High-fidelity study of three-dimensional turbulent transonic buffet on wide-span infinite wings

Abstract

Turbulent transonic buffet is an aerodynamic instability causing periodic oscillations of lift/drag in aerospace applications. Involving complex coupling between inviscid and viscous effects, buffet is characterised by shock-wave oscillations and flow separation/reattachment. Previous studies have identified both 2D chordwise shock-oscillation and 3D buffet/stall-cell modes. While the 2D instability has been studied extensively, investigations of 3D buffet have been limited to only low-fidelity simulations or experiments. Due to computational costs, almost all high-fidelity studies to date have been limited to narrow span-widths around 5% of aerofoil chord length (aspect ratio, $AR = 0.05$), which is insufficiently wide to observe large-scale three-dimensionality. In this work, high-fidelity simulations are performed up to $AR=3$, on infinite unswept NASA-CRM wing profiles at $Re=5\times 10^{5}$. At $AR \geq 1$, intermittent 3D separation bubbles are observed at buffet conditions. While previous RANS/stability-based studies predict simultaneous onset of 2D- and 3D-buffet, a case with buffet that remains essentially-2D despite span-widths up to $AR=2$ is identified here. Strongest three-dimensionality was observed near the low-lift phases of the buffet cycle at maximum flow separation, reverting to essentially-2D behaviour during high-lift phases. Buffet was found to become three-dimensional when extensive mean flow separation was present. At $AR \geq 2$, multiple 3D separation bubbles form, in a wavelength range of $\lambda=\left[1c-1.5c\right]$. SPOD and cross-correlations were applied to analyse the spatio/temporal structure of 3D buffet-cells. In addition to the 2D chordwise shock-oscillation mode (Strouhal number $St \approx 0.07-0.1$), 3D modal structures were found in the shocked region of the flow at $St \approx 0.002-0.004$.