Micrometeorological Measurements in a Street Canyon during the Joint ATREUS-PICADA Experiment

Abstract

In order to investigate the microclimatic conditions in a street canyon, a physical model was used to conduct the Joint ATREUS-PICADA Experiment (JAPEX) in situ experimental campaign. Four lines of buildings simulated by steel containers were installed to form three parallel street canyons at 1:5 scale, with width/height aspect ratio approximately 0.40. The reference wind and atmospheric conditions were measured, as well as the flow velocity and direction in the street. Preliminary results concern street canyon ventilation and thermal effects on in-canyon airflow, and show that vortical motions appear for reference wind directions perpendicular to the street axis. The presence of adjacent rows of buildings did not appear to significantly influence the flow character within the canyon for the case of a low aspect ratio corresponding to a skimming flow regime. The flow structure was not significantly affected by the thermal effects although some slight interference occurred in the lower part of the canyon. An analysis of horizontal temperature gradients indicated that a thin boundary layer develops near the heated facade. These facts imply that the thermal effects are considerable only very close to the wall.

Large-Eddy Simulation of Dispersion from Surface Sources in Arrays of Obstacles

“…Due to a lack of flow measurements, the detailed dynamics within these thin layers are not understood but the effect of the temperature differences on the larger-scale flow dynamics within the street canyon was nonetheless found to be negligible. Similar conclusions were drawn by Idczak et al (2007) from their joint ATREUS-PICADA experiment (ATREUS: Advanced Tools for Rational Energy Use towards Sustainability; PICADA: Photo-catalytic Innovative Coverings Applications for Depollution, Demilecamps and André 2005). The wind tunnel study by Kovar-Panskus et al (2002) on a square section cavity likewise demonstrated a very weak secondary flow near the ground at very low Froude number (based on the freestream velocity, canyon height and the difference between freestream reference temperature and the heated wall temperature) but found no evidence of large wall heating effects on the canyon flow field.…”

A Method for Monitoring the Heat Flux from an Urban District with a Single Infrared Remote Sensor

“…The boundary-layer thickness δ depends on the surface roughness and on the fetch. The value of δ is not well known for urban canopy surfaces, especially for building walls: our recent velocity measurements close to a flat vertical wall show that the dynamic boundary layer along a facade does not present the typical features of a wind-tunnel boundary layer over a horizontal rough plate (Maro et al 2013) while the temperature measurements of Idczak et al (2007) showed a very thin thermal boundary layer along a vertical facade, about 50 mm in thickness. On the other hand, it is probable that thick boundary layers may develop rapidly along real facades with obstacles such as windows, blinds and balconies, but experimental evidence is lacking.…”

“…Rather than a theoretically-based modelling this h c computation must be seen as a pragmatic method empirically based on the results of the urban campaigns BUBBLE (Christen 2005) and JAPEX (Idczak et al 2007). …”

“…Christen (2005) summarized the previous findings by splitting the mean wind profile into three regions: in the canyon layer it is similar to the classical logarithmic profile above the ground, between the mean roof level z b and z = 1.4z b it presents strong vertical gradients separated by an inflexion point, and above z = 1.4z b the profile presents the classical quasi-logarithmic shape of an atmospheric surface layer over a very rough surface. Finally, the measurements of Idczak et al (2007) provide evidence of the above-mentioned flow structures in a 1:5 scale street canyon (H/W ≈ 2.5) but they also showed that the influence of thermal transfer on the flow dynamics is limited to a very short distance from the walls (<0.4 m), and that for U ref > 1.5 m s −1 the mean wind speed varies little with height within the canyon, with values close to U ref when the flow is parallel to the street axis, and as low as 0.3U ref when it is perpendicular (U ref was measured at a height z ref = 1.9H = 10 m, outside of the building array).…”

Computational Fluid Dynamics Modelling of the Diurnal Variation of Flow in a Street Canyon

“…Recently, more attention has been paid to the impact of thermal forcing on street-canyon flow. Field measurements have shown the temporal variation of street-canyon flow with a diurnal solar cycle (Nakamura and Oke 1988;Louka et al 2002;Eliasson et al 2006;Offerle et al 2007;Idczak et al 2007). Laboratory experiments (Uehara et al 2000;Kovar-Panskus et al 2002) and two-dimensional numerical experiments (Sini et al 1996;Baik 1999, 2001;Xie et al 2005;Cheng et al 2009) were performed to examine the effects of solar heating on street-canyon flow.…”