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Building Downwash

Building downwash is induced by building structures (i.e. tiers) located up-wind of point sources (i.e. stacks).  As wind passes over structures, it first ascends up the front of the structure and then descends into a turbulent wake zone down-wind of the structure.  The dispersion of emissions from short stacks, located in close proximity downwind of structures will be influenced (i.e. subjected to downwash effects) inside these structure-induced  turbulence zones.

The EPA's Building Profile Input Program (BPIP)

The EPA's BPIP processor is used to compute maximum GEP (Good Engineering Practice) stack heights, the maximum HWE (Height of Wake Effect) values, and the BPIPs (Building Profile Input Parameters) for all combinations of single-tiers, tier-groups, stacks, and wind directions.  Dispersion models then use the BPIPs (associated with each stack for 36 wind directions) to compute the plume downwash (i.e. adjusted plume center line and turbulent dispersion)  downwind of tier-influenced stacks.

Good Engineering Practice (GEP) Stack Height for a Single Tier

BPIP first computes the GEP Stack Height for each stack.  The GEP height for a stack with no tiers in it's vicinity is zero.  The GEP for a stack with only one nearby tier is a function of the single tier's geometry and wind direction.  For a single wind direction, the down-wind length of the single tier's GEP-SIZ (Structure Influence Zone) is the lesser of the structure's (i.e. the tier's) width or height times five (5L) .  A stack located downwind, within the bounds of the tier's projected width and it's influence zone 5L length, should have a height (i.e. a GEP height) of 2.5 times the lesser of the tier's width or height, to effectively place the stack's emissions above (i.e. outside of) the tier's HWE.

For each single-tier, BPIP computes downwind GEP-SIZ geometry for 1440 wind directions (360 degrees in quarter degree increments).  For each wind direction, BPIP determines if one or more stacks are located within the tier's GEP-SIZ and, if so, computes a GEP value for each such stack.  BPIP iterates in this way for all combinations of single-tiers, stacks, and wind directions looking for the tier and wind direction that computes the highest GEP height for each stack.

Tier Groups - The Complex Part of  BPIP

BPIP also recognizes that the wind can encounter multiple tiers before it reaches a stack.  To accommodate this situation BPIP creates tier groups based on the relative geometry of neighboring tiers.  It computes the minimum distance between all tiers and, for each of 1440 wind directions, it computes GEP-SIZ geometry for all combinations of "sufficiently close" tier groups, and again computes GEP values for all stacks within these tier-group GEP-SIZs.  And to complicate things even further, BPIP iterates through all possible combinations of tiers-groups, computing different profile and GEP-SIZ geometry for tier-groups with different "focal-tiers" and "common-height-tiers".  All things considered, BPIP performs an exhaustive process of locating stacks in all possible GEP-SIZs and computing all possible BPIPs that produce the maximum GEP height for each stack and wind direction.

Grouping Tiers into Buildings

Advanced user's will also be aware that BPIP allows tiers to be grouped into 'buildings'.  Unfortunately, this is a widely misunderstood BPIP feature that often leads to modeling errors.  Grouping tiers into a building accomplishes only one thing:  it prevents BPIP from creating any of all the possible combinations of tier groups that would otherwise have been created using all the tiers that compose the building.  It effectively accomplishes the opposite of what the novice modeler would expect:  it prevents the tiers in a building from being grouped.  So why group tiers into a building?  One reason:  it reduces the run time by reducing the number of tier group combinations.

There are important things to understand when using multi-tier buildings.  A proper multi-tier building is composed of only tiers that are completely contained by the X-Y bounds of one of the tiers.  It turns out that, in such a configuration, the maximum computed GEP and HWE values will always be determined by one, and only one, of the tiers in the building.  Also, for those using BREEZE AERMOD, the 'elevation' of each tier must be the building elevation (i.e. the ground elevation).  Do not set the elevation of a taller tier to be the roof-top elevation of a shorter tier.

Maximum Height of Wake Effect (HWE) and Building Profile Input Parameters (BPIPs)

After computing the maximum GEP stack heights, BPIP computes the BPIPs that produce the maximum HWE for each stack, in all wind directions (i.e. 36 wind directions in 10 degree increments).  These values are written to the BPIP output file and subsequently used by the PRIME Plume Rise and Downwash Model within AERMOD.  This second process uses information from the previous maximum GEP process, and is very similar in structure.  The major differences are  1)  larger HWE-SIZs, with a larger width (1/2L on both sides), an extended 2L up-wind boundary, and the same 5L downwind boundary, are used, and  2)  only 36 wind directions, in 10 degree increments, are used.  The same exhaustive process of computing HWE-SIZs for all single-tiers and tier-groups, locating stacks within the HWE-SIZs, and then computing an influence height (GEP in the first process and HWE in this second) is performed.  The single-tier or tier-group BPIPs that produce the maximum HWE for each of the 36 wind directions is saved to the BPIP output file for each stack.

BREEZE Downwash Analyst - A Quick and Easy Way to Identify How Tiers Influence Stacks

Needless to say, the BPIP process becomes difficult to understand when numerous tiers and stacks are present in a model.  The BREEZE Downwash Analyst is designed to visually display how  1)  BPIP searches for tiers in the vicinity of stacks,  2)  computes GEP-SIZ geometry,  3)  identifies stacks within GEP-SIZs,  4)  computes GEP heights,  5)  computes HWE-SIZ geometry,  6)  locates stacks within HWE-SIZs,  7)  computes HWE values, and finally  8)  saves the BPIPs for all wind directions around each stack.  BREEZE Downwash Analyst is designed to help modelers quickly and easily answer the following questions:

Looking at the BPIPs in a table cannot answer these questions.  However, BREEZE Downwash Analyst can.

PRIME Plume Rise and Building Downwash Model

The BPIPs produced by running BPIP are used by the PRIME Plume Rise and Building downwash Model.  The model modifies the plume center line, concentrations, and dispersion values to simulate the effects of turbulence in the 'cavity' and 'far wake' regions.  Visit the EPA's SCRAM web site (http://www.epa.gov/scram001/) for more detailed information about the PRIME model.

BREEZE Downwash Analyst - A Quick and Easy Way to See Downwash-Influenced Plumes

Downwash Analyst's fifth analysis mode, PRM Plume - Select Stack & Time, provides an easy way to observe the downwash plumes that are computed by AERMOD whenever the wind is blowing toward a stack in a direction that has non-zero BPIPs.  To use this analysis mode, first run BREEZE AERMOD with the AERMOD_BREEZE_07026.exe (the 07026 designator might be a larger number as future versions of the executable are released by EPA) model and with the Input Menu | Output Options | Other File & Options | Generate Plume Downwash Centerline File option checked.  This tells BREEZE AERMOD to generate an Aermod.prm file in the .amz results archive file.  After the AERMOD Model Run, launch Downwash Analyst from the Tools Menu.  Both the BPIP input data (i.e. the building and stack geometry) and the Aermod.prm data will be loaded into Downwash Analyst and all five analysis modes will be made available.  Select the 'PRM Plume' analysis mode to see the AERMOD computed, downwash-influenced plume geometry with the BPIP geometry (i.e. building profile box), the near-wake (i.e. cavity) boundary, and the far-wake boundary.  View this information in either the 3D Display or the side-view Chart display.

 

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