Three-Dimensional Velocity and Concentration Measurements of Contaminant Release in a Scaled Urban Array

Abstract

Turbulent atmospheric flows present many challenges to modeling the spread of pollution from manufacturing, accidental exhaust contaminants, and airborne chemical or biological attacks. Computational fluid dynamics (CFD) simulations and reduced order atmospheric dispersion models are used to predict transport and concentration of atmospheric contaminants, but these techniques require validation against experimental data sets. The present work applies magnetic resonance (MR) techniques to measure contaminant flows in an urban array to generate robust data sets for model validation. Magnetic resonance velocimetry (MRV) and concentration (MRC) methods were employed to generate time-averaged, three-dimensional, three-component velocity field and concentration data sets at sub-millimeter resolution — a volume of data far greater than can be measured in large-scale field tests. A scaled urban array comprising 13 buildings at an angle of 26.2° relative to the incoming flow was placed in a water channel test section within an MRI system. A dilute aqueous solution of copper sulfate (CuSO4) was used as the working fluid to achieve a Reynolds number of 20,000 based on the test section’s hydraulic diameter. The test section design included a roughness section to ensure turbulent flow within the scan volume; two vertical injectors created plume-plume and plume-building interactions at a blowing ratio of 0.6. This work is the first of its kind to employ dual injection and buildings oriented at angles other than 90° or 45° to the main flow. Contaminant concentrations of 0.0125M, 0.0625M, and 0.3M were used and the data combined to increase the signal-to-noise ratio in regions of low concentration. Results illustrate flow and concentration details near building wake regions, injector plume mixing, the influence of building blockage on velocity gradients and concentration penetration, and contaminant flux throughout the flow region. Measurement uncertainty was estimated to be approximately ±5% for velocity and ±5% for concentration.