Approaching Wind Direction Research Articles

Estimating omnidirectional urban vertical wind speed with direction-dependent building morphologies

A main external factor that limits the energy assessment of high-rise buildings in urban areas is the vertical wind conditions. Conventional buildings' energy assessment tools apply a universal vertical wind profile estimation equation in all directions and only consider four typical terrain types. However, the various shape and configurations of buildings in cities can induce the unique wind speed in each direction, and the vertical wind speeds in the same terrain type can be different even in the same city. To get more realistic vertical wind conditions, this study developed an omnidirectional urban vertical wind speed estimation method with direction-dependent building morphologies. A pie-shaped segment method is applied to calculate building morphological features in approaching wind direction. Machine learning methods are developed to calculate parameters of the modified direction-dependent power law urban wind function to convert the Typical Meteorological Year (TMY) wind speed to urban vertical wind speeds. To validate the proposed method, wind tunnel data in Hong Kong are collected for a case study. The results indicate that the proposed method with morphological inputs can effectively estimate the vertical wind speed at different wind directions and heights, and deep neural networks and support vector regression have better performance.

Wind-tunnel analysis of turbulent flow in cross-ventilated buildings with windcatchers: Impact of surrounding buildings

This paper presents a detailed experimental analysis of ventilation flows in a building with a windcatcher surrounded by building arrays. The surrounding building arrangements were in two forms: inline and staggered, representing an abstract variety of suburban morphology. Ventilation measurements were performed for five approaching wind directions: two normal to the building opening(s) at 0° and 180° orientations, two oblique at 40° and 140° orientations, and one parallel to the building opening(s) at 90° orientation. Time-averaged and instantaneous data analyses were carried out. The time-averaged approach showed the overall ventilation flow interaction with the building. Meanwhile, the instantaneous data analysis helped investigate the unsteady airflow characteristics. Hence, the room pressure fluctuations were considered in the frequency domain. Additionally, the coefficient of variance and probability density functions were obtained to identify the nature of ventilation flow fluctuations.The results showed that the maximum reductions in ventilation rate were 30% and 42% for the inline and staggered arrays, respectively, compared to an isolated building case. The highest obstructions of the onset flow to the openings occurred at direct wind incidences for the inline array and oblique wind incidences for the staggered array. The nearby buildings enhanced the fluctuating ventilation flows at 0° and 180° incidences. However, the isolated building case showed the highest fluctuations at 90° incidence since the turbulent flow was mostly dissipated along the surrounding street canyons for the arrayed cases. The room interior pressure fluctuations and spectra showed prominent peaks associated with the Helmholtz frequency in all case studies.

Morphological changes and dynamic mechanism of blowouts on fixed dunes in the Otingdag sandy land, China

<p indent="0mm">Blowouts are a major indicator of fixed dune reactivation, and their continuous occurrence and expansion aggravate the process of regional desertification. Therefore, research on the morphological evolution and dynamic processes of blowouts is paramount not only to understanding reactivation processes in stabilized and semistabilized dune fields but also to accurately assessing regional desertification trends. At present, much research has focused on blowouts in coastal dunes and flat grasslands, and little research has focused on the evolution mode of blowouts that have developed on fixed dunes in semiarid inland areas. Moreover, the morphological changes and airflow dynamics of blowouts are usually analyzed separately, and research integrating the two is still insufficient. In this paper, a fixed dune on the southern edge of the Otingdag sandy land was selected as the research object. A high precision real-time kinematic global positioning system (RTK-GPS) and 17 2D ultrasonic anemometers were used to measure the topography and airflow of four blowouts with different morphologies on the fixed dunes, and the multiperiod imaging data and multiyear meteorological data were used to monitor the morphological changes of each type of blowout and the regional meteorological conditions. The goal of this research is to reveal the morphological evolution, erosion-accumulation characteristics and airflow distribution patterns of each type of blowout and to discuss the relationship among the topography of fixed dunes, morphology of various blowouts and airflow variation. The results showed that the length, width and area of each type of blowout continued to increase from 2010 to 2021, and the overall shapes of each type of blowout tend to be oval or trough-shaped. The expansion direction of the blowout shape has a general corresponding relationship with the erosion-accumulation pattern in the deflation basin. The eastern and southeastern edges of the deflation basins are the most severely eroded, and their overall shapes expand from west to east. Relative to the wind direction of the field reference station, the approaching wind direction of the blowouts hardly deflects at the foot of the windward slope of the fixed dune, while it deflects significantly at the middle and top of the fixed dune, and the deflection angle changes with the regional wind direction. Both the blowout shapes and the approaching wind directions determine the airflow pattern within the blowouts. Based on a detailed analysis of both the regional climate and the morphological changes in blowouts, this study considered that the effect of the topography of fixed dunes on regional airflow could be the fundamental reason for the morphological differences in blowouts. The shape of blowouts determines the airflow pattern in the deflation basin, and the change in the wind speed and wind direction in turn affects the erosion-accumulation pattern and finally changes the evolution direction of blowouts. The research results can act as a good supplement for studying blowouts in semiarid inland areas.

