Study on airflow characteristics of underground transportation buildings with sunken squares using particle image velocimetry experiments in a wind tunnel

Recently, sunken squares have been integrated into underground transportation buildings in an effect to improve the urban environment and provide people with a novel urban space experience. Since these sunken squares are directly connected to the indoor environment, when air enters the underground transportation building through the entrance, it has a non-negligible impact on indoor thermal comfort and energy consumption. In this study, we combined PIV technology with wind tunnel experiments to elucidate the airflow characteristics of underground transportation buildings that contain sunken squares. We focused on three different heights of underground transportation buildings and sunken square models and evaluated the influence of the height of underground transportation buildings, the height of sunken squares and the number of sunken squares on the airflow characteristics of underground transportation buildings. We identified the dimensionless average wind velocity at the large openings of the sunken square ranges from 0.013 to 0.082, and that the distribution of the wind velocity at the large opening on the leeward side was more uniform compared to windward side. We also determined, the air flow rate first increases and then decreases with the height of underground transportation buildings when the underground transportation building has the same number and connection forms of sunken squares. Finally, we were able to develop an airflow prediction model for large openings in a sunken square of an underground transportation building. The results of this study provide data and technical guidance for the design of sunken squares and underground transportation buildings.