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

above and within either real canopies or wind tunnel canopy samples. We have to
consider that in the “canopy” definition even urban canopies, such as buildings, are
included.
1.1 Literature review
A first important step in order to understand canopy flows behaviour was made by
Raupach and Thom (1981) since they stated that turbulent flows in canopies are far
fromrandom. Theyunderstoodthatthesefluxesareratherdominatedbylargecoherent
structuresofwholeplantscale, resultingfrominstabilityassociatedwiththemeanshear
at the top of the canopy. Later on, Raupach et al. (1996) attributed that instability
to the Kelvin-Helmholtz instability. It has been shown how these organized motions
are responsible for a large fraction of mass, momentum and energy transport across
the canopy-atmosphere interface (e.g. Gao et al., 1989; Novak et al., 2000; Finnigan
and Shaw, 2000). Quadrant Hole analysis and other techniques brought to a widely
recognizedknowledgeofthemechanismsandthephenomenologythathidebehindthose
large coherent structures, at least in the simplest cases of steady flow conditions and
homogenous canopies. That statement has been achieved thanks to many experiments,
performed both in situ and in laboratory, by Cionco (1978), Green et al. (1995),
Raupach et al. (1996), Finnigan and Shaw (2000), Poggi et al. (2004) and several
others.
Another remarkable progress has been introduced by Raupach et al. (1996) since
they developed the concept previously anticipated by Brunet et al. (1994): the mix-
ing layer analogy. A mixing layer is by definition a turbulent shear layer that forms
when two flow streams of different velocities, initially separated by a splitting plate,
merge downstream of the trailing edge of the plate (Fig. 1.1). This analogy is based
on the idea that canopy flows generate coherent eddies and active turbulence in the
upper part of the canopies that make flow more similar to a plane mixing layer than a
boundary layer. The characteristic strong inflection point of the mean velocity profile
is the main cause of instability associated with the coherent structures in the resulting
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