| Summary: | 碩士 === 國立臺灣大學 === 土木工程學研究所 === 97 === The purpose of this thesis is to investigate the vortex structure brought by a solid sphere dropped in a liquid and to observe the coupled motions between the solid sphere and a liquid. When a solid sphere moved downward with some specific speed, the wake behind the sphere produced vortices. Under some different conditions, the trajectories of vortices are similar but a little different, especially in high Reynolds numbers. Vortex is generated because of the viscosity from the boundary layer, so we also want to observe the interactions between the boundary and the liquid.
There are two parts in the experiment: in the first part we controlled the displacement of the sphere which dropped in different constant speeds, and then observed the flow field and vortices surrounding the sphere. In the second part we let the sphere drop freely. The movement of the sphere was interfered because the turbulence was more evident in high Reynolds numbers. In that situation, the trajectory of the sphere was not a straight line.
Due to the experiment was a three dimensional experiment, the problem of refractive index was solved by using specific solid and liquid, so that we could observe and analyze the two dimensional phenomena in a three dimensional space.
We used image processing methods to capture the trajectories of these bright spots seeding in a liquid. In the long exposure image, the trace of particles could stand for the flow field almost, and then use particle tracking velocimetry (PTV) to calculate the velocity field and vorticity field. So that we can understand the more detail information about the interaction between the solid sphere and the liquid. We needed numerical simulation to compare with the experiments to complete the investigation.
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