Experimental And Numerical Study On Flow Behaviors Behind Multiple Stationary Cylinders And Vortex-induced Vibrations Of Oscillating Cylinder

Multiple-pipelines or cylinders are widely applied on the offshore engineering, especially for the exploration of deep-sea oil and gas fields. However, due to tight arrangement, they could be easily damaged under the combined effects of the wave and current. Therefore, more and more attentions have been paid on the flow behavior behind multiple cylinders and the phenomenon of vortex-induced vibration (VIV). Additionally, a piggyback pipeline which comprises of one small pipe and a large pipe, is usually applied in offshore oil and gas explorations due to the technical requirements and economical considerations. However, there has not been extensive research on flow around two cylinders of different diameters. Most of the reported studies on two-cylinder configurations are concerned with two cylinders of an identical diameter. The aim of this study is to investigate the flow behaviors around multiple cylinders with unequal diameters based on experiments and numerical simulation.Two sets of experiments are carried out using the water channel and particle image velocimetry (PIV) technique to investigate the flow characteristics behind multiple cylinders: one is focused on flow around two stationary circular cylinders of unequal diameters with side-by-side and tandem arrangements; the other is focused on the study of an oscillating cylinder in the wake of an upstream stationary cylinder, where the larger cylinder is fixed at both ends while the downstream cylinder is mounted elastically by springs to limit its motion to cross-flow direction only.Instantaneous and time-averaged wake patterns of velocity, streamline topology, vorticity and normalized Reynolds stress contours are obtained from the cross-correlation analysis results of the images captured by the CCD camera. For two side-by-side unequal cylinders, instantaneous wake patterns are investigated at an intermediate spacing ratio to provide a plausible interpretation for the generation mechanism of gap flow deflection. For two tandem unequal cylinders, special attention is paid on the occurrence of bi-stable regimes for which reattachment or co-shedding can be observed at the same spacing ratio. Additionally, the interferences of the upstream stationary larger cylinder on the oscillation response of the downstream smaller cylinder are investigated, where the downstream cylinder is limited its motion to cross-flow direction only. The effects of gap ratios on vibration amplitude, vortex shedding frequency and flow patterns are examined.A series of numerical simulations according to the experimental sets have been accomplished. For the flow around multiple stationary cylinders, 2-D simulations at low Reynolds number are conducted to investigate the wake characteristics. For side-by-side unequal cylinders, the transition process of in-phase vortex shedding to anti-phase vortex shedding is investigated to explain the synchronized vortex shedding phenomenon. For two tandem unequal cylinders, biased flow phenomenon is studied. In the numerical study of an oscillating smaller cylinder in the wake of an upstream larger cylinder, the effects of gap ratios on the oscillation response of two tandem cylinders are investigated.The experimental and numerical results show that the gap ratio has a large effect on the wake patterns behind multiple unequal cylinders. For two side-by-side unequal cylinders, two wake patterns are observed at different spacing ratios, such as biased flow and asymmetrical vortex shedding. Three wake patterns are observed behind tandem cylinders of unequal diameters at different spacing ratios, which include single vortex street, bi-stable flow regime and co-shedding. It is found that the range of the bi-stable regimes for tandem unequal cylinders is related to the diameter ratio of the upstream to downstream cylinder, which decreases with increasing of diameter ratio.When the oscillating cylinder is placed in the downstream of a stationary larger one, the wake behind the downstream oscillating cylinder is quite different from the one behind a single oscillating cylinder. The results show that the vibration response of the smaller cylinder is largely affected by the presence of the upstream larger cylinder, and resulted in greatly reduced vibration amplitudes. With increasing gap ratios, the vibration amplitudes are increased, however, these magnitudes are still lower than that of a single cylinder. Additionally, the vortex shedding frequencies behind two unequal cylinders increases with the gap ratios as well.