Turbulent flows over dense filament canopies

© © The Author(s), 2020. Published by Cambridge University Press. Turbulent flows over dense canopies consisting of rigid filaments of small size are investigated using direct numerical simulations. 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 , the background turbulence is essentially precluded from penetrating within the canopy. As the elements are 'tall', with height-to-spacing ratios , the roughness sublayer of the canopy is determined by their spacing, extending to above the canopy tips. The dispersive velocity fluctuations are observed to also depend mainly on the spacing, and are small deep within the canopy, where the footprint of the Kelvin-Helmholtz-like instability dominates. The instability is governed by the canopy drag, which sets the shape of the mean velocity profile, and thus the shear length near the canopy tips. For the tall canopies considered here, this drag is governed by the element spacing and width, that is, the planar layout of the canopy. The mixing length, which determines the length scale of the instability, is essentially the sum of its height above and below the canopy tips. The former remains roughly the same in wall units and the latter is linear with for all the canopies considered. For very small element spacings, , the elements obstruct the fluctuations and the instability is inhibited. Within the range of of the present canopies, the obstruction decreases with increasing spacing and the signature of the Kelvin-Helmholtz-like rollers intensifies. For sparser canopies, however, the intensification of the instabilities can be expected to cease as the assumption of a spatially homogeneous mean flow would break down. For the present, dense configurations, the canopy depth also has an influence on the development of the instability. For shallow canopies, , the lack of depth blocks the Kelvin-Helmholtz-like rollers. For deep canopies, , the rollers do not perceive the bottom wall and the effect of the canopy height on the flow saturates. Some of the effects of the canopy parameters on the instability can be captured by linear analysis.