Development and Evaluation of Improved Building Downwash Algorithms for Solid and Streamlined Structures

In December of 2006, AERMOD¹officially became the EPA preferred model for regulatory dispersion modeling applications. The Plume Rise Model Enhancements (PRIME) algorithm incorporates enhanced plume dispersion due to the turbulent wake behind sharp-edged rectangular buildings and reduced plume rise due to descending streamlines behind these obstacles and entrainment of the plume in the building cavity. PRIME calculates fields of turbulence intensity and wind speed, as well as the local slope of the mean streamlines as a function of the building dimensions, which, coupled with a numerical plume rise model, determines the change in plume centerline location with downwind distance. Cimorelli¹ provides a brief description of PRIME and references Schulman² which is the only detailed documentation available on PRIME. No improvements to the downwash algorithms in PRIME have been made in more than 15 years since that original publication. Because building downwash often causes concentration predictions that exceed ambient standards, it is critical that these estimates be as accurate as possible. Recent field and wind tunnel studies³ have shown that AERMOD/PRIME can overpredict concentrations by factors of 2 to 8 for certain building types. On the other hand, for certain building and terrain configurations, AERMOD/PRIME can underpredict concentrations⁴.

Petersen and Guerra^3,5 documented several theoretical flaws in PRIME (the building downwash formulation in AERMOD) that likely account for the model overprediction tendendency for certain building configurations. Based on this initial work, three industry groups funded a research study in late 2016 with the following overall objectives: 1) correct the known problems in the theory; 2) incorporate and advance the current state of science; 3) expand the types of structures that can be accurately handled (e.g., streamlined, porous, long, wide); 4) properly document and verify the model formulation and code for the updated PRIME (PRIME2); and 5) collaborate with EPA to work toward implementing the improved model.

The industry groups include the American Petroleum Institute, the Corn Refiners Association and the American Forest & Paper Association. In January of 2017, the Electric Power Research Institute joined the research funding group. The goal of the current research is to submit a fully operational version of AERMOD with improved building downwash algorithms by the end of 2017.The updated building downwash algorithms, PRIME2, will include the following features:

• Building wake effects that decay rapidly back to ambient levels above the top of the building versus the current theory that has these effects extending up to 3 building heights;
• Reduced wake effects for streamlined structures; and
• Reduced wake effects for high approach roughness.

To help advance this research, a PRIME2 subcommittee under the A&WMA APM committee was formed to: (1) establish a mechanism to review, approve and implement new science into the model for this and future improvements; and (2) provide a technical review forum to improve the PRIME building downwash algorithms. Collaboration and cooperation from the EPA Office of Research and Development (ORD) has been on-going during the research project with the expectation that the new model can be implemented in a more expeditious manner.

This paper presents the results from the wind tunnel study used to develop a data base of wind speed and turbulence intensity measurements downwind of various solid, porous and streamlined structures. Based on those measurements new equations were developed to estimate the velocity deficit and turbulence intensity increase in the building wake as a function of downwind distance, height, building shape, and approach turbulence intensity. The turbulence enhancement equations are ultimately used in PRIME to compute the horizontal and vertical dispersion coefficients and plume rise. Hence, by correcting these calculations, the plume rise and dispersion calculations should also be more accurate.

Figure 1 shows an example of the predicted and observed turbulence intensity increase ratios versus normalized height for one rectangular building configuration. Also shown is the value determined using the current theory in PRIME (annotated AERMOD in the figures). It should be noted that similar figures were developed for all roughness and building configuration evaluated at all downwind distances. These will be available in the technical report for this project.

In general, the predicted i_z/i_zo values agree well with observations whereas the predictions obtained using the existing theory in PRIME do not agree well. The graphs in the figure show that i_z/i_zo decreases back to ambient values (i.e., 1) at approximately 1.5 building heights for the rectangular structure; in contrast, i_z/i_zo is constant with height with the current PRIME theory well above building top. Below the building top the new equation gives higher and lower i_z/i_zo predictions than the current PRIME theory depending upon building shape and surface roughness. The examples in the figure only show higher values with the new theory. The figure also shows that the maximum i_z/i_zo value decreases as roughness increases.

1. Cimorelli, A.J.; S.G. Perry; A. Venkatram; J.C. Weil; R.J. Paine; R.B. Wilson; R.F. Lee; W.D. Peters; and R.W. Brode. 2005. AERMOD: A dispersion model for industrial source applications. part I: general model formulation and boundary layer characterization. JAM. 44, 682-693.
2. Schulman, L., D. Strimaitis, J. Scire. 2000. Development and evaluation of the PRIME plume rise and building downwash model. J. Air & Waste Manage. Assoc., 50, 378-390. See https://www3.epa.gov/scram001/7thconf/iscprime/tekpapr1.pdf
3. Petersen, R.L., S.A. Guerra, A.S.Bova. 2017. Critical review of the building downwash algorithms in AERMOD. Journal of the Air & Waste Management Association. http://www.tandfonline.com/doi/full/10.1080/10962247.2017.1279088
4. Petersen, R.L., A. Beyer-Lout, R. J. Paine. 2012. Evaluation of monitored SO2 NAAQS exceedances due to the corner vortex. Technical Paper 451 Presented at the 105th Annual Conference and Exhibition of the Air &Waste Management Associate, San Antonio, TX, June 19-22, 2012. See http://www.cppwind.com/wp-content/uploads/2014/01/AWMA-2012-Petersen.Paper-451.Monitored-Exceedance.pdf
5. Petersen, R.L. Building Downwash – Problems, Solutions and Next Generation, Presented at the 11^th Conference on Air Quality Modeling, EPA Research Triangle Park, NC, August 12-13, 2015.

Figure 1. Predicted and observed vertical turbulence intensity increase versus height for three surface roughness configurations at x/H = 4.0 and for one rectangular buildings where H is the building height and x is the distance from the lee edge of the building.