Atmospheric Boundary Layer Wind Tunnel Research Articles

Velocity-pressure admittance for structural analysis of large cooling towers against strong winds

To facilitate dynamic structural analyses of large cooling towers subjected to strong winds in structural design and safety assessments, the auto-spectra of the local wind pressures on the tower shell are required. To avoid expensive and laborious physical model tests, Pirner provided empirical normalized pressure spectra for direct use, which imply that the concept of velocity-pressure admittance can be applied in the windward region on the tower shell. However, Pirner's empirical results are subjected to the fact that the dependence of pressure spectra on height or turbulence intensity of the oncoming flow is wrongfully neglected. To this end, velocity-pressure admittances are measured at various heights on a 1:200 scaled tower model in an atmospheric boundary layer wind tunnel, and unified formulae for velocity-pressure admittance on windward region of large cooling towers are fitted out for use considering the turbulence intensity effects. The unified formulae are further validated using data measured on a 167 m high full-scale cooling tower in China. The full-scale/empirical formulae comparison indicates that the unified formulae for velocity-pressure admittance proposed are applicable on the windward side [0 degrees, 70 degrees]; while a constant of 1 should be utilized as the normalized power spectral density in the circumferential region [70 degrees, 180 degrees] in structural analyses.

Floating wind turbine motion signature in the far-wake spectral content – a wind tunnel experiment

Abstract. The growing interest in floating offshore wind turbines (FOWTs) is rooted in the potential source of increased offshore energy production. As the technology is still in a pre-industrial state, several questions remain to be addressed where little field data are available. This study uses physical modelling at a reduced scale to investigate the signature of the floating motions into the wake spectral content of a simplified FOWT model. A wind turbine model based on the actuator disc concept is placed in an atmospheric boundary layer wind tunnel and subjected to a range of surge, heave and pitch motions. The signatures of idealised sinusoidal motion and realistic broadband motion on the model’s wake at distances of 4.6 D (D being the disc diameter) and 8 D are measured through the use of a rake of single hot wires. The spectral analysis shows that harmonic motion leaves clear signatures in the far wake's energy spectra, mainly in the top tip region, while broadband motion does not leave easily detectable signatures.

Full-Scale/Model Test Comparisons to Validate the Traditional Atmospheric Boundary Layer Wind Tunnel Tests: Literature Review and Personal Perspectives

For this paper, full-scale/model test comparisons to validate the traditional atmospheric boundary layer (ABL) wind-tunnel simulation technique performed until now by the wind engineering community are systematically reviewed. The engineering background includes some benchmark low-rise buildings specifically established for use in wind engineering research (the Aylesbury experimental buildings, the Texas Tech University experimental building, the Silsoe buildings, etc.), several high-rise buildings in North America and East Asia, long-span bridges, large-span structures, and cooling towers. These structures are of different geometries, are located in different wind environments, and are equipped with various transducers and anemometers. By summarizing the different articles in the literature, it is evident that notable discrepancies between the full-scale measurement and the model test results were observed in most full-scale/model test comparisons, which usually took certain forms: the mean and/or the peak negative pressures at the flow separation regions on buildings were underestimated in the wind tunnel; differences in the root-mean-square (rms) values of the acceleration samples between the full-scale measurements and the force balance model tests were non-negligible; the vertical vortex-induced vibration amplitudes of bridges measured using section models and aero-elastic models were much lower than those observed on the prototypes, etc. Most scholars subjectively inferred that inherent technical issues with the ABL wind tunnel simulation technique could be responsible for the observed full-scale/model test discrepancies, including the Reynolds number effects, the turbulent flow characteristics effects, and the non-stationarity effects. However, based on the authors’ years of experience and after discussion with experienced researchers, it was found that some of the full-scale measurements performed in earlier research were inherently less accurate and deterministic than the wind tunnel experiments they were supposed to validate, which could also be a significant cause of the full-scale/model test discrepancies observed. It is suggested herein that future studies in this field should regard full-scale measurements only as benchmarks, and that future works should focus on synthesizing the results from different schools of physical experiments and formulating universal empirical models of high theoretical significance to properly validate future wind tunnel tests.

