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

Ron Shnapp,Eyal Fattal,Alex Liberzon,Erez Shapira,Yardena Bohbot-Raviv,David Peri

doi:10.1038/s41598-019-43555-2

Ron Shnapp, Eyal Fattal + Show 4 more

Open Access

https://doi.org/10.1038/s41598-019-43555-2

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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.

Highlights

Understanding turbulent transport and mixing in urban and plant canopy flows is important for modeling urban air pollution[1] and for correctly estimating mass and momentum exchange rates (e.g., CO2, H2O) between the Earth surface and the atmosphere[2]
We used the 3D Particle Tracking Velocimetry (3D-PTV) extension to conduct flow measurements in a wind-tunnel based canopy flow model, the result of which form a unique dataset of Lagrangian trajectories both inside and above the modeled canopy layer 0.5H ≤ z ≤ 1.75H
We report on an extension to the 3D-PTV method which is based on an extensive real-time image analysis on dedicated hardware and software

Summary

Introduction

Understanding turbulent transport and mixing in urban and plant canopy flows is important for modeling urban air pollution[1] and for correctly estimating mass and momentum exchange rates (e.g., CO2, H2O) between the Earth surface and the atmosphere[2]. We present a novel solution to overcome the 3D-PTV limitations of high-speed imaging, long recording times and unwanted bottlenecks by developing a real-time image analysis on hardware for the open source software, OpenPTV36 This new system extends 3D-PTV by allowing to perform complex image analysis at difficult experimental conditions. It extracts the centroid positions and sizes of tracer particles from images in real-time, in cases with uneven image backgrounds, reflections, or solid surfaces, just a few to mention. This provides a solution to the central problem of software-based methods that require data transfer (state-of-the-art systems generate 9.6 Gb/s) to the computer.

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