CFD simulation of a data centre/IT suite showing airflow paths and predicted temperatures.
Simulation the way to best designs
Computer simulation in building design can provide extraordinary value for money resulting in lower capital expenditure, more efficient systems and a better design solution. David Stribling of Buro Happold's specialist simulation group, CoSA, in Scotland, discusses the advantages.
01 October 2002
The idea of virtual prototyping in engineering design has been around since the 1970s when the aerospace industry started to use computational fluid dynamics (CFD) to predict airflow over aerofoil sections and thus predict the performance of a wing design prior to building expensive models and testing in wind tunnels.
Over the years, this sort of technology has extended to many engineering applications and CFD is now applied in a wide ranging field of applications from the design of F1 racing cars to the cooling of electronic systems such as PCs and mobile phones.
Moreover, the concept of virtual prototyping does not apply only to modelling airflow using CFD but maybe something like 3D CAD which can be used in services co-ordination or conveying a better understanding of the building layout for testing access or addressing ergonomic considerations.
This article describes the most popular forms of virtual prototyping used in the construction industry today and gives examples where its use can provide significant advantages for both the design team and end client.
Dynamic thermal modelling
For the last 15 to 20 years building services consultants have used computers to carry out load calculations based on simple steady state heat loss calculations for individual zones.
Dynamic thermal modelling (DTM) takes this several steps further by taking a holistic view of the building and simulating how this reacts dynamically with the external conditions.
Using detailed historical weather data this far more detailed analysis allows calculation not only of what the peak loads on the building may be, but also at what time of the day they will occur.
This may well be outside the hours of normal occupancy, raising the question of whether they should be included in the sizing of the plant. Clearly this is a potential saving on cost of plant that could not have been realised using conventional steady state calculations.
Global attempts to reduce carbon dioxide (CO2) emissions means that sustainability is gaining increased popularity, leading to a move towards more naturally-ventilated buildings.
This raises the question of precisely what level of summer time temperature will be achieved in the space with no mechanical cooling. A DTM simulation of the building can go a long way to answering this question as well as testing innovative strategies such as night-time cooling, core activation or automatically opening vents.
Other possible areas for testing may also be strategies for solar shading such as high-performance glazing or brise-soleil. This would not only allow a comparison of the peak solar gain but also a calculation of the number of hours a certain temperature criterion may be exceeded during the course of a year. This information can then be combined with financial information in order to arrive at a cost effective design solution.
The latest versions of DTM software packages even allow the testing of a new building against Part L of the UK's Building Regulations providing the ability to test compliance of the building even at the conceptual or planning stage.
Computational fluid dynamics
Whereas DTM provides an overview of the dynamic response of the building together with the prediction of loads, computational fluid dynamics (CFD) provides a far more detailed analysis of the airflow characteristics and temperature distribution in specific rooms or spaces. CFD modelling can be employed at any stage of the design process. For example, it could be used at the conceptual design stage to demonstrate the relative merits of different strategies, at detailed design to optimise the chosen strategy, or employed on an existing installation as part of a building diagnostic review.
Typical applications of CFD might be:
The value of CFD analysis is different for each of these applications, as are the reasons driving such simulation work. For example in the field of data centres, reliability is the key driver and becoming more of a challenge as heat densities increase. Once the goal of reliability has been achieved, however, energy efficiency also becomes an issue, having an immense impact on the operating costs of such a facility.
In the field of contamination control, the issues would be more related to productivity by increasing yield in a silicon wafer fab. In pharmaceutical production, health and safety takes over, whether this be protecting operators from hazardous chemicals or maintaining the sterility of products.
Clearly, whatever the design issues needing to be addressed, CFD can and should be used to gain confidence in a particular strategy. Furthermore, the analysis should not stop at a simulation of the design in question.
The real value in a virtual prototype of a building is that performing 'what if' scenarios costs very little. This encourages innovation and the scope to reduce capital expenditure and increase energy efficiency.
In the early 1990s, British Telecom saw this potential when it began to explore the possibilities of eliminating refrigeration from its exchanges.
CFD analysis allowed the company to develop a free cooling strategy for which it won a Business Commitment to the Environment Award, reducing CO2 emissions by more than 22 tonnes per exchange per year as well as the total elimination of CFC (chlorofluorocarbons) and HCFC (hydrochlorofluorocar-bons) refrigerants in new installations.
Lighting simulation
Good lighting design is a careful blend of art and science, with the final design facilitating an activity or task to be undertaken easily and comfortably whilst both respecting and harmonising with the form of the space which surrounds it.
Lighting simulation has two functions in helping to achieve these goals. The first is the simulation of artificial lighting to ensure illuminance levels at the working plane and glare indices meet health and safety criteria.
Similar techniques can also be used in architectural lighting to give a virtual image of the effects of different lighting arrangements.
The second use of lighting simulation is in the prediction of daylighting levels within a building. The higher the amount natural light admitted into a building, the lower the cost of artificial lighting.
This point is made even more significant with the development of automatic lighting controls which sense the level of daylight and location of occupants, thus reducing energy costs.
Once again, testing of different lighting strategies is one of the most beneficial aspects to lighting simulation with room configuration, fenestration, light shelves and internal finishes all impacting on the levels of daylighting achieved in a space.
People flow modelling
Although not something immediately associated with building simulation, the modelling of people flow is becoming increasingly popular.
Exodus is a specifically written software application jointly developed by Buro Happold and the University of Greenwich.
Applications for people circulation modelling are almost limitless but the sorts of projects it has been applied to in the past include schools, airports, theatres and stadia where the efficient movement of people is essential.
Exodus is also extremely useful in evacuation modelling and was used extensively on the New Millennium Experience project to support the design of all evacuation routes.
Modelling of evacuation flows is of significant value in community and assembly buildings, offices and shopping complexes where it can be used to compare evacuation times with the time for hazard development.
This determines whether the characteristics of the building would allow for extended travelling distances or reduced door sizes when used in conjunction with the recent guides such as Draft BS99999 "Means of Escape".
There is no doubt that with increased emphasis on our health and comfort both at work and leisure, and a more focused approach to energy efficiency and sustainability, performance based design is the future of the construction industry.
Rules of thumb are great tools for checking calculations and are in the right ballpark, but as buildings evolve, each one becoming unique, design methods must follow suit.
Building simulation is the common sense use of the technology at our fingertips to deliver the best, most efficient and suitable designs for our clients.
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