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This paper presents a vortex‐induced vibration (VIV)‐based piezoelectric energy harvester that performs well for all wind directions, a so‐called omnidirectional wind energy harvester. The kinetic energy of this harvester stems from wind‐induced vibrations of a circular cylinder mounted on an orthogonal bibeam system, rather than a traditional single beam. Wind tunnel testing results show that compared to the traditional single‐beam energy harvester, the proposed harvester substantially enhances the effectiveness, in most cases that the beam is skew to the incoming flow. The reasons for the enhancement are explained in detail by examining the wind‐induced displacement response components of the cylinder identified by the image processing technique. For all wind directions, both the maximal output energy and the range of effectively working wind speed of the proposed bibeam wind energy harvester are significantly improved with respect to the single‐beam system, indicating excellent performance of the proposed omnidirectional harvester in a natural wind environment.

Sopite syndrome, centered around the drowsiness, lethargy, and irritability associated with motion sickness, can be induced by exposure to low-frequency motion. It is known that the vestibular apparatus plays an important role in the pathogenesis of motion sickness, which features several autonomic responses, and we have previously documented increased vestibular modulation of skin sympathetic nerve activity (SSNA) and an increase in skin blood flow associated with nausea. Here, we assessed whether imperceptibly slow sinusoidal motion, sufficient to induce sopite syndrome but not nausea, also modulates SSNA and skin blood flow. Participants were seated upright and exposed to a randomized set of sinusoidal linear accelerations, ranging from 0.03 Hz at 0.5 mG to 0.2 Hz at 5 mG, via a motorized platform. At all frequencies vestibular modulation was greater than the cardiac modulation of SSNA, but cardiac modulation and skin blood flow were both significantly lower during the motion than at baseline. We conclude that sopite syndrome is associated with a marked modulation of sympathetic outflow to the skin and cutaneous vasoconstriction.NEW & NOTEWORTHY Little is known about the autonomic consequences of sopite syndrome-the drowsiness that can be induced by low-amplitude cyclic motion. We recorded skin sympathetic nerve activity (SSNA) in seated participants exposed to slow sinusoidal linear acceleration (0.03-0.2 Hz), which preferentially activates hair cells in the utricular part of the otolithic organs, at amplitudes that generated no sensations of motion. At all frequencies, there was a clear vestibular modulation of SSNA and cutaneous vasoconstriction.

This paper proposes a computational fluid dynamics (CFD) model, along with dimensionless quantitative assessment standard—air pollution residual time (APRT) for the evaluation of local haze-fog (HF) dispersion in a built environment. A low APRT value ensures good ventilation. A building group model that comprises high-rise business building, mid-rise office buildings, low-mid-rise residential buildings (at the center of the building group), a mid-rise recreational center, and a local road (open terrain), was scaled down (1:100) to simulate the HF dispersion process. The orientation of the building group was numerically modified to generate a wind incidence normal to the high-rise building, mid-rise buildings, recreational center, and road. The results showed that the orientation of the building group largely determines the APRT. The most favorable orientation can reduce APRT by more than 50%. Our results strongly suggested that in order to reduce the consequential negative effect of air pollution, future urban designs should undergo a comprehensive ventilation assessment to ensure a low APRT value.

Gas cyclones are widely used to purify waste gases but are not effective in collecting fine particles. The cloud-air-purifying (CAP) technology was recently developed to overcome this deficiency by unitizing the supersaturated vapor in the cyclone. This paper presents a CFD study on the CAP cyclone. The condensational growth of particles is implemented in the CFD model. Its positive effect on the collection efficiency is confirmed and more clearly understood from microscopic investigations. Particles with the same initial size can grow into a narrow size distribution, yet the average growth ratio is increased with the increase of the supersaturation ratio and the decrease of the initial size, resulting in a significant improvement of the collection efficiency for fine particles, particularly in controlling those escaping through the middle-reverse flow. A predictive model is developed to consider the effect of the vapor. These results can help understand and optimize the CAP cyclone.

Detailed investigation of fire-wind interaction is highly instrumental in understanding the cause of the devastating consequences of major fire events in windy weather conditions. Enhancement of wi...

Cladding damage of tall buildings often occurs during hurricane events, and the damage is not only costly but also impacts the economy, and more importantly, threatens lives. One solution to reduce the cladding pressure is the application of modified double-skin facade (DSF) which is already of great interest to architects due to improving building energy efficiency. This paper investigates the effects on the cladding pressures of a building fitted with a DSF with vertical openings in the external skin mounted in front of the windward face of the building. The effects of the vertical openings on the building's surface pressure were investigated by conducting pressure measurements in the wind tunnel. The associated flow mechanisms were revealed by using particle image velocimetry technique. The DSF without opening increases both the mean suctions and fluctuating pressures on both side and leeward faces, and hence this common facade configuration leads to an undesirable effect on building claddings under strong winds. In contrast, the DSF with the opening(s) can effectively reduce wind pressures on the side and leeward faces. Therefore, creating vertical openings on the external skin of DSF is a practical approach to enhance the wind-resistance performance of building claddings.

