On the effect of twist in a hexagonal cylinder

A combined experimental and numerical investigation was conducted to examine the mechanisms of aerodynamic noise reduction for twisted hexagonal cylinders at Reynolds numbers ( $ \textit{Re} = 2\times 10^4$ – $10^5$ ) and twist angles per unit span $\gamma ^*\in \mathbb{R}[0,1/3]$ . It reveals a non-monotonic dependence of noise reduction on $\gamma ^*$ , optimised for $\gamma ^* = 1/6$ , where a tonal noise reduction of 15 dB and a total sound reduction of 11 dB at $ \textit{Re} = 2\times 10^4$ were achieved. This was consistent across all Reynolds numbers tested. Additionally, dual tones were observed in the noise spectra for cases with $1/18\leqslant \gamma ^* \lt 1/6$ , 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 $ \textit{Re} = 2\times 10^4$ to support the acoustic measurements. Spanwise variations in flow separation gave rise to two distinct regimes: separation (RI) and reattachment (RII). For Pattern I ( $1/5.4 \leqslant \gamma ^* \leqslant 1/3$ ), the spanwise variation of shear layer separation induced wavy vortex shedding, contributing to a moderate noise reduction. For Pattern II ( $1/18 \leqslant \gamma ^* \leqslant 1/7.2$ ), differences in vortex shedding frequencies between RI and RII regimes led to vortex dislocation, forming C- or X-type vortex structures. The $\gamma ^* = 1/6$ 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 $\gamma ^* = 1/6$ stems from the combined effects of frequent vortex dislocation and modulation, which reduces spanwise coherency and increases wake three-dimensionality.