In-line flow-induced vibration of rotating elliptical cylinders

Abstract

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 $ϵ=2b/2a$, 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 $\alpha =\left|\Omega \right|D/\left(2U\right)$, 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 $U^{\ast }=U/\left(f_{n}D\right)$, 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 $\alpha \in \{0.2,0.5,1\}$, 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 $ϵ=0.25$ and $ϵ⩽0.75$ 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.