Understanding the aerodynamics associated with the interaction of fire and cross-wind flow is of great importance because the consequence may have major implications in building design against bushfire (or wildland fire) attacks. However, a fundamental understanding of how the interaction of fire and wind can alter free stream flow aerodynamic properties has remained elusive. The scope of this study is to examine the pool fire and wind interaction under fixed wind velocity condition. This study dissects the fundamental mechanisms of how the interaction of horizontal momentum flow with a vertical buoyant plume leads to enhancement of wind velocity in the horizontal direction at a certain elevation. Changes in flow aerodynamics caused by the interaction of fire and wind were analysed using the computational fluid dynamics approach. The mechanisms causing the changes were explained. A module was developed and added to the FireFOAM solver to evaluate flow acceleration due to the pressure gradient, gravity, and viscous effects. The chosen computational model was validated against two sets of experimental data, namely, a buoyant diffusion fire plume in still air and the other in cross-wind condition. The numerical simulation revealed that due to the interaction of fire and wind, there is a negative longitudinal pressure gradient across the plume axis, causing the flow to accelerate and the velocity profile to alter. It was also shown that the distortion in velocity profile depends on the location downstream of the fire plume. The height of the distortion increases whilst the magnitude of the distortion diminishes as the longitudinal distance from the fire source increases. Investigation of the effects of heat release rate on wind enhancement further showed that fire with a higher heat release rate causes a greater pressure gradient and a lower density, culminating in higher flow acceleration and consequently increase of wind enhancement.

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.

This study aims to quantify the effects of exposure to wind-induced building motion on office workers. 55 participants, across 8 buildings in Wellington, reported their comfort, health, cognitive performance, work performance and response behaviours, via an on-line survey over a period of 18 months. Each building was instrumented with a small accelerometer that measured building accelerations across the entire study period. The analysis used acceleration dose and mean exposure to building accelerations to predict occupant responses. Participants reported stronger perceptible motion with increases in acceleration dose; moderated by the natural frequency of the building. Increases in building accelerations were associated with higher levels of motion sickness and sopite syndrome (sleepiness), which caused large reductions in task-based effort, consequently leading to reductions in work performance; however, cognitive performance was only mildly affected. These findings provide additional evidence showing the adverse effects of exposure to building motion. The limitations of field research and implications for experimental motion simulator studies and future serviceability criteria are discussed.

[Abstract] The wind flow around twin-box decks is particularly complex due to the very important role played by the slot distance between girders and the presence of the downwind box that is immersed in the wake of the upwind box. Detailed experimental studies on the three-dimensional flow features of this deck typology are certainly challenging, and CFD simulations have not tackled this problem so far. The goal of this 3D LES piece of research is to shed some light on the complex flow features around twin-box decks, particularly when the deck geometry is fully three-dimensional due to the presence of the transversal beams linking together the deck girders. Integral parameters, mean and instantaneous flow structures and spanwise correlations are studied for a segment of the Stonecutters Bridge. It has been found that the presence of the transversal beam plays a key role in the flow correlation loss, identifying, for instance, the out-of-phase shedding of vortices at different sides of the transversal beam. The numerical results agree well with experimental data in the literature, and offer a richer and more comprehensive explanation for the identified aerodynamic responses. This research has been funded by the Spanish Ministry of Economy and Competitiveness in the frame of the research project with reference BIA2016-76656-R and the Galician regional government with reference ED431C 2017/72. The first of the authors has been funded by the Spanish Ministry of Economy and Competitiveness in the frame of the National Program for Promotion of Talent and Employability through the BES-2014-068418 predoctoral contract grant, associated with the BIA2013-41965-P research project Xunta de Galicia; ED431C2017/72

[Abstract] The phenomenon of turbulence is present in almost every type of flow in practical applications. Depending on its level of intensity and length scale, it can modify both the aerodynamic and aeroelastic performance of a body under flow action. In wind tunnel tests, the desired turbulence level is achieved by placing obstacles, spires, grids and extra roughness generators upwind the tested model. On the other hand, when trying to reproduce turbulence effects by means of a computational fluid dynamics (CFD) approach, two options have usually been considered: synthetic turbulence generation and the reproduction of velocity and pressure fluctuations recorded from previous simulations or wind tunnel tests. Another option, whose feasibility in CFD applications is addressed in this work by means of a 2D URANS (unsteady Reynolds averaged Navier–Stokes) consists of placing a rod upstream of the studied body, near the stagnation line. This approach is based on the generation of small scale turbulence upstream of the studied body, so that the turbulent wake generated by an upwind rod impinges on the body located downwind. In the present study, by means of 2D URANS simulations, the smooth flow over a circular cylinder (the upwind rod) is studied focusing on its wake turbulence characteristics. Furthermore, the aerodynamic performance of a square cylinder, first under smooth flow, and later immersed in the turbulent wake of the upstream rod, are analysed. A substantial effort has been devoted in the verification studies of the numerical models. It has been found that the adopted numerical approach is able to reproduce the turbulent characteristics of the rod wake and assess the impact of the turbulent flow on a square cylinder, providing a promising agreement with experimental data. Xunta de Galicia; ED431C2017/72

This study investigated mean flow characteristics and structure of flow inside through-building openings with five different layout configurations for the purpose of wind energy harvesting. Two-dimensional Particle Image Velocimetry (PIV) was employed and validated against a series of Cobra probe measurements. The mean flow characteristics including streamwise and vertical mean velocity were investigated at the wind direction parallel to the corridors of the through-building openings. The coherent structure of flow was studied, using two-point cross-correlation and a simple flow visualisation. The results showed that creating recessed regions at inlet and outlet of the openings and using curved walls at corners increase the mean velocity at the middle of the corridor by approximately 25%. When using converging-diverging passages for openings, the mean velocity increases by approximately 33%. The two configurations exhibit better performance in attuning the flow to be more inform and steady in a larger portion of the corridor, and hence these configurations have the capacity to enhance the performance of a potential wind turbine inside the corridor. The confined area of corridors contributes to the progressive reduction of undesirable characteristics of the cavity flow including vertical mean velocity, and streamwise and vertical velocity fluctuations along the corridor.

