Measurements of Three-dimensional Transient Dispersion in the Urban Canopy using Magnetic Resonance Imaging: A Green's Function Approach
Abstract
Predicting turbulent scalar dispersion from street-level sources within the urban canopy is important for determining emergency response, assessing air quality, and providing input to neighborhood and regional scale pollution models. However, computationally efficient models based on the gradient diffusion hypothesis are inaccurate because the flow is driven far from equilibrium by the complex building geometry. Model developments are currently challenged by the available validation data: either due to uncertainties in the boundary conditions of field measurements, or because conventional laboratory techniques are limited to point-wise and planar data. This work uses magnetic resonance imaging to obtain three-dimensional, ensemble-averaged velocity and concentration fields for a transient release in a 1:2206 scale model of Oklahoma City circa 2003. A test section encompassing several blocks of the downtown business district is 3D-printed using stereolithography. A passive scalar contaminant is injected at ground level according to a 40-percent duty-cycle square wave. Data are obtained on a volumetric Cartesian grid within the urban canopy at 12 temporal phases and a typical spatial resolution of 10 voxels per street width. The data are used to extract the impulse response, or Green's function, for the source at each voxel using a regularized optimization procedure. The Green's function is used to generalize the results to different release profiles, and statistics such as the plume residence time can be determined independently of the injection protocol. The results highlight the fact that three-dimensional flow patterns, driven by the local building geometry, can produce non-intuitive plume spread. Health metrics, such as maximum and total contaminant exposure, vary strongly throughout the urban canopy. Maximum exposure occurs along unobstructed flow paths where advection dominates turbulent diffusion, while total exposure is significant in specific regions between buildings that fill rapidly with contaminant but wash out slowly. Plume residence time generally increases with distance from the source and can be substantially longer than the characteristic advection time scales in street canyons and corners between buildings, especially near the ground level.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMA080...08B
- Keywords:
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- 3307 Boundary layer processes;
- ATMOSPHERIC PROCESSES;
- 3322 Land/atmosphere interactions;
- ATMOSPHERIC PROCESSES;
- 3339 Ocean/atmosphere interactions;
- ATMOSPHERIC PROCESSES;
- 3379 Turbulence;
- ATMOSPHERIC PROCESSES