FLUID-ELASTIC INSTABILITIES OF CLAMPED-CLAMPED ALIGNED AND INCLINED CYLINDERS IN TURBULENT AXIAL FLOW

Fluid-elastic instabilities arise due to the coupling of structural motion and fluid flow. In the specific case of a clamped clamped cylinder in axial flow, it will buckle at a sufficiently high flow velocity and start to flutter at even higher flow velocities. This dynamic behavior is of importance to nuclear reactor core design, undersea pipe lines and devices for energy harvesting. In this contribution, the fluid forces and the dynamics of a flexible clamped-clamped cylinder in turbulent axial flow are computed numerically. In contrast to present analytical approaches, this numerical model does not require to tune parameters for each specific case or to obtain coefficients from experiments. To provide insight in the way viscous fluid forces affect the dynamics of a cylinder in axial flow, fluid forces are computed on rigid inclined cylinders, mimicking the damping force experienced by the same cylinder moving perpendicular to the axial flow. The computations showed the existence of two different flow regimes. Each regime gave rise to a different lift force behavior, which will also influence the damping of the coupled system. Furthermore it is shown that the inlet turbulence has a non negligible effect on these forces and thus on the dynamics of the cylinder. Next, the dynamics of a flexible cylinder clamped at both ends in axial water flow are computed by means of a methodology developed earlier. The results are successfully compared with dynamics measured in experiments available in literature. Computationally it was found that the cylinders natural frequency decreases with increasing flow velocity, until it loses stability by buckling. The threshold for buckling is in quantitative agreement with experimental results and weakly nonlinear theory. Above this threshold, the amplitude of the steady deformation increases with increasing flow speed. Eventually, a fluttering motion is predicted, in agreement with experimental results. It is also shown that even a small misalignment (1 degrees-2 degrees) between the flow and the structure can have a significant impact on the coupled dynamics.