Experimental Study on Turbulent Canopy Flow by means of Laser Doppler Anemometry

Since several decades, specific researches are set in order to understand the behaviour of flows over urban or plant canopies, as they are of great importance for many environmental processes. Most of the analyzed flows are turbulent, therefore their characterization is developed with statistical approach and parameters. Statistics involving fluid dynamics of turbulent canopy flows significantly differ from the surface layer one, since the vegetation is seen like a roughness. It is common knowledge that the turbulent motions hidden in these kind of flows have coherent structures. Many experimental surveys showed that the mean velocity profile is inflected, second moments are strongly heterogeneous, skewness factors are high and second-moment budgets are far from local equilibrium. Literature already reports the characterization of flow streams through and above dense and homogenous canopies, where is widely accepted that the flow may be assimilated to a mixing layer.
Earliest studies, considering fully developed turbulent flow both in wind tunnel and open field, brought to light that if the canopy vegetation density decreases flow behaviour gradually switches from a mixing layer model to a boundary layer perturbed by interaction between element wakes.
The goal of this work is to better understand and describe aspects of this particular transition, which occurs when canopy passes from moderate sparse to very sparse densities.
Considering two homogenous spacing (1h and 2h, with h element height) and two different ground arrangement (aligned and staggered), we have a total of four patterns under exam. Wind tunnel measurements of the 3D velocity field are performed with a Laser Doppler Anemometry (LDA) technique, within and above coniferous canopy samples, in order to derive statistical moments and Reynolds stresses. Multiple profiles of measure, obtained from a square region of vegetation, allow to perform spatial average and thus to contemplate heterogeneity of the field. From previous studies, evidences of the manifesting transition emerged by the evolution of the skewness factor, the shear length scale and the mixing length. Present work proposes to highlight the relation between the anisotropy component of the Reynolds stress tensor and this transition, showing that influences of density and ground arrangement may be even found in the Lumley invariant map.
With the support of available literature, statistics show a strong effect of density and a weaker, but present, effect of canopy disposition. Clues of the investigated transition are found in the Lumley diagram, especially for the staggered scheme. Certain characteristic turbulent motions appear to displace along the vertical axis with changes of either density or configuration. Furthermore, dispersive stresses, arising from a planar average and evaluated for the sparser staggered configuration, agree with Poggi et al. (2004) data.