Wind-driven cross ventilation in long buildings

The rule of thumb for effective wind-driven cross ventilation suggests that the building length L should be less than five times of ceiling height H. This study uses a Large Eddy Simulation model and wind tunnel experiments to investigate the mechanism behind this rule of thumb. The numerical results reveal that the ventilation rate decreases as the building length increases. This is partly due to the pressure difference between the windward and leeward façades of long buildings (aspect ratio L/. H≥2.5) is smaller than that of a short building (L/H=1.25). The other reason is owing to the internal friction, which can produce a sluggish zone with low wind speed inside the building. For buildings with aspect ratio L/. H≥5, the ventilation rate will be over-estimated, as much as about 20%, by ventilation models that do not consider the internal resistance. The location of the external openings can also influence the ventilation rate. When the openings are located in the opposite corners of the windward and leeward walls, also due to the internal friction, the ventilation was 15.5% less than that of openings on the centerline of the building. The mitigation effect of internal resistance on the ventilation rate can be quantified by a resistance model.