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CYLINDER VIBRATION EXCITATED BY SHEAR LAYERS FROM AN UPSTREAM CYLINDER

Md. Mahbub Alam
Department of Mechanical Engineering, The Hong Kong Polytechnic University Hung Hum, Kowloon, Hong Kong; Institute for Turbulence-Noise-Vibration Interaction and Control Shenzhen Graduate School, Harbin Institute of Technology Shenzhen, 518055, China

Yu Zhou
Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong; Institute for Turbulence-Noise-Vibration Interaction and Control Shenzhen Graduate School, Harbin Institute of Technology Shenzhen, 518055, China

Аннотация

This paper presents the free vibration of a cantileversupported circular cylinder of diameter D placed behind another of smaller diameter d. The diameter ratio d/D is 0.24 ~ 1.00 and cylinder spacing L/d is 1 ~ 2, where L is distance between the centre of the upstream cylinder to the forward stagnation point of the downstream cylinder. In this range of L/d, the shear layers separating from the upstream cylinder reattach on the downstream cylinder. An unusual violent vibration was observed at d/D = 0.24 ~ 0.8 for L/d = 1 or d/D = 0.24 ~ 0.6 for L/d = 2, but not at d/D = 1. It is proposed that, at a small d/D, the upstream cylinder wake narrows, and the shear-layer reattachment position on the downstream cylinder approaches the forward stagnation point, and hence the high-speed slice of the shear layer could impinge upon alternately the two sides of the cylinder, thus exciting the downstream cylinder. The violent vibration occurs at a reduced velocity Ur (= U/D/fn, where U is the free-stream velocity and fn is the natural frequency of the fluid-structure system associated with the downstream cylinder) ~ 13~22.5, depending on d/D and L/d, and grows rapidly, along with the fluctuating lift, for a higher Ur. To our knowledge, this phenomenon has not been reported previously and may have important implication in engineering applications. It is further noted that the flow behind the downstream cylinder is characterized by two predominant frequencies, corresponding to the cylinder vibration frequency and the natural frequency of vortex shedding from the downstream cylinder, respectively. While the former persists downstream, the latter vanishes rapidly.