Experimental Force Database From Controlled In-line and Cross Flow Cylinder Motion

摘要

Vortex induced vibration (VIV) is an important problem for offshore structures, where flow-induced vibrations can lead to fatigue damage. Prediction of the coupled forces and motions from VIV are critical in estimating fatigue for design purposes and also for estimating potential motions in offshore operations. Recent work has demonstrated the importance of considering combined in-line and cross-flow motions in prediction, however semi-empirical prediction methods that rely on force coefficient databases are difficult to implement with combined in-line and cross-flow motions due to the sheer number of experiments required to cover the relevant motion parameter space. This paper builds on previous forced motion experimental work to construct a force coefficient database based on combined in-line and cross-flow motions of a cylinder in a free stream. Hydrodynamic forces are measured over a wide range of normalized in-line and cross-flow amplitude, reduced velocity, and phasing between the in-line and cross-flow motion, while Reynolds number is held constant at a value of 7620. In-line and cross-flow motions are prescribed to be sinusoidal in order to limit the number of variable experimental parameters. Decomposed forces such as added mass and lift in phase with velocity are mapped over the range of parameters to investigate how these quantities change as a function of the combined motion. One interesting finding that was not previously recognized in earlier experiments of this type is the presence of large mean lift forces over some values of motion amplitude and phasing between in-line and cross-flow motion. In some cases, the mean lift is very large and comparable to oscillating lift forces at other motion parameter combinations. The presence of a mean lift is significant as it implies an asymmetry to the wake that may occur due to forced motions. On long, slender structures, if this type of forced motion were to occur at locations along the length of the structure, this could lead to kiting or mean deflections of the structure perpendicular to the direction of the flow.