A circular cylinder attached by a rigid splitter plate of different lengths was tested to examine its effects on the control of flow-induced vibration. Tests were carried out in a closed-loop water channel. A cylinder of diameter D = 20 mm and a mass ratio m* ≈ 50 was installed to oscillate in the transverse direction. A wide range of splitter length was considered, i.e., L/D = 0–3.5, at a range of reduced velocity Ur = 1–25 and the Reynolds number Re = 800–11 000. Numerical simulations were also conducted to reveal the flow structures associated with the vibration modes observed in the experiment. It is found that, as L/D increases from 0 to 0.25, the peak value of cylinder oscillation amplitude increases and appears at higher reduced velocities. When the splitter length continues to rise, galloping-type oscillations occur at L/D = 0.5 and 0.75. The transition stage has been found at L/D = 1.0. Oscillation is then significantly suppressed when the splitter length is larger than L/D = 1.5.

A flexible plate vertically clamped at the free-end leading edge was used to modulate the aerodynamic forces on a wall-mounted finite square cylinder. The side width (d) of the cylinder was 40 mm and the aspect ratio (H/d) was 5. The flexible plate was made of low-density polyethylene, with a width of d and thickness of 0.04 mm. The length of the flexible plate ranged from d/8 to d. All measurements were carried out in a low-speed wind tunnel with the free-stream velocity (U_∞) ranging from 4 to 20 m/s, corresponding to a Reynolds number ranging from 10 960 to 54 800. It was found that the flexible plate behaves distinctly depending on its length and has significant effects on the aerodynamic forces on the finite square cylinder. When U_∞ is smaller than the critical velocity U_cr, which is closely related to the length of the plate, the plate statically deforms, having a negligible influence on the aerodynamic forces on the cylinder. When U_∞ exceeds U_cr, the plate flaps periodically, resulting in a significant reduction in the aerodynamic forces. The maximum reduction in the mean drag, fluctuating drag, and fluctuating lateral force reaches approximately 5%, 25%, and 60%, respectively. The reduction in the aerodynamic forces is insensitive to both the plate length and flapping frequency. Flow visualization and particle image velocimetry results point out that the flapping plate induces large-scale vortices in the free-end shear flow, which suppress the formation of spanwise vortex shedding and make the upper part of the near wake symmetrical. The flapping configuration of the flexible plate and the corresponding pressure fluctuation on the free end were also addressed.

Due to a rapid decrease in fossil fuel resources and ever-growing carbon emissions, clean energy is urgently needed as a vital solution. In the past two decades, harvesting clean energy from ambient environment has attracted much attention. Flow induced vibration and energy harvesting performance of a cylinder with fins attached were investigated. Four configurations were studied: a plain cylinder, a cylinder with two windward fins, a cylinder with two leeward fins, and a cylinder with four fins. These four cylinders were tested in a water channel with a reduced velocity ranging between 2 and 25 and a Reynolds number ranging between 1500 and 11 400. It was found that the two-windward-fin cylinder underwent galloping, exhibiting much larger vibration amplitudes and a much broader operational velocity range, whereas the two-leeward-fin cylinder only underwent weak vortex-induced vibrations. By attaching both two windward and two leeward fins to the cylinder, a bi-directional flow-energy harvester was implemented, which outperformed the plain cylinder with much larger vibration amplitudes and a much broader velocity range. More importantly, due to the geometric symmetry, it is able to harvest flow energy from two opposite directions. A tuned-mass-damper system was then attached to the four-fin cylinder for the purpose of demonstration. Within the current flow speed range, the maximal voltage and power outputs are about 7.37 V and 1.81 μW, respectively, about 2.7 and 7.2 times the plain cylinder's peak values. The effects of flow incident angle and fin length were also studied. Numerical simulations were also conducted to provide a detailed information of flow and pressure to uncover the underlying physics. This bi-directional flow-energy harvester is a suitable candidate to operate at sites where the flow periodically switches its directions, such as in tidal flows.

In this study, the wake of a cylinder was actively controlled by the cylinder's oscillatory morphing surface. Experiments were conducted in a closed-loop water channel. A cylinder of diameter 36 mm was placed in 0.09 m/s water flow, resulting in the Reynolds number 3240 and the vortex shedding frequency around 0.5 Hz. The cylinder's morphing surface oscillated at four different frequencies, i.e., 0.5, 1, 2, and 4 Hz. It was found that, compared to the rigid circular cylinder, the cylinder with oscillatory morphing surface can generally produce a smaller vortex formation length, especially at intermediate oscillation frequencies. The shear layers developed from the cylinder transit and roll up earlier due to enhanced flow instabilities. With the highest-frequency oscillations, the shear layer develops into a train of many small vortices that follow the trace of undisturbed shear layer. This study reveals some physical insights into this novel flow control method, which could be useful in future engineering applications.