Buildings that have been designed for natural ventilation will save on energy bills whilst keeping their occupants thermally comfortable. For a building to be effectively natural ventilated, it has to be designed to be one in the first place. CFD modelling and simulation is a powerful technology employed to provide guidance to architects and engineers with respect to the design and construction of highly effective naturally ventilated buildings.
When modelling ambient wind flowing around a building, our fluid domain will include the development of interest and the far field boundary which will be located far enough from the building model to avoid artificial acceleration of the flow.Our standard practice of simulating natural ventilation is to model and simulate the building of interest including the neighbouring buildings, structures and/or natural landscapes because they have the potential to partially block the wind and thus alter the flow field resulting in increased pressure drop, tunnelling effect (localised high velocity), decreasing or increasing the amount of wind entering the building of interest. Hence, the wind profile depends on the size and arrangement of neighbouring buildings.This inclusion of the neighbouring buildings, structures and/or natural landscapes is performed to ensure a more realistic scenario is simulated. To be specific, the extent of the surrounding buildings to be explicitly modelled shall be within the proximity of minimum 3 times the length of the longest distance measured across the boundary of the development, or within 500m distance from the edge of development of interest, whichever that is smaller. In the event that the building and surrounding development are located within hilly terrain, the topography information shall also be included in the simulation models. The domain height shall be extended, approximately 6 times the height of the tallest building within the defined vicinity.
To determine the most effective building design and layout for natural ventilation, we performed macro and micro CFD modelling and simulation for the site development. The macro simulation involved large scale ventilation simulation to determine the wind flow condition around the proposed building development and neighbouring buildings. On the other hand, micro simulation involved simulating wind flow condition of all living spaces inside the units. During the simulation, all windows and doors were assumed to be fully opened as designed except for the main door which was assumed to be closed at all time.
Performance-Based Fire Safety Design
The objectives of performing CFD modelling and simulation of performance-based fire safety design were to characterise the behaviour of fire, smoke and ultimately determined the robustness of the proposed fire safety design. Our CFD simulation results addressed the following issues:
Enclosure: defined the enclosure being studied in the CFD modelling. The enclosure details included the identity of the enclosures that belonged in the fire model analysis, the physical dimensions of the enclosures included in the fire model, and the boundary conditions of each enclosure.
Fire Locations: the fire locations used in the CFD modelling followed the proposed fire locations established in the Singapore Fire Safety Engineering Guidelines (SFSEG) 2015, published by SCDF.
Fire Characteristics: the source fire was the critical input for the fire scenario and was often described as the ignition source. The source fire was typically characterised by a heat release rate although other important aspects included the physical dimensions of the burning object, its composition, and its behaviour when burning. The heat release rate could have been specified as a continuous function of time or it might be an array of heat release rate and time data. The fire characteristics used in the CFD modelling followed the fire hazards as specified in Table 3.1 of the SFSEG 2015.
The variables employed to describe the fire were:
a) Heat release rate (HRR)
b) HRR per unit area
c) Fire perimeter
d) Elevation from finished floor level
e) Soot yield
f) Heat of combustion
g) Toxic substances (for fractional effective dose, FED)
h) Radiative heat flux (for FED analysis)
CFD Software: SCDF only recognised 3 simulation software for this purpose. The software we employed, ANSYS Fluent, was one of these recognised software.
Computational Domain: the computational domain was as close as reasonably practicable to the actual enclosure. Where inlet and/or exhaust vents were located at the domain boundaries, we included an additional 5m buffer outwards to account for the aerodynamics of the vents. Where wind effects were being modelled, the domain was extended correspondingly to take this into consideration.
Temperature: in general, ambient temperature in air-conditioned spaces was taken to be 24 deg Celcius while that of non air-conditioned spaces, 32 deg Celcius.
Smoke Management Systems: relevant smoke management system i.e. engineered smoke control system, smoke purging system or smoke vents was included in the CFD modelling.
