Modelling of cowl performance in building simulation tools using experimental data and computational fluid dynamics

Exhaust cowls are used in conjunction with hybrid ventilation systems to efficiently convert wind energy into negative pressure and thus minimize the electrical energy required by the extract fan. Yet the fact that cowl performance is largely dictated by operating conditions imposes particularly stringent demands on modelling. This paper demonstrates, by way of a concrete example, the need for and potential benefits of a new methodological approach to the modelling of cowls. The study focuses on a specific modelling strategy, applied within a building simulation program, for a cowl used in a hybrid ventilation system. The method is progressively simplified to produce four variants, which chiefly vary according to their level of detail and, hence, the associated modelling effort. Wind pressure coefficients at facade, above roof and in the cowl are needed for all model variants. Some of the investigated variants rely on CFD computations of airflow around the building to determine these values. This study uses the example of a single-family house (SFH) to identify those criteria requiring particular attention in the performance of CFD numerical flow analyses. All four variants are examined on the basis of this example to determine which simplifications to the model are appropriate and permissible without unduly compromising the accuracy of the results.