Numerical Investigations of Urban Pollutant Dispersion and Building Intake Fraction with Various 3D Building Configurations and Tree Plantings.

Rapid urbanisation and rising vehicular emissions aggravate urban air pollution. Outdoor pollutants could diffuse indoors through infiltration or ventilation, leading to residents’ exposure. This study performed CFD simulations with a standard k-ε model to investigate the impacts of building configurations and tree planting on airflows, pollutant (CO) dispersion, and personal exposure in 3D urban micro-environments (aspect ratio = H/W = 30 m, building packing density λp = λf = 0.25) under neutral atmospheric conditions. The numerical models are well validated by wind tunnel data. The impacts of open space, central high-rise building and tree planting (leaf area density LAD= 1 m2/m3) with four approaching wind directions (parallel 0° and non-parallel 15°, 30°, 45°) are explored. Building intake fraction <P_IF> is adopted for exposure assessment. The change rates of <P_IF> demonstrate the impacts of different urban layouts on the traffic exhaust exposure on residents. The results show that open space increases the spatially-averaged velocity ratio (VR) for the whole area by 0.40–2.27%. Central high-rise building (2H) can increase wind speed by 4.73–23.36% and decrease the CO concentration by 4.39–23.00%. Central open space and high-rise building decrease <P_IF> under all four wind directions, by 6.56–16.08% and 9.59–24.70%, respectively. Tree planting reduces wind speed in all cases, raising <P_IF> by 14.89–50.19%. This work could provide helpful scientific references for public health and sustainable urban planning.

Field measurements and CFD simulations of wind characteristics at the Yellow River bridge site in a converging-channel terrain

This paper is aimed at investigating the airflow over unusual converging-channel terrain that includes a deep valley on the up-river side and a flat plain on the down-river side. A computational fluid dynamics (CFD) simulation with standard k–ε model was applied to study the variation of wind structure from the important approach wind directions. A long-period of field measurements provided supplementary data from a Sodar system and also 2D ultrasonic anemometers. Broad agreement was found between the observations and the CFD simulations. The results show that a wind speed-up was generated in the valley and its exit, whose magnitude was sensitive to the approaching wind direction, and that terrain-induced effects on wind direction are significant within the height range up to 200 m. It is also found that the wind speeds and directions along the entire bridge structure, including the spanwise deck and the bridge towers, is non-homogeneous due to the effects of the complex terrain. Moreover, through comparing the predicted wind speed at this bridge site, it was found that the Chinese national code recommends a lower speed than CFD simulations for the two worst approach directions which are normal to the bridge axis.

Experimental investigation of wind pressure characteristics and aerodynamic optimization of a large-span cantilevered roof

Cantilevered roof is particularly vulnerable to wind because high suction loads often occur on the rooftop due to flow separation. However, wind load characteristics associated with cantilevered roof have not been fully understood. In this paper, a 1/100 scale model of a cantilevered roof, over a grandstand, has been tested in a turbulent boundary layer flow, with particular emphasis on characterizing the wind pressure distribution on the cantilevered roof, and assessing the performance of several aerodynamic optimization devices. The results indicate that large wind suction generally occurs near the windward edge on rooftop surface. The effect of approaching wind direction is evident, which can influence the distribution pattern and value of wind pressure. For the effect of roof inclination, the down-sloping roof is found to be more appropriate since it results in most significant reduction in mean wind pressure, as well as the net lifting and bending moment coefficients. Furthermore, it is shown that adding a vented leading edge slot can effectively reduce the rooftop wind suctions. The maximum reduction in mean and peak suction loads is respectively 30% and 45%.

