Numerical investigation of the unsteady flow past an iced multi-element airfoil

Unsteady flow characteristics are investigated using wall-modeled large-eddy simulation for a 30P30N threeelement airfoil with streamwise ice and horn ice accreted at its slat leading edge, respectively. The ice accretions cause massive flow separation above the slat, which results in a surface pressure plateau on the slat and further unloads the downstream wing elements. The above mentioned two ice shapes lead to lift decreases of 10.66 and 16.45% and drag increases of 44.58 and 59.42%, respectively. Ice-induced vortices first roll up due to Kelvin– Helmholtz instability above the slat with ice accretions, and the subsequent vortex pairing halves their characteristic motion frequency. The developed vortices, composed of multiscale structures, dominate the slat upper surface and are convected through the main element to aggravate the flow separation over the flap. Moreover, extremely strong acoustic resonance occurs in the iced slat cove, and its tonal frequencies are consistent with theoretical model predictions. These intense tones are believed to result from the elevated pressure fluctuations along the cavity free shear layer, which are in turn related to the propagation of the ice-induced pressure fluctuations from above the slat because their most dominant frequencies identified by proper orthogonal decomposition are nearly equal to the resonant frequencies.