Vortex-Shedding Response of Long Cylindrical Structures in Shear Flow

The response of cylindrical structures to vortex shedding in a vertically sheared cross flow is analyzed. In contrast to the uniform cross-flow case, shear flow can excite more than one modal frequency at a time. Thus, the net response of the structure is a superposition of several vibration modes. The amplitude of each mode is determined by a balance between energy fed into the structure over a “locked-on” region of the structure and energy dissipated by fluid damping over the remainder of the structure. A solution method based on random vibration analysis is developed that uses an empirically derived lift coefficient and correlation length models. The technique is capable of handling both uniform and sheared (depth-varying) current profiles. Good quantitative agreement is found between the present method and the very limited field data available for shear flows, although it is concluded that the shear conditions in the tests were not sufficiently strong to validate the theory conclusively. The results show how using uniform-flow approximations to treat shear flow cases can significantly overpredict vibration amplitudes caused by vortex shedding.