Flow Structure Around A Transversely Oscillating Square Prism At A Low Reynolds Number

Flow round bluff-bodies is associated with large phenomenon in engineering applications,such as suspension bridges,chimney,off-shore platform and high-rise buildings and so on.As the asymmetric formation of vortices in the wake,it may result in vibration of structures,which aggravates the failure of structures.As a result,the research on flow around oscillating structures is essential.The thesis mainly investigated the vortex shedding structure and fluid characteristic of a single forced oscillating square prism,based on the condition of 2-dimensional,low Reynolds number?Re=100?.The normalized oscillation amplitude A*?=A/D?is varied from 0.1 to 1.0 with intervals of 0.1,and from 1.0 to 1.2 with intervals of 0.2;and the normalized oscillation frequency f*?=f_e?f₀?is changed from0.2 to 2.0 with intervals of 0.1,where A is the oscillation amplitude,and D is the width of a square prism,f_e is the oscillation frequency of the prism,f₀ is the vortex shedding frequency of the fixed prism.The main purpose of this research is to find vortex shedding modes at every oscillation state and summarize them to a complete amplitude-frequency?A*-f*?plane.Five different vortex modes are on the plane:vortex mode NP,vortex mode 2S,vortex mode P+S,vortex mode C?2S?and vortex mode T?Transition?which exists between two distinct vortex modes.In addition,there exists no vortex mode regime?1.4?A*?2.0,1.2?f*?2.0?.Multiple relation exists in NP mode regime,while it disappears in other vortex mode regimes.One of the vortex in 2S mode splits into two vortices forming the asymmetry P+S mode.C?2S?mode is the consequence of vortex mergence deriving from several oscillation cycles.For the similar shedding vortex modes,such as classical 2S mode and 2S mode with“tails”;P+S mode and anti P+S mode;2S mode and C?2S?,future comparisons of their similarities and differences are investigated through analyzing the velocity power spectrum density of the flow field behind the prism.When A*?0.8,the lock-in range will increase as A*increases and when 0.8<A*?1.2,the lock-in range will decrease.2S mode and P+S mode are mainly located in the lock-in range.For a fixed A*,the trend of energy transfer C_E*from fluid to the square prism,phase difference?between lift force coefficient C_L and motion y*,added mass ratio m_a*,added damping ???_a show difference as vortex mode changes from one to another.C?2S?mode among them which causes the sudden jump.As the value of C_E* keeps positive except for one oscillation state,the results of present research can be applied to free vibration.