Large-eddy simulations are carried out to study the combined
effects of roughness and favourable pressure gradient in boundary layer flows,
where the high acceleration (on smooth walls) may cause flow reversion to the
quasi-laminar state. A sand-grain roughness model is used, with the
no-slip boundary condition modeled by an immersed boundary method.
The properties and accuracies of the scheme are studied,
the roughness model is validated, and the spatial-resolution requirements are
determined.
The roughness model is applied to boundary layers subject to mild or strong
acceleration, with simulations carried out underlining the effects of
three parameters: the acceleration parameter, the roughness height,
and the inlet Reynolds number.
The roughness effects are limited to the roughness sublayer; the outer layer is
affected indirectly only, through the changes that roughness causes in the
relaminarization and retransition processes.
The roughness significantly affects the inner-layer
quantities like the friction velocity and the friction coefficient, while
the local Reynolds number, the outer-layer mean velocity, as well as the Reynolds
stresses beyond the roughness sublayer, are not sensitive to the roughness.
The acceleration decreases the Reynolds stresses in the
overlap region and promotes a laminar-like velocity profile.
The acceleration leads to stabilization of near-wall structures and causes
one-dimensional turbulence. The roughness generates small-scale structures
at the bottom wall, which disturb the larger structures originally
stabilized by the pressure gradient, leading to a decrease in the Reynolds-stress
anisotropy. Roughness increases the Reynolds stresses in the roughness sublayer
and tends to restore the fully turbulence flow early. The inlet Reynolds number
affects the flow stability by determining the viscous length scale
compared to the roughness length scales, and by determining how far
the roughness effect extents into the boundary layer. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2011-10-17 11:19:08.063
| Identifer | oai:union.ndltd.org:LACETR/oai:collectionscanada.gc.ca:OKQ.1974/6846 |
| Date | 17 October 2011 |
| Creators | YUAN, JUNLIN |
| Contributors | Queen's University (Kingston, Ont.). Theses (Queen's University (Kingston, Ont.)) |
| Source Sets | Library and Archives Canada ETDs Repository / Centre d'archives des thèses électroniques de Bibliothèque et Archives Canada |
| Language | English, English |
| Detected Language | English |
| Type | Thesis |
| Rights | This publication is made available by the authority of the copyright owner solely for the purpose of private study and research and may not be copied or reproduced except as permitted by the copyright laws without written authority from the copyright owner. |
| Relation | Canadian theses |
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