Mechanisms of vortex-induced vibrations of a D-section prism at subcritical Reynolds number

The paper presents a systematic investigation of vortex-induced vibrations (VIV) of a D-section prism at subcritical Reynolds numbers (Re) through direct numerical simulation. To this end, we first report the critical Re values for flow over a stationary and an elastically supported D-section prism within the angle of attack α=0∘-180∘. We then focus on the VIV response and vortex dynamics across eight recognized regimes, classified based on the vibration amplitude and frequency responses, at subcritical Re. Further, the physical mechanisms for the excitation and sustenance of the VIV response are imparted, using the force partitioning method that allows for load estimation by different generation origins. It is found that the VIV response of the D-section prism can be excited and sustained by the viscous and/or pressure lift coefficient in phase with the prism velocity. Finally, we check the critical Re for the galloping onset and verify the absence of galloping in subcritical flow. The reasons behind the absence and occurrence of galloping are unraveled. It is found that galloping is excited and sustained by the motion-induced asymmetric pressure lift with a frequency identical to the dominant vibration frequency. The viscous lift is entirely low-frequency and motion-related but negative to galloping. The analysis of mode conditions indicates that galloping requires an unstable structural mode with a frequency close to the natural frequency of the prism and flow mode with the natural vortex shedding frequency. The flow mode should be sufficiently strong to guarantee galloping at infinite reduced velocity.