Large-Eddy Simulations of Accelerating Boundary Layer Flows Over Rough Surfaces
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Authors
Yuan, Junlin
Date
2011-10-17
Type
thesis
Language
eng
Keyword
Relaminarization , Roughness , Large-eddy simulation , Turbulent boundary layers , Free-stream acceleration
Alternative Title
Abstract
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.
Description
Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2011-10-17 11:19:08.063
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