During the past few years, large-scale motions (LSM) in turbulent boundary layers over smooth-walls have received renewed attention from the research community. Common features of the LSMs found in wall-bounded flows are that they consist in elongated low- and high-speed regions, the length of which scale with the boundary-layer depth (δ) and can reach several times δ, they populate the log- and outer layer, they are animated by a meandering motion in the horizontal plane (Hutchins & Marusic, JFM, 2007) and interact with near-wall turbulence through an amplitude-modulation mechanism (Mathis et al, JFM, 2009). At the same time, attention has been devoted to the structure of boundary layer flows developing over rough walls, at laboratory scales or in the framework of atmospheric flows over urban or vegetation canopies, demonstrating similarities between flows over smooth and rough wall. In particular, the presence of streaky patterns of low- and high-speed regions, of ejection and sweep motions associated to the hairpin model and the organization of hairpin vortices in packets have been evidenced (Jimenez, Ann Rev Fluid Mech, 2004; Finnigan et al, JFM, 2009; Inagaki & Kanda, BLM, 2010; Takimoto et al, BLM, 2011). The results obtained in flows over rough-walls suggest that, in spite of the strong disturbance of the flow at the wall, LSMs exist and interact with the canopy flow in a similar manner as in smooth wall boundary layers. However, in configurations representative of flows over urban canopies, it is likely that the arrangement, the shape and plan density of the roughness elements have a strong influence on the flow dynamics in the lower part of the boundary layer.

Building upon these recent results, the aim of the present work is to analyze the statistical and spectral characteristics of turbulent boundary layer developing over cubical roughness arrays of different plan densities, at high Reynolds number, which has never been fully properly documented. This study, conducted in an atmospheric wind tunnel, is based on single hot-wire measurements performed along a vertical profile across the boundary layer, in a flow configuration representative of the atmospheric boundary layer (ABL) developing over an urban canopy. Three different rough walls of plan area density (i.e the ratio between the area of the surface occupied by the roughness elements and that of the total surface) of 6,25%, 25% and 44% are studied. Each roughness configuration is composed of a staggered array of cubes of height of h = 50 mm distributed on the wind tunnel floor over a total length of 22 m and a width of 2 m. The experiments have been performed with two free-stream velocities Ue = 5.8 and 9.1 m/s corresponding to Reynolds numbers h+ = h.u*/ν ~ 1300, 2000 and Reτ = δ.u*/ν ~ 27000, 42000, respectively (where u* is the friction velocity). One-point statistics up to the fourth order are computed using hot-wire data acquired at 10000 kHz over a period corresponding to about 27000 turnover times, δ/Ue, to ensure proper statistical convergence of both large- and small-scale contents. It should me emphasized that the large-scales convergence criterion requires one hour and twenty minutes of acquisition per measured point. Besides the computation of mean velocity, standard deviation, skewness and kurtosis, this unprecedented database is also used to investigate the evolution of the spectral density content of the streamwise velocity component as a function of the wall-distance, the roughness element distribution and the Reynolds number. The ultimate goal of this ongoing study is to identify the mechanisms of interaction between the ABL and the canopy flow in order to contribute to the development of wall and/or canopy model.