PowerWindow is a Linear Cascade Wind Turbine (LCWT) capable of generating electricity in very low blade speed ratios, which makes it a safe power generator in built environments. The power generation mechanism and aerodynamic performance of PowerWindow were investigated using experimental prototyping and computational fluid dynamic simulations in a previous study. Due to the limited suitable locations for installing the device in grounded configuration, this study investigates elevated and ducted installation configurations. In the elevated configuration PowerWindow is installed on a tower or in between two tall buildings. In ducted configuration it is installed inside a ducted area such as a through-building opening. Aerodynamic performances of the elevated and ducted PowerWindow are investigated and compared, using computational fluid dynamic simulations. It is shown that the coefficient of performance of the ducted PowerWindow with 12% is about 50% higher than the elevated one with 8%. The effect of solidity is also investigated on the flow mechanism and power generation. The results show that increasing solidity results in greater increase in power generation of the ducted configuration compared to the elevated one. This study also investigates effect of increasing solidity on the required pressure gradient and recommends an efficient solidity for each configuration based on the installation circumstances.

Wake-induced vibration (WIV) of a two-degree-of-freedom (2-DOF) downstream square cylinder behind a stationary equal-size upstream square cylinder is numerically investigated at low Reynolds numbers by using Characteristics-Based Split (CBS) finite element algorithm. Due to the interference between the cylinders, the flow pattern and dynamic characteristics of become more complex than an isolated cylinder case. The spacing ratio of the two equal-sized square cylinders in the tandem arrangement is fixed as L/D = 5.0. The Reynolds numbers of this two-cylinder system have five types, varying between 40 and 200 with a unit step of △Re = 40. The reduced mass of the square cylinder is Mr = 2.0, while its reduced velocity changes between Ur = 3.0–18.0. The numerical results show that the reduced velocity and Reynolds number can affect the characteristics of the flow patterns, oscillatory frequency, maximum amplitudes, and X-Y trajectories of the downstream square cylinder. The predominant vortex shedding patterns are 2S, 2S*, 2P, P + S, 2T and steady mode. Additionally, the figure “8” and figure “dual-8” are observed in the X-Y vibrating trajectories of the downstream square cylinder. Finally, the interactions between cylinders underlying the vibration characteristics of the square cylinder behind a stationary square cylinder with different Reynolds numbers are revealed.

The performance of a circular cylinder-based piezoelectric wind energy harvester with two small-size rod-shaped attachments on the main circular cylinder was experimentally investigated. The rod had three different cross-sectional shapes: circular, triangular, and square. The two rods were attached on two sides of the main circular cylinder at a series of circumferential locations. They were parallel to the cylinder axis and symmetrical to the stagnation line. It was found that attaching these three rod types at the circumferential locations of 45° and 60°, harvested power continuously beyond the critical wind speed. It was, therefore, dramatically superior over both the plain circular cylinder and the cases with other attachment locations. Additionally, the output voltage for a circumferential location of 60° was greater than that 45° one over the whole working wind speed range. Furthermore, attaching the triangular rods on the main circular cylinder showed a better performance than the other two rod types. The force measurement results indicated that the circular cylinder with triangular rods at the circumferential location of 60° had larger transverse force coefficients than the other two rod type cases, verifying the findings in the energy harvesting tests. Therefore, it is recommended to attach two triangular rods at the circumferential location of 60° on the main circular cylinder in order to enhance the performance of a circular cylinder-based wind energy harvester.

Lift-up buildings are advantageous in improving the wind circulation in a congested and compact city. However, the wind conditions in the void underneath a lift-up building, also known as the lift-up area, are vital for wind comfort of occupants of lift-up buildings. This study tested 28 lift-up buildings in a boundary layer wind tunnel to assess the influence of key design parameters; height and width of the main structure, and height, width, depth, and the shape of the central core on the wind conditions in the lift-up area. The results of the analyses show a significant influence of building height on the magnitude of wind speeds in the lift-up area while the width of the central core controls the area with low wind speeds. Tall buildings with short lift-up cores have small areas with acceptable pedestrian wind comfort, which can be increased by adopting corner modifications for the central core. The area of acceptable pedestrian wind comfort increases in oblique wind directions as the areas of high (>3.5 m s−1) and low (, Highlights • This study evaluated the influence of design of lift-up buildings on pedestrian wind comfort. • A total number of 28 lift-up buildings were tested in a boundary layer wind tunnel. • Dimensions of the building and the central core differently influenced pedestrian wind comfort. • A second-order, non-linear multivariable regression model was developed to predict pedestrian wind comfort.

The proper selection of a turbulent model greatly influences the prediction of wind in an urban environment. The purpose of this study is to evaluate the capabilities of the improved delayed detached eddy simulation (IDDES) model in a wind flow simulation. A cube (Silsoe cube, 6 m in dimension) with a normal wind direction under typical atmospheric conditions was simulated. Some computational parameters were thoroughly evaluated and analyzed, including the grid resolution, the discretization time step and the sampling time. The selected time-step (0.002 s) and sampling time (10 min) are the most economic and effective. The experimental results and large eddy simulation (LES) results were used to evaluate the performance of the IDDES model. The simulation results include the mean, maximum, minimum, and standard deviation of the pressure coefficients, and the reattachment length. The IDDES model shows results similar to those obtained using the LES model. The distributions of instantaneous velocity field and pressure field around the cube were also discussed, we concluded that the IDDES model is able to provide the acceptable prediction of the mean and unsteady features compared with full-scale experimental data around the Silsoe cube. Our study suggests that the IDDES model could be able to simulate the wind-structure interaction around a Silsoe cube. For the simulation of wind flow over an urban area, IDDES model could be potentially suitable but need to be further verified because of more complex nature flow than cube.

