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Detailed models

In recent years, CFD software has become more mature, and commercial products geared for building design are appearing on the market to ease the complexities of creating models. In addition to their CFD analysis engines, a handful of vendors are now marketing user-friendly interfaces for those products that are designed to simulate building conditions. These include Airpak, by Fluent. In time, CFD will become easier to use and economical enough to be applied to more mainstream designs, Srebric believes.

The Tropical Forest Building at the Phipps Conservatory lets in plenty of natural light for its permanent botanic residents. The simulation software Radiance was used to create this rendering of lighting conditions, and the results were used in the CFD analysis.

The analysis process typically begins by importing a CAD file with a proposed design into a commercial CFD program, which then plots the geometry of the space in 3D and the materials that could affect airflow and temperatures. The model also estimates boundary conditions—the temperature of surrounding structures or the amount of sunlight streaming into the design space. It’s at this point that the program creates the 3D grid of cells that will allow engineers to look at the area in fine detail. A grid for a relatively small 20-by-30-foot office space, for instance, may contain more than 100,000 of these cells, each of which may require nine different complex mathematical equations to properly model airflow and thermal properties, according to Srebric.

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The results are then sent to the analysis portion of the CFD software, which crunches the numbers to develop graphical representations of airflow and surface temperatures. The process of building models, running simulations, and analyzing results may take anywhere from a couple

of weeks to a period of months, depending on the size of the space and the number of design iterations the project goes through, Holt says.

No matter how sophisticated the CFD models may be, it takes more than just plugging some numbers into a software application to end up with an effective design. “Reality is always more complicated than any computer can ever simulate,” Nall says. “The best we can do is create a simplification of reality

by picking out those factors that are most important.” For instance, the configuration of walls within a particular floor plan may matter more than the material those walls are made of. These sorts of judgments take time and experience, and for that reason Srebric advises caution when architects choose consultants to conduct analyses. “Anyone giving me CFD results must convince me of their accuracy,” she says. “There must be some validation through benchmark comparisons with similar projects. Never trust anybody who is just delivering results [for a current project].”

This work doesn’t come cheap. CFD analysis may add up-front costs of anywhere from $10,000 for smaller projects to $100,000 or more for large buildings, Srebric says. “It’s too time-consuming and expensive for every project,” she concedes, which is why CFD has made little penetration into residential or small-scale work. But if a client wants to build something with unconventional forms or materials, CFD analysis may mitigate the risk of ending up with an uncomfortable interior. “The technology helps make up for the fact that you’re building something that no one has ever done before,” says Nall. He adds that for projects characterized by innovation, a CFD analysis is often included as part of Flack + Kurtz’s basic engineering services.

Science meets art

Fortunately, when CFD is done correctly, it takes on a life of its own that goes beyond reams of inscrutable statistics. “Once you’re over the simulation hurdle, using the results is a pure pleasure,” says Srebric. The resulting graphical models exhibit an almost artful display of colors, contours, and movement that together can turn the calculations into visual treats. More importantly, these representations help architects gain confidence in the validity of the design they offer a client. “The beauty of CFD is it allows us to test a building’s performance before it’s built,” Holtz says. “If all your failures happen during design, you can deliver a successful solution when it’s time to do the actual building."