Combined Effect of Courtyard Aspect Ratio and Wind Direction on Natural Ventilation in Educational Buildings Using a Porous-Media CFD Model

Natural ventilation is an important passive strategy for improving the indoor environmental quality of educational buildings. However, ventilation performance in courtyard buildings is strongly influenced by the interaction among building morphology, wind direction, and dense classroom occupancy. This study investigates the qualitative interaction of courtyard aspect ratio and inlet wind direction on indoor natural ventilation using an occupancy-aware computational fluid dynamics (CFD) approach. Classroom occupants were represented as a porous region with additional momentum source terms to reduce computational cost while retaining their aerodynamic effects. Six courtyard aspect ratios and five inlet wind angles (0°, 22.5°, 45°, 67.5°, and 90°) were evaluated using indoor wind velocity, age of air (AOA), and the area ratio of age of air (ARAA) as performance indicators. The cross-validation was performed on the 1.5 m evaluation plane of a representative classroom under the prevailing wind direction. The porous media model reproduced the occupant-resolved simulations with errors of only 5.8% for wind velocity and 3.4% for AOA, whereas neglecting occupants resulted in errors of 22.1% and 30.1%, respectively. The results reveal a clear qualitative interaction within the investigated cases between courtyard geometry and wind direction. Courtyards with aspect ratios below 1.5 achieved better ventilation at inlet angles of 45–67.5°, whereas the best-performing inlet angle shifted toward approximately 22.5° when the aspect ratio exceeded 2.5. At an inlet angle of 45°, ventilation performance showed the lowest sensitivity to changes in courtyard aspect ratio. In addition, classroom position relative to the courtyard, corridor, and open space substantially affected local airflow characteristics. The identified trends (e.g., best-performing inlet angle shifting from 67.5° toward 22.5° with increasing aspect ratio) were consistently reproduced across the six investigated aspect ratios. These findings demonstrate that courtyard geometry and building orientation should be coordinated rather than considered independently during early-stage design. The proposed porous media CFD approach provides an efficient and realistic method for evaluating natural ventilation and offers practical guidance for passive and sustainability-oriented educational building design for the investigated conditions.