This study examines the influence of building envelope features on interunit dispersion around multi-story buildings, when the presence of an upstream interfering building is also considered. Validated CFD methods in the steady-state RANS framework are employed. In general, the reentry ratios of pollutant from a source unit to adjacent units are mostly in the order of 0.1%, but there are still many cases being in the order of 1%. The influence of envelope features is dependent strongly on the interaction between local wind direction and envelope feature. In a downward dominated near-facade flow field, the presence of vertical envelope features forms dispersion channels to intensify the unidirectional spread. Horizontal envelope features help induce the dilution of pollutant to the main stream and weakens largely the vertical interunit dispersion. The large influences caused by the presence of envelope features extend the existing understanding of interunit dispersion based on flat-facade buildings.

This paper presents a novel and cost-effective method to replicate modified pedestrian level wind (PLW) fields in twisted winds using a conventional wind profile with similar wind speeds and turbulent intensities. The novel method, namely the Equivalent Wind Incidence Angle (ϕEq) method was developed using data of PLW fields near five isolated buildings with aspect ratio (height:width) ranging from 4:1 to 0.5:1 subjected to two twisted wind profiles with the maximum clockwise yaw angles of 13° and 22°. The magnitude of ϕEq was found to be 6° and 14° for the two twisted wind profiles with maximum yaw angles of 13° and 22° and for any clockwise wind incidence angle ϕ, ϕEq is ϕ +6°, and ϕ +14°. The ϕEq method was applied for an Air Ventilation Assessment (AVA) done for an urban site in Hong Kong, and the outcomes were compared with two similar AVAs conducted using the two twisted wind profiles and a conventional wind profile without ϕEq. The comparison revealed that velocity ratios (VRs) and wind speeds with a 50% probability of exceedance are similar or slightly different from those in twisted winds if the conventional wind profile is combined with the ϕEq method.

The aerodynamic damping of forward and backward inclined prisms has been identified through a series of forced vibration wind tunnel tests. The test results show that the amplitude of vibration and reduced wind velocity both have a significant effect on the aerodynamic damping of the inclined prisms, especially at reduced wind speeds where von Karman vortex lock-in occurs. The effect of forward and backward inclination on the characteristics of aerodynamic damping in inclined prisms is dissimilar to a vertical prism, and the characteristics have been discussed in terms of the unsteady aerodynamic force and Strouhal number in this study. This study not only advances our understanding on aerodynamic damping of inclined prisms, but also provides values that can be used to estimate aerodynamics and aeroelasticities of inclined prisms.

The increasing global concern about climate change and energy crisis has necessitated the development of techniques to reach and exploit renewable energy in unexplored regions. As such, decentralized small-scale wind energy harvesting in urban environments has gained momentum in recent years. In this study, a methodology has been developed to assess the performance of a special Double Skin Facade (DSF) system for wind energy generation using CFD simulations and local wind data. As a case study, a story-high corridor-type DSF system equipped with an array of wind turbines was integrated into a high-rise building, and its Annual Energy Production (AEP) within the context of four Australian cities was evaluated. The results showed that the free-stream wind speed can be amplified up to a maximum of 1.8 times inside the corridors of the DSF system. It was concluded that the benefit of the DSF system can be exploited the most in cities with strong bi-directional wind characteristics. Finally, it was shown that wind turbines inside the DSF system can annually generate up to 50% more energy at open terrain and 22%–45% more energy at dense urban and suburban terrains as compared with the same turbines in the free-stream condition.

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., Highlights • Two twisted and a conventional wind profiles were used for an Air Ventilation Assessment (AVA). • The AVA outcomes are considerably different for twisted and conventional wind profiles. • The accuracy of three empirical correction methods was evaluated using AVA results. • Correction methods tend to overpredict AVA results compared to twisted wind profiles.

To maximise the potential of decentralized micro-grid wind energy generation in urban environments, it is vital to develop techniques to enhance the flow characteristics in urban environments. This paper reports on an investigation of a new method of exploiting wind energy by utilizing a modified innovative Double Skin Facade (DSF) system. Computational Fluid Dynamics (CFD) simulation has been employed to investigate the effect of aerodynamic modifications, including recessed regions and curved walls, on the flow characteristics inside an empty corridor-type DSF for different wind directions. CFD results were validated against a series of wind tunnel tests. The results indicate that, compared with the original DSF, the modifications are capable of significantly enhancing the flow velocity inside the DSF at all wind directions, especially when the direction of the upstream wind aligns with the flow direction inside the DSF. The results also show the contribution of the confined area in diminishing the variations in velocity and reducing turbulence by a maximum of about 30%. It is concluded that the average available wind power density of an empty corridor-type DSF can be increased by a factor of 2 and 4.2, respectively, by creating recessed regions and curved walls.

This study investigated vortex-induced vibrations of a slender square-section cylinder inclined from the vertical direction by a series of angles experimentally. Both aeroelastic tests and pressure measurements were performed on the cylinder with forward inclinations (inclined to the upwind direction), a vertical attitude, and backward inclinations (inclined to the downwind direction) in a wind tunnel. The cylinder was kept stationary in the pressure measurement tests. The aeroelastic test results show that vortex-induced responses of the cylinder decrease considerably as increasing the forward inclination angle. By contrast, the effect of the backward inclination on the vortex-induced vibration varies. Not all the backward inclined cylinders exhibit crosswind responses smaller than those of the vertical cylinder does. The responses of the cylinder with a small backward inclination are significantly larger, whereas the cylinder with a large backward inclination exhibits lower responses than those of the vertical case. The variation in the vortex-induced vibration response with the inclination is explained by performing power spectral analyses on the pressure data on the side face over the cylinder span, which reveals local vortex shedding characteristics. The findings provide significant guidance to the wind-resistant design of an inclined slender structure.

