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.

Raw data will successively be uploaded here for the study "Wind tunnel and numerical study of wind pressure coefficients on amedieval Swedish church".

Air infiltration has great importance regarding energy, comfort, moisture and dust deposition in naturally ventilated historical buildings like churches. In these buildings, air infiltration is driven by stack effect and wind. When assessing the wind effect, reasonable estimation of wind pressure coefficients is crucial. Local wind pressure coefficients are significantly affected by the shape of the building and turbulence generated by its geometrical features, which – for moderately large churches – typically consist of main building body and window recesses, gable roof, tower, apse, and sacristy. To yield practically useful pressure coefficients, this study conducted wind tunnel and numerical investigations at a church, with gradual addition of these features. Wind pressure was measured on a small-scale model in a wind tunnel, while computational fluid dynamics (CFD) simulations were carried out for both small-scale and full-scale models. By comparing the experimental and numerical results, the SST k–ω turbulence model showed the best accuracy, yielding CFD results in good agreement with wind tunnel measurements. Most challenging was predicting the strong negative wind pressures occurring on a flat roof. Building structures like tower, apse, and sacristy had significant impact on the wind pressures on the central, main building body. This study adds knowledge on wind pressure effects by commonly occurring building components, with particular reference to churches and similar buildings. It was an important basis for developing further models for calculating the wind pressure coefficient.

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 Reynolds number independence (Re independence) criterion of Re > 11,000 is widely adopted to fulfill the dynamic similarity between the urban flow modeling and the down-scale measurements. However, for 2D street canyons with H/W ≥ 1.5, experiments, numerical simulations, and in-situ observations have reported different vortex-flow regimes with similar building configurations but at different scales. This study uses both wind tunnel experiments and numerical simulations to revisit the Re-independent flow regimes and Re independence criteria with an extensive Re range for idealized 2D street canyons with various aspect ratios (H/W = 1.1, 2.4, 3, 4, and 5). We introduced an optimized ratio of relative changes (RRCs) to evaluate the flow regimes’ similarity. The wind tunnel experiment confirms that the cavity flow with H/W = 1.1 meets the Re independence when reference building Re (Reref) exceeds 11,000. Simulations validated by the experiment results are conducted to investigate detailed flow regimes and the critical Re (Rec) range for each aspect ratio. The canyons with H/W = 2.4, 3, and 4 are dominated by a single asymmetric vortex when the Re independence is satisfied, while there are two vertically-stacked counter-rotating vortices in the canyon with H/W = 5. The value range of Rec increases with aspect ratio from 1.9 × 104–2.6 × 104 (H/W = 2.4) to 1.3 × 105–2.1 × 105 (H/W = 3), and 2.1 × 106–6.4 × 106 (H/W = 4 and 5). Our results indicate that the fully Re-independent flow regimes in deep canyons have fewer vortices than the literature value with down-scale experiments and simulations. The variant Rec with different aspect ratios suggests the requirement to conduct the Re-independence test for different model configurations.

Validated by wind tunnel data, this study numerically investigates the integrated impacts of wind and thermal buoyancy on urban turbulence, ventilation and pollutant dispersion in full-scale 2D deep street canyons (aspect ratio AR = H/W = 3 and 5, W = 24 m). Isothermal urban airflows for such deep street canyons can be Reynolds-number-independent when reference Reynolds number (Re) exceeds the critical Re (Rec~106,107 when AR = 3, 5), i.e. AR = 5 experiences two main vortices and one-order smaller NEV* (~10−3, the normalized net escape velocity) than AR = 3 with a single main vortex (NEV*~10−2). With sufficiently large Re (Re > Rec) and the same air-wall temperature difference (Ri = 2.62, 4.36 when AR = 3, 5), four uniform wall heating patterns were considered, including leeward-wall heating (L-H), windward-wall heating (W–H), ground heating (G-H), and all-wall heating (A-H). Various indicators were adopted to evaluate street ventilation and pollutant dilution capacity (e.g. age of air (τ,s), NEV*, pollutant transport rates (PTR)). Full-scale wall heating produces a strong upward near-wall buoyancy force, which significantly influences flow patterns and improves street ventilation for most cases. When AR = 3, L-H strengthens the single-vortex airflow. When AR = 5, L-H converts the isothermal double vortices into a single-clockwise vortex. For both ARs, W–H reverses the main clockwise vortex to an enhanced counterclockwise one, moreover G-H and A-H cause a more complicated multi-vortex pattern than isothermal cases. Overall, when AR = 3, L-H and W–H increase NEV* by 68% and 40% than the isothermal case. When AR = 5, four wall heating patterns all raise NEV* considerably (by 150%–556%). For both ARs, the L-H, W–H and A-H amplify the contribution of mean flows on removing pollutants but reduce that by turbulent diffusion compared with isothermal cases.

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. 

Improving city breathability has been confirmed as one feasible measure to improve pollutant dilution in the urban canopy layer (UCL). Building height variability enhances vertical mixing, but its impacts remain not completely explored. Therefore, both wind tunnel experiments and computational fluid dynamic (CFD) simulations are used to investigate the effect of building height variations (six height standard deviations σH = 0%–77.8%) associated to building packing densities namely λp/λf = 0.25/0.375 (medium-density) and 0.44/0.67 (compact) on city breathability. Two bulk variables (i.e. the in-canopy velocity (UC) and exchange velocity (UE)) are adopted to quantify the horizontal and vertical city breathability respectively, which are normalized by the reference velocity (Uref) in the free flow, typically set at z = 2.5H0 where H0 is the mean building height. Both flow quantities and city breathability experience a flow adjustment process, then reach a balance. The adjustment distance is at least three times longer than four rows documented in previous literature. The medium-density arrays experience much larger UC and UE than the compact ones. UE is found mainly induced by vertical turbulent fluxes, instead of vertical mean flows. In height-variation cases, taller buildings experience larger drag force and city breathability than lower buildings and those in uniform-height cases. For medium-density and compact models with uniform height, the balanced UC/Uref are 0.124 and 0.105 respectively, moreover the balanced UE/Uref are 0.0078 and 0.0065. In contrast, the average UC/Uref in height-variation cases are larger (115.3%–139.5% and 125.7%–141.9% of uniform-height cases) but UE/Uref are smaller (74.4%–79.5% and 61.5%–86.2% of uniform-height cases) for medium-density and compact models.