technical & practiceat the input stage as well as at theinterpretative stage. Similarly, it isnot uncommon for a study to be con-tracted-out to a CFD expert who,although familiar with the conceptsand workings of the program, mightlack specific expertise in wind engi-neering. Lack of expertise might leadto uncertainty in the production andinterpretation of the results.

Computational wind engineeringis generally performed using com-mercially available CFD programs –which today are remarkably flexiblein terms of the problems that theycan address – but special care shouldalways be taken not to underestimatethe complexity of the proceduresinvolved – not just in obtainingresults, but also in assessing them.Wind engineering has its own char-acteristics and peculiarities thatmake it difficult to model virtually.

For example, it is difficult to rep-resent the incoming windcharacteristics correctly – in particu-lar, the air turbulence, which isinfluenced by the type of terrain sur-rounding the buildings. Flowpatterns, even around a simple blockbuilding, typically comprise:● a horseshoe-shaped vortex on thewindward side of the building;● jets of air at the corners of thebuilding that may extend for aconsiderable distance downstream ofthe building;● regions of ‘flow separation’ on thetop and sides of the building wherethe flow pattern ‘separates’ from thebuilding;● a highly turbulent wake region onthe downstream side of the building,where wind speeds are relatively lowdue to the sheltering effect of thebuilding.

Incorrect representations of theseflow features will lead to erroneouspredictions of wind speed and sur-face pressures around the building.

Another fine meshThere are two other important issuessurrounding the quality of a CFDprediction: the treatment of turbu-lence and the resolution of thenumerical mesh, which is a concep-tual diagram of predictive velocitiesand pressures at given points. Theseare predictive because all CFD pro-grams currently have to make

There has been a surge in the use ofcomputational fluid dynamics(CFD) to predict the impact of windon buildings and their surroundingareas. While CFD is well establishedin certain fields, such as industrialengineering, transportation or aero-nautics, it is still in its infancy inarchitectural terms. But why arewind studies necessary?

As buildings become taller andmore complex, it is becomingincreasingly important for designersto understand the effect that localwind forces will have on a develop-ment, especially at ground level.

Typically, wind tunnel tests areundertaken at the planning stage, inorder to demonstrate to local author-ities that a development will notcause unacceptable wind speeds tooccur at pedestrian level. Or theymay be left until later in the designprocess, when data is required for thedetailing, where they can help withthe cladding and/or building ser-vices’ design.Armed with an accuratepicture of the wind environment, thearchitect or engineer can predict theimpact on:● structural strength/safety;● pedestrian comfort/safety;● environmental performance(HVAC systems, natural ventilation,smoke exhaust);● innovative schemes such asalternative energy systems usingwind turbines;● pollutant dispersion (traffic,boiler flues and industrial processes).

Flow diagramsThe two most commonly used toolsfor predicting the behaviour of windaround buildings are wind tunneltesting and CFD. The question is,which assessment method should beused, and when?

Wind tunnel testing is well estab-lished and has a proven record ofproducing reliable results. However,the design team will need to secureaccess to a specialist test facility

where the work can be undertaken byskilled engineers and technicians.

CFD, in contrast, is widely avail-able. Software packages can bebought off the shelf and the workundertaken in-house or outsourcedto an external consultant. But despitebeing apparently easy to use, CFDrequires specific specialist knowledge

It’s a breeze Studying the actual air flow over a physical building model canhave advantages over computer simulationsBY PHILIPPA WESTBURY AND STEWART MILES

40 l the architects’ journal 30 May 2002

The quality ofmodel moves on,but air flowsremain the same

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approximations and assumptions inthe treatment of turbulence due tothe practical limitations on the num-ber of data points – and so meshresolution – that can be handledwithout resorting to very large andpowerful computers.

The most commonly used modelssolve the time-averaged equations,making use of empirical and semi-empirical turbulence sub-models torepresent the variations that are ‘lost’in the averaging process. But becausethe equations are averaged, it is notpossible to analyse peak wind loads,peak pollutant concentrations orgustiness.

This makes CFD unsuitable forstudying structural responses. Itcould be used to assess the impact onbuilding ventilation systems andpedestrian comfort, but only if infor-mation about gustiness is notrequired. With respect to numericalmesh diagrams, an inappropriatechoice can lead to poor predictions.This is true particularly in wind engi-neering applications, wheresufficient resolution around thebuilding envelope is critical if com-plex flow patterns are to be captured.

Like CFD, wind tunnel predic-tions require expert handling. Here,an important issue is the appropriateuse of the scaling laws that allow theresults obtained at model scale to berealised at full scale. A physical windtunnel model may not need the samelevel of detail as an architectural pre-sentation model; typically, onlyfeatures greater than about 0.5mneed to be represented. However, it isquite possible to use an architecturalmodel rather than building a new

one if it is at an appropriate scale.Furthermore, wind tunnel tests canbe carried out as quickly as computa-tional modelling (and sometimesquicker, if data for numerous winddirections is required).

It is important to be aware of thebenefits and limitations of each tool,and their suitability for differentapplications. CFD can be successfullyapplied to predict internal flows andto assess thermal comfort and airquality, but some designers mayrequire the confidence that can onlybe gained from full-scale mock-uptesting of complex solutions. Windtunnel testing is still probably themost appropriate tool for examiningexternal flows around the buildingand its impact on structural safety,pedestrian comfort and HVAC per-formance. In many instances, themost accurate and cost-effective,long-term solution may be to useboth together.● For more information contactPhilippa Westbury, head of environ-mental wind engineering at theBuilding Research Establishment, onwestburyp@bre.co.uk, or telephone01923 664300.

COMPARISON OF APPRAISAL METHODS

Method Advantages DisadvantagesWind tunnel Well established and validated Relatively few centres with suitable facilitiestesting Mainly performed by specialist contractors Measurements at discrete points – not the entire flow field

Suitable for safety critical issues Reliable model-making may sometimes be an issueFlexible – can be used for most applicationsNumerous wind directions can be tested,quickly and easilyFlow visualisation possible

CFD Can be done in-house Not well established for wind engineeringFull flow field predicted Not suitable for structural studies and safety-critical issuesFlow visualisation possible Time-averaged results only

Results difficult to interpret – specialist knowledge is essentialReliability of results can be uncertain

FURTHER READING

● BS 6399: Part 2: 1997 Code of prac-tice for wind loads. This standardreplaced BS 6399: Part 2: 1995, whichin turn replaced CP3 Chapter V: Part 2:1972. BS 6399 is only appropriate ifthe response of the structure can beconsidered to be static; structures witha dynamic response are not covered.● BRE Digest 436 part 2,Wind loadingon buildings:BS 6399-2:1997,N Cookand R S Narayanan,1999,£10.50.Worked examples effective windspeeds for a site,and loads on a two-storey house.● Wind loading:a practical guide to BS6399-2,1999,£40,Nicholas Cook,Thomas Telford.

ReferencesThe Foundationfor the BuiltEnvironment(FBE) issponsoring theBRE to considerthe current statusof wind testingmodels and toestablish therelative benefitsand limitations ofwind tunnelanalysis and CFDmodelling;and to assessquantitativelytheir respectiveaccuracy.

30 May 2002 the architects’ journal l 41

Left: groundsurface mesh,showingpredictivevelocities andpressures at eachpoint.Bottom:complex flowpatterns on theleeward side maybe inaccurate.Right:surface mesh

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