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Are Urban-Canopy Velocity Profiles Exponential?

¹Aerodynamics and Flight Mechanics, Faculty of Engineering and the Environment, University of Southampton, Highfield, Southampton, SO17 1BJ UK.

Boundary-Layer Meteorology

|February 7, 2020

Summary

Urban canopy models often assume an exponential velocity profile, but this study finds it invalid for urban-type canopies. This research examines drag forces and stresses in urban flows, revealing key differences from vegetative canopies.

Keywords:

Canopy flows Urban environment Velocity profiles

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Area of Science:

Fluid dynamics
Urban meteorology
Computational fluid dynamics

Background:

Understanding airflow within urban canopies is crucial for predicting pollutant dispersion and wind loads.
Existing models often rely on assumptions derived from vegetative canopies, which may not apply to complex urban geometries.

Purpose of the Study:

To analyze sectional drag forces, Reynolds stresses, and dispersive shear stresses in urban-type canopies.
To evaluate the validity of common assumptions used in analytical models for canopy velocity profiles.
To investigate the implications for predicting roughness length and canopy flow behavior.

Main Methods:

Analysis of data from direct numerical simulations (DNS) and large-eddy simulations (LES).
Examination of spatially-averaged mean velocity profiles within canopies of varying roughness densities.
Deduction of canopy mixing length and sectional drag coefficient from flow data.

Main Results:

Common assumptions about drag forces and stresses within urban canopies are generally invalid.
The spatially-averaged mean velocity profile within urban canopies typically does not follow an exponential shape.
Dispersive shear stresses play a significant role in urban canopy flow dynamics.

Conclusions:

The exponential profile assumption is inappropriate for urban-type canopies, unlike vegetative canopies.
Despite the invalidity of the internal profile assumption, current models for predicting above-canopy roughness length remain effective.
Further research is needed to develop more accurate models for flow dynamics within urban canopies.