The interaction between the natural ambient winds found in the atmospheric boundary layer (ABL) and buoyant flames is crucial in broad applications in the scientific and engineering fields. Unlike the buoyancy-driven pool fires in still air that have been studied extensively, the complexity of physics changes significantly in wind presence. This study aims at analysing the characteristics of boundary-layer turbulence in the presence of large fires. The eddy dissipation concept, finite volume discrete ordinate method, and one k-equation model are used for combustion, thermal radiation, and sub-grid scale closure using the Large-Eddy Simulation (LES), respectively. A numerical model on simple cases is validated first to assess its capability to reproduce available experimental observations for a purely buoyant fire in still air. A forced-flow boundary layer combustion in a small chamber with a smooth inflow is further considered. In general, good agreement between the simulation results and available experimental data was achieved for temperature and velocity profiles. The unsteady inflow condition used to consider incoming atmospheric turbulence is generated through a precursor simulation. The wind interaction with the line fire changes the atmospheric boundary layer profile affecting the heat transfer ahead of the flame, thereby creating counter-rotating structures downstream. It is shown that the buoyancy-dominated flow due to the flame reaction induced local pressure variation and perturbed shear flow near the ground, thereby altering the wind speed through which the plume rises. Richardson number was also used as a dominant non-dimensional group to analyse the variation of enhanced flow vertical velocity with distance from the fire source. Thus, it is understood that the pronounced longitudinal shear spreading at the surface affects the behaviour of short term or puff releases, suggesting the shedding of small eddies during the combustion process.

In conventional modeling of air pollution dispersion, pollutants are treated as passive scalars or inert species even though most of them are chemically reactive [1]. Chemical reactions contribute to pollutant dispersion via the generation and depletion of pollutants, in addition to other two mechanisms: advection and turbulent diffusion. This study investigated how chemical reactions affect near-field pollution dispersion by integrating the simple NOx-O3 chemistry into RANS-based computational fluid dynamics (CFD) simulation. CFD simulation was used to model a mixed emission of NO and NO2 from a short stack attached to a building into ambient O3, prompting chemical reactions between the NO, NO2, and O3. Various degrees of chemical reactivity were modeled by varying the Damkholer number (Da) between 0.073 and 4.363. The results showed significant chemical reactivity for cases where Da [NO] > 1, while cases with Da [NO]

Slender structures placed close to each other could greatly magnify the dynamic response due to interference effects. This paper presents the results of experimental studies of the proximity interference induced vibration of two identical square prisms in a turbulent boundary layer to clarify its excitation mechanism. The upstream interfering prism was static, while the principal prism was dynamic and free to vibrate only in the crosswind direction. The divergent vibration behavior at the critical interfered location was examined in detail. Based on the time-resolved particle image velocimetry (TR-PIV) technique and unsteady pressure measurement, the vortex shedding process and corresponding pressure distribution of the dynamic principal prism were described by examining their correlation with the motion time histories. The biased gap flow between the two prisms was found to control the vibration of the dynamic principal prism. The interaction between the vortex shedding in the wake of the dynamic principal prism and the gap flow is dramatically intensified with increasing wind velocity. Such effects create a large fluctuating lift force on the dynamic principal prism at high reduced wind velocity, compared with measurement taken with two static prisms, which drives the prism to undergo a divergent crosswind response.

This paper presents an approach to predict crosswind force spectra and associated response of tall buildings with rectangular cross-section based on machine learning (ML) technique and random vibration-based response analysis. An efficient ML algorithm, light gradient boosting machine (LGBM), was trained to predict crosswind force spectra of the tall buildings by using the database from the Wind Engineering Research Center at the Tamkang University embedded in the aerodynamic database of NatHaz Modelling Laboratory. Furthermore, an unsupervised ML algorithm, K-means clustering, was employed to advance the understanding of the crosswind force spectrum characteristics of the tall buildings. The effects of three factors, i.e., ground roughness, aspect ratio and side ratio, on the force spectra were discussed based on clustering. To predict the crosswind response of tall buildings, case studies were carried out to validate the predictive accuracy of the LGBM model combined with random vibration-based response analysis. The results demonstrate that the proposed method combined with the multiple database-enabled design module for high-rise buildings developed by the NatHaz Modelling Laboratory at the University of Notre Dame is effective and computationally efficient to provide fast and accurate predictions of the crosswind force spectrum and associated crosswind responses of rectangular tall buildings.

The wind velocity field around buildings provides deep insights into the aerodynamic characteristics of buildings and indicates the pedestrian-level wind environment around buildings. Particle image velocimetry (PIV) is usually employed to measure the wind velocities around building models. Due to laser-light shielding, measuring instantaneous wind velocities at some shielded locations around a building model remains difficult. As a result, analyzing the wind flow pattern with these unmeasured wind velocities is difficult. Using machine learning techniques to impute unmeasured values allows for a comprehensive study of wind flow patterns with laser-light shielding. Unmeasured velocities around building models were imputed in this study using machine learning (ML) models such as the generative adversarial imputation network (GAIN), multiple imputations by chained equations (MICE), and neighbored distanced imputation (NDI). GAIN was the best model with a minimum variance and standard deviation of 1.508 and 1.228, respectively. Compared with experimental wind velocities, GAIN produced the minimum average mean squared error of 2.4%. The correlation between the experimental and predicted wind velocities was 98.2%. Thus, the validated GAIN model is recommended to be integrated into the PIV study to impute the unmeasured wind velocities to obtain a complete wind flow pattern.

