Turbulent flows over dense filament canopies

Abstract

Turbulent flows over dense canopies of rigid filaments of small size are investigated for different element heights and spacings using DNS. The flow can be decomposed into the element-coherent, dispersive flow, the Kelvin--Helmholtz-like rollers typically reported over dense canopies, and the background, incoherent turbulence. The canopies studied have spacings $s^{+}=3$--$50$, which essentially preclude the background turbulence from penetrating within. The dispersive velocity fluctuations are also mainly determined by the spacing, and are small deep within the canopy, where the footprint of the Kelvin--Helmholtz-like rollers dominates. Their typical streamwise wavelength is determined by the mixing length, which is essentially the sum of its height above and below the canopy tips. For the present dense canopies, the former remains roughly the same in wall-units, and the latter, which scales with the drag length, depends linearly on the spacing. This is the result of the drag being essentially viscous and governed by the planar layout of the canopy. In shallow canopies, the proximity of the canopy floor inhibits the formation of Kelvin--Helmholtz-like rollers, with essentially no signature for height-to-spacing ratios $h/s\approx 1$, and no further inhibition beyond $h/s\approx 6$. Very small spacings also inhibit the rollers, due to their obstruction by the canopy elements. The obstruction decreases with increasing spacing and the signature of the instability intensifies, even if for canopies sparser than those studied here the instability eventually breaks down. Simple models based on linear stability can capture some of the above effects.

arXiv comment37 pages, 24 figures. Changes made to address reviewer comments. Appendix with details of numerical method added

References in corpus (5)

• Turbulent drag reduction by anisotropic permeable substrates $-$ analysis and direct numerical simulations
• Direct numerical simulation of high aspect ratio spanwise-aligned bars
• Modulation of near-wall turbulence in the transitionally rough regime
• Monami as an oscillatory hydrodynamic instability in a submerged sea grass bed
• Scaling and dynamics of turbulence over sparse canopies

Cited by in corpus (13)

• Dynamics and fluid-structure interaction in turbulent flows within and above flexible canopies
• Urban boundary layers over dense and tall canopies
• Forced and natural dynamics of a clamped flexible fiber in wall turbulence
• Examination of outer-layer similarity in wall turbulence over obstructed surfaces
• Turbulent flows over porous lattices: alteration of near-wall turbulence and pore-flow amplitude modulation
• Towards decoupling the effects of permeability and roughness on turbulent boundary layers
• The wind-shade roughness model for turbulent wall-bounded flows
• Turbulent flows over porous and rough substrates
• Effects of porous substrates on the structure of turbulent boundary layers
• Metamaterials and Fluid Flows
• Resolving turbulence and drag over textured surfaces using texture-less simulations: the case of slip/no-slip textures
• Edge-effects in the turbulent flow over flexible aquatic vegetation
• Higher-order homogenised riblet boundary conditions