Magnetic Resonance Imaging measurements of scalar dispersion for a scaled urban transient release

Abstract

Atmospheric dispersion in urban environments is highly complex, and a fundamental understanding requires a combination of field tests, laboratory experiments, and simulations. In this study, a transient release dispersion experiment was performed involving a scaled model of Oklahoma City, with the intention of being relevant for Joint Urban 2003 (JU2003) related studies. Flow over the city model was fully turbulent and housed in a water channel, with a characteristic building Reynolds number of 6200. The transient release was achieved by injecting a passive scalar at street level following a square waveform with a frequency of 1 Hz. Magnetic Resonance Imaging (MRI) techniques were leveraged to obtain phase-averaged velocity and concentration measurements over 12 intervals throughout the transient release cycle, with average uncertainties of 5.4% and 2.2% of the bulk velocity and injected concentration, respectively. Velocity and concentration measurements are obtained on three-dimensional grids of roughly 2 million data points each. An investigation of three-dimensional flow and dispersion features is presented, and a control volume encompassing an intersection within the city model is analyzed by applying the scalar transport equation to the experimental data. The phase-averaged measurements enable the calculation of scalar flux time series as well as the transient storage term. Scalar flux and transient storage indicate complex dispersion characteristics, and can potentially be used to infer the turbulent diffusion of contaminant. The MRI datasets can provide a reference for other atmospheric dispersion modelers and experimentalists, and the corresponding analyses demonstrate particular advantages of MRI data for model evaluation.