A joint velocity-intermittency analysis reveals similarity in the vertical structure of atmospheric and hydrospheric canopy turbulence

Abstract

Turbulent flow through and over vegetation continues to draw significant research attention given its relevance to a plethora of applications in earth and environmental science. Canopy flows are characterized by three-dimensional coherent vortical motions not directly accessible from single-point measurements, which pose a challenge to formalizing links between vegetation structure and turbulent motion. A joint velocity-intermittency technique is applied to velocity data collected within and above aquatic vegetation in a hydraulic flume and above a forested canopy. The approach reveals behavior that provides greater insight into canopy flow dynamics than may be inferred from the vertical profiles of mean velocity, turbulence intensity and Reynolds stresses, which are the quantities usually studied. There is a remarkable similarity in the structure of such flows between the forest canopy and the flume study despite large differences in morphology and stem rigidity. In particular, these results determine an outer flow type arising above 1.5 canopy heights, while turbulent in-rushing events are most significant at the zero-plane displacement. The approach also implies ways in which improved models for canopy turbulence may be developed.

On small-scale and large-scale intermittency of Lagrangian statistics in canopy flow

“…This suggests that the changes in statistics of A T are due to increased/decreased levels of the canopy drag that fluctuated due to large-scale flow structures in the shear layer and the boundary layer above the canopy. This is in qualitative agreement with Keylock et al (2020) who recently associated streamwise velocity and intermittency in a canopy flow.…”

“…The non-local character of the turbulence in canopy flows leads to large-scale intermittency inside the canopy, which is expressed by a velocity skewness, sparse extreme events of momentum and scalar fluxes or time-varying Hölder exponents (e.g. Finnigan 1979;Gao, Shaw & Paw U 1989;Louka, Belcher & Harrison 2000;Keylock et al 2020). Therefore, canopy flows provide a fruitful ground for observing the two phenomena in conjunction, which is the aim of this work.…”

“…Other than that, there is a lack of studies that focus on large-scale intermittency in the Lagrangian framework. In particular, there are no empirical investigations of intermittency in Lagrangian statistics in canopy flows despite its importance to Lagrangian stochastic models with applications for dispersion and mixing in the environment (Wilson & Sawford 1996;Reynolds 1998;Duman et al 2016;Keylock et al 2020;Shnapp et al 2020;Viggiano et al 2020).…”

“…Finnigan 1979; Gao, Shaw & Paw U 1989; Louka, Belcher & Harrison 2000; Keylock et al. 2020). Therefore, canopy flows provide a fruitful ground for observing the two phenomena in conjunction, which is the aim of this work.…”

An investigation of the impact of turbulence intermittency on the rotor loads of a small wind turbine

“…In a comprehensive review, they note that the definition of intermittency varies from author to author. They also refer to many authors [16][17][18][19][20] who suggest that intermittency is caused by the presence of strong coherent vortices with diameters of the order of ten times the Kolmogorov length scale 1 . The general consensus, however, is that intermittent behaviour in flows is observed in the inertial-subrange and is characterised by "bursts" or sharp heterogeneous fluctuations in a few dense patches amongst a wide range of low-density patches.…”