Sensitivity-aided active control of flow past twin cylinders

Abstract

The flow-induced vibration (FIV) of twin structures always experiences significantly larger vibration amplitude than that of a single one, which causes severe structural safety problems. However, FIV control methods specifically for twin structures are still lacking. This study investigates the FIV and fluid forces of twin square cylinders under a jet flow control for a laminar flow. The control effects are studied for stationary and free-oscillating cylinders with a gap distance of L = 6.0D in co-shedding regimes (D is the side length of the cylinder). Global linear instability, adjoint method, and sensitive analysis are conducted to find a position where the jet flow should be placed. It was found that the most sensitive region is located in the near-wake region, and we thus set at jet flow at the rear of the cylinders. The control effects show that for stationary cylinders, UC achieves its maximum mean drag (CD) reduction of 11% when Uj > 0.3, meanwhile the CD of the downstream cylinder (DC) and fluctuating lift (CL') ≈ 0. UC plays a primary role in determining the control effects on fluid forces compared to DC, which coincides with the sensitivity analysis. For the free-oscillating case, much larger control energy is required to completely suppress vortex-induced vibration (VIV) with Uj > 2.4. In addition to the splitting effects of the jet flow, the neutralization effect of jet-induced vorticity was found plays a significant role in suppressing VIV, which can neutralize the vorticity generated from the side edge of the cylinders. High-order dynamic mode decomposition (HODMD) is also applied to uncover the underlying control mechanism. It demonstrates that the global energy of the dynamic mode energy is significantly decreased, meanwhile, the primary dynamic mode (i.e., the mode shape introducing VIV) is significantly weakened by the jet flow due to splitting and neutralizing effects.