On the validity of the cosine projection in wind-driven rain calculations on buildings

Wind-driven rain (WDR) is one of the most important boundary conditions governing the hygrothermal performance and the durability of building facades. Information concerning the quantity of WDR falling onto building facades is an essential requirement as a boundary condition for Heat–Air–Moisture transfer analyses and for building facade design. The quantity of WDR can be calculated with either semi-empirical methods (such as the WDR relationship) or numerical simulation methods that are based on Computational Fluid Dynamics (CFD). The WDR relationship is most often used. It applies the cosine projection to take into account the effect of varying wind direction on the WDR quantity or intensity. Up to now, the validity of the cosine projection for WDR calculations has not yet been investigated. Its use was suggested in the past and it has been adopted for all semi-empirical WDR calculations since then. Also, in the recently developed numerical simulation methods, it is tempting to apply the cosine projection to reduce the computational expense. In the present paper, the validity of the cosine projection is investigated based on 3D numerical simulations of WDR with CFD. It will be shown that the cosine projection, although generally accepted, is not valid and that it can give rise to significant errors.