LES Study on the Structure of Coherent Eddies Inducing Predominant Perturbations in Velocities in the Roughness Sublayer over Plant Canopies

Abstract

Large-eddy simulations (LESs) were performed for neutrally-stratified turbulent flows within and above a homogeneous plant canopy. 100 realizations of a three-dimensional turbulence field obtained from each of four LES runs, which differ in the driving force of the flow and the size of computational domain, were used in the present study. A conditional sampling technique was used to construct ensemble-averaged images of coherent eddies that induce predominant perturbations in the streamwise and vertical velocities near the canopy top. To reduce subjectivity, wavelet analysis was adopted for triggering the conditional sampling. Synthesis of the present study and numerous previous studies indicated that, in the canopy turbulence, the spatial scale of eddies that induce predominant perturbations in the streamwise velocity is generally three times larger than that of eddies that induce predominant perturbations in the vertical velocity, irrespective of whether concerning field observations, wind-tunnel experiments or numerical simulations. Therefore, scales of both eddies are mostly determined by a mechanism inherent in the roughness sublayer. An analysis of the ensemble-averaged results and each realization revealed several findings: (1) the smaller eddies that cause predominant perturbations in the vertical velocity are vortices that accord with the so-called mixing-layer (ML) analogy, which is widely accepted as a mechanism of coherent eddies developing near the canopy top; however, (2) the larger eddies inducing predominant perturbations in the streamwise velocity are not vortices and are much larger than expected from the ML analogy; (3) these eddies are streamwise-elongated motions of high-speed downdraft and low-speed updraft, having characteristic features such that the high-speed downdraft penetrates into the canopy and cross-streamwise spreads inside the canopy thus inducing low-speed updraft to the sides of the downdraft and that the low-speed updraft produces a lifted (higher than the canopy top) shear zone beneath an overriding high-speed motion thereby enhancing the shear instability in that area; (4) the high-speed and low-speed motions aligning side-by-side bear a close resemblance to streaky patterns observed in a near-surface region of planetary boundary layers, although the spatial scales are quite different.