Study of Realistic Urban Boundary Layer Turbulence with High-Resolution Large-Eddy Simulation

Mikko Auvinen,Leena Järvi,Topi Tanhuanpää,Antti Hellsten,Simone Boi

doi:10.3390/atmos11020201

Abstract

This study examines the statistical predictability of local wind conditions in a real urban environment under realistic atmospheric boundary layer conditions by means of Large-Eddy Simulation (LES). The computational domain features a highly detailed description of a densely built coastal downtown area, which includes vegetation. A multi-scale nested LES modelling approach is utilized to achieve a setup where a fully developed boundary layer flow, which is also allowed to form and evolve very large-scale turbulent motions, becomes incident with the urban surface. Under these nonideal conditions, the local scale predictability and result sensitivity to central modelling choices are scrutinized via comparative techniques. Joint time–frequency analysis with wavelets is exploited to aid targeted filtering of the problematic large-scale motions, while concepts of information entropy and divergence are exploited to perform a deep probing comparison of local urban canopy turbulence signals. The study demonstrates the utility of wavelet analysis and information theory in urban turbulence research while emphasizing the importance of grid resolution when local scale predictability, particularly close to the pedestrian level, is sought. In densely built urban environments, the level of detail of vegetation drag modelling description is deemed most significant in the immediate vicinity of the trees.

Highlights

In a rapidly urbanizing world, it is important to improve our understanding of the turbulent dispersion and exchange processes influencing the microclimate and air quality of our cities
This poses a challenge because urban boundary layer (UBL) flows are complex flow systems that arise from the interaction between atmospheric boundary layer (ABL) turbulence and varying arrangements of urban obstacles [1]
Considering the difficult nature of real urban flows, we must ask what is the meaningful level of detail with which the urban roughness elements, buildings, and vegetation must be described within the model? This study examines issues relating to local scale predictability and the observability of changes in realistic UBL flows where the situation is further complicated by nonideal conditions

Summary

Introduction

In a rapidly urbanizing world, it is important to improve our understanding of the turbulent dispersion and exchange processes influencing the microclimate and air quality of our cities. This poses a challenge because urban boundary layer (UBL) flows are complex flow systems that arise from the interaction between atmospheric boundary layer (ABL) turbulence and varying arrangements of urban obstacles [1]. Large-Eddy Simulation (LES), a modelling approach in the field of Computational Fluid Dynamics (CFD) capable of temporally and spatially resolving the relevant turbulent processes governing the transport and exchange of momentum, energy, and materials between the urban surface and the ambient air, provides a well-established modelling framework with which the relevant turbulent processes in complex UBL flows can be simulated [8,9]

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