Turbulent flows over canopies
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In this thesis, turbulent flows over canopies in the sparse and dense regimes are examined using direct numerical simulation. The term `canopy' is used to refer to tall roughness elements in the flow. Sparse canopies typically have large element spacings and allow turbulent eddies to penetrate between the elements, whereas dense canopies have small spacings and preclude the penetration of turbulent eddies within them.
In sparse canopies, we consider layouts with rigid elements and spacings larger than the characteristic scales of near-wall turbulence,
The dense canopies studied consist of rigid, prismatic filaments with small spacings. The effect of the height and spacing of the canopy elements on the flow is studied. The flow is composed of an element-coherent, dispersive flow and an incoherent flow, which includes contributions from the background turbulence and from the flow arising from the Kelvin--Helmholtz-like, mixing-layer instability typically reported over dense canopies. For the present canopies, with spacings
Two approaches based on linear stability analysis are proposed to capture the Kelvin--Helmholtz-like instability over dense canopies. The first approach models the canopy as an anisotropic permeable substrate whose wall-normal permeability,