Abstract

Flow-induced vibrations (FIV) of a flexibly-mounted harbor seal whisker module, allowed to oscillate in the cross-flow direction, placed in tandem arrangement with an upstream circular cylinder is studied, experimentally. The FIV response of the whisker module in terms of amplitudes and frequencies of oscillation are studied for a reduced velocity range of U* = 3.2–24.2, corresponding to a Reynolds number range of Re = 279–2,077. Flow-induced vibration response is studied and characterized for a wide range of separation distances between the upstream cylinder and the whisker module, as well as the angle at which the whisker faces the oncoming flow (angle of attack).

Instantaneous volumetric flow field behind the whisker model was measured using Volumetric Particle Tracking Velocimetry (PTV) data processed by the “Shake The Box” (STB) algorithm that allowed for the time resolved, three dimensional (3D) 3 components (3C) measurement of flow field in the wake of the whisker module. The wake dynamics of the tandem pair of the whisker module and the upstream cylinder were studied at different angles of attack of the whisker module.

Our results show while the whisker module was placed at 0° angle of attack, it did not experience any flow-induced vibration. However, when an upstream cylinder was placed in tandem with the whisker module, the whisker experienced large amplitude oscillations over a wide range of flow velocities. The whisker module picked up the “footprints” left behind the upstream cylinder for center-to center distance up to 25 times the whisker’s diameter. No oscillation was observed when the upstream cylinder was placed at relatively large distance of 50 times the whisker diameter. When the whisker was placed at 90° angle of attack, both for the standalone configuration of the whisker, as well as the tandem arrangement with the upstream cylinder, large amplitude oscillation were observed over a wide, but limited range of reduced velocities. This type of large-amplitude oscillation over a limited range of the flow velocity, while the frequency of oscillation stayed around the natural frequency of the system resembled those classic vortex-induced vibration response observed in the case of an elastically-mounted circular cylinder. Volumetric flow field measurements revealed highly three-dimensional vortex shedding patterns in the wake of the whisker module, which were attributed to the undulatory spanwise structure of the whisker. When the gap size between the upstream cylinder and the downstream whisker module was small, the wake in the gap region was characterized by the presence of two shear layers that were reattached to downstream whisker module. As the gap size increased, well-developed, highly three-dimensional vortical structures were observed in the gap region.

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