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

TL;DR

This paper investigates small-scale and large-scale intermittency in Lagrangian velocity statistics within canopy flows, revealing similarities to homogeneous turbulence and identifying distinct mechanisms for different intermittency scales.

Contribution

It provides the first empirical evidence of dual-scale intermittency in canopy flow Lagrangian statistics, extending the concept of universality to inhomogeneous, anisotropic flows.

Findings

Small-scale intermittency aligns with homogeneous isotropic turbulence models.

Flow attenuation by canopy drag affects energy increments without altering structure function scaling.

First empirical evidence of dual-scale intermittency in canopy flows.

Abstract

The interaction of fluids with surface-mounted obstacles in canopy flows leads to strong turbulence that dominates dispersion and mixing in the neutrally stable atmospheric surface layer. This work focuses on intermittency in the Lagrangian velocity statistics in a canopy flow, which is observed in two distinct forms. The first, small scale intermittency, is expressed by non-Gaussian and not self-similar statistics of the velocity increments. The analysis shows an agreement in comparison with previous results from homogeneous isotropic turbulence (HIT) using the multifractal model, extended self-similarity, and acceleration autocorrelations. These observations suggest that the picture of small-scale Lagrangian intermittency in canopy flows is similar to that in HIT, and therefore, they extend the idea of universal Lagrangian intermittency to certain inhomogeneous and anisotropic flows. Second, it is observed that the RMS of energy increments along Lagrangian trajectories depend on the direction of the trajectories' time-averaged turbulent velocity. Subsequent analysis suggests that the flow is attenuated by the canopy drag while leaving the structure function's scaling unchanged. This observation implies the existence of large-scale intermittency in Lagrangian statistics. Thus, this work presents a first empirical evidence of intermittent Lagrangian velocity statistics in a canopy flow that exists in two distinct senses and occurs due to different mechanisms.