Hydrodynamic characteristics of an inclined slender flexible cylinder subjected to vortex-induced vibration

The hydrodynamic coefficients (e.g., lift coefficients, drag coefficients and added mass coefficients) are key parameters for predicting the vortex-induced vibration (VIV) of flexible cylinders. It is an experimental challenge to directly measure the forces/coefficients along a flexible cylinder during model tests without disturbing the flow field. However, such hydrodynamic features can be successfully obtained via an inverse analysis of the displacement responses based on the structural dynamics. In this paper, the hydrodynamic coefficients were calculated using the towing tank experimental results of an inclined flexible cylinder undergoing VIV at five different inclination angles (alpha = 0 degrees,15 degrees, 30 degrees, 45 degrees and 60 degrees, where alpha denotes the inclination angle defined as the angle between the cylinder axis and the plane orthogonal to the oncoming fluid flow). It is found that the influence of the inclination angle on the hydrodynamic coefficients is generally insignificant when the inclination angle is varied from 0 degrees to 15 degrees. In contrast, the hydrodynamic characteristics of the inclined cylinder with large inclination angles (alpha = 45 degrees and 60 degrees) are distinct from those of the vertical flexible cylinder (alpha = 0 degrees) and this difference becomes more evident as the inclination angle is increased. The RMS of the fluctuating force coefficients in the cross-flow (CF) and in line (IL) directions, mean drag coefficients, axial mean lift and varying drag coefficients in the case of alpha = 60 degrees are much larger than those in the normal case(alpha = 0 degrees). Meanwhile, the axial mean IL added mass coefficients in the cases of alpha = 45 degrees and 60 degrees demonstrate higher values compared to those in the normal case at certain reduced velocities. It can be speculated that different-wake-behaviors are associated with the disrinrt hydrodynamic behaviors of the inclined flexible cylinder at different inclination angles. Within the same mode synchronized region, the axial distributions of the hydrodynamic coefficients in the cases of alpha=15 degrees and 30 degrees resemble that of the vertical flexible cylinder, while the spanwise variations of the hydrodynamic coefficients for alpha = 45 degrees and 60 degrees agree well with each other.