Study on mitigation of automobile exhaust pollution in an urban street canyon: Emission reduction and air cleaning street lamps

Affected by the rapid growth of the number of fuel motor vehicles, the automobile exhaust pollution in the street canyon is becoming more and more serious. Different from the pollution source control technology usually used for outdoor air pollution control, an air cleaning street lamp is designed in this work. A 3D steady numerical model with line source pollution is established to simulate the air flow and pollutant diffusion in street canyons and validated. Then, the distributions of pollutants in 90 scenarios of street canyons are numerically simulated. The results show that wind direction and wind speed have great influence on pollutants diffusion in street canyons. Air quality in street breathing zones is improved with the reduction of emission; however, the air quality inside the street canyon is still worse than that outside the street canyon. Scenario with air cleaning street lamps with four 45° downward air outlets is shown to work better in improving air quality. In most of the studied cases, air cleaning street lamps are found to be more effective than reducing emissions from automobiles. A better method to improve the air quality in street canyons is to combine emission reduction measures and air cleaning street lamps. For the high-pollution breathing zone under perpendicular, oblique and parallel approaching wind directions, by this better method the mean PM concentration decreases by 14.38%, 7.68% and 17.78%, respectively; the mean NOx concentration decreases by 24.92%, 21.33% and 29.03%, respectively; the mean CO concentration decreases by 12.77%, 7.45% and 15.05%, respectively.

Research on the effect of different position on classroom ventilation in a “L” type teaching building

The ventilation effect in different places in the teaching building is different. The aim of this paper is to study the relative ventilation relationship of the rooms in “L” type middle school teaching building. Taking Xiamen in the subtropical region as a typical city, different aspect ratio for 1:1, 1:1.5 and 1:2 were set as variables, using the CFD simulation method. Combined and separated effects of the defined parameters on natural ventilation performance are evaluated using the ratio of age of air area as criteria. The result shows that the “L” type teaching building ventilated condition presents several characteristics as the approaching wind direction changed. Corner, long wing and short wing of building have some influences on the wind environment for classrooms. In addition, different aspect ratio “L” type buildings correspond to different optimal inlet wind angles. The research results in this paper can help architects to design buildings more scientifically and reasonably.

Study of wind-direction effects on flow fields over two-dimensional hills using large eddy simulations

Numerous studies have been performed to investigate the characteristics of flow fields over simplified topographies to understand the basic trend of the flow field evolution as a function of hill shapes, hill slopes, ground roughness, and oncoming flow stratifications. However, wind direction, which has a key function in determining flow structures, has rarely been examined. Accordingly, the effects of wind direction on flow fields over two-dimensional (2D) hills are investigated in this study using large eddy simulations. It is found that when the approaching wind direction is not normal to the ridge line, the wind direction behind the 2D hill is not parallel to the approaching wind direction. The recirculation bubble downstream of the hill is weakened, and a significant spanwise velocity behind the hill is observed as the wind direction angle increases. The main direction of the streamwise correlation is found to be normal to the ridge line when the wind direction is 45° rather than parallel with the oncoming wind. A distinct peak in the spectrum of streamwise velocity is identified when the wind direction is 45°. This spectral peak is caused by the periodic shedding of an organized spiral flow structure.

