Facade Air‐Gap Optimization for Aerodynamic Enhancement of Fog‐Harvesting Membrane Systems

Abstract Building‐integrated fog‐harvesting systems offer a promising pathway to transform atmospheric moisture into an alternative water resource within urban environments. Despite their tested efficiency in open field, when fog‐harvesting membranes are integrated into building facades, their aerodynamic performance is strongly influenced by geometric confinement and local flow redistribution, which remain insufficiently investigated. This paper develops a tier‐based CFD framework for facade‐integrated porous membranes with a ventilated air cavity. Tier‐1 establishes a verified baseline and a consistent probe/monitoring protocol. Tier‐2 shows that, in a 2D top‐vented cavity, the dynamic‐pressure‐normalized membrane pressure response collapses primarily with the air‐gap ratio g / H , supported by cavity‐flow statistics. Tier‐3 introduces finite width and lateral redistribution and demonstrates a systematic width effect: membrane‐normal throughput is edge‐enhanced for narrow modules and decreases with increasing W / H toward a quasi‐2D limit. Increasing the air gap enhances the area‐averaged membrane‐normal velocity, used here as an aerodynamic proxy for fog‐laden airflow passing through the mesh, but this gain is accompanied by a growing membrane pressure drop and diminishing returns. The Tier‐3 dataset is synthesized into design guidelines over ( W / H, g / H ) and a compact pressure–throughput relation to guide early‐stage envelope integration.