Stability characteristics of aerofoil flows are investigated by linear stability analysis<br/>of time-averaged velocity profiles and by direct numerical simulations with timedependent<br/>forcing terms. First the wake behind an aerofoil is investigated, illustrating<br/>the feasibility of detecting absolute instability using these methods. The time-averaged<br/>flow around an NACA-0012 aerofoil at incidence is then investigated in terms of<br/>its response to very low-amplitude hydrodynamic and acoustic perturbations. Flow<br/>fields obtained from both two- and three-dimensional simulations are investigated,<br/>for which the aerofoil flow exhibits a laminar separation bubble. Convective stability<br/>characteristics are documented, and the separation bubble is found to exhibit no<br/>absolute instability in the classical sense; i.e. no growing disturbances with zero group<br/>velocity are observed. The flow is however found to be globally unstable via an<br/>acoustic-feedback loop involving the aerofoil trailing edge as a source of acoustic<br/>excitation and the aerofoil leading-edge region as a site of receptivity. Evidence<br/>suggests that the feedback loop may play an important role in frequency selection of<br/>the vortex shedding that occurs in two dimensions. Further simulations are presented<br/>to investigate the receptivity process by which acoustic waves generate hydrodynamic<br/>instabilities within the aerofoil boundary layer. The dependency of the receptivity<br/>process to both frequency and source location is quantified. It is found that the<br/>amplitude of trailing-edge noise in the fully developed simulation is sufficient to<br/>promote transition via leading-edge receptivity
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