Influence of site and tower types on urban natural ventilation performance in high-rise high-density urban environment

This paper investigates the urban ventilation performance at the street and tower-level in the high-rise high-density urban environment of Hong Kong. By analyzing 218 residential developments and 659 residential towers from 2003 to 2013, this paper presented an innovative way of generating idealized urban forms that considers the regulatory and social-economic factors. Nine typical high-rise high-density residential urban design scenarios with different site division strategies and tower types were generated. The urban ventilation performance of these cases was studied using a validated CFD simulation technique under four different approaching wind directions. Two indices were applied to quantify the urban ventilation performance in this study, namely Wind Velocity Ratio (WVR) and Local Mean Age of Air (MAA). The results show that site and tower types have a great influence on urban ventilation. This paper concludes that extra-large sites (around 4000 square meters) have the potential of achieving the best performance in both street and urban level ventilation, while probably enduring poor indoor air quality. Therefore, a centralized tower plan without deep re-entrants is recommended for large sites. For the small (around 500 square meters) and medium (around 1000 square meters) sites that generally have poor street-level ventilation, adding service lanes can help to improve the street ventilation but may cause poor tower-level ventilation since two rows of towers are closely placed in one street block. For the small sites without service lanes, small and rectangular tower plans in one row could help to achieve better tower-level ventilation and indoor air quality potential.

Outdoor measurements of relationship between canopy flow and wall pressure distributions of a block within urban-like block array

Research on indoor ventilation is mainly conducted utilizing wind-tunnel experiments and computational fluid dynamics simulations under controlled flow conditions. However, with these techniques, the effects of the actual urban boundary-layer flow with variable wind directions and turbulence on the ventilation performance of a building surrounded by various buildings remain unresolved. Therefore, we conducted outdoor experiments at the Comprehensive Outdoor Scale Model experiment site to evaluate the mutual relationships between wall pressure of a building within a square array, in which cubical blocks are aligned in same spanwise position in a row, and atmospheric urban boundary-layer flow. The dataset consisting of the simultaneous measurements of the wall pressure acting on a target block and the air flow around it was analyzed statistically. The results demonstrate that the distributions of wind pressure coefficient significantly change with the approaching wind direction, while similar cavity flow patterns such as the downward flow at windward wall and the upward flow at leeward wall are observed regardless of the wind direction. Additionally, the pressure coefficient is more sensitive to the approaching wind direction at the side-edge than at the center of the block surface. Moreover, the temporal and spatial distributions of wall pressure are highly correlated with the fluctuating flow above the canopy rather than the turbulence generated within the canopy.

Effects of envelope features and upstream buildings on ventilation performance of naturally-ventilated building

In order to study the influence of building facade components (sunshade, wing wall and balcony, etc.) on indoor and outdoor air distribution and indoor natural ventilation in high-density cities being hot in summer and warm in winter, 18 different cases are established to stimulate the effect of natural ventilation frequency of different position in the room by using the computational fluid dynamics (CFD) numerical simulation method through changing the incoming wind direction (wind incidence angle <italic>α</italic>=0° and <italic>α</italic>=45°), the height width ratio of upstream turbulence buildings and vertical and horizontal facade components. The results show the target building generally has a better natural ventilation performance when the wind obliquely incidents on the building (45°). In addition, the effect on ventilation performance of wing walls is highly dependent on the environmental factors including the approaching wind directions, the orientation of the target building and height of the upstream building. The interaction of these parameters affects the flow field characteristics, and changes the effect of facade components on the natural ventilation efficiency of the target building. This study can provide reference for improving the indoor natural ventilation efficiency and air quality by the facade optimization design of the building.

Non-Gaussian Features of Local Wind Pressure for Typical Gable Roof of Low-rise Building

The non-Gaussian characteristics of local wind pressure of typical gable roof of low-rise building with a slop angle of 18.4° were monitored by means of the wind tunnel test under terrain roughness regimes of A,B and C. The probability density distribution features of wind pressure coefficient series are investigated, the characteristics of non-Gaussian zones distribution of wind pressure relative to different wind angles and different terrain conditions are analyzed and the relationships between non-Gaussian pressure and distinctions of flow field on roof are discussed. The studying results indicate that GEV distribution and Gamma distribution fit the probability density distribution of wind pressure coefficient better than Gaussian distribution and Lognormal distribution, but the fitting effect for long trailing tail part is undesirable, when skewness and kurtosis are large in magnitude (absolute value). The variation of terrain roughness and wind directions have significant influence on the distribution of non-Gaussian pressure regions on low-rise building roof. Under the oblique wind direction, the Gaussian pressure zone is divided into three sections along the approaching wind direction, along which the areas move with the change of terrain roughness regimes. The non-Gaussian features of wind pressure is positively related to mutual correlation coefficient of it’s time-history.

