Introduction

Computational modelling techniques are increasingly being used to successfully predict the internal and external conditions within and around buildings. Building services engineers, architects, developers and clients use the results to evaluate HVAC strategies to ensure specific project requirements are achieved on-site first time. Substantial savings can be made through low cost design and the avoidance of on-site re-design and re-commissioning.

Through a powerful combination of dynamic thermal simulation and CFD, both the energy performance and details of internal comfort of your design may be assessed. An analysis of the feasibility of low cost HVAC and natural ventilation strategies using real data may be performed. Once set up, design modifications may be assessed quickly and cheaply.

Similarly, the availability of daylight, artificial lighting strategies and shading/overshadowing may be rapidly assessed and optimised.

Dynamic Thermal Simulation

The external environment of a building changes month-by-month, week-by-week, day-by-day and hour-by-hour. Due to its thermal capacity, the fabric of a building responds slowly to these changes. This in turn leads to a more gradual dynamic response of the interior climate as felt by the inhabitants. The prediction of these interior conditions is a vital component in the design of heating and cooling strategies.

Dynamic Thermal simulation is a technique which can predict these changing internal conditions over a time period of up to 1 year. The technique predicts zonal (or room) values for parameters such as air temperature, radiant temperature and air-change rates, using real weather data.

However, to predict the internal climate in detail, CFD is the only choice. CFD alone can predict hot spots, cold spots, draught risk and analyse flow paths vital for the assessment of HVAC schemes and natural ventilation strategies.

CFD in Building Design (click here for 'what is CFD ?')

With ever improving computer technology, the use of CFD in the design of HVAC strategies is becoming more widespread. CFD is the only choice to obtain the important details of the internal climate which result from a particular HVAC design. Flow Analysis Ltd has the expertise necessary to fully utilise the latest CFD software for your design issues, be they internal air flows or external (wind-driven) air flows.

A great strength of CFD is the clear and accessible output formats: colour contours, vectors and streamlines, which are easily understood by the non-specialist. For complex 3-dimensional spaces, animations are a great asset to gain both further insight and bring the simulation to life.

Once the geometry of the building, room or zone has been set-up, it is the known state of the air at, for example inlets and heat sources, which determines the flow and temperature elsewhere. An important source of heat is of course solar radiation. The amount of solar radiation (and reflected long-wave radiation) incident on the surfaces of the building may be obtained from the dynamic thermal simulation described earlier. Other important determining factors, such as flow rate through apertures may also be obtained from a dynamic thermal simulation. Thus dynamic thermal simulation may be used to provide the boundary conditions for CFD.

Once the geometry and boundary conditions have been set up, CFD may be used to obtain a 'snap-shot' of the internal climate at a particular representative time-instant. Or, if preferred, a worst case scenario may be examined, such as conditions of zero wind for a natural ventilation scheme. Once the model is set-up, design modifications can be easily incorporated and the simulation re-run at a low cost.

With ever-increasing computing power, it is now feasible to simulate the external (wind-driven) air flows around buildings (see picture above, for example). These simulations may be used to determine pedestrian comfort (increasingly required for planning approval), and pollutant or stack-exhaust dispersal.

CFD can also be used for transient calculations. This may be utilised for HVAC assessment over a small time-period, or more commonly for smoke and pollutant modelling (see the animation at the top of the page).

The availability of adequate natural daylight is of vital importance to the well-being of occupants. State-of-the-art computer simulation software as used by Flow Analysis Ltd is capable of providing a full picture of the daylight factors, lux levels and daylight autonomy throughout your building with rapid turnaround times.

Of course, artificial lighting will almost certainly be required and this can be easily added to the model and optimised in terms of light levels, cost and energy useage.

Shading is also an important issue which may be fully addressed in the software used by Flow Analysis Ltd. For example, by adding surrounding buildings, the shading patterns cast by a new development on existing buildings may be determined.