Intercomparison of wind-driven rain deposition models based on two case studies with full-scale measurements

Three different calculation models for wind-driven rain (WDR) on buildings are compared for two case studies for which full-scale measurements are available. The models are the semi-empirical model in the ISO standard for WDR (ISO), the semi-empirical model by Straube and Burnett (SB) and the CFD model by Choi, extended by Blocken and Carmeliet. This paper builds further on two generic studies in which these models were compared based on model theory and based on their application for idealized building configurations and for constant wind and rain conditions. In the present study, the models are applied to calculate WDR on the facades of a low-rise test building and a monumental tower building for actual transient rain events. The spatial and temporal distributions of WDR at the windward facade are determined and the model results are compared with each other and with the full-scale measurements. The agreement between the CFD results and the measurements is on average 20-25%, whereas the ISO and SB models show large discrepancies at many facade positions, up to a factor 2-5. The identification of the reasons for the discrepancies is based on the previous generic studies and on the detailed information provided by the validated CFD simulations. The reasons include: (1) the ISO and SB models do not take into account the wind-blocking effect; (2) they do not model the variation of shelter by roof overhang as a function of the wind speed and; (3) they only provide information for a limited number of building geometries. In spite of these deficiencies, these models provide a strong basis for further development. The deficiencies can be addressed with CFD, and it is suggested that future research should focus on improving the semi-empirical models based on the detailed results of validated CFD simulations.