Pressure and flow patterns during multiple-order vertical vortex-induced vibrations of a 6:1 rectangular cylinder

Rectangular cross-sections have been typically used to investigate vortex-induced vibration (VIV) mechanism of bluff bodies, which provides important reference for wind-resistant design of engineering structures, such as high-rise buildings and long-span bridges. A numerical simulation was conducted in order to examine the multiple-order vertical VIV mechanism of a 6:1 rectangular cylinder, and the surface wind pressure distribution characteristics and surrounding flow field were particularly focused. This rectangular cylinder has two VIV lock-in regions. According to the surface pressure distribution characteristics, it is deduced from the proposed simplified vortex theory that there are significant vortices drifting on the upper and lower surfaces of the rectangular cylinder, and the vortex experiences approximate two and one vibration period drifting from the leading edge to the trailing edge in the first and second lock-in region, respectively. More characteristics of the flow field are revealed by dynamic mode decomposition (DMD) method. The front three non-zero DMD mode frequencies are about 1–3 times of the natural frequency of the rectangular cylinder, respectively. Multiple vortices are also observed on the upper and lower surfaces in the high-order DMD modes, especially in the second lock-in region, indicating the importance of vortex drifting phenomenon in the occurrence of VIV. This investigation provides a deeper viewpoint combining surface pressure distribution characteristics and surrounding flow patterns, which is of important reference for revealing the VIV mechanism of wind-sensitive structures with bluff body cross-sections.