The Control Mechanism of the Soft Trailing Fringe on the Flow Characteristics Over an Airfoil

Inspired by the owl's silent flight, we experimentally investigated the flow control mechanism of the soft trailing fringes (STFs) on the wake of the S833 airfoil at the Reynolds number of Re = 2 × 104. A high-speed Particle Image Velocimetry (PIV) system is employed to visualize and analyze the flow structures in the wake of the airfoil at different angles of attack (AOA). Furthermore, spectral proper orthogonal decomposition and bispectral mode decomposition are carried out to identify the coherent flow structures and reveal the control mechanism from the perspective of simplified models. PIV measurements' results demonstrate that the STFs evidently suppress the turbulent quantities including turbulent kinetic energy and Reynolds shear stress in the airfoil wake. On the one hand, the STFs at low AOAs prevent the interaction between the upper and lower shear layers, and the leading- and trailing-edge vortices (TEVs) are significantly suppressed, thus destructing the von Karman vortex streets. On the other hand, the STFs at high AOAs divide the lower shear layer into two parts, markedly attenuating the TEVs and modifying the vortical structures in the wake. Besides, the quadrant analysis reveals that the STFs can mitigate the high-amplitude wall-pressure peaks, indicating that the STFs may manipulate the trailing-edge noise. However, the control effect is limited at median AOAs because the region with high triadic interactions moves upward in the interaction maps, which limits the impact of the STFs.