ECOLI BR I U M • APR I L 201422

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Computational fluid dynamics (CFD), is the science of using numerical methods to analyse the complex flow and behaviour of fluids, including gases; as well as heat transfer including buoyancy, convection and radiation.

Although the basic methods of CFD were developed long before the modern age of computers, its evolution has largely mirrored the exponential increase in computational power over the decades.

In the 1920s, British mathematician, physicist and meteorologist Lewis Fry Richardson used finite differences and cells (see sidebar) to assist with weather prediction. Three decades on, and numerical methods processed by computers were beginning to simulate transient 2D flows, extending to 3D flows in the 1960s.

In commercial and industrial applications, the use of CFD is significantly more youthful. Its evolution as a useful, powerful tool has followed three main threads: the development of meshing techniques, the incorporation of multi-physics and the development of turbulence models.

The latter point has been the biggest hurdle to its development, as turbulence in a flow is extremely complex to predict due to its random nature. Therefore, it is modelled rather than calculated.

However, improvements made to turbulence models over the years have been such that they are now more representative of actual fluid behaviour.

Also, the availability of increased computer power means software can now rely more on calculations rather than the

models, making the overall simulation more realistic and accurate.

Today, the predictive outcomes achieved by CFD find practical application across a range of industries, including automotive, aerospace and the built environment.

For the HVAC engineer, CFD is generally used to analyse the performance of ventilation design by estimating the performance of systems too complex for analytical solutions. It can also be used to explore and gain new insights into new technology.

According to Dr Colin Allison, M. AIRAH, from thermal fluid modelling firm Simultude, the application of CFD in the HVAC field achieves what complex analysis using empirical equations simply cannot.

Fluid ideasComputational fluid dynamics seems to be playing a part in almost every new project.

Sean McGowan spoke to industry leaders about the evolution of CFD and how it is used

to influence the decision making of HVAC engineers.

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“For example, the pressure drop in a pipe or across an orifice plate is simple to calculate from basic equations; however, what would be the pressure drop across a butterfly valve that is partially open?” he asks.

“Even this simplest example is far too complex to calculate manually, but CFD would be able to calculate it very accurately.”

While CFD is particularly useful during the design phase, it is also finding relevance as a tool to diagnose and help address existing problems in buildings.

Allison says a significant proportion of the modelling he does is problem solving. In many instances, he says such problems could have been avoided if CFD had been applied in the first place.

“For example, in one case the location of multiple chiller condensing units caused hot air short circuiting, and required expensive acoustic baffling to solve the problem.”

APPLYING THE SCIENCEProvided the need justifies the expense, CFD can be used in a range of suitable applications.

In the context of the built environment, this can extend from the distribution of heat in a data centre to the performance of a ventilation strategy in a commercial office. It can also be used to assess levels of carbon monoxide in a carpark and to demonstrate compliance through fire and smoke modelling.

Increasingly, CFD is also used beyond the building envelope to analyse the effects of wind flows around buildings on pedestrian comfort.

For many built environment projects, CFD is used to check that an initial design concept is working as intended, before being used to refine the design through incremental improvement.

“While the initial design concept might be based on the designers’ past experiences, novel building forms and system types can introduce uncertainties to the design process,” says Jon Morgan, senior engineer with Arup.

“This is where CFD can be useful, and the visualisation of fluid flow becomes a very powerful communication tool.”

Another emerging market for CFD modelling is in data centre design, where it is used to show performance under

failure mode, so both the client and engineers understand the resilience of the system.

“If, for example, you can understand what the impacts might be if a cooling unit fails in a server room, you can put a plan in place that reduces the risk to the surrounding equipment,” says Nicki Parker, principal ESD consultant with AECOM.

GREEN STAR AND COMPLIANCECFD is also finding application in the pursuit of environmental ratings, with simulations currently used to obtain the Green Star IEQ-2 credit that measures air change effectiveness (ACE), and the IEQ-9 credit for thermal comfort.

“In Green Star, CFD modelling is used to measure the mean age of air within the building as a guide to how well the outside air is supplied to the occupants in both mechanical and naturally ventilated buildings,” says Phillip Cook, sustainability project engineer for Wood & Grieve Engineers.

ACE is a particularly ideal application for CFD. Alternatives, such as the tracer gas technique, are often impractical to apply and cannot be performed in the design stage.

CFD has also found application in demonstrating compliance with the Building Code of Australia (BCA), particularly where engineers want to deviate from the minimum requirements stipulated in standards, such as ventilation flow rates in carparks or smoke management in atria.

“CFD is often used to provide evidence for an alternative solution, and so in many cases 3D models can be created to demonstrate that a particular strategy can still perform to the minimum requirements, leading to savings in construction and operational costs,” says Parker.

