Global Stability Analysis and DNS of a Swept Airfoil Section in Subsonic Flow

Direct numerical simulations are run for a NACA0012 airfoil at Mach 0.4 and Reynolds number 50,000. The flow fields are characterised by transitional separation bubbles that occupy 50–60% of the suction surface and are followed by a region of attached turbulent flow. At \(20^\circ \) sweep the dominant structures are oblique relative to the leading edge, while at \(40^\circ \) they are parallel to the leading edge. The latter case is also characterised by the emergence of a broadband peak in the energy spectrum. The changes in the flow are further studied by a global stability analysis based on the time and span-averaged flowfields. At the largest sweep angle a strongly unstable global mode emerges, peaking at a Strouhal number \(\mathrm {St}=3\) and with a range \(1< \mathrm{St} <5\) that is in good agreement with the DNS spectra. From the global eigenmode it is possible to discover the mechanism responsible for the mode, which involves a combination of shear layer convective instability, leading-edge receptivity and trailing edge sound radiation. The consistency of the global analysis and the DNS is notable, considering that the base flow is not a steady solution of the governing equations.