A New Research Scheme for Full-Scale/Model Test Comparison of Wind Effects on Pengcheng Cooling Tower Based on Sinusoidal Flow Field Simulations

When examining the history of wind engineering, it is evident that many full-scale/model test comparisons have found noticeable differences between the results. Although understanding the causes of these differences is important for practical purposes, limited numerical and experimental conditions have often resulted in subjective explanations for full-scale/model test comparisons without scientific validation. To address this issue, this article suggests the use of the computational fluid dynamics technique or the multiple-fan actively controlled wind tunnel technique to quantitatively reveal the adverse effects that impact the reliability of the traditional atmospheric boundary layer wind tunnel tests for a large cooling tower, including not only the widely acknowledged influences (Reynolds number effects and turbulent flow characteristics effects) but also the non-stationarity effects that have potential influences. Established on the novel proposition, a new research scheme for future full-scale/model test wind effects comparisons for large cooling towers has been formulated based on the numerical or physical simulations of the sinusoidal flow fields. Using the Pengcheng cooling tower as a case study, the research recognized the very significant impact of Reynolds number effects, the non-stationarity effects that cannot be ignored, and the negligible effects of turbulent flow characteristics.

Effect of void space arrangement on wind power potential and pressure coefficient distributions for high-rise void buildings

High-rise void building complexes can augment the permeation of airflows over buildings, and thereby strengthen urban wind energy harvesting for realization of the sustainable development goals (SDGs). This paper considers the wind over a typical 2×2 compact high-rise building array with the openings to analyze the wind power potential at varying void sizes, locations of voids, and direction of incoming wind. The predicted mean wind speeds and turbulence intensities are compared against the measured data with 4 generic high-rise building models having voids in an atmospheric boundary layer wind tunnel for validation of the computational model. Based on the comparative results from four common Reynolds-averaged Navier-Stokes (RANS) turbulence models including the standard, realizable, renormalization group (RNG) k-ε, and shear-stress transport (SST) k-ω models as well as the Reynolds stress model (RSM), the RSM approach can provide the most accurate predictions of streamwise mean velocity and turbulence intensity. The present study then conducts CFD simulations to explore the effects of opening configuration and wind direction on the wind field around high-rise void buildings. Considering the variations of void sizes of 10 × 10 m2, 12.5 × 12.5 m2, 15 × 15 m2, void heights of z/H = 0.25, 0.5, 0.75 and wind directions of 0°, 45°, respectively, the simulated results indicate elevated wind power densities with acceptable turbulence intensities over the upstream gap passage and void channels to achieve prospective urban wind power harvest and mitigate wind load on structures of high-rise void buildings.

Experimental Simulation of Thunderstorm Profiles in an Atmospheric Boundary Layer Wind Tunnel

Thunderstorms have different features in comparison with synoptic events, including a typical nose-shaped mean wind speed profile and non-stationary characteristics in time intervals from 10 min to 1 h. The simulation of thunderstorms in traditional wind tunnels requires suitable devices in order to replicate their peculiar characteristics. Disregarding the non-stationary characteristics of thunderstorm outflows, this paper aims to study the possibility of adopting a passive device such as a specially designed grid in order to reproduce the nose-shaped mean wind speed profile. A widely adopted model of the mean wind velocity profile from the literature is employed as a target profile for the verification of the experimental findings. The results obtained show a good agreement between the measured and target mean wind speed profiles and an acceptable turbulence intensity level compared with full-scale and experimental measurements. The proposed device offers a practical and cost-effective solution to simulate the main characteristics of a thunderstorm event in a traditional atmospheric boundary layer wind tunnel, which could be adopted to assess the significance of thunderstorm loading on civil engineering structures and define the requirement for ad hoc specialist studies.

