Wind Tunnel Modeling of Complex Flow and Dispersion Phenomena in support of the Joint Urban Atmospheric Dispersion Study 2003

During July 2003, the extensive field campaign JOINT Urban 2003 took place in Oklahoma City, USA. Passive tracers were released from point sources at several inner-city locations and subsequently sampled at a large number of receptor points. Main objective of the field campaign was the generation of data for the validation of micro-scale dispersion models that can be used as fast-response tools for accidental releases of toxic material in an urban environment.

Hamburg University in combination with the University of Oklahoma was tasked to support the field campaign by corresponding wind tunnel experiments. Prior to the field tests, a scaled wind tunnel model of the central business district of Oklahoma City was built and a boundary layer with appropriate turbulent characteristics was developed in the large boundary layer wind tunnel 'WOTAN'. Numerous Laser light-sheet flow visualization experiments were performed which provided insight in the complex flow and dispersion characteristics of downtown Oklahoma City. Subsequently, flow and concentration measurements were carried out. The results were used to evaluate the source and sampling positions selected for the field tests.

The work is presently continued by replicating some of the dispersion episodes found during the field campaign. Utilizing sophisticated instrumentation, flow and dispersion measurements with high resolution in space and time are carried out. Time series of tracer concentration are sampled and analyzed with respect to transient dispersion phenomena. The results are used to quantify the natural variability and intermittency inherent to the field samples. Additional mean flow and turbulence measurements are carried out for several cross-sections downstream of the sources in a fine grid that corresponds to the spatial resolution of numerical models. These data will enable a direct comparison with modeled velocity fields to be carried out. The will describe the experimental strategy applied and exemplary results out of the extensive database compiled will be presented. For an interested audience, a video presentation containing exemplary video sequences from laser light sheet visualization experiments will be available.