A hybrid methodology for investigating plume downwash in complex environments: a refined meso-scale model, a gaussian plume model, and wind tunnel testing

It is a challenging matter to accurately evaluate the various influences leading to plume downwash from stacks in complex environments that entail terrain, varied meteorological conditions, and sites with intricate building geometries. No one tool is suitable to the task. A hybrid approach has been developed that unites a “refined” meso-scale model, a hierarchy of Gaussian-based dispersion models, and a boundary-layer wind tunnel. The meso-scale model employed is the Advanced Regional Prediction System (ARPS), developed at the University of Oklahoma. ARPS was modified to simulate the atmospheric surface layer with greater accuracy compared to most existing meso-scale models, and to provide enhanced analysis features specific to wind engineering pursuits. Particular attention was placed on accuracy of flows over complex terrain, where flow separation plays a major role in overall flow patterns critical to dispersion and related phenomena.

The hybrid approach to downwash analysis is specialized on a case-by-case basis in which each tool is employed in a structured sequence depending on the circumstances encountered. For example, through a process of grid nesting, the meso-scale model can assess critical meteorological variables from large to small scales, as influenced by synoptic events and local surface and terrain features. A large database of meteorological data (e.g., NCEP/NCAR Reanalysis data) enables the meso-scale model to recreate the past at locations where meteorological data is scarce or nonexistent. Thus, the wind speed at stack height and thermal inversions that evolve from the morning hours to early afternoon can be simulated for arbitrary study days, and effects such as terrain-induced downwash can be addressed directly. The output from the meso-scale model can then be used as input to the appropriate Gaussian plume model to evaluate the potential downwash effects of thermal inversions and stack height winds.

When detailed building geometries neighbor a stack, or if a stack plume is to be evaluated in fine detail, the wind tunnel becomes the primary analysis tool. However, the meso-scale model output can be used to establish the approach flow for the tunnel and to assess the potential impacts of thermal influences not modeled by the wind tunnel. (Note: the wind tunnel is the primary tool to evaluate terrain-induced downwash in complex situations in which neutral stability can be assumed.).

To elucidate the power of the hybrid approach in evaluating potential downwash of a stack plume, a case study is presented for a location in a river valley. Analysis revealed that thermal inversions, convection generated by surface heating, high stack top wind speeds, and terrain-induced turbulence effects create individual and combined conditions that lead to potential plum downwash.