Assessing wind field characteristics along the airport runway glide slope: an explainable boosting machine-assisted wind tunnel study

Aircraft landings are especially perilous when the wind is gusty near airport runways. For this reason, an aircraft may deviate from its glide slope, miss its approach, or even crash in the worst cases. In the study, we used the state-of-the-art glass-box model, the Explainable Boosting Machine (EBM), to estimate the variation in headwind speed and turbulence intensity along the airport runway glide slope and to interpret the various contributing factors. To begin, the wind field characteristics were examined by developing a scaled-down model of Hong Kong International Airport (HKIA) runway as well as and the surrounding buildings and complex terrain in the TJ-3 atmospheric boundary layer wind tunnel. The placement of probes along the glide slope of the model runway aided in the measurement of wind field characteristics at different locations in the presence and absence of surrounding buildings. Next, the experimental data was used to train the EBM model in conjunction with Bayesian optimization approach. The counterpart black box models (extreme gradient boosting, random forest, extra tree and adaptive boosting) as well as other glass box models (linear regression and decision tree) were compared with the outcomes of the EBM model. Based on the holdout testing data, the EBM model revealed superior performance for both variation in headwind speed and turbulence intensity in terms of mean absolute error, mean squared error, root mean squared error and R-square values. To further evaluate the impact of different factors on the wind field characteristics along the airport runway glide slope, the EBM model allows for a full interpretation of the contribution of individual and pairwise interactions of factors to the prediction results from both a global and a local perspective.

Estimating turbulence intensity along the glide path using wind tunnel experiments combined with interpretable tree-based machine learning algorithms

Wind fluctuations near airport runways can make aircraft landings risky. Consequently, an aircraft may veer off its glide path, missed approach, or crash. In this study, a scaled-down model of Hong Kong International Airport (HKIA) and the complex terrain in its vicinity were built in a TJ-3 atmospheric boundary layer wind tunnel to investigate wind turbulence intensity. Cobra probes were placed along the glide slope of the models' runway to compute turbulence intensity at various locations under different inflow wind directions, with and without surrounding terrain. Next, advanced tree-based machine learning algorithms, including Random Forest, Extreme Gradient Boosting, Adaptive Boosting, and Light Gradient Boosting Machine optimized via Bayesian Optimization, were used to estimate turbulence intensity along the glide path. The Bayesian optimized-random forest model outperforms all other models in prediction performance, as measured by MAE (0.521), MSE (1.046), RMSE (1.024), and R2 (0.934). Furthermore, according to SHAP analysis, "Terrain", "Distance from Runway", and "Wind Direction" significantly contributed to high turbulence intensity along the glide path. The most optimistic predictions for high turbulence intensity were the presence of complex terrain, a shorter distance from the runway, and an inflow wind direction between 125 and 200°.

Wind loading on a stepped roof building: Comparison of field measurements, wind tunnel data, and standard provisions

This study investigates wind loads on stepped roof buildings measured in the field and in an atmospheric boundary layer wind tunnel to verify the suitability of the North American codes and standards for the design of stepped roofs. Good agreement has been generally found between field and wind tunnel results. The field measurements and wind tunnel results were compared with the NBCC 2020 and ASCE/SEI 7–22 (2022) provisions. It was found that the Edge provisions of NBCC 2020 are currently underestimated and need to be increased. The study suggests merging the Corner and Edge zones into a single “Perimeter” zone in NBCC 2020 as this would resolve the current underestimation of the NBCC Edge wind loads and add simplicity to the Code provisions. Peaks of ASCE/SEI 7–22 (2022) were found conservative, except for the zone near the step wall on the lower roof; ASCE/SEI 7–22 (2022) positive peaks seem to be underestimated.

Frequency domain identification of modal characteristics and loads from output‐only measurements

AbstractThis paper proposes a frequency domain framework for concurrently estimating the modal structural parameters and modal loads from the measured output response. A mathematical load model is theoretically developed and compared to the load determined by force identification. Unlike conventional modal identification schemes, the proposed approach can effectively accommodate the spectral shape of the external load in the frequency domain, enabling the determination of the modal parameters without explicit white noise assumption. The proposed approach is verified by applying it to a single‐degree‐of‐freedom system system representing a modal response, an aeroelastic model in an atmospheric boundary layer wind tunnel, a multi‐degree‐of‐freedom system, and the measured acceleration response of a 40‐story building. The simulation results demonstrate that the proposed technique can reliably estimate the modal parameters and load model and is robust against the noise present in the response and the order of the load model. Additionally, the proposed technique can be applied to various loading conditions such as vortex‐induced and buffeting effects since the modal parameters and the load model can be identified for any shape of the loading spectrum.