In such instances, the value of CFD becomes easier to quantify.

Parker offers an example where a carpark design included the proposal for 15 jet fans to assist with ventilation. By applying

MAKING SENSE OF ITThepost-processingpartofCFDmodellingisavisualmedium,withresultstypicallypresentedinaseriesofgraphicsorsnapshotsoftheflowoccurring.Thismightrangefromcutplanesoftemperaturetopressureorvelocitycontours.

Butlikeanytypeoftechnicalinformation,theresultscanbepresentedverywell,orverypoorly.

Asmostaretypicallypresentedbacktotheengineeringdesignteam,theirtechnicalnatureistypicallynotabarriertocommunication.Butpresentingtotheclientisquiteadifferentbeast.

“Thepurposeofaclientpresentationismorelikelytobeaboutthenarrativeofthedesignprocess,”saysArup’sJonMorgan.“Andclarityofinformationiskey.

“Todothiswell,theCFDanalystneedstohaveaveryclearunderstandingoftheclients’driversandthedesignproblemincontext,andthisrequiresgoodintegrationoftheanalystwiththedesignteam.”

Morgansaysthenextchallengeisinselectingtherightstyleofcommunication.

“Forquiteawhile,CFDhashadtheupperhand–thevisualisationsweregenerallymoreengagingthanwhattherestofthedesignteamwasgenerating.Butthisischanging,andwenowfindthattechnicalresultsneedtobepresentedusingthelatestcommunicationstoolssuchasvirtualreality,augmentedrealityandinteractiveweborPDFdocuments.”

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CFD, her team was able to demonstrate that compliant design could be achieved with just three fans.

PICKING THEIR MARKGenerally the cost of setting up a CFD model is complex and quite expensive, so it is typically only applied where the results can justify the expense; and where the outcomes are not intuitive.

There are three typical cost components with CFD: software, hardware and labour.

Software, more commonly referred to in CFD-speak as “codes”, varies greatly in cost. While the use of free CFD tools such as Open Foam may remove one of the cost components, its use generally results in more man-hours to reach a suitable resolution.

Conversely, the more expensive and capable commercial CFD codes are easier to use, are more reliable and have much better support, resulting in fewer man-hours.

Here, the old adage that you get what you pay for rings true.

“Unfortunately there seems to be a perception that all CFD codes are equal,” says Allison. “But this is like saying a Toyota and Mercedes are equal.

“A lot of the work I do is to repeat work that was previously botched up by using a rudimentary CFD code. Even worse, I have seen atrocious CFD results presented to clients who have to then draw conclusions from this.”

In such instances, Allison says clients become discouraged by the poor CFD simulation results and therefore see the costs as unjustified. Ultimately, this negative outcome inhibits confidence and further use of CFD.

The other major expense relating to CFD is time or man-hours. In this regard, specialist training is required to use CFD modelling competently. As a window into the future, many engineering university

students now do courses in CFD as part of their degree.

The final cost component is computer power. Although small CFD models can be conducted on a desktop PC, a cluster of computers is generally required for more complex models.

Though cost is an obvious barrier to the broader use of CFD, Morgan says that like any other form of analysis its purpose is to reduce uncertainty and hopefully improve the efficiency or effectiveness of a design solution.

Therefore if used well, it should result in an overall cost saving.

“When applied to industrial processes, for example, the cost of analysis is likely to be relatively small when compared to the potential savings with respect to reduced manufacturing time, materials, operational costs and improved reliability,” Morgan says.

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“For environmental problems, the use of CFD may be integral to a life-safety design, in which case the cost of analysis would need to be weighed against human safety.”

CELLS AND GRIDSA CFD model is built to accurately reflect the real-world environment. In an HVAC situation, the starting point is generally constructing a 3D model of the portion of the building to be simulated. This includes walls, doors, windows and all the architectural elements like staircases and beams that complete the structure.

The air conditioning or ventilation system is then constructed.

“Fortunately, the more powerful CFD codes have libraries of pre-made parts for diffusers, fans, grilles and louvres – indeed, everything you need to construct a typical ventilation system,” says Allison.

Integral to the model is the conservation of the mass within the space, and that

the 3D geometrical model is water-tight with no holes (as can often occur when architectural models are built). This volume is then divided into millions of smaller volumes, called cells.

The collection of all cells is known as the mesh.

In much the same way as a television is divided into pixels that make up a clear picture, the smaller the cells in a CFD model the more accurate the outcome. This comes at a price, however, since meshes with higher volumes of cells take longer to complete. Therefore there is a balance between getting an accurate result and achieving a solution in a reasonable period of time.

“Think of it this way,” says Parker. “Is there any requirement to know what is happening in every cubic centimetre volume when you are looking at wind flows around buildings and the impact on pedestrian comfort?

