Linear modal instabilities around post-stall swept finite wings at low Reynolds numbers

Linear modal instabilities of flow over untapered wings with aspect ratios $AR=4$ and 8, based on the NACA 0015 profile, have been investigated numerically over a range of angles of attack, $\alpha$ , and angles of sweep, $\varLambda$ , at chord Reynolds numbers $100\le Re\le 400$ . Laminar base flows have been generated using direct numerical simulation and selective frequency damping, as appropriate. Several families of unstable three-dimensional linear global (TriGlobal) eigenmodes have been identified and their dependence on geometric parameters has been examined in detail at $Re=400$ . The leading global mode A is associated with the peak recirculation in the three-dimensional laminar separation bubble formed on the wing and becomes unstable when recirculation reaches $\textit {O}(10\,\%)$ . On unswept wings, this mode peaks in the midspan region of the wake and moves towards the wing tip with increasing $\varLambda$ , following the displacement of peak recirculation; its linear amplification leads to wake unsteadiness. Additional amplified modes exist at nearly the same and higher frequencies compared to mode A. The critical $Re$ has been identified and it is shown that amplification increases with increasing sweep, up to $\varLambda \approx 10^\circ$ . At higher $\varLambda$ , all global modes become less amplified and are ultimately stable at $\varLambda =30^\circ$ . An increase in amplification of the leading mode with sweep was not observed over the $AR=4$ wing, where tip vortex effects were shown to dominate.