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Distinct response branches of a freely vibrating square cylinder
Rajesh Bhatt, MD MAHBUB ALAM

Last modified: 2018-04-16

Abstract


Flow-induced transverse vibrations of an elastically mounted square cylinder are numerically studied for a cylinder mass ratio of m* = 3.0, Reynolds number Re (=UD/n = 100 – 200) and reduced velocity Ur (=U /fnD = 1.0 - 30), where D is the cylinder width, Uis the free stream velocity, fn is the natural frequency of the cylinder system, and n is the kinematic viscosity of the fluid. The focus is given on (i) identifying different branches of flow-induced vibrations, (ii) flow characteristics at different branches, and (iii) dependence of different branches on Re. It is found that the dependence of Strouhal number St on Ur can distinguish different branches more appropriately than that of the vibration amplitude on Ur. With increasing Ur, the cylinder undergoes three response branches at Re = 100: initial (Ur < 4.5), lower (4.5 < Ur < 10), and desynchronization (Ur ≥ 10). Akin to those at Re =100, the initial and lower branches at Re = 200 appear at Ur < 12 while a galloping branch occurs at Ur > 12. It is discovered that, in the galloping branch, the vibration response at 12 < Ur < 24 involves the characteristic of both initial and lower branches while that at Ur > 24, of both initial and desynchronization branches. The phase lag between force and displacement changes from 0° to 180° at the commencement of the lower branch. For Re = 200 at Ur = 5, force and displacement signals comprise two intermittent modes corresponding to high and low oscillation frequencies.

 


Keywords


Flow-induced vibrations, square cylinder, galloping

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