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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.
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.
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