almost monotonically increasing reduction is found with increasing frequency, in a wide frequency range for which turbulence-impingement noise dominates, for all radiation directions. This extends observations reported in previous studies in the midspan plane only. The straight-edge results are also used to validate an analytical prediction model in a three-dimensional context. This part includes a novel correction method to account for sound refraction through the shear layers of the nozzle jet, leading to a remarkably good agreement of predictions with measurements even at shallow observer angles where the effect of the jet shear layers is significant and needs to be accounted for.