Fluidelastic stability of a rotated square array with multiple flexible cylinders subject to cross-flow

Over the past decade, theoretical investigations have revealed the possible existence of two distinct mechanisms, a fluid-damping controlled one requiring only a single degree-of-freedom system and a fluid-stiffness controlled one requiring two or more degree-of-freedom system, instrumental in causing fluidelastic instability of cylinder arrays subjected to fluid cross-flow. As yet, the existence of these mechanisms has not been verified experimentally, and some researchers tend to neglect one or the other of these mechanisms in their theoretical studies.In this thesis, with the objective of obtaining further insight into the nature of fluidelastic instability mechanisms, experimental and theoretical studies have been performed on a rotated square array with $P/d$ = 2.12. Previous theoretical and experimental studies on this array have established the fact that a single flexible cylinder, in an otherwise rigid array, is fluidelastically stable. However, multiple flexible cylinder dynamic (vibration) experiments undertaken in this study show that fluidelastic instability develops when the array incorporates three of more flexible cylinders. This result verifies the duality of the instability mechanisms and suggests that the cylinder motion in the present array is dominated by the fluid-stiffness controlled mechanism, rather than the fluid-damping controlled mechanism.Involved dynamic (vibration) experiments have been undertaken to elucidate the effect of various parameters such as, number of cylinders, cylinder position, cylinder mass, frequency detuning and fluidelastic coupling on the instability threshold of this array, in which the fluid-stiffness controlled mechanism prevails. It has been determined that varying mechanical damping has a small effect on the critical velocity, whereas, varying cylinder mass generates, relatively, large changes in the critical velocity. A "Connors type" instability equation, or versions of it, are shown not to be applicable in this array, mainly due to the strong dependence of the mass exponent on the actual value of the non-dimensional mass.Frequency detuning of adjacent cylinders is also shown to have a significant effect on the critical velocity. Further dynamic (vibration) experiments revealed the co-existence of dynamic and static instabilities within close proximity to each other. It was possible to switch from one type of instability to the other, by varying one, or more, of the mechanical properties of the flexible cylinders.Next, the time averaged fluid forces acting on static cylinders were measured as a function of monitored, and surrounding, cylinder displacements at different Reynold numbers, to attain a physical understanding of the flow pattern in the array. The results complemented and verified the various dynamic and static instability findings of the vibration (dynamic) experiments.Finally, the fluid forces were incorporated in a quasi-steady, multiple degree-of-freedom model for comparison with experimental results