The influences of building layouts on pollutant dispersion within urban street canyons have been widely studied, although they have been rarely considered with envelope features. Different envelope features, such as wing walls, balconies and overhangs, have not yet been quantitatively assessed for their effects on the wind flow patterns around street canyons to a certain degree. Adopting the evaluation indicators of personal intake fraction and daily pollutant exposure, this study aims to investigate the potential influences of building layouts and envelope features that affect pollutant exposure risks for pedestrians and near-road residents via computational fluid dynamics (CFD). The turbulence modelling approach and numerical methods are validated by reported experiments in the literature. Asymmetric street canyons with an aspect ratio of 2 and two typical building layouts, namely, step-up and step-down notch configurations, are further investigated to test their effects. The results indicate that the step-down configuration provides worse situations for wind environments and pollutant dispersion than symmetric and step-up configurations. More specifically, the presence of overhangs has the greatest impact on the personal intake fraction (P_IF) change ratio, followed by balconies. The largest P_IF change ratio occurs on the fifth floor of an upstream building when overhangs are applied to the step-down street canyon. The aforementioned findings are helpful to understand the influences of building layouts together with envelope features on the wind environment as well as pollutant exposure risks experienced by pedestrians and near-road residents.

This paper presents a CFD study on the air ventilation assessment (AVA) in urban topology with twisted wind profiles (TWP). At present, most AVAs were conducted through wind tunnel experiments, and conventional wind profile (CWP) with no deviation of wind direction along the heights was employed. However, in practice, the hilly terrain around cities such as Hong Kong results in twisted wind flow that wind directions vary along the heights. This paper presents an LES-based CFD model based on the best practice guidelines (BPGs). Two twisted wind profiles (TWP) were introduced, and six simulation cases were performed. The CFD predicted wind speeds were validated against wind tunnel measurements in the literature, and good agreement was achieved. With the aid of the CFD model, wind conditions over heights were investigated, and the effects of twisted wind were evaluated. The results indicated a significant influence of TWPs on the AVA’s outcomes and the wind speeds’ vertical distribution. In addition, with the CFD results, it is possible to investigate the wind-structure interactions in specific urban areas and provide a better understanding of the effects of buildings on the local wind conditions, which could be useful to the urban plan for sustainable cities.

Galloping is characterized by large and periodical oscillations which may lead to collapse of slender structures. This study is the first attempt of a comprehensive experimental and theoretical investigation of galloping of transversely inclined prisms. A modified quasi-steady model is proposed with a constant term to estimate the galloping of a transversely inclined prism, which is later experimentally investigated by conducting a static Synchronous Multi-Pressure Sensing System (SMPSS) test and an aeroelastic test in a boundary layer wind tunnel. The galloping responses of the prisms were measured in the aeroelastic test, while the aerodynamic force coefficients were determined from the SMPSS test. These experimental results were subsequently utilized to validate the quasi-steady model. Based on the proposed model, the galloping responses of the prisms were predicted and compared with the experimental results. The experimentally measured and theoretically predicted galloping responses are discussed with respect to aerodynamic damping ratios, onset galloping wind speeds, distributed pressure coefficients, point pressure spectra and vortex shedding frequencies. Interesting findings are summarized.

The unsteady aerodynamic forces acting on a slender prism were investigated using a forced vibration technique. The prism was driven to oscillate by an actuator and the unsteady distributed pressures, under different wind velocities and oscillation amplitudes, were measured. The measurement was calibrated with respect to driving oscillation, aerodynamic force coefficient, as well as motion-induced force coefficient. Then, the generalized and local aerodynamic force coefficients and the motion-induced force coefficients of the prism, which are functions of reduced wind velocity and oscillation amplitude, were analyzed. It shows that the effects of structural motion on the coefficients are significant in the crosswind direction while the effects are slight in the along-wind direction. Furthermore, in the crosswind direction, the coefficients tend to increase with oscillation amplitudes at low wind speeds while they are at a quasi-steady state at high wind speeds. These characteristics were analyzed from the perspectives of generalized and pointwise spectra, force-response coherences and Strouhal numbers of the prism. The study advances the understanding of the effect of structural motion on three-dimensional prisms, which can be utilized to improve response predictions of the prisms.

To date the engineering community has seen double-skin facade systems as non-structural elements on buildings for the aesthetic desire, improving the indoor environment, reducing the energy use, and even improving acoustics in buildings. In this study, the effects of a double-skin facade system on the wind-induced responses of a tall building were investigated via a program of wind tunnel tests. Two types of wind tunnel tests, i.e. aeroelastic test and pressure test, were performed. It was found that a double-skin facade with/without vertical openings installed in front of the windward face of the building model results in very different effects on the wind-induced responses in alongwind and crosswind directions. In the alongwind direction, the facade with/without openings induces negligible effects on the wind-induced response. However, in the crosswind direction, the facade with opening(s) reduces the wind-induced response significantly, whereas the facade without any opening increases the response, compared to the bare tall building model. Therefore, in addition to achieve the purposes of the aesthetic desire, improving indoor environment and reducing energy use, the double-skin facade system with vertical opening(s) can also be used to effectively reduce crosswind responses without inducing larger alongwind responses.

