Abstract
An experimental field campaign is designed to unveil mechanisms responsible for turbulent exchange processes when mechanical and thermal effects are entwined. The focus is an urban street canyon with a mean aspect ratio H/W of 1.65 in the business centre of a mid-size Italian city (H is the mean building height and W is the mean canyon width). The exchange processes can be characterized by time scales and time-scale ratios specific to either mechanical or thermal process. Time scales describe the mixing caused by momentum and heat exchange within different canyon layers, while their rates are surrogates of their efficacy. Given that homogeneous mixing does not always occur within the canyon, several time scales are estimated at different levels, showing that mechanical and thermal processes may both contribute to enhance mixing. By computing mechanical time scales, it is found that the fastest mixing occurs at the canyon rooftop level for perpendicular or oblique wind directions, while slow mixing occurs for parallel directions. Thermal processes are faster than the mechanical ones and are particularly efficient for perpendicular wind directions. By calculating the time-scale ratios, exchange processes are found to facilitate mixing for most wind directions and to regulate the pollutant-concentration variability in the canyon. This variability can be associated with the local-circulation regime, demarcated as thermally driven or inertially driven using a buoyancy parameter, i.e., the ratio between thermal and inertial forcings. Using this approach, a generalization of the results is proposed, enabling the extension of the current investigation to different street-canyon aspect ratios.
Highlights
Exchange processes between the urban canopy layer and an idealized inertial layer above have been addressed previously (e.g., Barlow and Belcher 2002; Bentham and Britter 2003; Harman et al 2004) using data from laboratory, field, and numerical experiments
In urban canopies characterized by the skimming-flow regime (Oke 1987), local atmospheric circulation (Britter and Hanna 2003) and exchange processes are driven by turbulence
For an in-canopy thermally-driven circulation, i.e. when the differential heating between opposite building facades is larger than the unperturbed inertial flow (Dallman et al 2014), exchange processes are modified by the turbulent heat transport (Nazarian et al 2018), and their efficacy scales with the level of mixing within the canyon and the thermal stratification above (Nazarian et al 2017)
Summary
Exchange processes between the urban canopy layer and an idealized inertial layer above have been addressed previously (e.g., Barlow and Belcher 2002; Bentham and Britter 2003; Harman et al 2004) using data from laboratory, field, and numerical experiments. For an in-canopy thermally-driven circulation, i.e. when the differential heating between opposite building facades is larger than the unperturbed inertial flow (Dallman et al 2014), exchange processes are modified by the turbulent heat transport (Nazarian et al 2018), and their efficacy scales with the level of mixing within the canyon and the thermal stratification above (Nazarian et al 2017). This efficacy is modified by the heat release from the ground (Li et al 2012)
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