Investigation on Characteristics of a Cylinder with Normal Slit Experiencing Flow-Induced Vibrations

• Main Authors: Jun-Yun Wu, 巫均運
• Other Authors: Cheng-Hsiung Kuo
• Format: Others
• Language: zh-TW
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/cgi-bin/gs32/gsweb.cgi/login?o=dnclcdr&s=id=%22107NCHU5311048%22.&searchmode=basic

碩士 === 國立中興大學 === 機械工程學系所 === 107 === In this study, the numerical method is used to investigate the behavior of a vertical slit cylinder subjected to fluid induced vibration only in the vertical direction. We first simulate the induced vibration results of a single cylinder to verify the code of the present study and to ensure the suitability for analyzing the induced vibration for a cylinder with a normal slit. The simulation results show that: (1) The responses (displacement amplitude, lift force) and the types of wake flow structure are roughly consistent with the results of the related literature for a single cylinder. For the vortex-induced vibration of a slit cylinder, the major difference is the hysteresis character near the boundary of the initial and the upper branch regions. The hysteresis response can be captured by using the transient inflow conditions and proper turbulence parameters; (2) In the II and III regions, the dominant frequencies of displacement and total lift are close to but not exactly equal to the natural frequency of the system. In other words, as U* increases, the dimensionless frequency collapses onto a straight line with positive slope close to St = 0.11. While U* = 9.32, the dimensionless frequency is close to 1.0, and the displacement amplitude has the maximum value (=1.2). (3) For a single cylinder, across U* = 4.8, the vibration pattern changes from 2S to 2P corresponding to a  phase difference between the cylinder displacement and the vortex induced lift force. However, in the case of a slit cylinder, the flow structure transition from 2S to P+S patterns occurs across U* = 9.32, and also there exists a π phase difference between the vortex-inducing force and the cylinder displacement. (4) The blowing/suction at the slit exit can be regarded as a periodic disturbance to the boundary layer. When the amplitude of the dominant frequency is close to the natural frequency of the system, the displacement has a phase difference of π between the wake structures, and the amplitude has a maximum value. The above result is similar to the flow field characteristics under active flow control leading to the primary lock on state. However, a single cylinder does not have this feature; (5) Except for the initial branch region, the vibrating amplitudes of the displacement of the slit cylinder are much larger than those of a single cylinder.