Report: the report included output quantities for visibility, temperature and velocity. Additional output quantities e.g. carbon monoxide, heat flux, FED could also be included in the report but this depends on the scope of the performance-based design. For slice parameter, slices in all three planes (x, y and z-planes) were included. A cut along the centreline of the fire origin and slices at critical areas in the model were included. More slices could be included if a spill plume forms part of the analysis. At least 2 slices of Z-plane at 1.7m (estimated human height level) and 2.5m above the finished floor level were included. Additional slices at critical areas in the model could be included if required. An example of such a project we delivered was the Jakarta AEON shopping mall.
Pollutant (plume) Emission
The objectives of performing CFD modelling and simulation of plume emission were to determine the effectiveness of the pollutant discharge equipment and to optimise the system so that the emission would not recirculate to fresh air intake units of the building and affect neighbouring buildings.
In addition, there were other exposure limits set out by OSHA PEL, NIOSH REL, ACGIH which defined the permissible exposure limits for a given exposure period, as well as other critical concentrations such as odour threshold limit for the case of ammonia smell nuisance. Thus, it was prudent to ensure that the designed pollutant discharge was able to meet the required emission & exposure limits.
Recirculation into air intake units: in building and construction industry, it was common to locate both fresh air intake units and plume discharge equipment in close proximity such as on a building roof. Hence, there was possibility that discharged pollutant was drawn backed into the building through the air intake units. This could be the result of insufficient plume height and locations of the equipment.
Impact on neighbouring building: due environmental wind conditions, pollutant dispersion could be carried over to neighbouring buildings, which might have air intake units on their roofs or occupants with rooms exposed to the plume. If the plume height was not high enough to overcome neighbouring buildings, these plumes would have negative consequences on these buildings. This was especially so for taller neighbouring buildings or neighbouring building on a higher topography.
Simulation scenarios: multiple scenarios were considered to study the impact of different variables. These variables included wind direction, wind speed, variable performance level of discharge equipment and ambient temperature conditions. An example of such a project we did was the Singapore LKC school of medicine.
AUSTRALIA SINGAPORE ASIA PACIFIC REGION
The objective of performing wind-driven rain (WDR) simulations was to determine the effectiveness of a building’s weather protection features whilst still maintaining the effectiveness of its natural ventilation and smoke evacuation design capabilities. Our CFD simulation results answered 3 important questions:
Wind-Driven Rain (WDR): was the weather protection effective against wind driven rain? If rain droplets did penetrate the interior spaces, where was the exact location, what was the extent, distant and quantity? Which façade was affected the most? The objective was to ensure the safety of the building users and to minimise, if not prevent, rain water-induced damage to furniture and building structure.
Thermal Comfort: the weather protection might be effective against WDR but did it compromise the natural ventilation of the building? How effective were the louvres in allowing sufficient air to enter the interior space? Did the wind meet the recommended velocity of 1.15 m/s for 31 deg Celcius or 1.5m/s for 32 deg Celcius? The objective was to minimise the overdesign of weather protection feature which ultimately reduced natural ventilation.
Smoke Ventilation: in the event of a fire breakout in the station, what was the natural movement of smoke and the likely places of it getting accumulated? How effective were the louvres in allowing the smoke to escape to the atmosphere? How effective was the natural ventilation in evacuating smoke? The objective was to ensure the building design did compromise building fire safety even if the design was effective against WDR and promotes Thermal Comfort.
Bottom Line: In several projects, all of the above areas of study (WDR, Thermal Comfort and Smoke Ventilation) must now concurrently meet relevant government agency requirements. An example of such a project we undertook was the Singapore Canberra MRT station.
