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

There are several equations that have been developed to describe pressure difference due towind-driven flow. The equations below describe a case with a constant wind speed creating asituation where wind pressure does not fluctuate with time. However, for single-sided ventilationfluctuations in wind speed may be important. A diagram portraying wind-driven ventilation, theairflow direction and resulting pressure versus height is presented in Figure 8. If the openings onopposite sides are identical, the pressure differences across the openings are equal to half thepressure difference across the building when it is assumed that there is negligible pressuredifferential through the interior of the building. The Bernoulli equation applied between a pointat some distance from the face of the building containing the window and the façade thenreduces to the ideal equation:Pw2U POO O(2.10)2where P w is the pressure due to wind at the façade and P O is the pressure away from the building,and U O is a reference velocity away from the building. Any height differential for flow along aparticular streamline is neglected, so both gz terms are zero, and the velocity at the face of thebuilding is zero as it is the stagnation point. A pressure coefficient, c p , is used in the actual case,and is a function of wind direction and location of the measurement on the building façade. Theresulting equations are:2UPOw PO CPO2(2.11)CQ CPdAUO2(2.12)where Q is the flow entering or leaving through the openings. The value of c p depends on thegeometry of the building and the location on the façade, and values are often obtained throughthe use of wind tunnel experiments (Orme 1999). The pressure on the exterior of the building inFigure 8 is assumed to not vary significantly with height.Figure 8. Wind Driven Ventilation: Airflow Direction and Pressure versus Height29