Spatio-temporal dynamics of the flow in the flap cove of a high-lift airfoil

This study investigates the flow field in the flap cove of a two-element high-lift airfoil, focusing on mechanisms responsible for noise generation. The main-element trailing-edge wake mixes weakly with the flap-cove shear layer, producing a persistent velocity deficit along the flap suction surface. This contrasts with the slat region, where stronger mixing between the shear layer and slat wake at low angles of attack leads to higher broadband noise levels. Within the flap cove, a turbulent separation bubble (TSB) bounded by a shear layer is identified. Strong correlations between flap leading-edge (LE) pressure fluctuations and cove flow support the use of LE noise models. However, significant correlation near the shear-layer reattachment point indicates additional acoustic scattering from the main element, suggesting that LE noise is not the sole contributor. The shear layer reattaches on the main element approximately three step heights downstream, where the step corresponds to the separation lip at the main-element cusp, and the step height represents the characteristic depth of the cove. This reattachment is driven by high-speed flow through the flap gap, shortening the TSB relative to canonical backward-facing step flows. Spectral analysis of hot-wire and wall-pressure data reveals dominant mid- and low-frequency peaks associated with shear-layer vortex shedding and TSB breathing. The shedding frequency agrees with predictions of an analytical slat-noise model, despite the absence of strong flow–acoustic coupling. Proper orthogonal decomposition shows that low-frequency breathing drives large-scale oscillations of the stagnation line and reattachment location, likely triggered by Görtler-type centrifugal instabilities.