Building downwash: what the wake does to a plume
A building in the wind is an obstacle course for a plume. Air accelerates over the roof, separates at the edges, and forms a cavity of recirculating air behind the structure with a turbulent wake extending far downwind. A stack that releases into — or gets pulled into — that disturbed flow can put its plume on the ground within a few building heights of the source. This is building downwash, and it is why the Good Engineering Practice (GEP) stack-height rules from the Regulatory review part exist. This page covers the physics as AERMOD models it.
PRIME: the downwash algorithm inside AERMOD
AERMOD handles building effects with the Plume Rise Model Enhancements (PRIME) algorithms. The model formulation document describes the whole treatment in one passage:
PRIME partitions plume mass between a cavity recirculation region and a dispersion enhanced wake region based upon the fraction of plume mass that is calculated to intercept the cavity boundaries. These boundaries are established from estimates of the locations of the lateral and vertical separation streamlines. Dispersion of the recirculated cavity mass is based on building geometry and is assumed to be uniformly mixed in the vertical. At the boundary of the cavity region, cavity mass is emitted into the wake region. Here, it is combined with plume mass that was not captured by the cavity and dispersed at an enhanced rate based on source location, release height and building geometry. The enhancement of turbulence within the wake decays gradually with distance, allowing for a smooth transition to ambient levels of turbulence in the far-field.
Source · AERMOD Model Formulation (EPA-454/B-26-003) §5.5.3 — quoted word-for-word from the source document.
Read the passage as three fates for plume material:
- Captured by the cavity — mixed top-to-bottom in the recirculation zone right behind the building. This is the worst case: emissions brought to ground level essentially at the fence line.
- Dispersed in the wake — not captured, but spread at an enhanced rate by building-generated turbulence that decays with distance.
- Escaped — far enough above the disturbance to disperse normally.
Downwash also steals plume rise
The wake doesn't just spread the plume faster — it holds the plume down:
Plume rise, for sources influenced by a building, is estimated using a numerical model that includes effects from streamline deflection near the building, vertical wind speed shear, enhanced dilution from the turbulent wake and velocity deficit. In general, these building induced effects act to restrict the rise that the plume would have in the absence of the building.
Source · EPA-454/B-26-003 §5.5.3.
This is the double penalty that makes downwash cases severe: the plume is both mixed downward more aggressively and prevented from climbing away.
A smooth hand-back to the ordinary model
Inside the wake, PRIME computes the concentration exclusively (fed with AERMOD's own turbulence values rather than the older schemes PRIME was first built on). Beyond the wake, the model blends:
…concentrations beyond the wake are estimated as the weighted sum of the two calculations.
Source · EPA-454/B-26-003 §5.5.3 (eq. 89: total = γ × PRIME estimate + (1 − γ) × ordinary AERMOD estimate, with γ decaying exponentially with distance from the wake in all three directions, eq. 90).
There is no cliff where the building "switches off" — the building's influence fades exactly the way the physics says it should.
When a downwash analysis is required
The Guideline ties the obligation to the GEP formula height (see GEP stack height): for stacks below the formula height H + 1.5L, "air quality impacts associated with cavity or wake effects due to the nearby building structures should be determined," and downwash "should also be considered" even for stacks at or above formula height, since the excessive-concentration definition (a 40 percent downwash-caused excess) can still be met — 40 CFR 51 App. W §7.2.2.1(b). The same paragraph names the tools: AERSCREEN for screening estimates "based on the PRIME downwash algorithm," AERMOD for refined ones.
The building dimensions all of this runs on are not typed in by hand — they come from a dedicated preprocessor, covered next.
In PlumeSmart
Buildings drawn or imported with a project generate the direction-specific building inputs automatically, and stacks flagged by the processor as wake-affected carry that flag into the run — so a downwash-driven result can be traced to the building and direction that caused it.