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Model answers

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Our feature on wind tunnel testing (AJ 30.5.02) has prompted a reappraisal of the merits of CFD assessments

In seeking to analyse a building's environmental performance and the suitability of natural ventilation, standard calculation procedures can, in the majority of cases, provide all the necessary information. However, in certain buoyancy driven schemes, where stack effects or air flows are important in the ventilation design, a more detailed assessment would help optimise the performance to reduce carbon emissions and provide added credibility to the design.

This is particularly true for cases involving rooms with high ceilings and large glazed areas or even in smokeshafts (see next week's AJ Technical and Practice). This is where Computational Fluid Dynamics (CFD) comes into its own.

Although the technology has tended to be considered as something of an expensive luxury, rather than a necessary and invaluable tool of the trade, this is set to change. The cost of high-powered computing has come down dramatically and improvements in the functionality and ease of use of commercial software mean that CFD is becoming standard practice in many industries.

Whereas in the past, CFD tended to be available only to those companies with significant resources to construct and solve representative models within a realistic timescale, such as in the automotive, process and aerospace industries, the technology can now be considered as a valuable and practical design tool for analysing performance. Models can now be created in a matter of hours and solved overnight, providing airflow characteristics within a simple 3D space, quickly and relatively inexpensively. The results provide comprehensive information on flow velocity, temperature, humidity and many other required variables. This can be used to predict the occupant comfort levels within a building before construction has even begun.

For those wanting to use CFD in their design, the first steps in applying it are actually quite simple. The process begins with a 3D CAD model, which can be supplied by the designer or architect in a suitable format. If this is unavailable, the layout can easily be built from drawings using the geometry creation tools available in most commercial CFD codes.

Hard cell Having constructed or imported the geometry of the building design, the air-filled space is broken down into hundreds of thousands of tiny cells to generate a computational mesh.

Each cell represents a point in space where all of the variables of interest will be calculated.

The next step is to specify the boundary conditions that will describe the physics of the building, along with any appropriate models - turbulence or thermal radiation, for example). The boundary conditions specify variables such as flow rates at openings and heat sources from occupants and equipment.

Careful consideration needs to be given to determining the most appropriate inputs at this stage. CFD has a wide range of applications, so its application needs to be correctly targeted. It is important that the models selected should be physically representative and reflect reality, otherwise the data produced will be of little value. From here on, the computer takes over the process as, iteratively, it solves the equations of energy and fluid motion in each individual cell, taking into account the physical laws of conservation.

When the programme has produced a solution, the user can interrogate the results using the range of post-processing options to determine airflow and thermal characteristics of the building, as a whole, or just in local zones. This will provide an accurate and realistic virtual picture of how the building design will perform.

A variety of outputs allows the user to obtain any information they may need, whether graphical or numerical. Point data provides output comparable to conventional probes (such as at smoke detectors and airflow feedback controls). Average, minimum or maximum values of any quantity can be determined for the whole building or regions of interest, eg the average temperature and humidity in occupied spaces.

Contour plots provide graphical output over a selected region, such as air speed within a slice of a room.

Comfort criteria can be calculated and plotted directly throughout the domain. The use of advanced options enables the user to produce animations, such as the flow path lines from diffusers or walkthroughs of the geometry, allowing the designer easily to visualise the fluid flow physics at work. These powerful tools ensure that the complex results of a CFD study can be analysed and understood by all the team members involved in a building design - not only those with specialist technical knowledge - to the benefit of the whole project.

In recent times, a number of building designs have aimed at attaining higher levels of energy efficiency, reducing a proposed building's energy needs and giving a competitive edge over rival designs. CFD can often be a key factor in achieving this efficiency, allowing the designer to play with possibilities - and has built a healthy track record of success. The potentials for CFD include:

atria, auditoria and sports halls;

fire and smoke modelling to determine if mechanical smoke extraction is required;

assessing car-park ventilation for exhaust and smoke extract;

clean rooms and contamination control for pharmaceutical and electronic manufacturing;

data centres and telecom switching facilities;

offices and commercial premises;

environmental wind analysis.

Models v modelling Having outlined what CFD can do, it is important to also be aware of what it cannot. CFD technology provides extensive capabilities but it does not offer those involved in building design a solution for all problems. For example, when predicting how a building reacts dynamically with external conditions, wind tunnel testing can play a vital role in providing data such as wind pressure coefficients at window openings.

However, Dynamic Thermal Modelling remains the best method for predicting how the building will react over the longer period. By basing its analysis on historical weather data over a long period of time, the method allows calculation of a building's thermal response, showing where and when peaks will occur and energy performance over a representative year.

The strength of CFD in such an example would be in providing a more comprehensive assessment and understanding of the thermal and airflow characteristics around and within the building, at the times when normal or peak loads occur.

In terms of practical usage, there are technical limitations too. The capability of CFD remains restricted by the speed and memory size of any given computer. For large buildings that require complex models, a cluster of computers to solve the problem within a realistic timescale may still be required.

However, current software allows standard desktop machines to be used in parallel, to achieve the necessary performance. Also, as with all modelling tools, the results are only as good as the boundary conditions that are applied. It is essential that only someone fully trained in the application of CFD uses the software;

an inexperienced user is likely to generate a converged solution that is not true to life.

By allowing the designers to obtain a detailed insight into the behaviour of a building's physics that other designers may not have, a CFD capability can deliver a significant competitive edge. This previously impossible insight into how an integrated building and ventilation design will perform can then free the designer to be more knowledgable and creative in their design.

Daniel Burton is business development manager and Steve McCormick is senior CFD engineer at Fluent Europe, tel 0114 281 8888, email daniel@ fluent. co. uk or visit www. fluent. com

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