Extraction of vortical structures in boundary layer next to wavy surface
| Autor: | Po-Chen Chen, 陳柏丞 |
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| Rok vydání: | 2017 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 106 In this thesis we study the coherent vortical structures within the boundary-layer flow next to a wavy surface. Both aqueous flow beneath a free-propagating surface wave and turbulent air flow above a prescribed propagating wavy boundary are considered. Four vortex identification schemes based on the characteristics of velocity-strain rate are adopted to extract the vortical structures. These schemes are also compared with traditional methods based on vorticity vector. Conditional averaging technique of the flow field is also applied to extract the horseshoe vortices. In both aqueous and air flow, four vortex identification schemes derive similar results. For the aqueous flow underneath a free-propagating surface wave, counter-rotating vortex pairs that elongate along streamwise direction are observed. By comparing vortex identification schemes with methods based on vorticity vector, it is found that observing vortical structures is a more intuitive way to study the flow field. For the air flow above a prescribed propagating wavy boundary, horseshoe vortices and quasi-streamwise vortices are found, the regeneration of forward horseshoe vortices are observed, and the larger scale vortical motions in the flow field above wavy surface are discovered. In addition, the conditional averaging results shows that out of the two sampling criterions, the quadrant analysis sampling technique is more feasible than the VISA sampling technique. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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