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

Abstract

Linear modal instabilities of flow over untapered wings with aspect ratios and 8, based on the NACA 0015 profile, have been investigated numerically over a range of angles of attack, and angles of sweep, at chord Reynolds numbers. 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. 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. On unswept wings, this mode peaks in the midspan region of the wake and moves towards the wing tip with increasing, 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 has been identified and it is shown that amplification increases with increasing sweep, up to. At higher, all global modes become less amplified and are ultimately stable at. An increase in amplification of the leading mode with sweep was not observed over the wing, where tip vortex effects were shown to dominate.