WITHDRAWN: Flow and Noise Control Mechanism of Twisted Hexagonal Cylinder

This study investigates the aerodynamic noise characteristics of twisted hexagonal cylinders at subcritical regime (Re = 2 × 104 to 1 × 105), with a focus on the influence of twist angle ( from 1/18 to 1/3) on noise reduction performance. Far-field acoustic measurements reveal that all twisted configurations achieve varying degrees of noise reduction, with the overall sound pressure level (OASPL) showing a non-monotonic dependence on the twist angle. The configuration with ∗ = 1/6 yields the most effective noise suppression, achieving up to 15 dB reduction in tonal peaks and an 11 dB decrease in OASPL at Re = 2 × 104. Dual-peak features in the acoustic spectra are observed in several cases, leading to a classification of flow patterns into three types: pattern I (single peak), pattern II (dual peaks), and a transitional pattern. To elucidate the underlying mechanisms, numerical simulations were conducted for selected twist angles (1/9, 1/6 and 1/3). The results show that spanwise geometric variation induces three-dimensional vortex shedding by creating distinct non-reattachment and reattachment regions along the span. Particularly for twist angle equals to 1/6, the shear layers are more stable and extend further downstream, resulting in fragmented, less coherent vortex structures identified as a key factor in the observed noise reduction.