CFD study of air flow in cavities in ventilated facades

The service life of building components and building materials strongly depends on their hygrothermal conditions. In this perspective, ventilated facades are employed in which a – sometimes very narrow – air gap or cavity between the rain screen and the back wall is introduced. Hygrothermal modelling of such facades requires knowledge of the air flow patterns, velocity magnitudes and ventilation rates in these cavities. These variables are determined by the complex interaction of several parameters. The following influencing parameters are distinguished: building and cavity geometry, environment topography, position of the cavity inlet and outlet openings, reference wind speed and wind direction. In the past, information on air flow in such cavities for hygrothermal modelling has been mainly obtained by simplified theoretical and/or semi-empirical expressions, which provide an estimate of the air flow rate as a function of the surfaces pressures at the inlet and outlet opening. Usually, a uniform velocity pattern is assumed in the entire cavity volume. Little is known about the actual mean air speed and turbulence intensity inside the cavity. To the knowledge of the authors, no detailed study of the relationship between these variables and the influencing parameters has yet been performed. Such detailed study would require either detailed full-scale measurements or numerical simulation with Computational Fluid Dynamics (CFD). Wind tunnel tests are considered inappropriate due to Reynolds similarity problems for the narrow cavities and the small inlet and outlet openings. This extended abstract presents preliminary results of a comprehensive CFD study on wind-induced air flow through the cavities of an exposed low-rise test house. The house exhibits different cavity assemblies for typical Scandinavian wooden claddings. A specific feature of these simulations is the full numerical simulation of both the air flow pattern around the building and the resulting flow patterns inside the cavities. The simulation results provide the relationship between cavity air change rate and cavity design, facade position, wind velocity and wind direction, and this information will be made available as input for future hygrothermal modelling efforts.