Advanced Building Simulation

New perspectives on CFD simulation 143
transported contaminants from outdoors to indoors. Dilution was thus developed as
a response to nineteenth-century concerns about the spread of disease in interior
environments (Addington 2001, 2003). The HVAC system emerged at the beginning
of the twentieth century as the ideal technology for diluting the multiple sources of
heat and mass typically found in an interior. No other system was capable of simultaneously
mitigating these diverse sources to provide for temperature, humidity,
velocity, and air quality control in the quest to provide a homogeneously dilute
interior. Both as a technology and a paradigmatic approach, the HVAC system has
maintained its hegemony ever since. All other technologies, including those for passive
systems, are fundamentally compared against the standard of the dilute interior
environment.
While the technology remained static over the course of the last century, the understanding
of the physics of air and heat has undergone a radical transformation. Heat
transfer and fluid mechanics, the two sciences that govern the behavior of the interior
environment, were the last branches of classical physics to develop theoretical
structures that could adequately account for generally observable phenomena. The
building blocks began with the codification of the Navier–Stokes equations in the
mid-nineteenth century, and they fell into place after Ludwig Prandtl first suggested
the concept of the boundary layer in 1904. Nevertheless, the solution of the nonlinear
partial differential equations wasn’t applicable to complex problems until iterative
methods began to be employed in the 1950s, leading to the eventual development of
CFD in the late 1960s to early 1970s. If the standard definition of technology is that
it is the application of physics, then the HVAC system is clearly idiosyncratic in that
it predates the understanding of the governing physics. Many might argue that this is
irrelevant, regardless of the obsolescence of the technology—it is and will continue to
dominate building systems for many years.
The use of CFD has been constrained by the normative understanding of the
building technology. This contrasts significantly with its use in other fields in which
CFD simulation is supra the technology, and not subordinate. For example, in building
design, CFD is often used to help determine the optimal location of diffusers and
returns for the HVAC system. In engineering, CFD is used to investigate autonomous
physical behaviors—such as vortex shedding in relationship to fluid viscosity—and
technologies would then be developed to act in accordance with this behavior. Existing
building technologies heavily determine the behavior of air, whereas the extant air
behavior does not determine the building technology. Many who understand the critical
importance of this conceptual distinction would argue, however, that it is of little
relevance for architecture. Building technologies have long life spans and do not
undergo the rapid cycles of evolution and obsolescence that characterize technological
development in the science-driven fields. Given that one cannot radically change building
technologies, and, as a result, cannot investigate building air behavior without the
overriding influence of HVAC-driven air movement, then how can the discipline of
architecture begin to exploit the possibilities inherent in numerical simulation?
6.2 Determining the appropriate problem (what to model)
The methods currently available for characterizing transient fluid behavior—
theoretical analysis, empirical evaluation and numerical description—have yet to