A combined experimental and numerical investigation was conducted to examine the mechanisms of aerodynamic noise reduction for twisted hexagonal cylinders at Reynolds numbers (Re = 2 10⁴ – 10⁵) and twist angles per unit span ^* R0, 1/3. It reveals a non-monotonic dependence of noise reduction on ^*, optimised for ^* = 1/6, where a tonal noise reduction of 15 dB and a total sound reduction of 11 dB at Re = 2 10⁴ were achieved. This was consistent across all Reynolds numbers tested. Additionally, dual tones were observed in the noise spectra for cases with 1/18 ^* 1/6, leading to the identification of two distinct flow patterns (Pattern I and II) based on the number of tones in the spectrum. Large-eddy simulations were performed at Re = 2 10⁴ to support the acoustic measurements. Spanwise variations in flow separation gave rise to two distinct regimes: separation (RI) and reattachment (RII). For Pattern I (1/5. 4 ^* 1/3), the spanwise variation of shear layer separation induced wavy vortex shedding, contributing to a moderate noise reduction. For Pattern II (1/18 ^* 1/7. 2), differences in vortex shedding frequencies between RI and RII regimes led to vortex dislocation, forming C- or X-type vortex structures. The ^* = 1/6 configuration leads to a transitional pattern between Pattern I and II, where modulation was predominantly observed in the RI regime. The superior noise reduction of ^* = 1/6 stems from the combined effects of frequent vortex dislocation and modulation, which reduces spanwise coherency and increases wake three-dimensionality.
Bao et al. (Thu,) studied this question.