Methodology for the Evaluation of Natural Ventilation in ... - Cham

Figure 7. Neutral Pressure Level for Buoyancy Driven VentilationWhen natural ventilation is used as a means to ventilate a building under the buoyancy drivencase, the airflow is not assisted with forced air from wind or mechanical systems. This is oftenconsidered the critical design situation, during warm summer months for applying this passivetechnique in buildings. In the buoyancy driven case, the following parameters are somewhatinterdependent, making the analysis of this ventilation scheme more complicated. These include: the size of inlets and outlets, the height of the space,the strength of the heat sources driving the airflow,the resulting temperature difference between the interior and exterior spaces due to theinterior heat source(s)Additionally, complex building geometries, such as multiple floors that are directly or indirectlyconnected, increase the difficulty of evaluating the forces that drive natural ventilation flow. It isin part this complexity combined with the lack of understanding of the physical mechanismsinvolved in both buoyancy- and wind-driven natural ventilation that reduces the effective use ofnatural ventilation in building design.2.2.2 Wind-Driven VentilationNatural ventilation is influenced by several environmental conditions, the most unpredictablebeing wind velocity, both its speed and its direction. Both of these factors are difficult to controland analyze, especially in a full scale building. In the actual environment, instantaneous windspeed varies with time, and the pressure difference varies with building geometry and location onthe building surface. In most wind-driven natural ventilation experiments a constant, uniformwind speed is used. These design wind speeds are often the mean wind speeds for a givenlocation over a specific period of time, often years or decades (Awbi 2003).28