High-fidelity explicit large eddy simulations of airflows inside buildings using the immersed boundary method and orthogonal grids

Large eddy simulation (LES) can be performed using general-purpose flow solvers such as Fluent or OpenFOAM. These solvers typically require regular grids with high orthogonality and low skewness for explicit LES. Consequently, most existing LES studies of indoor airflows rely on structured grids. Given this grid constraint, this paper presents a novel framework that demonstrates how the unique characteristics of indoor airflows make orthogonal, non-body-conformal grids a suitable choice for high-fidelity explicit LES in buildings. First, orthogonal grids enable more accurate spatial discretization than general-purpose flow solvers, enhancing the precision of scale-resolving simulations. Second, staggered grid arrangements ensure pressure–velocity coupling without introducing artificial numerical dissipation. Third, indoor airflow simulations often involve relatively moderate Reynolds numbers and localized geometrical complexity, making the immersed boundary method (IBM) particularly suitable for handling solid boundaries. IBM eliminates the need for complex re-meshing techniques required for body-conformal grids, thereby facilitating simulations of airflow disturbances caused by moving objects, such as doors or human movement. Our main contribution is to define and validate this framework, as well as to test it by modifying an existing incompressible flow solver, REEF3D, originally designed for hydrodynamics. We evaluate the performance of this method through a series of benchmark tests relevant to indoor airflows, including assessments of airflows generated by sliding doors.