Description of Survey AFN and CFD Calculations

A Survey on the Utility of Natural Ventilation Performance Metrics

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Airflow Networks (AFN)

AFN Calculations

Airflow Network (AFN) calculations are conducted using EnergyPlus's implementation of Walton's (1989) AIRNET pressure-driven airflow model. AFNs are dynamic networks of space air volumes (nodes) and connections between them (doors, windows, holes, stairways, etc.). They conserve mass for flows between nodes using a set of nonlinear equations which are solved iteratively based on external pressures and internal temperatures. In this way, AFNs are much faster than computational fluid dynamics simulations (CFD, see later in this document) and can be calculated hourly within a dynamic thermal model such as EnergyPlus. These calculations are performed within the Toronto, ON, Canada climate, which is humid continental (Köppen climate type Dfa) or ASHRAE zone 6A. The Toronto IWEC weather data is used in all analysis.

Simulations use standard occupancy schedules, space conditioning requirements, and mechanical ventilation rates largely derived from the National Renewable Energy Laboratory’s 2014 Building America House Simulation Protocols (Wilson et al. 2014). Space occupant densities (for a two-bedroom unit), equipment power densities, and lighting power densities are portrayed in Figure 1. Occupancy, equipment, and lighting schedules are shown in Figure 2. When windows are closed and natural ventilation is not occurring, 0.23 L/s/m² plus 7.0 L/s/person of fresh air is provided through mechanical ventilation. Also when natural ventilation is not occurring, spaces are conditioned to be between 21°C and 26°C.

Figure 1. Thermal Load Densities

*Note: Occupant densities are for a 2-bedroom unit and change as per the Building America peak load densities for different numbers of bedrooms.

Figure 2. Daily Schedules

Materials are typical for Toronto and are derived from the US Department of Energy Commercial Reference Buildings' Midrise Apartment for climate zone 6A. Ceilings, floors and walls adjacent to neighbouring units are maintained as adiabatic, not transferring thermal energy. Heat transfer coefficients (SI U-values, W/m²-K), the visible light transmittance (VLT, %), and solar heat gain coefficient (SHGC, %) are included in Table 1.

Table 1. Unit Fabric Performance Criteria*

Element	U-Value (W/m²-K)	SHGC (%)	Tvis (%)
Exterior Walls	0.361	-	-
Exterior Glazing	1.66	46	70

*Note: Ceilings, floors, and shared walls are modelled as adiabatic.

External windows are opened provided outdoor weather conditions are appropriate for natural ventilation (outdoor temperature less than 35°C and greater than 15°C, relative humidity less than 90%). External windows are closed when indoor temperatures drop below 20°C. In addition, internal openings are controlled in a semi-realistic manner accounting for whether they are between two public spaces (public-public, such as a kitchen and dining area), between a public and a private space (public-private, such as a bathroom and circulation space), or are a bedroom door. The three types of opening schedules are included in Figure 3. Discharge coefficients are modelled as 0.65, a 35% pressure loss, across any opening threshold. All opening ratios are modelled at 80% operability.

Figure 3. Fractional Opening Schedules for Internal Openings

Annual Carbon Dioxide concentrations in the outdoor urban environment as modelled are depicted in Figure 4. These are based upon data collected in Vancouver (Lee et al. 2016) and increased by 10 ppm to account for global atmospheric changes since 2015. Occupants generate 3.82e-8 m³/s-W of CO2 based on their metabolic rate, which is fixed at 125.3 W/person. No other CO2 sources are modelled within the units.

Figure 4. External CO2 Concentration

AFN Results Processing

Three metrics of natural ventilation performance are calculated using AFNs:

Percent of thermally comfortable hours
Peak, 1st and 5th percentile Carbon Dioxide concentrations
Mean air changes per hour (ACH)

All AFN-based measures account only for the time period during which natural ventilation is possible (see AFN Calculations above). Thermally comfortable hours are determined based on the ASHRAE-55 Adaptive Thermal Comfort standard. Measures are calculated at the individual thermal zone level before being area-weighted into single numerical values for numerically-ranked survey questions. Mean air changes per hour are also visualized spatially at the thermal zone level. All calculated results can be found and visualized at the end of this document.

Computational Fluid Dynamics (CFD)

CFD Calculations

The CFD calculations are constructed using Butterfly and simulated using the OpenFOAM CFD simulation engine. For each residential unit, the CFD calculations are performed on the middle unit of a three-wide by three-high aggregate as depicted in Figure 5. All CFD simulations were carried out under isothermal conditions. The geometry of the model units for CDF analysis is simple using rough door and window openings without complex detailing and fenestration. The wind tunnel dimensions are depicted in Figure 5 as well as relative measures. Meshing is refined based on the input geometry using the SnappyHexMesh algorithm. Models ranged between 2.7 million (Figure 6, building B) and 3.6 million (Figure 6, building C) mesh cells. A standard K-epsilon turbulence model is used.

Figure 5. CFD Wind Tunnel Relative Dimensions

Input wind velocity is 1.2 m/s (a reasonable urban velocity) at a height of 10 m coming directly from the south (or -Y axis) and is vertically stratified as per ASHRAE Fundamentals. The simulation solver is run until velocity reaches normalized residuals <0.0001 and pressure, K, and epsilon reach normalized residuals <0.005. In addition, probe values were checked for stability before considering a solution to be converged.

CFD Results Processing

Three metrics of natural ventilation performance are calculated using CFD:

Mean air velocity
Velocity vectors visualization
Particle streamlines

Velocity probes were created on a 0.3 m by 0.3 m grid across all internal spaces, 1.2 m above the finished floor. These probes were used in calculating the mean air velocity (m/s) and in displaying velocity vectors. Particle streamline displays were generated using sources at the centroid of each external window or door in Paraview. Probes were also placed across all window surfaces and were used to calculate volumetric flow rates, which are not used in the survey. All calculated results can be found and visualized in the Calculated Results section.

A Note on Results Processing

For all whole-building measures, the AFN and CFD simulations were run in eight orientations: N, NE, E, SE, S, SW, W, and NW. The results are presented as of the eight separately-orientated area-weighted means.

For graphic visualizations—ACH, velocity vectors, and streamlines—the South-oriented results are displayed.

Calculated Results

Figure 6. Visuals of the Six Simulated Apartment Units*

*Note: Units are in meters.

Table 2. Tabular Numerical Whole-Building Results During Naturally Ventilated Hours

Building	Mean ACH	Comfortable Hours (%)	Peak / 1% / 5% CO2 Levels (ppm)	Mean Velocity (m/s)*
A	27.7	80.4	717 / 525 / 459	0.18
B	26.9	77.4	886 / 599 / 464	0.09
C	155.4	73.2	678 / 490 / 447	0.30
D	45.4	73.1	722 / 520 / 473	0.27
E	23.8	83.7	585 / 485 / 455	0.28
F	20.4	78.8	837 / 569 / 477	0.23

*Note: All results are calculated using AFN except for Mean Velocity.

Figure 7. Mean ACH During Naturally Ventilated Hours (South Orientation)

Figure 8. CFD Velocity Vectors (South Wind, 1.2 m/s)

Figure 9. CFD Particle Streamlines (South Wind, 1.2 m/s)

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