The effect of simple spiral fin around a bare tube on Karman vortex shedding was experimentally investigated. We measured the distributions of the mean velocity, the intensity of velocity fluctuation, the spectrum of velocity fluctuation and the coherence distribution of Karman vortex in the spanwise direction. And the flow around a fin tube was visualized by using smoke wire technique. As a result it was made clear that the periodic velocity fluctuation caused by Karman vortex was remarkably observed in the wake of the fin tube although the fin was mounted around a bare tube with a large pitch along the tube axis. The vortices in the near-wake of the fin tube make cells structure with the scale of the pitch length of the fins. The rotation axis of each vortex was inclined against the tube axis. The large scale vortex was formed by the coalescence of the vortex cells. Therefore, the spanwise scale of the vortex in the wake of the fin tube was larger than one pitch of the fins.

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