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Session: Control of separated/unsteady flows II
Session starts: Thursday 06 November, 09:40
Presentation starts: 09:40
Room: Commission roon 2

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Sei Haishi (Kyushu Institute of Technology)
Masaki Fuchiwaki (Kyushu Institute of Technology)

Abstract:
When a moving body generates a fluid force, a vortex ring is generated, and when the fluid force generated by the moving body changes, the structure and behavior of the vortex ring also change [1]. In other words, the vortex ring generated by the moving body is a history of generating a fluid force, and it is possible to estimate the fluid force generated by the moving body from the structure and behavior of the vortex ring [2-4]. Therefore, if we can capture the overall picture of the vortex ring that changes with the motion of the moving body in detail, it will be possible to know the change in the acquired fluid force in more detail. However, since the flow field generated by the moving body changes in a complex manner, it is not easy to capture the change in the entire vortex ring. In addition, an elastic moving body not only deforms itself due to its motion, but also changes the flow field, and furthermore, the pressure acting on the elastic body changes from the flow field, and the elastic body is further deformed, which is known as a fluid-structure interaction phenomenon (Fluid-Structure Interaction). Elastic moving wings are also one of the representative elastic moving bodies, and although their flow field is a very complex phenomenon, it has been reported that they exhibit better performance than rigid moving wings [5-6]. In recent years, elastic membrane wings, which are very thin, have attracted attention. It is known that elastic membrane wings can generate greater fluid forces in the low Reynolds number range than rigid flat plates. Furthermore, elastic moving bodies are not only found in engineering products, but also in parts of the bodies of living organisms in nature. Fish fins, bird wings, and butterfly wings are typical examples, and they generate the fluid forces necessary for swimming and flying. A typical example is the wing of a butterfly, which flies by flapping its wings [7]. The flapping motion of a butterfly's wings also generates the fluid forces necessary for flight, and a vortex ring is generated as a result of this history. It is also very difficult to directly measure this unsteady fluid force, but if the dynamic behavior of the vortex ring can be captured, it is possible to roughly estimate the unsteady fluid force. In this study, we quantitatively visualize the dynamic behaviors of a pair of vortex rings generated by the flapping butterfly wings, and clarify the dynamic lift generated by the butterfly. In particular, we estimate the dynamic lift generated during one cycle of the flapping motion of the wings from the dynamic behavior of the vortex rings visualized by two-dimensional PIV measurement, and further clarify it from dynamic lift measurement synchronized with the two-dimensional PIV measurement to verify the accuracy.