Measurements and simulations of airflow in mechanically ventilated active

The cavity in active facades can be mechanically or naturally ventilated, or by a combination of both, referred to as hybrid. The airflow in mechanically ventilated active envelopes is generally assumed to be more controllable than in naturally ventilated variants. However, due to the presence of blinds and asymmetric inlet or outlet openings, simplified methods are often not able to predict the flow patterns in the cavity. A better understanding of the behaviour of active facades is necessary to be able to predict the performance of active facades in practice. In this paper, a set of measurements and simulations is reported of low-speed airflow patterns in a mechanically ventilated active facade with and without blinds. The airflow is measured in a mechanically ventilated active envelope at the laboratory of the Royal Military Academy in Brussels, Belgium. Two-dimensional Particle Image Velocimetry (PIV) and one-dimensional hotfilm anemometry (HFA) are applied under isothermal conditions. Several variants are studied. CFD simulations are performed to investigate whether they are capable of predicting the airflow. The results of PIV measurements, HFA measurements and CFD simulations are compared. The measurements of both techniques (PIV and HFA) show high deviations, therefore it is likely that these deviations are present in the airflow and are not due to measurement errors. The measurement results indicate that the airflow is three-dimensional. Although the airflow is quite unstable, one could say that accurate measurements are obtained. A two-dimensional numerical model for a single storey facade with mechanical ventilation is developed. The comparison between the measurements and the 2D simulations show that the CFD model is capable of predicting the measurements within the range of the measurement fluctuations. Three-dimensional modelling is also applied, because of the threedimensional character of the flow. The results of the three-dimensional steady-state simulations do not show vortices in the third direction. The measurements and simulations also illustrate that the boundary conditions (e.g. dimensions and position of the inlet) have a major influence on the flow pattern in the facade. This study has provided more insight and a better understanding of the behaviour of isothermal airflow in active facades. A numerical model which can predict the tendencies of the isothermal airflow in the facade is developed. Detailed information about the boundary conditions is needed for a better understanding and simulation of the behaviour of the facade.The steady-state 3D simulation do not show vortices in the third direction. In future research, transient simulations are probably needed to show these vortices.