Natural Ventilation Design Using CFD

In an effort to make buildings healthier and more energy efficient, architects are increasingly incorporating natural ventilation into their design strategies in order to take advantage of free, available wind power. The extent to which natural ventilation can replace forced ventilation in a given building depends on the local climate and specific site utilization. The ASHRAE Standards 55 and 62.1 that cover natural ventilation establish minimal requirements for climate and building openings but also concede that the ultimate responsibility for proving the effectiveness of this technique lies with the design team and the specific requirements of local codes. But how does a design team prove that air is flowing according to plan without actually creating the structure and taking measurements? Only two possibilities exist — regard each room as a very large ratio conduit and apply conventional equations to those spaces, or do a 3-dimensional numerical analysis of the flow path. Numerical analysis, known as Computational Fluid Dynamics (CFD), is now being recognized as the only reliable way to predict natural airflow through a building and assure that adequate air quality and comfort is provided at all points of each room before construction begins. CFD computer programs allow designers to divide a volume into a large number of small regions and calculate the air and heat transfer between each region, minimizing the assumption-related errors that would otherwise occur. Minimizing computational error at the beginning of the design process reduces the risk of costly post-construction order changes that can occur as substandard air quality is discovered. CFD software can vary in its level of sophistication. While the most basic Navier-Stokes heat and mass transfer equations are essential and can be of great use, a proper natural ventilation analysis tool should include calculations for buoyancy, turbulent convection, and the ability to do open boundary modeling. Other features such as local solar loading and transient analysis are also desirable. A comprehensive CFD package can be particularly useful for modeling the complex airflow found in mixed-mode designs and identifying regions of stagnant air, high heat loss or gain, short-circuited airflow, and other conditions that inhibit good building performance and limit the potential for sustainability.