Quantifying the performance of a top–down natural ventilation Windcatcher™

Estimating the performance of a natural ventilation system is very important if one is to correctly size the system for a particular application. Estimating the performance of a Windcatcher™ is complicated by the complex flow patterns that occur during the top–down ventilation process. Methods for predicting Windcatcher™ performance can currently be separated into simplistic analytic methods such as the envelope flow model and the use of complex and time consuming numerical methods such as CFD. This article presents an alternative semi-empirical approach in which a detailed analytic model makes use of experimental data published in the literature for 500 mm square Windcatchers™, in order to provide a fast but accurate estimate of Windcatcher™ performance. Included in the model are buoyancy effects, the effect of changes in wind speed and direction, as well as the treatment of sealed and unsealed rooms. The semi-empirical predictions obtained are shown to compare well with measured data and CFD predictions, and air buoyancy is shown only to be significant at relatively low flow velocities. In addition, a very simple algorithm is proposed for quantifying the air flow rates from a room induced by a Windcatcher™ in the absence of buoyancy effects.