Cooling Tower Exhaust Ventilation
The objectives of performing CFD modelling and simulation of cooling tower exhaust were to determine the extent of exhaust recirculation into the cooling towers and to establish the temperature profile around the vicinity of cooling towers. Our CFD simulation results addressed the following issues:
Recirculation into air intake units: in building and construction industry, due to the space constraint on the building roof, it was common to locate the cooling towers in close proximity to one another. Moreover, trellis were installed above these cooling towers for aesthetic reason. Because of such a scenario, there was a possibility that the exhaust from a cooling tower recirculated into its own air inlet or inlets of other cooling towers. This might eventually affect the performance of the cooling towers. Thus, our CFD simulations predicted whether recirculation occurred based on the proposed arrangement and locations of the cooling towers.
Temperature profile: a temperature profile around the cooling towers was also established to determine whether elevated temperature affected the cooling towers performance or whether they were acceptable to the efficient operations of the cooling towers.
Impact by neighbouring buildings: due environmental wind conditions, the performance of a group of cooling towers might be affected by neighbouring buildings. This was especially so for shorter buildings being surrounded by taller neighbouring buildings which might act as obstacles to wind flow.
Simulation scenarios: multiple scenarios were considered to study the impact of different variables. These variables included wind direction, wind speed, variable performance level of discharge equipment, ambient temperature conditions and height of trellis. An example of such a project we did was the Singapore OUE building.
Within the building & environment industry, we provide simulation services in the following areas:
- Building acoustics analysis
- Building performance
- Cooling tower exhaust ventilation
- Data centre room layout optimisation
- Generator exhaust extent of recirculation and HVAC ducts optimisation
- Natural ventilation simulation study
- Optimisation of solar chimney design
- Performance-based fire safety design
- Pollutant (plume or particles) emission
- Wind-driven rain
Buildings consume much energy and contribute to greenhouse gas emissions. The quests are thus to reduce the carbon footprints of buildings and their energy usage. Since buildings are relatively expensive to build, their performance must be known and ensured prior to construction.
Our simulation consultants performed CFD simulations on Jakarta, Indonesia T3 Airport to analyse impact of changes, answered 'what if' scenarios, provided comprehensive data which were difficult to obtain from experimental tests and allowed alternatives to be studid before optimum designs were confirmed.
As building simulation consultants with expertise in building performance analysis, our in-depth simulation analysis services have assisted our clients to achieve optimum building performance in the areas of thermal comfort analysis (velocity, temperature, RH), design of natural ventilation, solar and daylighting analysis, embodied energy, energy usage, architectural design, room layout, HVAC sizing for occupants comfort and optimising the locations of diffusers and returns.
Data Centre Room Layout Optimisation
The objectives of performing CFD modelling and simulation on Reserve Bank of Australia data centre were to: (1) determine the effectiveness of the thermal cooling system by analysing the temperature, air flow and pressure, (2) optimise the data centre room layout design, (3) predict the uniformity of air flow coming out from the tile diffusers and (4) predict if a CRAC unit fails, how much time was available for the operators to react.
Our CFD simulation results answered 4 important questions:
Cooling system – we determined the presence of temperature hotspot, air flow dead zones and cooled air under or oversupply. The effectiveness of a data centre thermal management system was determined by its ability to maintain a consistent homogeneous room temperature profile and prevented the room temperature from exceeding certain temperature.
Optimisation of layout – answered ‘what if’ scenarios such as the relocation of tiles, server racks, server rack maximum height and CRACs returns. It was always so much financially cheaper to investigate the impact of changing a variable in the ‘virtual model’ than to do so in the real physical world. For example, the impact of adding server racks on the overall cooling capacity was simulated first to ensure sufficient cooling would still be available and no additional hot spots were being introduced. After investigating the relevant variables, the data centre was modified with confidence and cost effectively.
Uniformity of air flow – predicted the air flowrate and temperature coming out from each tile based on sub-plenum design and features such as piping, cable trays and support columns. Knowing the flowrate coming out from each tile enabled the tile to be positioned in appropriate locations which in turn prevented the oversupply or undersupply or cooled air to the server racks.
Response time – if a given CRAC unit fails, predicted the duration available for the operators to react. Knowing the response time available meant a contingency plan that factored in this response time could be developed to prevent the room from overheating and damaging the server equipment.