Fluid flow across a bluff body induces alternating lift and drag forces, which can produce adverse effects on many engineering applications. For a successful engineering design, it is essential to know how to suppress the fluid forces acting on the body and what is the effect of flow structure on the fluid forces. The subject of fluid forces reduction has been receiving a great deal of attention recently. A number of studies have shown that flow control is one of the effective means to suppress the forces acting on the body. However, due to numerous parameters involved, at present a detailed knowledge about how to use flow control method to suppress fluid forces is far from complete. The present study is to investigate the effects of a control cylinder on flow characteristics and fluid forces acting on the main cylinder in a two-dimensional time-dependent flow by using numerical technique. For the purpose of reducing the fluid forces acting on the main cylinder, a smaller-sized control cylinder is positioned centrally upstream. Both the main cylinder and the control cylinder are square cylinders. Steady and fluctuating fluid forces acting on the main cylinder, flow characteristics, and Strouhal numbers are investigated for different size-ratios and gap-ratios of the cylinders at a Reynolds number of 8000. Two size-ratios, 0.3 and 0.6, are used in this study. Gap-ratio is ranged from 0.5 to 3.5. It is observed that for small gap-ratios, there are two quasi-stationary vortices between the cylinders. The fluid forces acting on the main cylinder, not only the drag but also the fluctuating lift, are significantly reduced by the control cylinder. However, the fluid forces are suddenly increased when the Karman vortex street is formed between the two cylinders for larger gap-ratios. It can be concluded that in order to suppress the fluid forces acting on the main cylinder, the control cylinder must be placed at a position where the Karman vortex street would not be formed between the cylinders.

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