In-line flow-induced vibration of rotating elliptical cylinders

This study numerically investigates the in-line flow-induced vibration (FIV) of elastically mounted elliptical cylinders undergoing forced rotations in a free-stream flow. The two-dimensional numerical simulations were conducted at a Reynolds number of 100. The cross-sectional aspect ratio (or elliptical ratio) of the cylinders varied from 1 to 0.25. The aspect/elliptical ratio is defined by epsilon = 2b/2a, where 2a and 2b are the streamwise and cross-flow dimensions, respectively, of the cross-section of a cylinder placed at zero incidence angle. The Reynolds number is defined by Re = UD/nu, where U is the free-stream velocity, nu is the kinematic viscosity of the fluid, and D is the major axis length (i.e. D = 2a). The dimensionless rotation rate, defined by alpha = |Omega|D/(2U), is varied at values of 0.2, 0.5, 1 and 2, where Omega represents the angular velocity of the body rotation. The FIV response is examined as a function of reduced velocity, defined by U-* = U/(f(n)D), with f(n) being the natural frequency of the system. Interestingly, two synchronisation modes were identified: a "rotation-dominated" (RD) mode and a "wake-dominated" (WD) mode. For alpha is an element of {0.2, 0.5, 1}, the RD mode was found to be associated with significantly high-amplitude vibration, while the WD mode was associated with low-amplitude vibration. However, as alpha increased to 2, the WD region exhibited a higher amplitude peak compared to the RD region. The maximum vibration amplitude in the present study was observed to be approximately 0.5D, occurring for alpha = 0.2. A further analysis of the wake structure revealed that vortex feeding or merging behaviour occurred at alpha = 0.5, 1 and 2 for epsilon = 0.25 and epsilon <= 0.75 for alpha = 0.5. Increasing the rotation rate or the aspect ratio could weaken the impact of rotation on vibration, resulting in a reduction in the peak vibration amplitude of RD region. Notably, harmonic frequency components exceeding the rotation frequency were observed for alpha = 0.2. Further investigation with the fixed body revealed that the wake pattern of the rotating elliptical cylinder undergoes a transition when alpha < 0.3, exhibiting significant instability characterised by the superposition of high-order harmonic components.