Focusing on a two-stage variable-pitch axial-flow fan, this study investigates a bionic-inspired flap modeled after the Shortfin Mako shark's caudal fin. The effects of flap height, installation configuration, and angle were first numerically analyzed on the aerodynamic performance, internal flow, and noise of the first-stage rotor to determine an optimal single-stage configuration, which was then applied to both rotor stages for further evaluation. The underlying mechanisms were explored from the variations in the flow field and vortex structures. Results show that the bionic flap significantly increases the pressure differential across the mid-to-aft chord of the blade, thereby enhancing its work capacity. For the single-stage setup, a flap height of 3%c is optimal. A trade-off exists between the backward-facing and forward-facing installation configurations: the former significantly enhances total pressure rise and efficiency at the cost of increased noise from flow separation, while the latter is quieter but less effective. A comprehensive evaluation identified the backward-facing Shortfin Mako bionic-inspired flap (BF-SMBF) with α = 75° as the best single-stage blade design. Applying this design to both rotor stages further boosts the total pressure rise and broadens the high-efficiency range relative to the single-stage application, without significantly altering the overall sound pressure level or spectral characteristics. Thus, installing BF-SMBFs with h = 3%c and α = 75° on both rotor stages is recommended for superior overall performance.
Wu et al. (Thu,) studied this question.