Stability and receptivity characteristics of a laminar separation bubble on an aerofoil

Abstract

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

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Identifiers

ORCID for R.D. Sandberg: orcid.org/0000-0001-5199-3944

ORCID for N.D. Sandham: orcid.org/0000-0002-5107-0944

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