Experimental Study On The Characteristics Of Flow-induced Motion Of Cylinders With Different Cross-section

The vortices shedding from a bluff body is a kind of widespread physical phenomena in nature, which contains a lot of complex physical mechanisms. When the flow around an elastically mounted cylinder, the alternating vortex shedding leads to the cylinder experiences periodic fluctuating pressure, thus inducing the cylinder to vibrate. The vibration of the cylinder further changes the vortex formation and vortex shedding patterns. The interaction between the fluid and structure is called Flow Induced Motion(FIM). In the field of engineering, the sustained oscillation caused by FIM may lead to structural fatigue damage. As a result, the structural integrity and the reliability of performance is severely affected. Therefore, exploring and revealing the complex physical mechanisms of FIM is of important significance in theory and application. When the flow around cylinders with different cross-section, the vortex formation and vortex shedding patterns may differ, resulting in the lift and drag forces changing, further leading to the FIM responses and wake characteristics altering.In order to investigate the characteristics of the FIM of cylinders with different cross-section, in this thesis, the experiment study on FIM of elastically mounted square cylinder, trapezoid cylinder, and triangular cylinder are conducted in wind tunnel. The cylinders are constraint to vibrate only in the transverse direction, which is perpendicular to the flow velocity. By analyzing the FIM responses of the cylinders, such as the amplitude, vibrating frequency, vortex shedding frequency, and the phase angle, the relationship between the FIM branches and the amplitude, frequency, and phase angle are investigated. The effects of the velocity, mass ratio, and damping ratio on the characteristics of FIM, as well as the mechanisms of FIM of cylinders with different cross-section are explored.Firstly, the FIM of elastically mounted square cylinders with low mass ratio, moderate mass ratio, and high mass ratio is experimented in wind tunnel. The effects of mass ratio and damping ratio on the characteristics of FIM of square cylinder are investigated. The results show that the mass ratio and damping ratio have significant influence on the FIM responses and branches. When the damping ratio is lower, for square cylinders with low mass ratio and high mass ratio, three branches are observed and which are the VIV initial branch, upper branch, and lower branch. The beating phenomenon is captured in the lower branch. While, for the FIM of square cylinder, except the initial branch and upper branch, a higher amplitude branch is observed. With the increase of damping ratio, for the square cylinders with low mass ratio and moderate mass ratio, only the initial branch and higher amplitude branch are clearly observed. However, for the square cylinder with high mass ratio, the initial branch and upper branch are observed obviously and the higher amplitude branch is not observed.Secondly, the experiments on FIM of the trapezoid cylinder and triangular cylinder are conducted in wind tunnel. The effects of damping ratio and velocity on the amplitude, vibrating frequency, vortex shedding frequency and phase angle are deeply investigated. The results show that for the trapezoid and triangular cylinder, only the VIV initial branch and lower branch are observed. Both the vibrating frequency and the vortex shedding frequency of the trapezoid cylinder are lower than the natural frequency. The results show that for lower damping ratio, the FIM of the trapezoid cylinder may be induced and the corresponding branches may be observed in low velocity range. The vibrating frequency and vortex shedding frequency of the triangular cylinder are slightly less than the natural frequency in the initial branch. However, in the latter part of the lower branch, the vortex shedding frequency is greater than the vibrating frequency and natural frequency. For the trapezoid cylinder and triangular cylinder, the beating phenomenon is not observed in the lower branch.In order to get a better understanding of the characteristics of the FIM branches of cylinders with different cross-section, the relationship between the branch and phase angle is also investigated, the results show that when the phase angle is below 90 degree, the FIM of the cylinder is in the initial branch and upper branch(for square cylinder), the vortex shedding enhances the FIM of the cylinder. When the phase angle is greater than 90 degree, the FIM of the cylinders is in the lower branch and the vortex shedding weakens the FIM of the cylinder, leading to the amplitude decreasing.Compared with the FIM of the square cylinder, trapezoid cylinder, and triangular cylinder, for the circular cylinder with the same feature size, there is no obvious vibrating amplitude response in the whole tested velocity range and the cylinder remains almost static. Which indicates that under the same condition, the FIM of cylinder with sharp corners is prone to be induced. Furthermore the sharper corners of the cylinder, the FIM phenomenon of cylinder is more prone to induced.