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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. • Experimental study related to JU2003 with measurements at 2 million locations. • Magnetic resonance imaging data provide novel insight into transient release plumes. • Local urban geometries strongly influence contamination and 3D ventilation patterns. • Regions between buildings exhibit longer residence times and asymmetric responses. • Measurements provide a means to calculate scalar flux time series. [ABSTRACT FROM AUTHOR]

Magnetic resonance imaging techniques were used to collect time-averaged three-component velocity and concentration data across a scaled model of 2003 Oklahoma City. The experiments included the release of a passive scalar designed to mimic the field tests of the Joint Urban 2003 (JU03) study conducted at the same site and provide measurements throughout the entire three-dimensional domain in nearly 10 million locations. The flow was in a fully turbulent water channel with a Reynolds number of 36,000 based on the channel hydraulic diameter. The geometrically similar cityscape was on the bottom wall of the channel. A low-momentum injector was designed to release the contaminant at the intersection of Broadway and Main street – a location that closely matches the Intense Observation Period 8 of JU03, which has been previously examined in several studies. Magnetic Resonance Velocimetry (MRV) and Magnetic Resonance Concentration (MRC) techniques measured the time averaged field with experimental uncertainties of 5.5% and 4% respectively, providing a candidate validation data set for atmospheric flow models. The inflow velocity boundary condition was measured at the inlet plane of the test section. Detailed analysis is provided of the flow field and concentration distribution, focusing on major features in the downtown area. In addition, a comparison with the available field test data near surface locations is conducted. The measurements are believed to be amongst the first of their kind and suggest that MRV/MRC techniques may be an effective means to study the flow and scalar dispersion within urban environments. • Lab study of Oklahoma City Field dispersion tests with 9.5 million measurements. • Lower level flows follow streets, whereas flows above buildings remain streamwise. • Tallest buildings strongly influence the downstream flow field. • Preferential concentrations west of Broadway occur. [ABSTRACT FROM AUTHOR]

Momentum, advection, diffusion, and turbulence are component physics relating to fire simulation tools like computational fluid dynamics (CFD). Magnetic Resonance Velocimetry and Magnetic Resonance Concentration MRV/MRC techniques can produce heretofore unrivaled detailed measurements of three-component velocity and concentration fields in turbulent flows. This study exhibits 3D flow comparisons between velocity and concentration fields obtained using MRC/MRV and SIERRA/Fuego for an urban geometry based on a section of downtown Oklahoma City. A 1:2500 scale water flow scenario provides 0.8 mm resolution data. Various techniques are employed to quantify the accuracy of the simulation results. The techniques all generally suggest a good comparison between the model and experiments throughout the compared volume. The selected metrics provide benchmark accuracy measures that can be used to indicate quantitative accuracy of the simulations, as well as for targets for future simulation improvements. [ABSTRACT FROM AUTHOR]