| Summary: | 碩士 === 國立臺灣大學 === 應用力學研究所 === 101 === In this study, the force element theory proposed by Prof. Chang C. C. (1992) is used to analyze three-dimensional unsteady aerodynamics for hovering flapping flight of fruit-fly at low-Reynolds-number flows. The theory enables us to quantify the contributions to the forces exerted on the wing in terms of fluid elements with non-zero vorticity, and extract potential forces such as added mass and inertial forces from the total forces. The variations of the lift force and its constituent components for three different type motions, including symmetric, advanced and delayed rotations, are carefully examined. In conjunction with the previous results of Hsieh, Chang and Chu (J. Fluid Mech, 2009, vol. 623, pp. 121–148), we further compare each force contribution with the results under simplified two-dimensional assumptions. It is shown that the lift is almost supported by vorticity in the flow field, and the added-mass forces have positive contribution for these three-type motions.To understand the lift force generation relative to three-dimensional vortex structures, such as leading-edge vortex (LEV), trailing-edge vortex (TEV), tip vortex (TV) and Root vortex (RV) during different hover motion stages, we divide whole flow domain into annularity column regions with same center of circle, which is the rotation center of the wing. Except a well-known high lift mechanism generated, the delayed-stall vortex, during the midstroke for three different types of rotation, the insect wing will take advantage of wake vortices to gain extra lift force, termed as “riding on lift elements” at two different time instant, as performing turning stage.
Besides, the line of force analysis of the pressure force analysis (PFA) and the vorticity force analysis (VFA) is pursued by dividing flow domain into some regions along spanwise direction of the wing. From comparison of differences between PAF and VFA, we could isolate two-dimension characteristics from three dimensional flows, further survey feasibility of two-dimensional analyses on flapping motions in unsteady flow.
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