Stable and convective boundary-layer flows in an urban array

Abstract

In this paper non-neutral approaching flows were employed in a meteorological wind tunnel on a regular urban-like array of rectangular buildings. As far as stable stratification is concerned, results on the flow above and inside the canopy show a clear reduction of the Reynolds stresses and an increment of the Monin-Obukhov length up to 80%. The roughness length and displacement height were also affected, with a reduction up to 35% for the former and an increment up to 12% for the latter. A clear reduction of the turbulence within the canopy was observed. In the convective stratification cases, the friction velocity appears increased by both the effect of roughness and unstable stratification. The increased roughness causes a reduction in the surface stratification, reflected in an increase of the Monin-Obukhov length, which is double over the array compared to the approaching flow. The effect on the aerodynamic roughness length and displacement height are specular to the SBL case, an increase up to 50% of the former and a reduction of the same amount for the latter.

Dispersion in an array of buildings in stable and convective atmospheric conditions

“…Above the canopy, however, both the total and turbulent flux appear to be reduced by stratification. In the CBL case the vertical velocity fluctuations are enhanced everywhere (Marucci and Carpentieri, 2019b). On the other hand, the concentration levels are reduced inside and above the canopy until a point (that in the case of Fig.…”

“…The channelling is caused by the presence of the small street canyons and it is even more evident in the first 2H downstream of the source, where the plume axis is almost coincident with the long street centreline. Above the canopy the plume axis still presents a deflection from the free-stream wind direction, despite the fact that the flow field is already completely aligned with the tunnel axis (Marucci and Carpentieri, 2019b). The angles are slightly different, though (8.6 • for NBL and 10.8 • for SBL).…”

“…Surface aerodynamic (friction velocity u * , roughness length z 0 , displacement height d, BL detpth δ) and thermal (scaling temperature θ * = − w θ 0 /u * , thermal roughness length z 0h , thermal displacement height d h ) were estimated as described in details by Marucci and Carpentieri (2019a) and Marucci and Carpentieri (2019b), by fitting the logarithmic profiles and the vertical shear stress profiles. Other values reported in the table are a reference temperature close to the floor (Θ 0 ), the temperature at the boundary-layer height (Θ δ ), a velocity scale valid on the mixed layer of a CBL, defined as (Kaimal and Finnigan, 1994):…”

“…the Monin-Obukhov length (L), the bulk Richardson numbers measured at the boundary-layer depth (Ri δ ) and building height (Ri H ), the Reynolds number (Re δ ) and roughness Reynolds number (Re * ). A full analysis of the boundary layer flow, turbulence and temperature fields over the urban array in the five stratification cases considered here is reported by Marucci and Carpentieri (2019b).…”

“…Initially, new methodologies were developed and optimised to simulate either stable or convective conditions in a meteorological wind tunnel, producing a boundary layer that was thick enough for urban studies (Marucci et al, 2018). The non-neutral boundary layers produced in that first phase were then applied to a single heated/cooled street canyon (Marucci and Carpentieri, 2019a) and to an array of rectangular buildings (Marucci and Carpentieri, 2019b). The latter, in particular, studied the effects of several incoming SBLs and CBLs on the flow over and within the urban array (using a wind direction of 45 • ), finding that the modifications on the flow and turbulence fields caused by even the weak stratification levels tested were significant.…”