Stability characteristics of different aerofoil flows at [formula omitted] and the implications for aerofoil self-noise

Abstract

Three compressible-flow direct numerical simulations (DNS) of aerofoils were conducted at a chord based Reynolds number of Rec=150,000: a Controlled-Diffusion (CD) aerofoil at two incidences and an untripped NACA 6512-63 aerofoil at 0∘ angle of attack (AoA). The aim of the simulations was to help improve the understanding of the generation mechanisms of aerofoil self-noise. To this end, the time-averaged flows around the aerofoils were investigated using a multi-dimensional global stability analysis based on the response to very low-amplitude hydrodynamic initial pulses. For the CD aerofoil at 5∘ AoA and the untripped NACA 6512-63 aerfoil at 0∘ AoA, the base flows are found to be globally unstable and an acoustic feedback loop occurs regardless of the perturbation locations. For the CD aerofoil at a slightly higher incidence (8∘ AoA) where a laminar separation bubble appears at the leading edge and a turbulent attached boundary layer is present at the trailing edge, the initial perturbations decay with time for all initial pulse locations indicating that the base flow is globally stable. The results confirm that for cases where a laminar separation bubble is present on the aerofoil suction side at the trailing edge, independent of whether tones in the DNS exist or not, an unstable feedback loop exists. The laminar separation bubble at the aerofoil trailing edge on the suction side seems to be the only required prerequisite for triggering the acoustic feedback loop.