Particle image velocimetry measurement and CFD simulation of pedestrian level wind environment around U-type street canyon

The configuration of an urban street canyon plays an important role in determining local microclimate including pedestrian level wind environment (PLWE). The semi-closed U-type street canyon has been widely used in high-density trans-oriented-development urban design for providing private space. However, a quantitative evaluation on PLWE of the U-type street canyon is still absent to date. This study quantitatively assesses PLWE of the U-type canyon via particle image velocimetry tests in a wind tunnel. A range of canyon aspect ratios (the ratio of the canyon height to width) and length ratios (the ratio of the canyon length to height) were investigated. The wind flow inside and in the vicinity of the street canyons was measured and analysed under the perpendicular, oblique and parallel approaching wind directions. It was found that the U-type canyon shows a higher PLWE inside canyon on the open side while a lower PLWE inside canyon on the closed side. Compared to parallel canyon, there is a significant enhancement of PLWE in the centre of the U-type canyon and in some spots on the leeward side of the canyon. This is further strengthened with increasing the canyon aspect ratio and length ratio. Nevertheless, the U-type canyon exhibits a lower wind speed at the pedestrian level than that of the parallel canyon both inside and in the vicinity of the street canyons in general especially under parallel wind direction. It is believed that these findings are greatly beneficial to build sustainable cities with a high quality PLWE.

The impact of semi-open settings on ventilation in idealized building arrays

Semi-open settings (or hanged walls) above the pedestrian level in street canyons can provide protection to pedestrians from heavy-rain and strong sunshine, but may also influence street ventilation and thus pollutant dispersion. By performing computational fluid dynamics (CFD) simulations and employing ventilation concepts of net escape velocity (i.e. diluting rate for exhale effect) and the age of air (i.e. freshness of air for inhale effect), this paper investigates the effects of low-level semi-open settings on ventilation in idealized block arrays with rectangular or square form (building height/street width H/W = 1.25) under neutral atmospheric conditions. Four upstream wind directions of 0° (parallel), 15°, 30°, 45° between the approaching wind and the main street axis and four widths of semi-open settings (Ws = 0 m, 2 m, 4 m, 6 m) are investigated. Results indicate that the increasing width of semi-open settings produce greater age of air (i.e. older air) and smaller net escape velocity (i.e. less diluting rate) than those without semi-open settings. The wider semi-open settings experience the worst ventilation. In addition, the approaching wind directions of 0° and 15° lead to worse ventilation than 30° and 45°, for both rectangular and square forms.

The impact of urban open space and ‘lift-up’ building design on building intake fraction and daily pollutant exposure in idealized urban models

Sustainable urban design is an effective way to improve urban ventilation and reduce vehicular pollutant exposure to urban residents. This paper investigated the impacts of urban open space and ‘lift-up’ building design on vehicular CO (carbon monoxide) exposure in typical three-dimensional (3D) urban canopy layer (UCL) models under neutral atmospheric conditions. The building intake fraction (IF) represents the fraction of total vehicular pollutant emissions inhaled by residents when they stay at home. The building daily CO exposure (Et) means the extent of human beings' contact with CO within one day indoor at home. Computational fluid dynamics (CFD) simulations integrating with these two concepts were performed to solve turbulent flow and assess vehicular CO exposure to urban residents. CFD technique with the standard k-ε model was successfully validated by wind tunnel data.The initial numerical UCL model consists of 5-row and 5-column (5×5) cubic buildings (building height H=street width W=30m) with four approaching wind directions (θ=0°, 15°, 30°, 45°). In Group I, one of the 25 building models is removed to attain urban open space settings. In Group II, the first floor (Lift-up1), or second floor (Lift-up2), or third floor (Lift-up3) of all buildings is elevated respectively to create wind pathways through buildings. Compared to the initial case, urban open space can slightly or significantly reduce pollutant exposure for urban residents. As θ=30° and 45°, open space settings are more effective to reduce pollutant exposure than θ=0° and 15°.The pollutant dilution near or surrounding open space and in its adjacent downstream regions is usually enhanced. Lift-up1 and Lift-up2 experience much greater pollutant exposure reduction in all wind directions than Lift-up3 and open space. Although further investigations are still required to provide practical guidelines, this study is one of the first attempts for reducing urban pollutant exposure by improving urban design.

