Turbulence Characteristics Across a Range of Idealized Urban Canopy Geometries

Blunn, Lewis; Coceal, Omduth; Nazarian, Negin; Barlow, Janet F.; Plant, Robert S.; Bohnenstengel, Sylvia I.; Lean, Humphrey

doi:10.1007/s10546-021-00658-6

Cited by 30 publications

(16 citation statements)

References 72 publications

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“…Various investigations seek to improve the turbulence closure formalism through one-dimensional multilayer urban canopy models: large eddies are analyzed through numerical simulations in a variety of idealized geometries and flow regimes. The study reveals a good charac-terization in the declared boundary conditions [29]. Other investigations have carried out wind tunnel experiments [30] in four urban morphologies: two tall canopies with uniform height and two super tall canopies with a great variation in the height of the elements (where the maximum height of the element is more than double the average height of the canopy, h max = 2.5 h avg ).…”

Section: Dissipation and Complex Systemsmentioning

confidence: 75%

Urban Densification Effect on Micrometeorology in Santiago, Chile: A Comparative Study Based on Chaos Theory

2022

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The concentration distribution of anthropocentric pollutants is favored by urban densification, affecting the micrometeorology in big cities. To examine this condition, chaos theory was applied to time series of measurements of urban meteorology and pollutants of six communes of the Metropolitan Region of Santiago de Chile, in two periods: 2010–2013 and 2017–2020. Each commune contributes, per period, six different time series: three for the meteorological variables (temperature, relative humidity, and magnitude wind speed) and three for the atmospheric pollutant concentrations (PM10, PM2.5, and CO). This qualitative study corroborates that each of the time series is chaotic through the calculation of chaotic parameters: Lyapunov exponent, correlation dimension, Hurst coefficient, correlation entropy, Lempel–Ziv complexity and fractal dimension. The variation in the chaotic parameters between the two periods can be interpreted in relation to the roughness change due to urban densification. More specific parameters, constructed from the Kolmogorov entropies and the fractal dimensions of the time series, show modifications due to the increase in the built surface in the most current period. This variation also extends to micrometeorology, as is clear from the Lempel–Ziv complexity and the Hurst coefficient. The qualitative picture constructed using chaos theory reveals that human interaction with nature affects diversity and sustainability and generates irreversible processes.

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Section: Dissipation and Complex Systemsmentioning

confidence: 75%

Urban Densification Effect on Micrometeorology in Santiago, Chile: A Comparative Study Based on Chaos Theory

2022

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“…Here, we focus on the flow field within the urban canopy layer (UCL) and discuss the performance of geometric factors in predicting flow variability among different configurations with spatially averaged flow properties in Section 4.1, canopy‐averaged flow properties in Section 4.2.1, and drag parameterization in Section 4.2.2. These analyses further contribute to the development of urban canopy parameterization through the calculation of turbulent length scales (Li et al., 2020) and drag coefficients (Santiago et al., 2013) using both idealized (Blunn et al., 2022) and realistic urban configurations.…”

Section: Resultsmentioning

confidence: 99%

Novel Geometric Parameters for Assessing Flow Over Realistic Versus Idealized Urban Arrays

Nazarian

Hart

et al. 2023

J Adv Model Earth Syst

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Urban heterogeneity, such as the variation of street layouts, building shapes, and building heights, cannot be fully represented by density parameters commonly used in idealized urban environmental analyses. To address this shortcoming and better model flow fields over complex urban neighborhoods, we propose two novel descriptive geometric parameters, alignedness and building facet entropy, which quantify the connectivity of inter‐building spaces along the prevailing wind direction and the variation of building facet orientations, respectively. We then conducted large eddy simulations over 101 urban layouts, including realistic urban configurations with uniform building height as well as idealized building arrays with variable heights, and evaluated the resulting bulk flow properties. Urban canopy flow over realistic neighborhoods resembles staggered building arrays for low urban densities but becomes similar to aligned configurations beyond λp ∼ 0.25 where the realistic flow is less sensitive to changes in density. We further show that compared to traditional density parameters (such as plan and frontal area densities), the mean alignedness, a measure of connectivity of flow paths in street canyons, better predicts canopy‐averaged flow properties. Furthermore, for realistic urban flow, the dispersive momentum flux shows a clear increasing trend with building density, and a decreasing trend with alignedness, which is in contrast with idealized cases that exhibit no clear trend. This distinct behavior further highlights the necessity of evaluating flow over realistic urban layouts for flow parameterization. This study provides an improved method of describing urban layouts for flow characterization that can be applied in neighborhood‐scale urban canopy parameterization.

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“…For spatial averaging, we consider the intrinsic averaging approach (Mignot et al ., 2008) as discussed in Section 2.2, where averaging is performed over the part of the domain occupied by air. Such an approach causes discontinuities in flow profiles (e.g., TKE profiles in Figure 2) at the heights where building density changes but can be corrected by introducing additional terms to account for density variations (Blunn et al ., 2022).…”

Section: Resultsmentioning

confidence: 99%

Representing the effects of building height variability on urban canopy flow

Lu,

Nazarian,

Hart

et al. 2023

Quart J Royal Meteoro Soc

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We conducted large‐eddy simulations over 98 urban arrays with varying building densities and height distributions. Compared to uniform‐height urban arrays, the influence of height variability on urban flow is pronounced and acts differently in two idealized urban configurations: the low buildings induce higher wind speed and stronger turbulence over staggered arrays but act inversely over aligned building configurations. The flow motions around tall buildings generate strong dispersive fluxes, which are sometimes of similar magnitude to the turbulent momentum flux and responsible for a persistent isolated roughness flow pattern in the upper canopy regardless of the urban density. Tall buildings further contribute disproportionately to the form drag of the urban surface, reaching up to 3.9 times the form drag induced by buildings of height equal to the average building height, in dense layouts. The flow inflection points ‐ i.e., the largest wind speed gradient that defines the aerodynamic interface between the urban canopy flow and the surface layer flow above, are found to be displaced to the maximum building height if less than 25% of buildings are below the mean building height. These findings provide critical insight for the development of urban canopy models, where the impacts of height variability on flow are often linked to the vertical variation in urban density alone. To address this deficiency, we provide a case study that considers the drag amplification due to the impact of vertical urban structures in the urban canopy model, enabling high‐resolution regional climate models to reproduce urban air flows better.This article is protected by copyright. All rights reserved.

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Turbulence Characteristics Across a Range of Idealized Urban Canopy Geometries

Cited by 30 publications

References 72 publications

Urban Densification Effect on Micrometeorology in Santiago, Chile: A Comparative Study Based on Chaos Theory

Urban Densification Effect on Micrometeorology in Santiago, Chile: A Comparative Study Based on Chaos Theory

Novel Geometric Parameters for Assessing Flow Over Realistic Versus Idealized Urban Arrays

Representing the effects of building height variability on urban canopy flow

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