  Linear modal instabilities of flow over finite-span untapered wings have been
investigated numerically at Reynolds number 400, at a range of angles of attack
and sweep on two wings having aspect ratios 4 and 8. Base flows have been
generated by direct numerical simulation, marching the unsteady incompressible
three-dimensional Navier-Stokes equations to a steady state, or using selective
frequency damping to obtain stationary linearly unstable flows. Unstable
three-dimensional linear global modes of swept wings have been identified for
the first time using spectral-element time-stepping solvers. The effect of the
wing geometry and flow parameters on these modes has been examined in detail.
An increase of the angle of attack was found to destabilize the flow, while an
increase of the sweep angle had the opposite effect. On unswept wings,
TriGlobal analysis revealed that the most unstable global mode peaks in the
midspan region of the wake; the peak of the mode structure moves towards the
tip as sweep is increased. Data-driven analysis was then employed to study the
effects of wing geometry and flow conditions on the nonlinear wake. On unswept
wings, the dominant mode at low angles of attack is a Kelvin-Helmholtz-like
instability, qualitatively analogous with global modes of infinite-span wings
under same conditions. At higher angles of attack and moderate sweep angles,
the dominant mode is a structure denominated the interaction mode. At high
sweep angles, this mode evolves into elongated streamwise vortices on higher
aspect ratio wings, while on shorter wings it becomes indistinguishable from
tip-vortex instability.