Urban building disposition plays an important role in determining local microclimate including air quality. Ancient cities normally have some special building dispositions to reduce the penetration of cold wind in winter, which, however, may impact adversely on air pollutant dilution today. This paper investigated the pedestrian-level air quality at a common building disposition in Chinese ancient cities, namely a U-type street canyon. On-site measurements were conducted comparatively at a U-type street canyon and a nearby open space in Xi'an China during January 2015. Three primary air pollutants (PM10, PM2.5 and NO2) as well as wind speed and direction, air temperature and relative humidity were measured continuously from 8:00 a.m. to 8:00 p.m. for a six-day period that covered both clean and hazy days. Pedestrian-level wind condition at the U-type street canyon is mostly independent of that above the canyon, where adverse dilution condition is clearly evident for pollutants. PM2.5/PM10 ratio at the street canyon reached up to 0.9, which is nearly twice that at the nearby observatory. Overall, air quality index (AQI) in the street canyon is, on average, higher by 20% than that at the open space. These findings suggest that this ancient design should be discouraged.

Owing to the void space at lower heights, lift-up buildings have high building permeability at ground level and subsequently improve the air circulation in congested urban areas. Despite this advantage, the lift-up design has been sparsely adopted for buildings in urban areas partly because of the lack of understanding of the combined effects of building dimensions and lift-up design on the surrounding pedestrian level wind (PLW) field. Therefore, this study aims to investigate the influence of lift-up buildings with different aspect ratios (height/width) on the surrounding PLW field and pedestrian wind comfort level. Five lift-up buildings with aspect ratios 4:1 to 0.5:1 were tested in a boundary layer wind tunnel and results were compared with those of five buildings with similar dimensions but without lift-up design. The results reveal a strong dependence of the maximum wind speed in lift-up areas with building height, which results subsequently a small area of acceptable wind conditions near tall and slender lift-up buildings. Lift-up designs adopted for short and wide buildings produce larger areas of pedestrian wind comfort. The central cores modified with corner modifications are effective in increasing the pedestrian wind comfort in the lift-up area of tall and slender buildings., Highlights • Tall and slender buildings generate high-speed wind flows in lift-up areas. • Modified corners of lift-up cores improve the pedestrian wind comfort. • Lift-up cores with modified corners are recommended for tall and slender buildings.

Efficient wind energy harvesting by utilizing small-scale wind turbines in the urban environment requires techniques to enhance the desirable flow characteristics, including velocity magnitude and uniformity, and diminish the unfavorable characteristics, including high turbulence and intermittence. This study proposed a Double-Skin Facade (DSF) system with strategic openings to harvest wind energy in the built environment. A series of wind tunnel tests and CFD simulations have been conducted to investigate the characteristics and related mechanisms of flow within the cavity of DSF integrated with a tall building model at different incident wind angles. The discrepancy between numerical and experimental results generally remains within an acceptable range of 15% which validates the capability and accuracy of the developed CFD simulations in predicting the flow characteristics. It was found that the flow becomes more uniform while the turbulence progressively decays as flow progresses through the cavity for all wind directions. Hence the regions in the middle of both the leading and trailing sides of the cavity are favorable locations for installing small-scale, building-mounted wind turbines. Overall, the DSF system with a strategic opening can effectively enhance the flow within the cavity for a wide range of incident wind angles and can be adapted for wind energy harvesting purposes.

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.

The objective of the study is to develop a new construction material as an alternative for expensive dampers. A Tuned Liquid Damper (TLD) system that is cast in a concrete masonry unit has been proposed. The block has been examined in its individual characteristics to consider the resonant effect of the building subjected to a wide range of water depth, dw, from 5 mm to 80 mm. The TLD block was designed to internal dimensions of 190 mm (length) × 60 mm (width) × 90 mm (height) in accordance to the Uniform Building Code (UBC) and Eurocode 8. Sine-sweep responses results were plotted in transmissibility ratio, ut/ug to frequency ratio, ω/ωn. The block at a depth of dw was simulated and empirically calculated. Tests for dw of 50 mm, 60 mm, and 70 mm have verified the fundamental frequency, fn, to be similar with the simulated and calculated eigenvalues. Further study of the combined structural model and TLD blocks test as a system has been compared with the numerical simulations. The results varied by approximately 2%. The system has recorded reduction of responses immediately at the test of dw, 5–80 mm. Optimum performance has been recorded at 60 mm. The test results employed half-power bandwidth to obtain the damping ratio. The damping has been contributed by both viscous damping component and inertia of the moving water body. The Rayleigh stiffness damping component has not been identified to participate in the forced excitation performance reduction. It was found that the new innovative block with tuned liquid can significantly increase the damping characteristics. Hence, the research is necessary to propose an alternative new construction material to withstand earthquake and wind disasters on buildings.

This paper identifies seven areas where an increase in our fundamental understanding of the human response to building motion will facilitate the design of next-generation serviceability criteria for wind-induced building motion. These advances in knowledge address: (1) understanding the effects of wind-induced building motion on occupants, (2) metrics for building performance assessment, (3) understanding of habituation to building motion, (4) potential and real benefits of education, (5) motion characteristics to predict adverse occupant effects, (6) differentiation between residential and office serviceability criteria, and (7) multidisciplinary research methods and measures used in occupant comfort research. Each is discussed with reference to engineering literature and incorporates a multidisciplinary perspective including psychological and physiological research. Finally, methodological issues in the occupant comfort literature are discussed and recommendations for future research are offered to facilitate the design of next-generation serviceability criteria for wind-induced building motion.

