Spatial distribution of wind driving rain and drying on façades and associated hygrothermal response: An external coupled CFD and HAM model.

Heat, Air, and Moisture (HAM) transfer models have proven their value in the renovation and restoration sector. They allow for analyzing the causes of moisture accumulation and related issues, as well as studying the impact of renovation measures. Wind-driven rain (WDR) is one of the most important moisture sources, while evaporation serves as the main drying mechanism; together, they determine the hygrothermal performance and durability of building components. In traditional HAM modeling, WDR and the convective heat transfer coefficient (CHTC) are often simplified by applying generic, uniform values across the façade. However, this approximation fails to account for the spatial and temporal variations in WDR and neglects the significant variations in CHTC across the windward façade due to surrounding wind flow conditions. These oversimplifications can limit the accuracy of hygrothermal performance predictions, particularly when assessing moisture-related risks for heritage structures or during building renovation planning. In this paper, the spatial distribution of the catch ratio and CHTC are implemented through an externally coupled computational fluid dynamics (CFD) model with a hygrothermal response model (HAM). Steady Reynolds-averaged Navier-Stokes (RANS) equations and Eulerian multiphase CFD simulations are performed for CHTC and WDR, including the turbulent dispersion of raindrops. The coupling model is tested on a cubic low-rise building, with an analysis of the effects on surface degradation phenomena, including frost damage, salt crystallization, and algae growth. Results indicate that conventional approaches tend to underestimate critical rain loads, while the drying capacity is overestimated. Furthermore, the specific combination of rain loads and drying patterns results in unexpected performance patterns.