Previous studies have rarely addressed single-sided ventilation driven by the external flow over the roof, which exhibits considerable potential owing to its highly turbulent nature and strong suction associated with leading-edge flow separation. In this study, wind tunnel experiments on single-sided ventilation through a roof opening were conducted using two isolated generic models: a cylinder and a rectangular prism, each with a set of replaceable openings. Both models were tested either flush- or floor-mounted. Two inflow conditions, each with three free-stream velocities, were considered. For both models mounted beneath the floor, the nondimensional ventilation rates (Q*) are comparable to values reported in the literature; for the prism, a slight increase in Q* with orientation suggests the development of a mixing layer along the streamwise extent of the floor-level opening. In the floor-mounted configuration, body-induced flow disturbances tend to enhance ventilation. Three primary governing rooftop flow regimes are identified—recirculation zone, flow reattachment, and conical vortex—whose relative dominance over the opening depends on inflow turbulence, wind incidence angle, and model configuration. When the opening lies entirely within the recirculation zone, Q* is proportional to the normalized local fluctuation intensity, with a coefficient of about 0.16. For certain yaw angles, the marked increase in Q* strongly correlates with the presence of a conical vortex over the prism model roof, which features strong suction and intense fluctuations. Direct advection through the opening could occur with a favorable opening size and location, allowing deep penetration of the reattaching shear layer.

As a preliminary investigation of the wind-driven purging process for densely built environments through the canopy layer, the ventilation efficiency of standalone semi-enclosed models incorporating a horizontal opening in the roof façade was investigated in the wind tunnel. For comparison, two models with different geometries were constructed, and each model was tested individually. Both models were equipped with replaceable roof covers, enabling the adjustment to the opening size. The ventilation efficiency was evaluated by continuous releasing and sampling of the tracer gas, from which the normalized purging velocity (PFVn) was derived. Additionally, the flow condition over the opening was monitored using the Laser Doppler Anemometer. It was found that separation flows from the frontal edge(s) of the model could introduce secondary circulations across large openings, resulting in dramatic increases in PFVn. Both the rectangular prism model and cylinder model possessed higher PFVn compared to prior studies on single-sided ventilation, while close values were observed with cylinder model mounted under the wind tunnel floor. Besides, the vertical distribution of integral length scales of streamwise velocity indicated the stratification feature of separation flows under low-turbulent incoming flow conditions. Measurement results provide validation data for further simulation studies including more detailed structures.

This study adapted the mean age of air, a time scale widely utilized in evaluating indoor ventilation, to assess the impact of building layouts on urban ventilation capacity. To distinguish it from its applications in enclosed indoor environments, the adapted index was termed the effective mean age of air (T_E). Based on an experimentally validated method, computational fluid dynamic (CFD) simulations were performed for parametric studies on four generic parameters that describe urban morphologies, including building height, building density, and variations in the heights or frontal areas of adjacent buildings. At the breathing level (z = 1.7 m), the results indicated three distinct distribution patterns of insufficiently ventilated areas: within recirculation zones behind buildings, in the downstream sections of the main road, or within recirculation zones near lateral facades. The spatial heterogeneity of ventilation capacity was emphasized through the statistical distributions of T_E. In most cases, convective transport dominates the purging process for the whole canopy zone, while turbulent transport prevails for the pedestrian zone. Additionally, comparisons with a reference case simulating an open area highlighted the dual effects of buildings on urban ventilation, notably through the enhanced dilution promoted by the helical flows between buildings. This study also serves as a preliminary CFD practice utilizing T_E with the homogenous emission method, and demonstrates its capability for assessing urban ventilation potential in urban planning.

A tracer gas measurement method was developed to investigate the single-side removal process of pollutants through the roof level, conducted in a wind tunnel utilizing a deep box model with various openings at its top. A homogeneous emission was modelled by filling the box with well-mixed nitrous oxide released from a line source inside. The removal efficiency from the openings with box at different orientations was evaluated by normalized purging velocity (PV) and spatial-mean age of air (). The PIV system was adopted to visualize the flow field above the opening. Varying interactions between the perturbations from the openings and the deflected flow over them resulted in diverse PVs at different orientations. There are also clues showing that the Reynolds-number independent conditions for cavity flows, such as inside a street canyon, are also related to the scale of the open area at the roof height.

Here, it is presented a possible methodology and experimental model for benchmarking of air quality in cities. The concept behind the methodology is that a city’s inherent structure affects the potential for contaminant removal due to the resistance it poses to inflow. The approach is based on homogenous emission across the street surface network, representing a worst-case situation. Different levels of complexity can be used for benchmarking, making it valuable for evaluating different layouts. Additionally, an urban ventilation index suitable for these kinds of experimental studies has been suggested. 

The drag force generated by aligned arrays of cubes of different packing density and exposed to different wind directions in a wind tunnel is discussed. Results allowed to build a drag force rose which shows that the drag force increases with increasing packing density till λ_p = 0.25 for any wind direction. It is also shown that, independent of the packing density, the drag force increases with increases deviation of WD from the perpendicularity.

The purpose of the paper is to examine the relation between urban morphology, wind direction and air flow rates. In the study a highly idealized city model was used consisting of a circular block divided into two or four equally large sectors. Wind tunnel experiments and CFD predictions have been conducted. The interaction between the atmospheric boundary layer and a city is considered to be both a function of the overall shape and the internal resistance to the flow caused by the friction when the wind flows over the urban surfaces. Flow along the streets is generated by pressure differences. In the wind tunnel, velocity measurements have been recorded in the streets at several points and pressure on the ground was registered in 400 points. The wind tunnel measurements were used to validate the CFD model. The CFD predictions provided complete flow and pressure fields for different configurations and wind directions. The flow balance is presented considering both the horizontal air flow and the vertical air flow (subsidence and updraft). Special attention was on the pressure distribution at ground level (pressure footprint), which is believed to provide valuable information that can be used for qualitative city ventilation analyses.