Integrating twisted wind profiles to Air Ventilation Assessment (AVA): The current status

Twisted wind flows generated by the complex terrain of Hong Kong induce two types of complication to Air Ventilation Assessment (AVA), first, imposing a false boundary condition on the wind tunnel tests done for AVA and, second, creating an ambiguity in determining the approaching wind direction in calculating the probability of occurrence of winds. The latter issue is partially solved using correction methods in post-analysis of AVA but the accuracy of these methods is not yet accessed. This study employs two twisted wind profiles to test an urban area in a boundary layer wind tunnel to investigate the influence of twisted wind flows on the outcomes of AVA and to estimate the accuracy of three common correction methods: No-Shift, Threshold, and Proportional methods. The results reveal significant differences in wind speeds at the pedestrian level for twisted and conventional wind flows at locations with low building densities. The discrepancies in wind speeds are minimum at the locations where the density of buildings is high. The indicators calculated by the No-Shift method frequently deviate from those of the twisted wind flows, while the Threshold and Proportional methods routinely over-predict the indicators of AVA.

Effects of twisted wind flows on wind conditions in passages between buildings

The hilly terrain in Hong Kong detour winds and generate twisted wind profiles, which have varying wind directions along the profile's height. Twisted winds considerably influence the wind environments in built-up areas. Particularly the wind conditions around building arrays have paramount importance in Hong Kong as most of the building arrays are closely-spaced tall residential towers with some recreational areas located at buildings’ lower levels. This paper aims at the systematic investigation of the surrounding wind conditions of building arrays in twisted wind flows by a series of wind tunnel tests. There were 9 building models with different heights and passage widths tested in two twisted wind profiles at a number of wind incidence angles. Each model had three identical buildings and two similar passages among buildings. The observed wind environments in twisted wind flows were asymmetric near building arrays and wind speeds were dissimilar in the two passages. Compared to the building height, the passage width has more significant influences on wind conditions in passages in twisted wind flows. The effective wind direction, which is the vector summation of wind twist and wind incidence angle, is important in understanding the influence of approaching wind directions of twisted wind flows.

Pedestrian-level wind environment around isolated buildings under the influence of twisted wind flows

The influence of twisted wind flows on pedestrian-level wind environments was evaluated by using two twisted wind profiles (TWP) in a boundary layer wind tunnel. Simulated wind profiles had maximum yaw angles of 13° and 22° to represent ‘high’ and ‘extreme’ wind twist conditions, respectively. Five buildings with the aspect ratio (Height: Width) of 4:1 to 0.5:1 were tested for a number of wind incidence angles to assess the influences of building dimensions and approaching wind directions. All test cases were repeated in a conventional wind profile (CWP) with similar mean wind speeds and turbulence intensities for the purpose of comparison. The results reveal that pedestrian-level wind environments in TWPs are different than in CWPs owe to asymmetric wind fields, displaced flow features, and variations in areas of high and low wind speeds. The increased areas of low wind speeds and displaced downstream far-field low wind speed (DFLWS) zone are the two important flow modifications that would have negative impacts on outdoor thermal comfort and air pollutant dispersion in built-up areas where TWPs exist. Due to the embedded wind twist angles, wind incidence angles have different effects on flow modifications in twisted wind profiles than in conventional wind profiles.

Effect of lift-up design on pedestrian level wind comfort around isolated building under different wind directions

The pedestrian level wind environment is seriously worsen by moderated air flow in the built-up cities like Hong Kong. The lift-up design is therefore adopted in the building constructions to improve the weak wind condition. In order to evaluate the influence of lift-up design on the pedestrian level wind comfort, the wind flow around isolated buildings with and without Lift-up design are simulated respectively via CFD approach. The turbulence model and numerical method are firstly validated by comparing the simulated wind flow data with a wind tunnel test. Then the validated model is used to simulate the wind flows around the isolated buildings. Results show that the lift-up design can improve the wind comfort at pedestrian level and its effects are highly rely on the approaching wind direction. Specifically, the wind comfort is better under the oblique wind direction than the other wind directions.