On the influence of upstream boundary-layer state on low-frequency unsteadiness in transonic aerofoil shock buffet

Transonic buffet is a type of unsteady shock wave/boundary-layer interaction (SBLI) found in a wide range of aerospace applications. In this work, the focus is on the two-dimensional chordwise shock oscillations observed on span-periodic aerofoils at Strouhal numbers of italic St tilde left bracket 0.05 comma 0.1 right bracket St ∼ [ 0.05 , 0.1 ] St ~ [0.05, 0.1 ] . Previous experimental and computational studies have categorised ‘laminar’ or ‘turbulent’ buffet based on the absence or presence of tripping upstream of the interaction. However, the buffet response over a full range of upstream boundary-layer states has yet to be investigated. In this work, implicit large-eddy simulations are performed at active buffet conditions for fourteen levels of numerical tripping strength, in order to obtain a full range of laminar, transitional and turbulent boundary layers upstream of the SBLI. While laminar and turbulent buffet are found to share similar low-frequency unsteadiness, a narrow band within the transitional regime is identified for the first time with partially suppressed buffet, greatly reduced shock oscillations and a 4–6 times × × reduction in lift fluctuations. In the transitional cases, additional spectral energy content is identified over several distinct modes in the range italic St tilde left bracket 0.32 comma 1.9 right bracket St ∼ [ 0.32 , 1.9 ] St ~ [0.32, 1.9 ] . Spectral proper orthogonal decomposition analysis attributed these peaks to shock-foot oscillations coupled to boundary-layer streaks and a strongly enhanced vortex-shedding mode originating from the rear of the SBLI. The partial suppression of low-frequency unsteadiness in some transitional cases is interpreted as arising from the interaction between upstream boundary-layer streaks and the shock foot and separation line, regions known to be receptive to external forcing in the context of buffet.