Blended inflow boundary conditions: Addressing the displacement height in differing inertial and roughness sub-layer flows for urban areas

Accurately modelling urban airflow and pollutant dispersion requires inlet boundary conditions that reflect the complexity of urban roughness and its influence on the urban boundary layer (UBL). This study evaluates four computation fluid dynamics (CFD) inflow scenarios, focusing on improving the modelling of velocity and turbulence kinetic energy (TKE) profiles in urban environments using the Standard $k-\epsilon$ (SKE) turbulence model. Particular emphasis is placed on incorporating the displacement height and differentiating between the roughness sub-layer (RSL) and inertial sub-layer (ISL) regions.

A novel blended inlet profile is proposed to bridge existing gaps in UBL modelling. It enables better alignment of flow profiles with experimental data while addressing the transition between the RSL and ISL. Statistical evaluations demonstrate that the blended approach offers improved performance and consistency across the CFD domain.

The results highlight the critical role of accurate inflow conditions in street-scale analyses, where pollutant concentrations are highly sensitive to velocity and TKE profiles. The findings underscore the potential of the blended formulation provides as a robust method for modelling complex urban environments, adaptable to different turbulence models and varying morphometric features such as building height variability and density.