The Flow-induced Motion And Energy Harvesting Characteristics Of A Cylinder With Different Cross-sections

The flow around a bluff body is a canonical problem in nature which is related to a lot of complex flow mechanism. When fluid flow over the surface of the bluff body, it produces vortices shedding behind the bluff body. Those vortices will result in periodic pulsating force. When the cylinder is elastically mounted, the forces will cause vibrations, which is called the Flow-induced motion(FIM). The traditional engineering challenge is to reduce the excitation force on the surface of the structure and prevent structure damage. However, FIM is not always harmful and it can also benefit us. The concept of harvesting kinetic energy of fluids flow through FIM has received increasing attention during the recent years. The shape and size of a bluff body are the most important parameters affecting the exciting force, but the influence of the geometry of the oscillating cylinder is one that has received little attention with regard to FIM. Thus, the research of FIM of cylinder with different cross-sections has great values both on theory and engineering application.In the process of research, the paper mainly uses numerical modeling method, and the experimental method is a supplement. The FIM response of cylinders with different cross-sections are studied in the Reynolds number range of 1×104≤Re≤1.2×105, including the square cylinders, the trapezoid cylinders, the triangular cylinders and the rhombus cylinders. The mechanical power output of each oscillating cylinder is calculated using the energy transfer mathematical model. In this paper, the effect of cross-section on the near wake response is discussed, the mechanisms of FIM, such as VIV and galloping, are explored as well. The effect of the geometry character on energy conversion efficacy of the oscillating system is explored.Firstly, the calculation accuracy of the simulation tool is validated by wind tunnel test. Then the FIM of cylinders with different cross-sections are simulated in detail. Compared with the experiment result of a smooth circular cylinder, the simulation results shows that the amplitude of the studied cylinders is higher and the Reynolds number of high amplitude is also much wider than the smooth circular cylinder. With regard to square cylinders at different angle of attack, the mean position is deflected to below y=0. What’s more, the amplitude of the trapezoid cylinder is apparently higher than the other cylinders, which reaches 4.5D.The frequency ratio of square cylinders with small attack angle(α < 10°), triangular cylinders with small vertex angle(θ=53.13° and θ=60°) is much less than the frequency ratio of smooth circular cylinder. This is due to the happening of galloping. Except for the cylinders prone to galloping, the frequency ratio of other cylinders have a similar trend compared to smooth circular cylinder. With the increasing of Reynolds number, high frequency VIV is observed for square cylinders(α≥10°) and rhombus cylinders, among which the frequency ratio of rhombus cylinder with AR=1.0 is apparently higher than the other cylinders.In this paper, both VIV and galloping is observed for cylinder with different cross-sections. The overall trend is as below: the increasing attack angle of square, the increasing vertex angle of triangular cylinder and the decreasing axis ratio of rhombus cylinder can suppress the onset of galloping.The effect of attack angle on the wake structure of square cylinder is the most complex. As the Re and attack angle change, the wake response of square cylinder can be classified into 6 independent regions. The near wake structure is different for each region. By contrast, the effect of d/D on wake response of trapezoid cylinder is small. Besides, the vortex shedding mode of triangular cylinder will become more simplified with the increasing of vertex angle(θ), and the vortex shedding mode of rhombus cylinder will become diverse with the increasing of axis ratio(AR).Finally, the mechanical power output of oscillating cylinder with different cross-sections is calculated. The results show that the energy transfer efficacy of square cylinder and rhombus cylinder is evidently better than cylinders with other cross-sections. The higher the angle of attack is, the better energy transfer efficacy of square cylinder will be. The optimal energy transfer efficacy is obtained for rhombus cylinder with AR =1.0. By comprehensive analysis, the comparison of energy transfer efficacy for different cylinder is given below: rhombus cylinder>square cylinder>triangular cylinder>trapezoid cylinder. For the present investigation conditions, the rhombus cylinder has a greater potential to drive a generator. In other words, it is more suitable for harvesting energy from fluid.