Atmospheric boundary layer over urban roughness: validation of large-eddy simulation

TL;DR

This study validates wall-modeled large-eddy simulations (LES) for urban boundary layers over different roughness geometries, demonstrating accurate predictions of flow features and turbulence spectra, thus supporting LES as a reliable tool for urban flow analysis.

Contribution

The paper introduces high-order LES with spectral validation for urban boundary layers, improving accuracy over standard criteria and emphasizing spectral analysis in validation.

Findings

LES accurately predicts mean velocity profiles and wake regions.

LES outperforms standard validation criteria in urban flow simulations.

Turbulent energy cascade is well-resolved over two decades of frequency.

Abstract

The study presents wall-modeled large-eddy simulations (LES) characterizing the flow features of a neutral atmospheric boundary layer over two urban-like roughness geometries: an array of three-dimensional square prisms and the 'Michel-Stadt' geometry model. The former is an arrangement of idealized building blocks. The latter mimics a typical central European urban geometry. In both cases, the incident wind angle is $0^\circ$. The Reynolds number for each case are $Re_H = 5.0 \times 10^6$ and $8.0 \times 10^6$, respectively ($Re_H = U_{ref} H/\nu$ with $U_{ref}$ and $H$ denoting the reference velocity and building height, respectively, and $\nu$ the kinematic viscosity). The LES employs a high-order, low-dissipation numerical scheme with a spatial resolution of 0.75m within the urban canopy. An online precursor simulation ensures realistic turbulent inflow conditions. The simulations performed successfully captures mean-velocity profiles, wake regions, and rooftop acceleration, with excellent agreement in the streamwise velocity component. While turbulent kinetic energy is well predicted at most locations, minor discrepancies are observed near the ground. The analysis of scatter plots and validation metrics (FAC2 and hit rate) shows that LES predictions outperform the standard criteria commonly used in urban flow simulations, while spectral analysis verifies that LES accurately resolves the turbulent energy cascade over approximately two frequency decades. The Kolmogorov -2/3 slope in the pre-multiplied spectra has been well reproduced below and above the urban canopy. These findings reinforce the importance of spectral analysis in LES validation and highlight the potential of high-order methods for LES of urban flows.