A wind tunnel study of aerodynamic effects of façade and roof greening on air exchange from a cubic building

Ventilation through openings has long been considered an effective means of improving indoor air quality in living spaces. Although green roofs and façade greening are wide spread for urban buildings, their effect on ventilation is not yet fully understood. This paper presents a fundamental experimental investigation of the relevant aerodynamic effect on the air exchange of an isolated building exposed to wind. The study was performed in an atmospheric boundary layer wind tunnel using a reduced scale (M = 1:300) cubic model building exposed to 0° wind with openings on its two side faces. Vegetation of different permeabilities, heights and thicknesses were placed on the building, and tracer gas measurements were performed. These properties may correspond to different vegetation species e.g ivy or shrubby plants, commonly used for façade or rooftop greening. Experimental results show that, regardless of species, façade greening on the windward wall attenuates air exchange rate while rooftop greening enhances it, especially at the upper building levels. When applying façade or rooftop greening of the same properties, the building’s air exchange rate is reduced by up to 16% but increases by only up to 8%, respectively. The effect of vegetation thickness is more evident for the thicker layers of façade greening. It is notable however, that a layer of low-permeability facade greening has a minor influence on air exchange rate.

Investigation on favorable pressure gradient in an atmospheric boundary layer wind tunnel using large‐eddy simulation and its impact on wind field and surface pressure measurement

AbstractA constant cross‐section wind tunnel usually experiences a negative pressure gradient (ie, favorable pressure gradient, FPG), resulting in a certain degree of discrepancy between the simulated and idealized atmospheric boundary layer (ABL) flows. To investigate its impact, three large‐eddy simulation (LES) cases were conducted in this study: one simply reproduced a wind tunnel, one removed the FPG by adjusting the ceiling height, and one reduced the FPG by canceling unnecessary boundary layers associated with the tunnel sidewalls and ceiling. The required ceiling height in the second case was estimated based on the pressure gradient in the first case. The FPG was found mainly generated by the contraction effect induced by the boundary layers over the tunnel walls because it can be largely removed in the third case. In addition, we investigated the mean and standard deviation of the surrounding wind field and surface pressure coefficient of an isolated square‐section building model. Because the FPG in the macroscale ABL is negligible compared with the adverse pressure gradient in the microscale near‐wall boundary layers of the building, the influence of the FPG is negligible in a wind tunnel experiment conducted in a relatively large wind tunnel with a safe blockage ratio.

A New Research Scheme for Full-Scale/Model Test Comparisons to Validate the Traditional Wind Tunnel Pressure Measurement Technique

A new research scheme for a full-scale/model test comparison is proposed for effectively validating the wind tunnel pressure measurement technique in this article. With the new research scheme and using quantifiable data, future full-scale/model test comparisons are expected to reasonably disclose the main problem with the traditional atmospheric boundary layer wind tunnel pressure measurement technique with regard to the Reynolds number (Re) effects, the aero-elastic effects and the flow characteristic effects. Based on the engineering background of the Peng-cheng cooling tower, it was quantified by using the new research scheme that the average relative errors of the mean wind pressure coefficients are 52.39%, 1.87% and 35.91% for the Re effects, the aero-elastic effects and the flow characteristic effects, respectively. In view of the fluctuating wind pressure coefficients, the average relative errors are 97.58%, 21.14% and 20.20% for the Re effects, the aero-elastic effects and the flow characteristic effects, respectively. These suggest that Re effects are extremely significant for pressure measurement model tests of circular cylindrical structures, and flow characteristic effects are noticeable too. They should be mitigated using effective approaches, i.e., increasing the model surface roughness and placing the actively controlled devices at the beginning of the wind tunnel’s working section, respectively. Additionally, the key field measurement technique adopted (i.e., the calculation of the reference static pressure) and details of manufacturing the aero-elastic pressure measurement model for the present study are also expounded in the article for reference.

