Direct numerical simulations of airfoil self-noise

Direct numerical simulations (DNS) of airfoil self-noise were conducted. For the low Reynolds number airfoil flows accessible by DNS, the occurrence of laminar separation bubbles involving laminar-turbulent transition and turbulent reattachment leads to additional noise sources other than the traditionally studied trailing-edge noise. Cross-correlations of acoustic and hydrodynamic quantities in conjunction with rayacoustic theory are used to identify the main source locations for a NACA-0006 airfoil. It is found that the contribution of trailing edge noise dominates at low frequencies while for the high frequencies the radiated noise is mainly due to flow events in the reattachment region on the suction side. DNS have also been conducted of NACA-0012 airfoils with serrated and straight flat-plate trailing-edge extensions using a purposely developed immersed boundary method. Noise reduction for higher frequencies is shown and the effect of the trailing edge serrations on the acoustic feedback loop observed in previous simulations and the subsequent effect on the laminar separation bubble is studied.