Wake dynamics of two side-by-side rectangular cylinders

Flows around two side-by-side rectangular cylinders are researched via a lattice Boltzmann method. The effects of the side ratio (γ = 1/2, 1/4 and 1/8), gap ratio (g* = 0.2–3.0) and Reynolds number (Re = 10–160) on the wake structure, vortex shedding frequency and fluid force on the cylinder surface are investigated. A total of nine distinct coupled vortex wake modes involving the single bluff-body, flip-flopping, in-phase synchronized, anti-phase synchronized, deflected, steady and three transition modes are observed. In the flip-flopping mode, deflected mode and transition mode 1, the vortex shedding frequencies and fluid forces of two cylinders are typically separated, whereas they are always equal in the other modes. The wake transition process is dependent on the side ratio, gap ratio and Reynolds number and the transition mode 1 between the in-phase synchronized and anti-phase synchronized modes can only occur at low side ratio due to the vortex interference. The mechanisms for the observed phenomena have also been analysed based on the frequency spectrum of lift coefficient.