RANS CFD modeling of the flow around a thin windbreak fence with various porosities: Validation using wind tunnel measurements

In computational fluid dynamics (CFD) simulations of airflow in built environments, the porous-jump model is commonly used to represent extremely thin porous windbreak fences, which have much smaller sizes than the neighborhood computational cells. However, owing to the limited quantitative validation studies that include turbulence characteristics, no clear guidelines exist on modeling such thin porous fences in CFD simulations. In this study, pressure loss coefficients of porous materials with various porosity values and opening shapes were determined using a small wind tunnel while the velocity and turbulence statistics behind the fences were measured in an atmospheric boundary layer wind tunnel. Two separate CFD models based on the Reynolds-averaged Navier–Stokes equation and porous-jump models were used to mimic the airflow in each wind tunnel and then they were validated using the experimental results. The results showed that the pressure loss was accurately predicted by CFD simulations. For wire-mesh fences, a good agreement was obtained between experimental and numerical results for the mean streamwise velocity and turbulent kinetic energy (TKE), whereas the velocity deficit and TKE were overestimated and underestimated, respectively, for the non-wire-mesh fences. This study can be used to improve the performance of thin porous windbreak fences.

Experimental investigations into the effect of urban airflow characteristics on urban air mobility applications

Unmanned Aircraft Systems (UAS) operations in urban environment are significantly challenged by the complex airflow structures and high levels of turbulence owing to the densely built-up regions and the presence of tall buildings. This paper investigates the urban airflow characteristics that are expected to pose significant challenges to the flight of unmanned aircraft systems and impair their aerodynamic stability. Using experimental methods, a 1:400 scale city model was tested in an Atmospheric Boundary Layer (ABL) wind tunnel at various wind incidence angles to capture a representative range of wind speed changes, wind direction changes, wind shear and turbulence intensity flow features. The results can be used as a first step in informing regulations for UAS operations in urban flows, where the most severe flow conditions are those which contribute to develop operational limits. This paper reveals the severity of the flow conditions in representative urban environments, particularly around and between the buildings.

Codification study of wind-induced loads on canopies attached to mid-rise buildings

Wind-induced loads on canopies attached to low-rise buildings have been investigated in the past and formed the basis for design recommendations in the latest wind standards and building codes of practice. The scope of this study was to extend the investigation to canopies that are attached to taller buildings and generate additional recommendations on the design load calculation of such non-structural attachments.Various geometrical parameters such as canopy length, tilt angle, installation location and building height along with wind direction were considered in the experimental protocol. Therefore, several configurations of a 1:20 scale model were tested in an atmospheric boundary layer wind tunnel. Pressures on both the upper and lower surfaces of the canopy were measured for different wind directions and surface and net mean and peak pressure coefficients are presented. The results indicated that an important flow mechanism influencing the peak pressures on the attached canopies is the turbulence generated by the canopy and building edges at the wind directions parallel to the canopy's parent wall. From a codification point of view, the area-averaged coefficients versus effective area curves were obtained and enveloped, considering all model configurations, to generate design area-averaged coefficients curves. These codified design curves can be incorporated into wind standards and building codes of practice for the purpose of designing attached canopies to mid-rise buildings.

Wind tunnel experiments for investigating wake effects in atmospheric boundary layers using a simplified miniature model wind turbine

The clustering of wind turbines within a prospective site is prone to significant wake interactions, which have a negative impact in terms of loads and plant performance. Further improvements in the design and control of wind farms can be achieved through a detailed assessment of wake effects under site-specific conditions. The aim of this paper was to investigate wake effects in atmospheric boundary layer (ABL) flows using a simplified model wind turbine. First, we performed experiments under smooth- and turbulent-uniform inflows to highlight the wake features of the model. Second, we evaluated the interactions with two different neutrally stratified ABL: i) ABL Type-I (α = 0.11, TI = 6 %), ii) ABL Type-II (α = 0.23, TI = 13 %), where α is the incident wind shear and TI is the turbulence intensity. For each case, hot-wire anemometry was used to obtain wake mean velocity, turbulence intensity and power spectrum profiles at four different downstream positions. We also included a comparison with three analytical wake models to get further indications on the experimental measurements. First results showed that the simplified model is well-suited for its intended purpose. The largest discrepancies between experiments and model predictions were in the near-wake region and in cases with lower background turbulence. We also corroborated a strong influence of the incoming wind profile on how fast the wake recovers to the undisturbed conditions. Furthermore, this study provided an initial framework to explore the ABL wind tunnel at Universidade Federal do Rio Grande do Sul (UFRGS) as a viable tool for research in wind energy.