“Conversely, if you divided a car park up into 2m³ volumes, are you able to accurately predict what the concentration of carbon monoxide might be throughout the space?”

Once the mesh is confirmed, the fundamental equations of fluid flow, known as the Navier-Stokes equations, are then applied to each cell by the CFD model. Parker says this is conducted hundreds of times, using the previous estimate as the starting point for the next until the difference between the concurrent solutions only varies by a very small amount.

This is known as reaching convergence.

A converged solution does not necessarily mean it is a correct solution, but rather that it is a completed calculation according to the various inputs and assumptions. Therefore, an understanding of the underlying physics involved, and ability to loosely predict what might be expected to happen in the situation being modelled is important.

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Placing unfettered confidence in the results of CFD can be a rookie error.

“Upon observation, one can tell whether the results are credible or not,” Allison says.

“Inexperienced users tend to accept results unconditionally, which can have detrimental consequences [but] most experienced engineers can sense whether the CFD is credible or not. On the other hand, if the modelling is credible then it provides tremendous reassurance, making the design decision far easier.”

The time required to reach a solution is typically dictated by what the design team can tolerate. Though it is not uncommon to be able to set up and run a simple model in just a few hours, more complex problems naturally take more time.

“If we have a week to find a solution, then we can throw all our toys at the problem,” says Morgan. “If we only have a day then the solution must be simpler and consequently more uncertain.”

Using the example of carpark ventilation and contaminant modelling to demonstrate the advances made in CFD, Morgan says where once this would have been solved by using a simple turbulence model and fairly coarse mesh, it can now be solved with a finer mesh and more accurate representation of turbulence.

“The information is of much higher quality,” he says. “But the simulation time might not have changed all that much.”

Additionally he says the number of elements in a CFD domain is highly variable.

“For most buildings applications it is likely to be in the order of a million or so,” Morgan says. “But other industries regularly run one or two orders of magnitude greater than this.”

CONFIDENCE BUILDINGDespite being used across a wide range of industry sectors where it has become embedded as an essential design tool, CFD has taken longer to find application in building design. This is largely because the problems to be solved carry less risk and can be tackled in a variety of ways.

However, it appears to be on the verge of moving beyond a check-box exercise, with confidence in CFD and other simulation exercises growing.

Where CFD evolves next is largely at the mercy of the IT industry, with advances in

CFD codes continuing to be closely aligned to improvements in computing power.

For instance, the development of direct numerical simulation supersedes the turbulence models and instead applies the equations directly to every cell in the domain. Allison predicts this will typically require much finer meshes.

But this advance will require greater computing speed and more memory.

“As the industry realises the benefits offered by CFD,” Parker says, “its capabilities and applications will continue to expand.” ❚

CFD GLOSSARYBoundary conditions–theknowninputstoamodelsuchasinletflows,walltemperaturesandheatloadsthatsettheproblemup.

Boundary layer–thelayerorfluidnexttoasurfacewheretheeffectsoffrictionaresignificant.

Cells–theindividualsmallvolumesthattheairspacegetsdividedintoinordertoestimatetheflowpatterns.

Convergence–whenthereisaverysmalldifferenceintheconsecutivesolutionstothefundamentalfluidflowequationsacrossthewholedomain.

Domain–theairvolumethatisboundbysolidobjects(e.g.,aroom).

Instability–whenthesolutiondoesnotsettletoasinglevalueandchangesforeachiteration.

Iterations–eachtimethecomputersolvestheequationsoffluidflowandproducesasolution.Manyiterationsarerequiredtoreachconvergence,ofteninexcessof1,000.

Laminar –whenafluidflowsinparallellayers.

Mesh–thecollectionofsmallvolumes(cells)thattheairspacegetsdividedintoinordertoestimatetheflowpatterns.

Navier Stokes equations–thefundamentalequationsoffluidflowtodeterminepressure,velocityanddensity.

Residuals–thedifferencebetweenconsecutivesolutions.

Steady state–wheninputsandoutputsdonotchangeintime(i.e.conditionsareconstant).

Streamlines–theimaginarypaththataparticlewouldfollowifitwasreleasedatapoint.

Symmetric–whenamodelissplitbecauseonehalfisamirrorimageoftheother.Thissavessimulationtimebutcanonlybedonewheninputsandgeometryarethesameforbothhalves.

Velocity–theairspeedinaparticulardirection(i.e.inx,yorzcoordinates).

Thermal comfort –thecombinationoftemperature,airspeed,radiationandhumidityanditsrelationtooccupants

Turbulence–chaoticfluidflowwithlargechangesinpressureandspeed

Typical Navier-Stokesequationsforbuildingservicesapplications.

Source:NickiParker,principalESDconsultantAECOM