Numerical analysis of the street canyon thermal conductance to improve urban design and climate

Built environment is increasingly dependent on the scientific knowledge which integrates urban design and climate. In the work presented here, the canyon thermal conductance which quantifies the heat transported outside of a canyon street, is analyzed to improve on understanding of how to accomplish this integration. A two-dimensional, steady, k$k$–ε$\epsilon$ turbulence model is used to study the influence of a windward heated wall on the air flow circulation in a street canyon with building height-to-street width ratio (aspect ratio) from 0.7 to 1.5. The numerical results presented here suggest that the air flow regime is strongly affected by buoyancy and three configurations are predicted: (I) and (II) with high Froude numbers (≈10¹$\approx {10}^1$) result in one or two stable counter-rotating vortices, with an intenser upper vortex; air flow regime (III), with low Froude numbers (≈10^-1$\approx {10}^{-1}$), is dominated by the lower vortex whose intensity is enhanced by a strong upward current close to the heated surface confining the upper vortex to a strict leeward zone of the canyon. Transitional Froude numbers are found as a function of canyon aspect ratio for transitions between regimes. The relevance of the results for urban design are quantified and analyzed in terms of canyon thermal conductance. The main conclusion is that, for one vortex skimming air flow regime, the canyon thermal conductance linearly increases with wind intensity, being larger streets more exposed to thermal losses. Multiple vortices in the air flow regime significantly decrease the canyon thermal conductance and, therefore, narrow streets provide protection from heat losses on windy and cloudy days and nights.