Advanced Building Simulation

a different physical state variable. The generalized form of the conservation equation
is given by
�� �� �
� � ��j �
�t
�kj�� ��
� S
�kj� �k j
Integrated building airflow simulation 91
unsteady term � convection term � diffusion term � source term
(4.3)
CFD is a technology that is still very much under development. For example, several
different CFD solution methods are being researched for building airflow simulation:
direct numerical simulation, large eddy simulation (Jiang and Chen 2001), Reynolds
averaged Navier–Stokes modeling, and lattice Boltzmann methods (Crouse et al.
2002). In practice, and in the building physics domain in particular, there are several
problematic CFD issues, of which the amount of necessary computing power, the
nature of the flow fields and the assessment of the complex, occupant-dependent
boundary conditions are the most problematic (Chen 1997). This has often led to
CFD applications being restricted to steady-state cases or very short simulation periods
(Haghighat et al. 1992; Martin 1999; Chen and Srebic 2000). An application example
is shown in Figure 4.4.
Integration of CFD with building energy is also still very much in development
although enormous progress has been made in recent times (Bartak et al. 2002; Zhai
et al. 2002).
Hensen et al. (1996) analyzes the capabilities and applicability of the various
approaches in the context of a displacement ventilation system. One of the main
conclusions of this work is that a higher resolution approach does not necessarily
cover all the design questions that may be answered by a lower resolution approach.
Each approach has its own merits and drawbacks. An environmental engineer typically
needs each approach but at different times during the design process. The main
conclusion of this study is summarized in Table 4.1.
Notwithstanding the above, in the context of combined heat and airflow simulation
in buildings, it is the zonal method that is currently most widely used. The reasons
for this are threefold. First, there is a strong relationship between the nodal networks
that represent the airflow regime and the corresponding networks that represent its
Figure 4.4 Model of a historical building and CFD predictions of air velocity distribution in the central
longitudinal section at a particular point in time (Bartak et al. 2001).