What a data center leaves behind outside its own walls, where generator exhaust disperses, whether that exhaust re-enters its own intake or a nearby one, what wind load the facade and roof equipment are exposed to, and how pedestrians around the building experience wind, is a question set that sits apart from indoor (white space) cooling design. The external flow CFD study described on this page answers that question set numerically, under site-specific wind boundary conditions calibrated against long-term climate data.
In short: data center exhaust dispersion and re-ingestion CFD computes where generator exhaust disperses under the prevailing and worst-case wind directions, what fraction of it re-enters the air intakes, how pollutant concentration spreads around the site, what wind load the facade and roof equipment carry, and what pedestrian-level wind comfort looks like around the building, all under boundary conditions calibrated against more than 30 years of climate data.
Why a separate study
Indoor (white space) CFD models simulate airflow inside the building envelope and the temperatures at rack level; where generator exhaust goes once it leaves the building, and under which wind condition it returns to its own intake or a neighbouring one, sits outside that scope. Answering that question requires a separate external flow model that includes the surrounding building geometry, terrain roughness, and the prevailing and worst-case wind directions. It can be run as part of a comprehensive data center CFD and thermal safety analysis project, or as a standalone study.
The risk side is concrete: when a generator re-ingests its own exhaust, or that of a nearby source, the quality of its combustion air drops, turning a backup system that is expected to run at full load without interruption into an unplanned failure risk. The same return path can also degrade indoor air quality at fresh air intakes.
The process side points the same way: in some countries, planning and environmental permitting now ask for wind microclimate and exhaust dispersion to be submitted as a separate technical assessment for data center applications. Carrying out that assessment early, while site layout and massing can still change, keeps the cost of a later revision down.
The E1 to E5 external flow scenario set
We define the external flow scope as five standard scenarios, each answering a distinct question. Depending on project scope, all five or a subset can be run.
| Code | Scenario | What it determines |
|---|---|---|
| E1 | Dispersion under the prevailing wind | The exhaust plume’s spread pattern under the most frequent wind direction and speed |
| E2 | Re-ingestion under the worst-case direction | The most critical angle for the air intakes and the re-ingestion risk at that angle |
| E3 | Pollutant concentration map | The spread of NOx, CO and particulate matter around the building |
| E4 | Facade and roof wind loading | Pressure coefficients and equipment wind loads referenced to VDI 3783-12; see facade wind loading |
| E5 | Pedestrian comfort and safety | Comfort and safety classification against pedestrian-level wind speed criteria; see pedestrian-level wind comfort |
The boundary condition chain
The reliability of a CFD result depends on the quality of its boundary conditions. We apply the following four steps on every project; for the observation and climate data layer this chain rests on, see our meteorology and climate data service.
- ERA5 reanalysis, 1979 to present. More than 30 years of hourly wind direction, speed, temperature and atmospheric stability data are extracted for the site.
- Bias correction against the nearest observation station. The ERA5 series is corrected for systematic bias against the nearest weather station’s record; topography and urban roughness are additionally accounted for at site scale.
- ASHRAE n=50 design day envelope. Extreme temperature and humidity values are tied to the design day, including a safety margin.
- Calibrated atmospheric boundary layer profiles. Turbulence intensity, surface roughness (z0) and the wind profile are fed to the solver as scenario-specific input.
Outputs
Every study includes the following four output groups; scope can be narrowed or extended per project.
| Output | Content |
|---|---|
| Plume and dispersion map | The exhaust plume’s spread pattern under the E1 and E2 scenarios |
| Re-ingestion ratio and intake concentration | The recirculation ratio and pollutant concentration at each air intake |
| Facade and roof pressure coefficients | Local pressure coefficients and equipment wind loads referenced to VDI 3783-12 |
| Pedestrian-level comfort map | Wind speed classification and comfort/safety assessment at pedestrian level around the building |
Reference standards
| Standard | Where it applies in this study |
|---|---|
| VDI 3783-12 | Atmospheric dispersion calculations (E1 to E3) and facade/roof wind load values (E4) |
| ASHRAE Fundamentals (n=50 design day) | Boundary condition calibration and the extreme temperature/humidity envelope |
Where a client specification calls for an additional standard or acceptance criterion, the study is scoped to that criterion.
Data required for the study
- A massing/site plan of the building and its surroundings (a 3D model where available)
- Generator and exhaust stack location, height, flow rate and temperature
- Location of fresh air intakes and cooling equipment intake points
- Planned equipment layout on the facade and roof (where E4 is in scope)
- The boundary of areas with pedestrian traffic (where E5 is in scope)
- Observation records from a nearby weather station, where available
Missing data does not stop the study; it is defined as an assumption at the outset, and the report states clearly which result depends on which assumption.
Frequently asked questions
- Is this different from your data center CFD service?
- It is a distinct scope that can also be purchased on its own. Our data center CFD and thermal safety analysis service also covers white space, grey space and transient scenarios; the external flow set (E1 to E5) on this page can run as a subset of that scope, or as a standalone study focused only on exhaust dispersion, re-ingestion, facade/roof wind load and pedestrian comfort.
- What exactly is re-ingestion?
- It is the return of hot or polluted air discharged by a generator or cooling equipment, driven by wind, back into the intake of that same equipment or a nearby air intake. When a generator re-ingests its own exhaust, the quality of its combustion air drops, creating an unplanned failure risk in a backup system that is expected to run uninterrupted at full load.
- At which stage of the project should this be done?
- It returns the most value during site selection and early design, while building massing and the generator/intake layout can still change. Carried out before a planning or environmental permit application, it also produces the technical justification that application may require for wind microclimate. On a facility with a fixed layout, it is used to verify the current situation or to assess risk before a capacity uplift.
- Is pedestrian comfort (E5) required on every project?
- No. E5 is relevant where there is pedestrian traffic around the building, whether public access or staff spending time outdoors. Where the surroundings are enclosed and pedestrian access is restricted, this scenario can be left out of scope; scope is agreed together at the start of the project, based on the site layout.
- What is VDI 3783-12?
- It is a German engineering standard (Verein Deutscher Ingenieure) used for atmospheric dispersion and wind effect calculations. We report the dispersion calculations under E1 to E3 and the facade/roof wind loads under E4 against this standard.
- What input data is required?
- A massing model of the building and its surroundings, together with basic information on the generator and air intake locations, is enough to start. Missing data is defined as an assumption at the outset, and the report states which result depends on which assumption.
- In what format are the results delivered?
- As a technical report containing plume and dispersion maps, the re-ingestion ratio table, facade/roof pressure coefficients and the pedestrian comfort map; GIS layers and an executive summary are included on request.
Related content
- Data Center CFD and Thermal Safety Analysis: Our comprehensive data center CFD service, including white space, grey space and transient scenarios.
- Meteorology & Climate Data: The observation and climate data layer the boundary conditions rest on.
- Facade Wind Loading: Wind load analysis for building facades and roof-mounted equipment.
- Pedestrian-Level Wind Comfort: Pedestrian-level wind comfort and safety assessment around the building.