When a bluff body is placed in flow, vortices are shed downstream of the body. For the case of a bluff body with a circular cross-section (a cylinder) attached to a spring and a damper, when the frequency of vortex shedding is close to the natural frequency of the structure, the cylinder oscillates in a direction perpendicular to the flow. This is called Vortex Induced Vibration (VIV) and is a canonical problem in fluid-structure interactions. The majority of studies on VIV of a flexibly mounted rigid cylinder are for the cases where the flow direction is perpendicular to the long axis of the structure. However, in many engineering applications, such as cable stays in bridges, mooring lines of floating offshore wind turbines and undersea pipelines, the flow direction may not be perpendicular to the structure. The hypothesis is that the VIV in inclined cylinders is similar to a normal-incidence case, if only the component of the free stream velocity normal to the cylinder axis is considered. This is called the Independence Principle (IP). The IP neglects the effect of the axial component of the flow, which is legit for small angles of inclination, but not for large angles. In this Thesis, a series of experiments have been conducted on a flexibly-mounted rigid cylinder placed inclined to the oncoming flow with various angles of inclination (0° < θ < 75°) in a subcritical Reynolds number range of 500 – 4,000 to investigate how the angle of inclination affects VIV. In these experiments, a rigid cylinder was mounted on springs, and air bearings were used to reduce the structural damping of the system. The system was placed in the test section of a recirculating water tunnel and crossflow displacements were measured. Even at high angles of inclination, large-amplitude oscillations were observed. The IP was found to be valid for angles of inclination up to 55°. While for all inclinations the onset of lock-in was observed to be at the same normalized flow velocity, for angles of inclination larger than 55°, the lock-in region (the range of dimensionless flow velocities for which the cylinder oscillates with a large amplitude) was smaller. These results show that the influence of the axial component of the flow is non-negligible for angles of inclination larger than 55°.
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