Comparison of calculation models for wind-driven rain deposition on building facades

Wind-driven rain (WDR) is an important factor in the dry and wet deposition of atmospheric pollutants on building facades. In the past, different calculation models for WDR deposition on building facades have been developed and progressively improved. Today, the models that are most advanced and most frequently used are the semi-empirical model in the ISO Standard for WDR assessment (ISO), the semi-empirical model by Straube and Burnett (SB) and the CFD model by Choi. This paper compares the three models by applying them to four idealised buildings under steady-state conditions of wind and rain. In each case, the reference wind direction is perpendicular to the windward facade. For the CFD model, validation of wind-flow patterns and WDR deposition fluxes was performed in earlier studies. The CFD results are therefore considered as the reference case and the performance of the two semi-empirical models is evaluated by comparison with the CFD results based on two criteria: (1) ability to model the wind-blocking effect on the WDR coefficient; and (2) ability to model the variation of the WDR coefficient with horizontal rainfall intensity R_h. It is shown that both the ISO and SB model, as opposed to the CFD model, cannot reproduce the wind-blocking effect. The ISO model incorrectly provides WDR coefficients that are independent of R_h, while the SB model shows a dependency that is opposite to that by CFD. In addition, the SB model can provide very large overestimations of the WDR deposition fluxes at the top and side edges of buildings (up to more than a factor 5). The capabilities and deficiencies of the ISO and SB model, as identified in this paper, should be considered when applying these models for WDR deposition calculations. The results in this paper will be used for improvement and further development of these models.