Drag and Flow Field around Elasto-Flexible Cylinder in Uniform Flow.

Abstract

The purpose of this experiment is to clarify the mechanism of drag reduction in the natural swimming of fish, whales and dolphins. It is well known that the change in their shape and the local deformation of their skin contribute to drag reduction, especially in the case of dolphins. Therefore, in order to understand the interaction between a deformable surface and the local flow pattern, RFFRC (rigid front flexible rear cylinder) models of a simplified marine animal were studied in a water tunnel. The laser light sheet method was used to visualize the flow field around the models. The vortex pair shedding period from the models, the deformation of the models' cross-sectional form and models' drag were measured simultaneously. The following results were obtained. For some types of RFFRC which could be deformed easily in response to a change in the local flow pattern with vortex shedding, the increase in flow velocity in the wake was observed. In the case the St number was about 20% higher than that of a rigid cylinder. Although, the drag that some types of RFFRC had was higher than that of the rigid one. However, the other types of RFFRC which could not quickly deform show no difference in St number, whereas the mean drag of this type was about 20% lower and the time variation of the drag became smaller than that of the rigid cylinder.