Turbulence-Kinetic-Energy Budget in the Urban-Like Boundary Layer Using Large-Eddy Simulation
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Turbulence-Kinetic-Energy Budget in the Urban-Like Boundary Layer Using Large-Eddy Simulation Geng Tian1,2 · Boris Conan1,2 · Isabelle Calmet1,2 Received: 30 January 2020 / Accepted: 19 September 2020 © Springer Nature B.V. 2020
Abstract We describe and explain the turbulent processes at play in the lower part of the urban boundary layer through performing a large-eddy simulation of the flow over an urban-like canopy composed of a staggered array of cubes with a packing density of 25%. The simulation models neutral thermal conditions at a Reynolds number (based on both velocity at the top of the domain and the domain height) of Re = 50,000. A dynamic Smagorinsky model is implemented in order to allow for energy backscattering from subgrid scales. A wall refinement of the grid allows resolving the viscous sublayer. Turbulent statistics up to the third order, as well as each term of the turbulence-kinetic-energy budget, are computed individually everywhere in the domain. Results are discussed in relation to experimental and numerical data from the literature in order to describe turbulent energy transfers occurring in the roughness sublayer. The fine grid resolution close to surfaces serves to analyze in depth the three-dimensional distribution of turbulence production inside the urban canopy layer. This analysis in turn leads to discovering areas, never previously documented in an urban-like canopy, of highly positive and highly negative production close to the surface, away from the well-known high production area in the shear layer. Furthermore, evidence of a close link between high and low production areas near the surfaces and singular points in the mean flow is presented, thus laying the groundwork for a simple pre-diagnostic tool to detect turbulence-kinetic-energy production areas near surfaces. Keywords Large-eddy simulation · OpenFOAM · Turbulence-kinetic-energy budget · Turbulence production · Urban canopy layer · Wall-bounded flow
1 Introduction In the context of rapid urbanization, an understanding and accurate modelling of turbulent flow in the urban boundary layer (UBL) is of great importance in evaluating the influence of urban design on air quality and urban climate. The roughness sublayer (RSL) is the lowest
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Boris Conan [email protected]
1
Ecole Centrale de Nantes, LHEEA UMR CNRS 6598, Nantes, France
2
Institut de Recherche en Sciences et Techniques de la Ville (IRSTV), FR CNRS 2488, Nantes, France
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part of the UBL (Oke 1997) and where high momentum, heat and mass exchange between the urban canopy layer (UCL) and the atmosphere takes place (Rotach 1999). The RSL extends from the ground up to two to five times the average building height, depending on the geometric arrangement of the elements (Raupach et al. 1991; Cheng et al. 2007)whose presence gives rise to complex flows consisting of vortical structures and low-momentum regions (Coceal et al. 2007b). The UCL is the lowest part of the RSL, lying below the average building height, where the flow is directly in
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