Modern megacities are teeming with closely-spaced tall buildings, which limit air circulation at the pedestrian level. The resultant lack of air circulation creates poorly ventilated areas with accumulated air pollutants and thermal discomfort in the summer. To improve air circulation at the pedestrian level, buildings may be designed to have a ‘lift-up’ shape, in which the main structure is supported by a central core, columns or shear walls. However, a lack of knowledge on the influence of the ‘lift-up’ design on the surrounding wind environment limits the use of ‘lift-up’ buildings. This study aims to investigate the influence of ‘lift-up’ buildings and their dimensions on the pedestrian-level wind environments using wind tunnel tests. A parametric study was undertaken by using 9 ‘lift-up’ building models with different core heights and widths. The results were compared with the surrounding wind environment of a control building with similar dimensions. The results reveal that the ‘lift-up’ core height is the most influential parameter and governs the area and magnitude of high and low wind speed zones around such buildings. Based on wind tunnel test results and a selected comfort criterion, appropriate core dimensions could be selected to have acceptable wind conditions near lift-up buildings., Highlights • The effects of ‘lift-up’ buildings and their dimensions on the surrounding wind environments are studied. • The ‘lift-up’ core height is the most important parameter of a ‘lift-up’ design. • Appropriate lift-up dimensions are selected by combining wind speed data with a predetermined wind comfort criterion.

The pedestrian level wind environment is seriously deteriorated by moderated local wind flow in a densely built-up subtropical city like Hong Kong. In order to improve the weak wind condition, the lift-up design has been used for some time. However, there is a lack of understanding and quantitative assessment of its modification on the pedestrian level wind comfort around different building configurations under different wind directions. This paper aims to study the effects of lift-up design in four common building configurations on the wind comfort via computational fluid dynamics (CFD) simulations. The turbulence model and numerical method are firstly validated by comparing the simulated wind flow data with the wind tunnel test results. The validated model is then utilized to simulate the four building configurations, including the “─”, “L”, “U” and “□” shaped buildings. The mean wind velocity ratio ( MVR ) and mean wind velocity change ratio ( Δ MVR ) are employed to identify the wind comfort and to quantitatively evaluate the improvements due to the lift-up design. Results show that the lift-up design can improve the wind comfort in building surroundings and its influence is highly dependent on the incident wind direction. Specifically, the wind comfort is better under the oblique wind direction than the other two wind directions. These findings can provide us a better understanding of the lift-up design and will be helpful in better precinct planning.

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.

Fire-wind enhancement is a phenomenon associated with the increase of free-stream wind velocity due to the interaction of fire and free stream flow. The potential adverse effects of enhanced wind on the pressure loads around buildings highlight the necessity of investigating the phenomenon. The interaction of wind and fire has long been a subject of interest however, investigation of the factors affecting the phenomenon has not received due attention. One of the factors that affect this phenomenon is terrain slope. This paper used a Computational Fluid Dynamic solver called FireFoam to evaluate the effect of terrain slope on fire-wind enhancement. The results revealed that the enhancement of wind velocity due to fire increases with an increase in terrain slope. This is because fire-wind interaction leads to the generation of a favourable longitudinal pressure gradient whose magnitude increases with the increase of terrain slope.

Wind enhancement triggered by fire-wind interaction can potentially pose significant damage to structures built in bushfire prone areas. The effect of terrain slope is one of the parameters contributing to the enhancement of wind by fire that needs to be taken into account. This study employs a validated model of Computational Fluid Dynamics to assess the effects of terrain slope on this phenomenon. A module was developed and appended to the FireFOAM solver to output individual component of flow acceleration. Multiple analyses were used to explain the effects of terrain upslope and downslope on the phenomenon. The results reveal that although the enhancement of wind velocity due to fire increases with an increase in terrain upslope, a terrain downslope reduces flow enhancement by fire. The results also established that while an upslope terrain reinforces the Coanda effects and intensifies attachment of the plume to the ground, the downslope condition mitigates Coanda effects and reduces the flow's tendency to attach to the ground downstream of the fire source. Furthermore, under a constant heat release rate and upstream wind velocity, the maximum magnitude of wind enhancement linearly increases with the increase of upslope angle. Keywords: Upslope, Downslope, Wind enhancement, Fire, CFD, Coanda effects

Highlights • CFD model validated for predicting pollutant dispersion around a HRR building. • Effects of distance and position angle of emission source studied. • Critical range of position angle of low risk found for pollutant source. • Pollutant concentration on building as a function of source location modelled. • Overall view of risk of pollutant emission from regions nearby building obtained., This article investigates the dispersion of airborne pollutants emitted from different locations near a high-rise building. A Computational Fluid Dynamics (CFD) model for simulating the wind flow field and the pollutant dispersion was developed and validated by wind tunnel data. Then the spreading of the pollutant emitted from different locations to a rectangular-shaped high-rise residential (HRR) building was numerically studied. The pollutant source location was set in a wide range of the position angle and distance between the source and the building. It was found that the pollutant concentration on the building decreases with an increase in the emission distance whereas the effect of the position angle is more complicated. Interestingly, there is a critical range of the position angle from which the emitted pollutants will not spread to the building in a significant way. The effect of the source location was linked to the wind flow field around the building, particularly with several major flows. The vertical distributions of the pollutant concentration on different faces were also investigated, and it was found that these are more affected by the vertical flow near each face. Finally, a mathematical model was developed to evaluate the pollutant concentration as a function of the emission distance and position angle. These findings are helpful to the understanding of the dispersion of airborne pollutants around high-rise buildings and the related hazard management in urban design.