Natural ventilation assessment in typical open and semi-open urban environments under various wind directions

Semi-open street roofs protect pedestrians from intense sunshine and rains. Their effects on natural ventilation of urban canopy layers (UCL) are less understood. This paper investigates two idealized urban models consisting of 4(2 × 2) or 16(4 × 4) buildings under a neutral atmospheric condition with parallel (0°) or non-parallel (15°, 30°, 45°) approaching wind. The aspect ratio (building height (H)/street width (W)) is 1 and building width is B = 3H. Computational fluid dynamic (CFD) simulations were first validated by experimental data, confirming that standard k–ε model predicted airflow velocity better than RNG k–ε model, realizable k–ε model and Reynolds stress model. Three ventilation indices were numerically analyzed for ventilation assessment, including flow rates across street roofs and openings to show the mechanisms of air exchange, age of air to display how long external air reaches a place after entering UCL, and purging flow rate to quantify the net UCL ventilation capacity induced by mean flows and turbulence. Five semi-open roof types are studied: Walls being hung above street roofs (coverage ratio λa = 100%) at z = 1.5H, 1.2H, 1.1H (‘Hung1.5H’, ‘Hung1.2H’, ‘Hung1.1H’ types); Walls partly covering street roofs (λa = 80%) at z = H (‘Partly-covered’ type); Walls fully covering street roofs (λa = 100%) at z = H (‘Fully-covered’ type). They basically obtain worse UCL ventilation than open street roof type due to the decreased roof ventilation. ‘Hung1.1H’, ‘Hung1.2H’, ‘Hung1.5H’ types are better designs than ‘Fully-covered’ and ‘Partly-covered’ types. Greater urban size contains larger UCL volume and requires longer time to ventilate. The methodologies and ventilation indices are confirmed effective to quantify UCL ventilation. •Semi-open street roofs protect pedestrians from strong sunshine and heavy rains.•But they may affect airflows and ventilation in urban canopy layers (UCL).•Age of air & flow rates are analyzed under wind directions of 0°, 15°, 30°, 45°.•Walls fully or partly covering street roofs at z = H get the worst UCL ventilation.•Semi-open street roofs at z = 1.2H, 1.1H get good ventilation and are realistic designs.