Extended 3D-PTV for direct measurements of Lagrangian statistics of canopy turbulence in a wind tunnel

Abstract

Direct estimation of Lagrangian turbulence statistics is essential for the proper modeling of dispersion and transport in highly obstructed canopy flows. However, Lagrangian flow measurements demand very high rates of data acquisition, resulting in bottlenecks that prevented the estimation of Lagrangian statistics in canopy flows hitherto. We report on a new extension to the 3D Particle Tracking Velocimetry (3D-PTV) method, featuring real-time particle segmentation that outputs centroids and sizes of tracer particles and performed on dedicated hardware during high-speed digital video acquisition from multiple cameras. The proposed extension results in four orders of magnitude reduction in data transfer rate that enables to perform substantially longer experimental runs, facilitating measurements of convergent statistics. The extended method is demonstrated through an experimental wind tunnel investigation of the Lagrangian statistics in a heterogeneous canopy flow. We observe that acceleration statistics are affected by the mean shear at the top of the canopy layer and that Lagrangian particle dispersion at small scales is dominated by turbulence in the wake of the roughness elements. This approach enables to overcome major shortcomings from Eulerian-based measurements which rely on assumptions such as the Taylor’s frozen turbulence hypothesis, which is known to fail in highly turbulent flows.

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

“…The full experimental details are given in Shnapp et al. (2019), and Lagrangian statistics were analysed in Shnapp et al. (2020).…”

“…We recorded the trajectories using a real-time image analysis extension of the 3D-PTV method described in Shnapp et al. (2019). The PTV algorithms and the analysis were applied using the OpenPTV consortium (2014) open-source software and the Flowtracks package by Meller & Liberzon (2016).…”

“…This work presents an analysis of Lagrangian statistics in a canopy flow using empirical results from a recent wind tunnel experiment (Shnapp et al. 2019). The existence of small-scale intermittency is demonstrated in § 3.1, and the results are compared with previous studies from homogeneous isotropic turbulence (HIT) flows.…”

Lagrangian turbulence in the woods

“…The novelty of the analysis of Shnapp (2021) is to show that this aforementioned Lagrangian phenomenology, initially designed for isotropic turbulent flows, gives a fair account of the fluctuations of velocity along the trajectories obtained in his wind tunnel (Shnapp et al 2019). Despite the presence of a strong mean flow, and a model canopy laid out on the bottom of the tunnel, Shnapp (2021) evidences that the subset of particles flying just above this rough surface exhibits a statistical behaviour in quantitative agreement with the theoretical predictions for the velocity increment p.d.f.s and moments obtained within this formalism, with furthermore the same level of intermittency.…”

Stereoscopic PIV measurements using low-cost action cameras

“…Since its origin in the 1980s, PIV became a well-established velocity measurement technique that significantly augmented the study of fluid flows (Adrian 2005). Further improvements of the basic, two-dimensional technique allows to measure all three velocity components in a planar measurement domain which is known as stereoscopic PIV (Arroyo and Greated 1991) or even in a three-dimensional domain with techniques like tomographic PIV (Elsinga et al 2006) and 3D-particle tracking velocimetry (3D-PTV) (Maas et al 1993;Nishino et al 1989;Shnapp et al 2019;Cierpka et al 2013). A recent 1 3…”