Experiments were conducted in a low-turbulence wind tunnel to investigate the
efficacy of localised acoustic forcing upon the dynamics and stability of the
flow on a cambered, wall-bounded airfoil over a range of Reynolds numbers (Re)
where the flow state can switch between two limits — a low-lift state (SI)
where separation continues beyond the trailing edge and a high-lift state (SII)
where the separated flow is closed off to form a laminar separation bubble. The
switching between SI and SII can occur close to a critical angle of attack
($\alpha_{\textrm{crit}}$) which varies with $\textrm{Re}$. The most effective
forcing frequencies are found at a constant value of a rescaled Strouhal
number, $\textrm{St}^* = \textrm{St}/\textrm{Re}^{1/2}= 0.027$, which indicates
that though the primary unstable modes of the separated shear layer are of the
inviscid, Kelvin-Helmholtz type, these modes are seeded by length scales that
originate in the laminar (viscous) boundary layer. The most effective chordwise
forcing location varies with $\textrm{St}/\textrm{Re}^{1/2}$ and incidence
angle, $\alpha$, and is always upstream of the separation point. Although the
boundary layer flows are far from two-dimensional, forcing at a fixed chord
location across all spanwise locations is effective in controlling the SI —
SII transition. Strategies for active and passive feedback control are
suggested.

Este artículo explora los viajes en el tiempo y sus implicaciones.

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