Experimental study on wind-induced vibration and aerodynamic mitigation measures of a building over 800 meters

The new-generation high-rise buildings are taller and more flexible than before. Rigid model and aeroelastic model tests of a proposed steel super high-rise building with a height of 838 m were carried out in two simulated typical wind fields defined in the Chinese code [1] and the ISO standard (ISO 4354) in an atmospheric boundary layer wind tunnel. The wind pressure distributions of the building were obtained through the rigid model tests and then the wind-induced vibration responses of the building were calculated. Through the aeroelastic model test, the vibration responses of the building, which naturally include the wind–structure coupling effects, were directly obtained. The test results shows that strong crosswind vortex-induced resonance may appear at high wind speeds. Based on these results, the crosswind aerodynamic damping of the building was then studied. Finally, seven aerodynamic measures for controlling the wind-induced vibrations of the building were proposed and tested. The results show that the seven proposed aerodynamic measures can reduce the dynamic responses of buildings in different degrees.

Application and assessment of a GPU-based LES method for predicting dynamic wind loads on buildings

This study presents the assessment of a fast Large Eddy Simulation method for estimating dynamic wind loads on buildings using a GPU-based CFD software, which produces statistically converged results on a nine-million-cell mesh in approximately 6 hours. The surface pressure distribution of a cuboid building model was validated with experimental data obtained in an atmospheric boundary layer wind tunnel and compared with field measurements. Although due to the applied equidistant Cartesian grid the large gradients near the edges are not fully resolved, good overall agreement was found for the mean and fluctuating pressure distributions (correlation coefficient: 0.90/0.73, FAC2: 0.92/0.98, FB: −/0.06, MG: −/0.95, NMSE: −/0.10, VG: −/1.08). It was shown that the numerical model is able to produce matching turbulent spectra in an intermediate frequency range within the inertial subrange, limited by the domain size and the spatial resolution. Mesh refinement for capturing large gradients as well as for expanding the frequency limits can be achieved by using a GPU with higher VRAM capacity for the simulation. The continuing advancement of the presented model is a promising development for estimating dynamic wind loads on buildings and identifying design problems fast enough for the engineering practice, without high-performance computing.

Pumping ventilation of corner and single sided rooms with two openings

Corner rooms with two or more open windows in perpendicular facades can be naturally ventilated in cross-ventilation or pumping ventilation. These two airflow regimes also occur in rooms with two openings in the same façade, in the form of single sided pumping or cross sided ventilation. This paper presents an experimental and numerical simulation study of the scale and occurrence of these two flow regimes for rooms in a rectangular building exposed to wind. Flow visualization and tracer gas measurement of effective airflow were performed in an atmospheric boundary layer wind tunnel using a rectangular model of a three-story building (1/20 scale) with a ventilated middle floor. Experimental results show that pumping ventilation occurs when the wind is perpendicular to the façade (single sided rooms) or aligned with the building corner (corner rooms). In addition to these two perfectly aligned wind directions, pumping also occurs for a range of incoming wind angles: ±19° for single sided; and ±9° for corner rooms. As a result, for isolated rectangular buildings that have, at least, one single sided and two corner rooms in each facade, pumping ventilation can potentially occur in two or more rooms for 62 % of incoming wind directions. To investigate the transition between steady cross-ventilation and unsteady pumping ventilation, three-dimensional computational fluid dynamics large eddy simulations were performed to obtain wind generated pressures in the ventilation openings. Results show that the transition from cross-ventilation to pumping occurs when the steady pressure becomes smaller than the unsteady component. These results are used to develop a pressure based simplified model for corner ventilation that can predict effective airflow from external wind generated pressures with an average error below 10.2 %.