Wake–cylinder interactions of a hinged cylinder at low and intermediate Reynolds numbers

This paper is devoted to the study and classification of vortex-induced oscillation and the wake structure of flow past finite cylinders. Experiments were performed in a water tunnel using cylindrical particles hinged in the center of the test section while allowing them one degree of rotational freedom. The speed of flow and aspect ratio of the cylinders were used to vary the Reynolds number in the study between 100 and 5000. The cylinders display different responses to the fluid flow depending upon the Re, ranging from steady orientation, periodic oscillation to autorotation. A hydrogen bubble flow visualization technique was used to examine the vortex structure and supported by image analysis techniques. Specific features of the wake structure such as length of the primary vortex, its frequency and amplitude were analyzed as a function of Re. Our investigations indicate that the frequency of oscillation of the cylinder and the vortex shedding increases monotonically with the Reynolds number. Also, the length of the primary vortex versus Re shows interesting features and reveals possible critical points in the flow when vortex structure changes. In order to better understand qualitative aspects of the cylinder’s dynamics that go beyond experiments, a simplistic forced nonlinear pendulum toy model was employed and seen to capture qualitative aspects of the cylinder’s dynamics.