Spatial Characteristics of Roughness Sublayer Mean Flow and Turbulence Over a Realistic Urban Surface

Abstract

Single-point measurements from towers in cities cannot properly quantify the impact of all terms in the turbulent kinetic energy (TKE) budget and are often not representative of horizontally-averaged quantities over the entire urban domain. A series of large-eddy simulations (LES) is here performed to quantify the relevance of non-measurable terms, and to explore the spatial variability of the flow field over and within an urban geometry in the city of Basel, Switzerland. The domain has been chosen to be centered around a tower where single-point turbulence measurements at six heights are available. Buildings are represented through a discrete-forcing immersed boundary method and are based on detailed real geometries from a surveying dataset. The local model results at the tower location compare well against measurements under near-neutral stability conditions and for the two prevailing wind directions chosen for the analysis. This confirms that LES in conjunction with the immersed boundary condition is a valuable model to study turbulence and dispersion within a real urban roughness sublayer (RSL). The simulations confirm that mean velocity profiles in the RSL are characterized by an inflection point z γ located above the average building height z h . TKE in the RSL is primarily produced above z γ , and turbulence is transported down into the urban canopy layer. Pressure transport is found to be significant in the very-near-wall regions. Further, spatial variations of time-averaged variables and non-measurable dispersive terms are important in the RSL above a real urban surface and should therefore be considered in future urban canopy parametrization developments.

Numerical framework for the computation of urban flux footprints employing large-eddy simulation and Lagrangian stochastic modeling

“…The study features a spatial resolution of 1 m, which is unprecedented at this scale. Giometto et al (2016) found the same resolution to be sufficient to capture the relevant turbulence physics within a real urban roughness sublayer. However, the effect of grid resolution on the final result is not investigated in this work.…”

Interaction between turbulent flow and sea breeze front over urban‐like coast in large‐eddy simulation

“…This provides an understanding of the structures of turbulent flow and its interaction with SBF over urban‐like surface from a fundamental perspective. We note that uneven buildings or land use may impose a spatial contrast of dynamic and thermal forcing, which can influence the features of turbulent flow [e.g., Chen et al ., ; Giometto et al ., ]. Further introducing complex surface with variable block's shape and/or height is a potential work in the future [ Jurelionis and Bouris , ].…”

Perturbations to the Spatial and Temporal Characteristics of the Diurnally-Varying Atmospheric Boundary Layer Due to an Extensive Wind Farm

“…Treating a wind farm as an enhanced surface roughness is perhaps the most common concept used to represent the atmospheric flow over turbines. Modelling of large-scale (in comparison to surface roughness) momentum-absorbing elements near the surface as enhanced surface roughness has counterparts in studies of flows over vegetated canopies and urban environments (Tseng et al 2006;Giometto et al 2016). Note that, in the above framework, the ABL is partitioned vertically, and is dependent on neglecting entrance effects and the formation of an internal boundary layer that would be prominent features in a flow over a realistic finite-sized wind farm.…”