Investigation of flow mechanism of a robotic fish swimming by using flow visualization synchronized with hydrodynamic force measurement |
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Authors: | Guang-Kun Tan Gong-Xin Shen Shuo-Qiao Huang Wen-Han Su Yu Ke |
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Institution: | (1) Institute of Fluid Mechanics, Beijing University of Aeronautics and Astronautics (BUAA), Beijing, 100083, China |
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Abstract: | When swimming in water by flapping its tail, a fish can overcome the drag from uniform flow and propel its body. The involved
flow mechanism concerns 3-D and unsteady effects. This paper presents the investigation of the flow mechanism on the basis
of a 3-D robotic fish model which has the typical geometry of body and tail with periodic flapping 2-freedom kinematical motion
testing in the case of St = 0.78, Re = 6,600 and phase delay mode (φ = −75°), in which may have a greater or maximum propulsion (without consideration of the optimal efficiency). Using a special
technique of dye visualization which can clearly show vortex sheet and vortices in detail and using the inner 3-component
force balance and cable supporting system with the phase-lock technique, the 3-D flow structure visualized in the wake of
fish and the hydrodynamic force measurement were synchronized and obtained. Under the mentioned flapping parameters, we found
the key flow structure and its evolution, a pair of complex 3-D chain-shape vortex (S–H vortex-rings, S1–H1 and S2–H2, and their legs L1 and L2) flow structures, which attach the leading edge and the trailing edge, then shed, move downstream and outwards and distribute
two anti-symmetric staggering arrays along with the wake of the fish model in different phase stages during the flapping period.
It is different with in the case of St = 0.25–0.35. Its typical flow structure and evolution are described and the results prove that they are different from the
viewpoints based on the investigation of 2-D cases. For precision of the dynamic force measurement, in this paper it was provided
with the method and techniques by subtracting the inertial forces and the forces induced by buoyancy and gravity effect in
water, etc. from original data measured. The evolution of the synchronized measuring forces directly matching with the flow
structure was also described in this paper. |
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