Interference between an oscillating-cylinder wake and a neighbouring flow field

Abstract

This thesis presents an experimental study of fluid-structure interference. Two topics are coverd. Firstly, The flow behind two side-by-side tubes of identical diameter, d, one stationary and the other forced to oscillate in the lateral direction at an amplitude of A = 0.1 ~ 0.5d was examined. Two values of T/d, i.e. 2.2 and 3.5 were investigated, where T is the cylinder centre-to-centre spacing. The Reynolds number Re ranges from 150 to about 1000. The effect of forced oscillating amplitude A/d, spacing ratio T/d and frequency ratio fe / fs, (where fe is cylinder oscillating frequency and fs is the vortex shedding frequency of an isolated stationary cylinder), on the vortex shedding and the wake structure was examined. Specific attention was given to the occurrence of 'lock-in', where vortex shedding from the oscillating cylinder synchronizes with fe. Significant influence of these parameters has been observed on the flow behind the cylinders in terms of the predominant vortex patterns and interactions between vortices. It has been found that the shedding frequency associated with the oscillating and the stationary cylinder can be modified as fe/fs approaches unity. Furthermore, the flow regime may change under the conditions of T/d = 2.2 and A/d = 0.5 from two distinct coupled streets to the combination of one narrow and one wide street. Subsequently, the lock-in state is considerably extended probably because multiple dominant frequencies in the asymmetrical flow regime can all be locked in with the structural oscillation. Secondly, the effect of an oscillating cylinder on the flow in an alternately arranged 7-row array of 46 cylinders was investigated based on the laser-induced fluorescence flow visualization. The flow structure and vortex shedding frequency, without any cylinder oscillating, were in agreement with previous reports. As one cylinder in the first row was forced to oscillate laterally at an amplitude of A/d = 0.1 and 0.25 and frequency ratio fe/fs = 0.68 - 1.36, where fs is the vortex shedding frequency when the cylinder was stationary, the 'lock-in' phenomenon occurred, that is, the shedding frequency of vortices from the oscillating cylinder synchronized with the oscillation frequency. The cylinder oscillation appreciably changed the flow structure, promoting vortex shedding from the oscillating cylinder and those downstream. It has been found that vortex shedding from the nearest two rows of cylinders downstream can be also locked in with the cylinder oscillation. When the oscillating cylinder was placed in the third row, the vortex shedding and cylinder-oscillating frequencies were decoupled at the lower end of fe/fs but locked in at the higher end. The effect of Reynolds number was also examined. Two publications, including one refereed conference proceeding and one refereed journal papers, have been produced out of this work.