Effects of streamwise gust amplitude on the flow around and forces on two tandem circular cylinders

Three-dimensional large eddy simulations are carried out to investigate the effect of streamwise gust amplitude ratio A_R on the flow structure around and dynamic forces on two tandem circular cylinders at a subcritical Reynolds number of Re = 1 × 10³. The center-to-center space L between the two cylinders is fixed at 3.5D, where D is the cylinder diameter. The velocity U of sinusoidal streamwise gust is set as U = U₀ (1+A_Rsin2πf_ut), where the velocity amplitude ratio A_R is varied from 0 to 0.25, U₀ is the mean velocity, f_u is the frequency of the sinusoidal streamwise gust, and t is the time. In order to enhance the understanding of the streamwise gust effect on the physical properties of the flow around the two tandem cylinders, the shear-layer reattachment, flow separation, wake recirculation, Strouhal number (St), unsteady forces, and phase lag (φ) between the fluctuating lift forces of the two cylinders are explicated. The results show that the flow structure around the cylinders is sensitive to A_R. A change in A_R thus leads to modification of reattaching flow (A_R ≤ 0.15) to co-shedding flow (A_R ≥ 0.20). The St, fluctuating forces and φ thus jumps at the boundary between the reattaching and co-shedding regimes. A stronger gust effect appears on the fluctuating drag of the upstream cylinder than that of the downstream cylinder. Besides the dominant component of the fluctuating lift induced by the vortex shedding, two other components are observed with the frequencies equaling to the superposition and subtraction of the gust and vortex-shedding frequencies, respectively. In addition, two more cases with L/D = 1.5 and 6.0 are conducted for deeper understanding of A_R effects.