STUDY OF TURBULENT DISPERSION OF POLLUTANT PLUMES IN A STAGGERED ARRAY OF OBSTACLES Bing-Chen Wang Dept. of Mechanical & Manufacturing Engineering, Univ. of Manitoba Winnipeg, MB, R3T 5V6, Canada E-mail: bc wang@umanitoba.ca Eugene Yee Defence Research & Development Canada – Suffield P.O. Box 4000, Medicine Hat, Alberta, T1A 8K6, Canada E-mail: eugene.yee@drdc-rddc.gc.ca Fue-Sang Lien Dept. of Mechanical & Mechatronics Engineering, Univ. of Waterloo Waterloo, Ontario, N2L 3G1, Canada E-mail: fslien@mecheng1.uwaterloo.ca ABSTRACT In this research, we report a numerical and experimen- tal study of the turbulent dispersion of a passive scalar released from a continuous ground-level point-source in a staggered array of 16 × 16 cubic obstacles. The numerical simulation of the flow and scalar fields was based on the Reynolds-averaged Navier-Stokes method and experimental measurements of the flow and dispersion were obtained in a boundary-layer water channel. Results of a detailed com- parison between the water-channel experiment of flow and dispersion and model predictions of the mean flow, turbu- lence kinetic energy, mean concentration and concentration variance are presented. INTRODUCTION Turbulent dispersion of passive scalars in an environ- ment with complex geometries represents a challenging topic with vast applications in thermal-fluids engineering, chemi- cal processing, urban atmospheric pollution monitoring, and boundary-layer meteorology. The major challenge associ- ated with this subject involves obtaining a deeper under- standing of the interaction of the dynamically evolving flow structures with the complex boundaries, as well as the cou- pling of the momentum and scalar transport processes. In order to develop an effective methodology for pre- dicting turbulent dispersion in an urban complex, significant efforts have been made over the past decade based on both experimental and numerical approaches. Recent advances in the study of turbulent dispersion in an urban environ- ment involve a wide range of scales, including field trials conducted at very large regional scales and laboratory simu- lations conducted in small water channels and wind tunnels. Large-scale urban field studies in the United States have in- cluded the Mock Urban Setting Trial (MUST) conducted at U.S. Army Dugway Proving Ground in northwestern Utah in September 2001 (Yee and Biltoft, 2004), the Joint Urban 2003 Experiment conducted in Oklahoma City (Flaherty et al., 2007), and the Urban Dispersion Program (UDP) con- ducted in New York City over the period from 2004 to 2007 (Allwine et al., 2007). Owing to the need for high-quality data sets for validating numerical models for the prediction of passive scalar dispersion within an urban environment, a number of laboratory studies that measure urban flow and dispersion within idealized building arrays have recently been conducted in wind tunnels (MacDonald et al., 1998; Yee et al., 2006; Pascheke et al., 2008) and water channels (Yee et al., 2006). The development of numerical models for the concen- tration variance (second-order moment of concentration) for urban plumes have been undertaken recently, including the work of Andronopoulos et al. (2002), Hsieh et al. (2007), Milliez and Carissimo (2008), Wang et al. (2009) and Yee et al. (2009). In the model of Wang et al. (2009), the dissipation length scale for concentration variance is deter- mined by the characteristic motions of eddies smaller than the local plume scale in the initial meandering stage of plume development, and is limited by the integral length scale of turbulence when the local plume scale becomes larger than the energy containing eddies of the flow in the turbulent diffusive stage of plume development. In comparison with the model of Wang et al. (2009), the model of Yee et al. (2009) improves the formulation for the concentration vari- ance dissipation rate by relating it to an inner time scale associated with relative dispersion. To date, this new model of Yee et al. (2009) has been validated only against two sets of experimental data on a dispersing plume resulting from a continuous release of a passive tracer within regularly aligned arrays of rectangular obstacles (Wang et al., 2010). As a further advancement of our previous studies, we report a new set of high-quality water-channel data for tur- bulent dispersion of a passive scalar released from a localized source in a staggered array of cubic obstacles. In addi- tion, we apply a Reynolds-averaged Navier-Stokes (RANS) method to numerically simulate the physical processes of turbulent dispersion in this staggered array of obstacles, and compare these predicted results with the experimental data reported herein in order to provide further validation of the physically-based model of Yee et al. (2009) for the scalar variance dissipation rate. EXPERIMENTAL MEASUREMENTS The water-channel simulations of flow and dispersion in stag- gered obstacle arrays were conducted at Coanda Research & Development Corporation (Burnaby, BC, Canada). The water-channel experiment for various obstacle arrays is fully described in Hilderman and Chong (2007), and only the im- 1