POD-based Analysis of the Pressure Field of Elongated Hexagonal Cylinders

This paper presents the effects of aspect ratio (B/D, width to height ratio), Reynolds number (Re, based on free incoming flow U∞ and D), and vortex-induced vibration (VIV) on the pressure properties of elongated hexagonal cylinders through proper orthogonal decomposition (POD) analysis. The examined cases cover B/D = 4, 7, and 10, Re = [3.5 × 104, 4.2 × 105], and primary and secondary VIV resonances. The experimental results show that the mean pressure field is insensitive to Re and VIV, while the fluctuating pressure significantly depends on B/D, Re, and VIV. For the cylinder in fixed state, the fluctuating pressure field can be decomposed into three typical fluctuating modes, i.e., leading-edge fluctuating mode (LEFM), trailing-edge fluctuating mode (TEFM), and their coupling fluctuating mode (CFM). With an increase in B/D, the TEFM and CFM obviously decay, while the LEFM slightly intensifies. On the other hand, with an increase in Re, the TEFM obviously enhances, while the LEFM generally weakens. The above observations suggest that the effects of cylinder leading-edge vortices on trailing-edge vortices gradually reduce with increasing B/D and Re. For the cylinder in VIV state, the fluctuating pressure field can be decomposed into four typical fluctuating modes, i.e., LEFM, CFM, distorted TEFM, and motion-induced fluctuating mode (MIFM). Compared to the above fixed case, the two emerging modes, i.e., distorted TEFM and MIFM, are the underlying mechanisms of VIV. But in primary and secondary VIV resonances, the distorted TEFM and MIFM capture different energy percentages, which is the underlying physic of the cylinder multi-lock-in phenomenon.