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 ( Re = 2× 10⁴ – 10⁵ ) and twist angles per unit span γ ^*∈ R[0,1/3] . It reveals a non-monotonic dependence of noise reduction on γ ^* , optimised for γ ^* = 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 γ ^* < 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 γ ^* 1/3 ), the spanwise variation of shear layer separation induced wavy vortex shedding, contributing to a moderate noise reduction. For Pattern II ( 1/18 γ ^* 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 γ ^* = 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 γ ^* = 1/6 stems from the combined effects of frequent vortex dislocation and modulation, which reduces spanwise coherency and increases wake three-dimensionality.