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CFD Simulation for Sustainable Design
27 January 2020
Apurva Jain

How CFD Analysis Can Make Buildings Greener

The ‘green’ mantra in construction is no longer just a chant by new-age bohemian-type architects and engineers. Green buildings are becoming a necessity in many countries and an increasing number of building regulation authorities globally are insisting on the incorporation of green architecture or green energy initiatives as part of building design. Promoting and facilitating green energy use in construction, computational fluid dynamics, or CFD, involves simulations and calculations that are part of the larger mechanical engineering, plumbing and fire, or MEP design engineering, umbrella. Though CFD encompasses a range of study and calculations, intelligent use of CFD calculations can save energy costs and thus make buildings greener.

Looking at CFD, computational fluid dynamics is part of the study of fluid mechanics using numerical analysis and other data to analyse and predict issues concerning the flow of fluids (both liquids and gases). It’s also known as the study of heat transfer. Computational fluid dynamics can offer solutions to these issues. The full-stream flow of the fluid and interaction of different fluids with a choice of surfaces can be calculated and simulated to better understand the flow visually. Information gathered through CFD calculations typically help analyse the following conditions:

  • Exhausts and their effect on the environment
  • Smoke and fire risk
  • Quality of indoor environments
  • System of natural ventilation
  • Exterior wind loading
  • Heating/cooling load calculations

As far as green energy is concerned, CFD calculations help determine complicated HVAC requirements, such as:

  • Optimal occupant thermal comfort
  • Reduction of equipment required for air flow
  • Considering heat outside for passive heating
  • Analysing wind loading on a building
  • Analysing acoustic paths and sources of noise
  • Controlling contamination risks in sensitive areas
  • Increasing crop output in indoor agricultural locations

Though all of these conditions are important factors in making a building green, one of the first requirements to consider is wind speed, wind direction and natural ventilation. A clear picture of how the variations and flow of fluids affect the environment is most clearly seen through simulation. By studying fluid flow, heat transfer, chemical reactions and related phenomena through applied mathematics, physics and computational software, it is possible to create an accurate visual of the effect of fluid flow and its interaction with objects in its immediate surroundings. Additional factors considered are velocity, pressure, temperature and the density of the fluid in motion, in this case – the wind.

It is possible to ensure structural integrity under high wind loads by analysing wind speed and wind direction at different times of the day or year through CFD. Closely placed high-rise buildings in urban areas alter natural wind behaviour. Simulation aids created through CFD help design ventilation systems that effectively optimise natural and mechanical ventilation. Using CFD calculations can help identify where there is wastage or mixing of hot air and cold air. Wind and buoyancy can help push fresh air through a structure, and simulation can depict exactly how that is likely to happen. If CFD simulation can help promote natural ventilation in a building, the building becomes an energy-efficient space. Computational fluid dynamics can also analyse envelope/roof thermal transmittance to calculate heat gain or loss to improve wall, roof and floor configurations for optimal efficiency.

So, in effect, the advantages of considering CFD with the help of simulation results in:

  • affordable, reliable and accurate representation of design that helps architects make the right decisions
  • simulation of more than one possible situation, without a physical set
  • accurate data on velocity, pressure, temperature and density of moving fluid
  • ability to predict and modify fluid movement before actual implementation
  • analyse HVAC system efficiency and indoor air quality (IAQ)
  • optimise natural, or passive, heating, cooling and ventilation

The optimisation of HVAC performance (with the help of improved natural ventilation) can significantly improve the ‘green’ rating, or star rating, of a building. Simulations help decide the possible thermal comfort of a home. In Australia, the star rating is measured on a scale of zero to 10, where the higher number of stars indicates that occupants need less artificial cooling or heating for their comfort. Houses rated at 10 stars do not require heating or cooling using conventional energy sources and are thus as ‘green’ as can be.

One of the ways CFD simulations benefit architects is that it helps them understand site land form more clearly, and by simulating the wind environment, they are better able to comprehend how site surroundings can affect the wind environment. Architects can also conduct qualitative analyses and integrate natural ventilation into the design after studying how natural ventilation affects the site plan, the building’s shape and the building’s envelope interface through CFD.

On-site Planning with CFD Simulations

The relationship of a building with its site, other buildings around it, wind directions and building clusters can be optimised by studying CFD simulations. This will maximise natural resources and create opportunities for improved natural ventilation while keeping adverse winds in check at certain areas. Simulations consider wind directions in summer and winter and wind changes due to surrounding buildings, thus enabling the positioning of the building for optimum results. The simulations can also indicate how open spaces, such as roads, green sections, water bodies and plazas form air flues that can bring in prevailing winds during summers.

Wind speed, direction and airflow circulation varies at all times. In case of excessively high wind speeds, where the wind velocity is too high for pedestrian comfort, it occurs typically as a result of narrow wind tunnels within compact building layouts and upwind building blockages, so that it is difficult for downwind buildings to have cross ventilation. Since CFD simulations study average wind speed, wind shadow zone, size of calm zone (wind speed lower than 1m/s), size of strong wind zone (wind speed higher than 5m/s) and wind pressure, buildings can be designed to adjust to these factors and provide a comfortable wind environment for the occupants both inside and outside the immediate vicinity of the building.

Thermal pressure influences building ventilation in large indoor spaces. Designing building shapes, interiors and floor plans efficiently needs the architect to consider the following factors:

  • Reasonable room depths that incorporate functional elements
  • Forms can be created at different, suitable locations in a building to deflect wind and direct it through wind tunnels. This can be done by distributing building volumes and creating open spaces to enable natural ventilation.
  • Atriums, patios and alleys can be created to aid thermal pressure ventilation. Simulations using CFD can accurately predict wind pressure values so that surface wind pressure of rooms can be calculated, and natural wind pressure ventilation can be assessed. Temperature difference, height difference and the position of planes determine the thermal pressure ventilation of a space.
  • Coupling calculations made from a 3D model’s radiation and temperature readings can be combined with wind speed and wind temperature readings to estimate thermal pressure ventilation and further improve it.
  • Some buildings have a staggered layout. This results in the formation of a ‘calm zone’ during summer and affects natural ventilation unfavourably. After consulting CFD simulation visuals, the open spaces are assimilated into the building forms, particularly ground levels. Opening up the ground levels increases wind speed and creates favourable natural ventilation.
  • Envelope openings of buildings and wind deflection structures can be optimised using CFD technologies. Simulations and calculations through CFD also help direct ideal indoor airflow paths and avoid blocks.

Following the consideration of the above factors, an architect has further considerations to think about. To balance indoor wind speed and airflow distribution density, CFD simulations can help an architect decide on:

  • Appropriate position, size and method of opening doors and windows
  • Installation of wind deflection boards to improve airflow distribution
  • Effect of sun-shading components on indoor ventilation

These considerations, through their accuracy and detail, can improve the utilisation of natural ventilation, thereby reducing HVAC and electrical usage, resulting in greener buildings.

In a real-life example, an exhibition centre in China created suitable openings on its envelope structure to direct optimum airflow paths after taking into account outdoor wind pressure and wind velocity. The results of CFD simulation showed that a large calm zone existed in a glazed corridor with poor ventilation. Windows and doors were then modified so that east and west-facing windows were opened to increase the volume of natural ventilation in the corridor, resulting in optimum thermal comfort.

So, in effect, one of the key advantages of CFD simulation and calculation technologies is that it helps qualitatively analyse and evaluate the natural ventilation in a building so that green building design is optimised. Specific features of the site plan, building shape and envelope are studied using CFD wind simulations to improve natural ventilation, resulting in energy-efficient green buildings.