Quantifying the performance of a top down natural ventilation windcatcher

Measurements and smoke tests show that thequadrants of a Windcatcher with a positivepressure across them act as supply ducts, whilethose with a negative pressure across them actas exhaust ducts. However, analysis of the sideand leeward C, values shows that they do notnecessarily balance mass flow in and out of theWindcatcher, indicating that either the pressurein the supplied room drops or there is an amountof infiltration through the building fabricinitiated by the Windcatcher.In order to better understand Windcatcherperformance, a simple analytic model isdeveloped that utilises experimental data toestimate the losses in the system. Two differentscenarios are considered for the room adjoiningthe Windcatcher: (i) this room is perfectlysealed; and (ii) air infiltration is allowed into theroom so that the pressure in the room remainsatmospheric. Here, it is observed that, for thosevalues of C, reported for a square Windcatcherin the literature, the overall volume flow rate ofair out of the room always exceeds that cominginto the room. Based on this data, the analyticmodel may be used to estimate the losses in theWindcatcher, from which it is thenstraightforward to derive a simple relationshipbetween the overall area of the Windcatcher andthe volume flow rates into and out of theWindcatcher in order to predict Windcatcherperformance for a given application.