Turbulent Transfer Between Street Canyons and the Overlying Atmospheric Boundary Layer

Abstract

The turbulent exchange of momentum between a two-dimensional cavity and the overlying boundary layer has been studied experimentally, using hot-wire anemometry and particle image velocimetry (PIV). Conditions within the boundary layer were varied by changing the width of the canyons upstream of the test canyon, whilst maintaining the square geometry of the test canyon. The results show that turbulent transfer is due to the coupling between the instabilities generated in the shear layer above the canyons and the turbulent structures in the oncoming boundary layer. As a result, there is no single, unique velocity scale that correctly characterizes all the processes involved in the turbulent exchange of momentum across the boundary layer. Similarly, there is no single velocity scale that can characterize the different properties of the turbulent flow within the canyon, which depends strongly on the way in which turbulence from the outer flow is entrained into the cavity and carried round by the mean flow. The results from this study will be useful in developing simple parametrizations for momentum exchange in the urban canopy, in situations where the street geometry consists principally of relatively long, uniform streets arranged in gridlike patterns; they are unlikely to be applicable to sparse geometries composed of isolated three-dimensional obstacles.

Effect of upstream flow regime on street canyon flow mean turbulence statistics

“…Each of the following cases used roughness elements and canyons of equal height. The turbulent eddies defined by integral length scales within the skimming flow regime were found to be limited or suppressed within and above the roughness by the large λp [9,31]. This is further confirmed by Rafailidis [27] who noted that λp within the skimming flow regime has only a mild effect on the turbulence statistics at z/h = 1 and above.…”

“…Salizzoni et al [31] also noted that in the wake interference regime the turbulent structures and turbulence intensity are larger than in the skimming flow regime above the roughness. The shear stress was found to vary with λp within and above the roughness up to a height of approximately 5h [31]. When investigating the shear layer size no significant difference was found between the skimming and wake interference regimes.…”

“…On the contrary, the turbulence is increased at z/h = 1 by the flow impinging on the windward face in the isolated roughness regime [9]. Salizzoni et al [31] also noted that in the wake interference regime the turbulent structures and turbulence intensity are larger than in the skimming flow regime above the roughness. The shear stress was found to vary with λp within and above the roughness up to a height of approximately 5h [31].…”

Sensitivity Analysis of CALINE4 Model under Mix Traffic Conditions

“…(Macdonald et al, 2003). It is a function of the spatial density, orientation and height of obstacles to the wind and plays a significant role in interaction between airflow and landscape (Barratt, 2001;Salizzoni et al, 2011). Spatially variable roughness creates horizontal variations in turbulence and the local mean flow, both of which can affect pollutant dispersion (Barnes et al, 2014).…”

Transport processes in and above two-dimensional urban street canyons under different stratification conditions: results from numerical simulation

“…Huq et al [16] used the difference between velocities across the shear layer, U , to fit the non-dimensional mean velocity profile of a street canyon of h/b = 1.92, and found the profile fits well to the hyperbolic tangent form [35]. However, the study of Salizzoni et al [36] showed that it is not possible to use a single scaling parameter to scale the flow in all the three parts of street canyons. This is not surprising as the three parts of street canyon flows reflect quite different fluid mechanical phenomena and these need not scale in the same manner.…”