How can computer software aid the operation of MEP systems? Matt Glanville and Peter Bourke outline the applications of computational fluid dynamics in building design.
Computer aided design (CAD) in the construction industry has made giant leaps in recent decades. Computer aided drafting has all but replaced the traditional drafting board in most engineering design offices and numerical finite element methods are now routinely used for a range of engineering design applications.
In wind engineering, the wind tunnel is the traditional drafting board used by practitioners to understand the interaction of wind with the built environment. Wind tunnel testing on scale models of high-rise towers can be traced back almost half a century to 1964. At this time, Cermak Peterka Petersen founder Dr Cermak tested the design of the World Trade Centre Twin Towers for New York City, bringing wind engineering to the attention of architects and engineers worldwide.
Wind engineers reproduce the highly complex turbulent flows that occur in the atmosphere by generating scaled boundary layer turbulence within the wind tunnel. One of the fundamental tenets of wind tunnel testing is that the statistical properties of the atmospheric boundary layer will collapse to a single universal profile when non-dimensionalised using the proper length and velocity scales.
Put another way, the wind tunnel can be considered to be an infinitely powered analogue computer, whereby the peak wind loads measured on wind tunnel models will be closely representative of the actual wind loading on a full-scale building.
The emergence of CFD
Computational fluid dynamics (CFD) is the emerging digital technology that is perceived by some professionals to be an alternative to wind tunnel testing. CFD is a form of finite difference simulation that can be used to provide iterative solutions to the Navier-Stokes turbulent-flow equations, along with heat and material transfer equations.
CFD requires specialised software, significant processing power and trained specialists for its operation. Once the hardware and software are in place, the specialists recreate the environment and conditions that have to be tested. Running a CFD simulation can often take several days, depending on the number and power of processors used.
So what are suitable applications of CFD in building design and how close is this technology to replacing wind tunnel testing? The answer lies in our current understanding of a phenomenon that is part of all of our daily lives - turbulence. Practically all real fluid flows contain some degree of turbulence, including the wind that blows across our cities. And turbulence remains the most challenging unsolved problem of classical physics.
CFD has been shown to be most effective in building design applications where the level of turbulence is low and predicted levels of turbulence intensity are of less interest in gross flow predictions. CFD is particularly useful for many bounded flow applications in mechanical services design, such as detemining the air movement and natural ventilation within building spaces, where the wind tunnel has limitations with respect to viscous effects.
Air conditioning performance is well suited to CFD simulation as a predictor for the air distribution within enclosed building spaces between diffusers and return air where turbulence scales are low.
In fact, the distribution of conditioned air in large internal spaces such as atria that have fully-glazed facades is difficult to predict without the aid of CFD packages. This is due to the complex interaction of mechanically-cooled air with solar radiant and ambient heat loads. Smoke dispersion under fire simulation and indoor pollution dispersion studies are other applications where thermal stratification within the building space is a critical parameter that is well suited to CFD simulation.
Taking CFD outdoors
CFD has a potential application for the calculation of outdoor environmental gross flows around buildings. In such applications the mean component of the wind is of most interest, provided the limitations of the output are properly understood and applied judiciously.
For flow through semi-open buildings such as open-sided parking structures or natural-ventilation green buildings, the results from wind tunnel testing can be used to determine the input boundary conditions for the CFD model.
Specialist consultants such as CPP regularly conduct combined wind tunnel and CFD studies to determine the natural ventilation and space heat flushing potential of residential apartments. Important factors to be considered in these cases include the building massing; facade articulation and building orientation; the exposure to prevailing breezes if available; and, importantly, the unsteady turbulence characteristics of the approach flow.
Mean flows that interact with the exterior surfaces of buildings however are influenced by the mechanical turbulence created by the highly complex separation and reattachment processes.
These cannot be reliably predicted by CFD. It is the relatively high level of turbulence in the atmospheric boundary layer that gives rise to design peak wind loads on building structures. In typical atmospheric flows, turbulence levels are high and add another 50-200% to the mean load component to produce the design peak load.
Turbulence in atmospheric flows imparts complex and fluctuating loads on buildings that are computationally exhaustive even for simple and exposed geometries. In such cases, CFD predictions vary significantly depending on the input assumptions, such as the turbulence modelling algorithm and mesh geometry.
In dynamically sensitive structures, turbulence is the critical load component in activating dynamic response, such as wind-induced resonant accelerations in slender high-rise towers that can lead to motion sickness in some extreme cases.
Contemporary CFD research in wind engineering is focused on benchmarking results gained in CFD modelling against wind tunnel or full-scale measurements.
To date, there have been four international symposia that were dedicated specifically to the use of CFD in wind engineering and there is yet to be a consensus on a standard set of modelling assumptions to be employed in research, let alone the design office. At the most recent international conference in wind engineering held in Cairns, Australia during 2007, around 20 of nearly 300 papers related to CFD predictions of wind loads on buildings.
At an academic level the limitations of CFD are well known and current research continues to rely on the wind tunnel and some well controlled full-scale data as the only reliable method for assessing peak wind loads.
CPP is currently incorporating CFD into the design of a new boundary layer wind tunnel under construction in Australia. CFD is proving an ideal tool to predict the gross flow movement through the tunnel and to finetune critical components such as turning vanes, contractions, and a blockage tolerant section. The tunnel in turn will be used to predict structural and cladding peak wind loads on building models once complete.
Matt Glanville is director of wind engineering and air quality consultant Cermak Peterka Petersen (CPP), Peter Bourke is the firm's simulations and wind engineer.
CFD and wind tunnel modelling in brief
- Wind engineers reproduce complex atmospheric turbulent flows by generating scaled boundary layer turbulence in a wind tunnel. The peak wind loads measured on wind tunnel models are closely representative of those on a full-scale building.
- Computational fluid dynamics (CFD) is a digital technology that is perceived to be an alternative to wind tunnel testing. The software modelling can be used to provide iterative solutions to turbulent flow, heat and materials transfer equations.
- CFD is particularly useful for applications in mechanical services design, such as the air movement and natural ventilation within building spaces, where the wind tunnel has limitations with respect to viscous effects.
- CFD simulation is a good predictor of the air flow within enclosed building spaces where turbulence scales are low, such as to determine the distribution of conditioned air in large internal spaces like atria that have fully-glazed facades.
- Details of the CPP boundary layer wind tunnel project are available on www.cppwind.com