Abstract
Thermal convective flows are common phenomena in real urban canyons and strongly affect the mechanisms of pollutant removal from the canyon. The present contribution aims at investigating the complex interaction between inertial and thermal forces within the canyon, including the impacts on turbulent features and pollutant removal mechanisms. Large-eddy simulations reproduce infinitely long square canyons having isothermal and differently heated facades. A scalar source on the street mimics the pollutant released by traffic. The presence of heated facades triggers convective flows which generate an interaction region around the canyon-ambient interface, characterised by highly energetic turbulent fluxes and an increase of momentum and mass exchange. The presence of this region of high mixing facilitates the pollutant removal across the interface and decreases the urban canopy drag. The heating-up of upwind facade determines favourable convection that strengthens the primary internal vortex and decreases the pollutant concentration of the whole canyon by 49% compare to the isothermal case. The heating-up of the downwind facade produces adverse convection counteracting the wind-induced motion. Consequently, the primary vortex is less energetic and confined in the upper-canyon area, while a region of almost zero velocity and high pollution concentration (40% more than the isothermal case) appears at the pedestrian level. Finally, numerical analyses allow a definition of a local Richardson number based on in-canyon quantities only and a new formulation is proposed to characterise the thermo-dynamics regimes.
Highlights
The ongoing worldwide urbanisation and increasing population of the urban environments have raised awareness towards a series of environmental issues having adverse effects on public health [1,2]
The present study aims at providing further insight into the mixed-convection regime of a single infinitely-long urban canyon of unity aspect ratio
The present study aims to analyse the effects of a horizontal thermal gradient, while additional setups can be investigated in future dedicated studies
Summary
The ongoing worldwide urbanisation and increasing population of the urban environments have raised awareness towards a series of environmental issues having adverse effects on public health [1,2]. Where g is the gravity acceleration, β is the thermal expansion coefficient, ∆T is the characteristic temperature difference, H is the mean building height, and U0 free-stream velocity Such a quantity compares the buoyancy force per unit mass gβ∆T between canyon surfaces (e.g., street, building facades, rooftops) with the wind-driven mechanical force per unit mass Ur2e f /H. Highly-resolved large-eddy simulations (LESs) are employed to reproduce the non-linear interaction among the in-canyon inertial velocities induced by the surrounding flow, the natural convection arising from heated building facades, and the mechanisms of pollution removal. This required a careful choice of numerical schemes to realistically reproduce the coupling between inertial and thermal turbulence.
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