Primary instability, sensitivity and active control of flow past two tandem circular cylinders

The tandem two-cylinder configuration is frequently employed in engineering structures. Its fluid force and vortex shedding are significantly greater than those of widely studied single-cylinder because of the wake-gap interaction. To unveil the underlying mechanism and perform wake control, sensitivity analysis, the adjoint method, and global linear instability were applied to the flow past two tandem cylinders. Utilizing the sensitivity map, the optimal control location was determined. The results indicate that the most sensitive region is located behind the upstream cylinder; therefore, we implemented jet control in that location. The two-dimensional direct numerical simulation was conducted to evaluate the effects of control. The findings indicate that by applying a dimensionless jet flow velocity of 1.0 at the diagonal rear of the upstream cylinder, the fluid force can be effectively reduced with an L/D ratio of 6.0. Additionally, the control effects were evaluated at a range of Reynolds numbers between 75 and 200. Analysis using high-order dynamic mode decomposition (HODMD) reveals that the presence of the jet flow causes a backward shift in the global modes and a substantial reduction in the modal energy. This demonstrates that the jet flow effectively inhibits the occurrence of vortex shedding, resulting in a reduction in both fluid forces and vortex shedding.