The impact of wind on tall buildings can be calculated with cfd, but this isn't always the best way. We discuss the options available.
In recent years, cities have been moving up as much as out. In spite of the controversy over tall buildings (not including the September 2001 attack in New York) it seems that the only way is up for developers and architects alike.

At the same time, there has been a surge in the use of computational fluid dynamics (cfd) to predict the impact of wind on buildings and the surrounding areas. CFD has proved extremely useful in many ways, allowing engineers to try out different scenarios.

However, while cfd is well established in certain fields, it is still in its infancy in wind engineering terms and in some circumstances may not be the appropriate tool to use.

Researchers at BRE are currently working on a research project that will compare the performance of both cfd and the traditional tool, wind tunnel testing. On completion, the researchers will publish guidance that will help architects and engineers to make the right choice.

Why are wind studies necessary?
As buildings become taller and more complex in form, it is increasingly important for designers to understand the effect that local wind forces will have on a development. Armed with an accurate picture of the wind environment, the architect or engineer can predict the impact on:

  • structural strength/safety;
  • pedestrian comfort/safety;
  • environmental performance (hvac systems, natural ventilation, smoke exhaust);
  • innovative schemes such as alternative energy systems using wind turbines;
  • pollutant dispersion (traffic, boiler flues and industrial processes).

The tools
The two most commonly used tools for predicting the behaviour of wind around buildings are wind tunnel testing and cfd. The question is, which method should be used and when?

Wind tunnel testing is well established and has a proven record of producing reliable results. However, the design team will need to secure access to a specialist test facility where the work can be undertaken by skilled engineers and technicians.

CFD, in contrast, is widely available. Software packages can be bought off the shelf and the work undertaken in-house or outsourced to an external consultant. But despite apparently being easy to use, cfd requires specific specialist knowledge at the input stage as well as at the interpretative stage. Without this expertise, it can be used inappropriately and may produce unsafe results. Similarly, it is not uncommon for a study to be contracted-out to a cfd expert who, although familiar with the concepts and workings of the program, might lack specific expertise in wind engineering. This can result in uncertainty in the interpretation of the results and how to act on them.

While cfd is an effective tool in fields such as aeronautics and weather forecasting, the degree to which it can be applied reliably to wind engineering problems is less certain. Computational wind engineering is generally performed using commercially available cfd codes, which today are remarkably flexible in terms of the problems they can address.

However, special care is required when applying a general-purpose code to a specific problem. Wind engineering has its own characteristics and peculiarities that make it difficult to model. For example, the representation of the incoming turbulent boundary layer and the fine structure of the flow field around the building envelope are both critical, but difficult to model correctly.

There are two other important issues surrounding the quality of a cfd prediction: the treatment of turbulence, and the resolution of the numerical mesh. All cfd codes make approximations and assumptions in the treatment of turbulence because current computers are unable to solve fluid flow equations exactly. There are practical limitations on the number of data points, and hence mesh resolution, that can be handled without resorting to very large and powerful computers. The most commonly used models solve the time-averaged equations, making use of empirical and semi-empirical turbulence sub-models to represent the variations that are 'lost' in the averaging process.

Smoothing out peaks and troughs
However, because the equations are averaged, it is not possible to analyse peak wind loads, peak pollutant concentrations or gustiness. This makes cfd unsuitable for studying structural response. It could be used to assess the impact on building ventilation systems and pedestrian comfort, but only if information about gustiness is not required. With respect to numerical mesh, an inappropriate choice can lead to poor predictions. This is true particularly in wind engineering applications where sufficient resolution around the building envelope is critical if complex flow patterns are to be captured.

Like cfd, wind tunnel predictions require expert handling. Here, an important issue is the appropriate use of the scaling laws that allow the results obtained at model scale to be realised at full scale. However, the main advantage of wind tunnel testing over cfd is that the unsteady characteristics of wind and its interaction with buildings are properly represented in the wind tunnel. This is critical for obtaining information about peak structural loads and the gustiness of conditions for pedestrians and hvac performance.

To summarise, it is important to be aware of the benefits and limitations of each tool, and their suitability for different applications. Cfd can be successfully applied to predict internal flows and to assess thermal comfort and air quality.

However, even here some designers may require the confidence that can only be gained from full scale mock-up testing of complex solutions. Wind tunnel testing is still probably the most appropriate tool for examining external flows around the building and its impact on structural safety, pedestrian comfort and hvac performance.

In many instances, the best and most cost effective solution may be to use the two tools together, an approach used successfully at BRE when investigating a range of complex problems.

Developing guidance
Because there is much still to be understood about cfd's application as a wind engineering tool, the Foundation for the Built Environment (FBE) is sponsoring BRE to undertake research1. It will consider the current position and assess the comparative performance of the alternative tools. It will:

  • establish the relative benefits and limitations of wind tunnel analysis and cfd modelling;
  • assess quantitatively the accuracy of prediction for both wind tunnel testing and cfd against a number of test cases.

All relevant issues will be studied, including the comparative accuracy, reliability, cost, turn-around time, flexibility, detail of information and ease of use of the alternative methodologies. Specific cfd issues will be addressed, in particular the appropriate choice of turbulence model, wind boundary condition specification and mesh resolution. In addition to undertaking comparative wind tunnel and cfd studies, both approaches will be scrutinised against full-scale experimental measurements. On completion, FBE will publish guidance that will enable designers to make informed decisions on the most appropriate tool to use.

Until we have the results of this research, we must recognise that there are valid reasons to be concerned about the applicability of cfd to wind engineering problems.

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