Boundary Layer Wind Tunnel Tests Research Articles

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.

A wind tunnel investigation of yawed wind turbine wake impacts on downwind wind turbine performances and wind loads

In the current work we experimentally explored yawed wind turbine wake impacts on downwind wind turbine performances and wind loads. The lab-scale wind turbine model with a rotor diameter ( D) of 0.442 m and a height of 1 m (=2.26 D) was installed in a closed-circuit boundary layer wind tunnel (test section: 15 m long × 3.6 m wide × 2.0 m high) of the Research Institute for Applied Mechanics (RIAM) of Kyushu University. Power performance tests were initially conducted with a single rotor in isolation in order to characterize a rotor’s power output in stand-alone conditions. A detailed comparison of the tests revealed that the power output decreased rapidly as the yaw angle (γ) increased. It is presumed that the power output decrease in yawed cases is mainly due to the decrease in the effective rotor area and the change in the angle of the incoming wind flow with respect to the wind turbine blade. Next, using two wind turbine models aligned with the dominant inflow direction, the merging wakes behaviors caused by three different lateral separation distances were tested: (a) Case 1 ( y = 0), (b) Case 2 ( y = 0.5 D), and (c) Case 3 ( y = 1 D). Here, the separation distance between the two wind turbine models was fixed at 6 D in all cases. Extremely large power output deficits of 46%–76% were seen in the Case 1 configuration. This is mainly due to the significant wake velocity deficits induced by the upwind wind turbine model. In the Case 2 configuration with γ values of 20° and 30°, a significant increase in the power output of the downwind wind turbines was observed. Similar to Case 1 configuration, these results are considered to be mainly due to the upwind turbine-induced wake velocity deficits and wake deflection. Finally, in the Case 3 configuration, no significant difference was found in all of the results, and the tendency was almost the same. We show that the wake velocity deficits induced by the upwind wind turbine model had almost no effect on the power output of the downwind wind turbine model. We evaluated the total power output of the two turbines. As a result, in the Case 2 configuration with 20° yaw angle, the total power output of the two wind turbine models was the highest due to the increase in the power output of the downwind wind turbine model. In order to investigate the main cause of the significant increase in the power output of the downwind wind turbine model at 20° and 30° yaw angles in the Case 2 configuration, we measured the lateral wind speed distribution at the 6 D position on the downwind side of the upwind wind turbine model by using the ultrasonic anemometer. As a results, it was clarified that the peak of the wake velocity deficits induced by the upwind wind turbine model is clearly shifted away from the downwind turbine such that it experiences a smaller deficit due to wake steering. Also, with wake steering the upwind turbine-induced wake velocity deficits may be smaller due to the reduction in rotor area. Finally, it is extremely important to understand the wind load acting on the downwind wind turbine model operating within the wake region induced by the upwind yawed wind turbine model when the maximum power output is generated. It can be seen that as the yaw angle of the upwind wind turbine model increased, the power output generated by the downwind wind turbine model and the streamwise wind load acting on it also increased. However, it was also clarified that the streamwise wind load acting on the downwind wind turbine model in this situation did not exceed the stand-alone value.

Comparison and uncertainty quantification of roof pressure measurements using the NIST and TPU aerodynamic databases

The pressure coefficient of a low-rise building in the ASCE wind load design provision is usually obtained from boundary-layer wind-tunnel tests. Although these tests tend to be standardized, inconsistent results from different facilities have been acknowledged as a long-standing issue. This work compares roof pressure for a low-rise building model archived in the National Institute of Standards and Technology (NIST) aerodynamic database and the Tokyo Polytechnic University database, followed by quantifying the measurement and data reduction uncertainties of the NIST datasets. The Monte Carlo simulation propagates four elemental uncertainties to the mean Cp¯, standard deviation Cp′ and peak pressure coefficient Cp,peak. Results indicate that pronounced differences in the roof pressure from two datasets are attributed to different inflow characteristics (including the Jensen number variation) and the inherent measurement uncertainties. High measurement uncertainties of Cp¯ and Cp,peak are strongly correlated with vortical flow structures, either separated flow or conical vortices at the roof corner and windward edges. Two dominant measurement uncertainty sources are distinguished: the dynamic pressure ratio uncertainty and the surface pressure tap uncertainty. Alternative flow and pressure measurement techniques are noted to potentially reduce the two dominant uncertainty sources. This work is intended to clarify measurement uncertainty sources of obtaining pressure coefficients in wind-tunnel model tests and shed lights on why large differences exist from different tests.

Validation metrics and turbulence frequency limits for LES-based wind load evaluation for low-rise buildings

The accuracy of Large-Eddy-Simulation (LES) in modeling wind loads, especially components and cladding loads, does not currently fulfill its potential. The lack of extensive consideration for wind loading requirements and their direct influence on LES duration and mesh size are identified as research gaps. This study presents a validation metric for LES modeling of low-rise buildings, including the wind field and aerodynamics. Most of the proposed validation requirements are adapted from Boundary Layer Wind Tunnel (BLWT) testing requirements to highlight the challenges and opportunities for LES. The turbulence spectral frequency limits significantly contribute to the cost-accuracy trade-off in LES. A stationarity test of recorded time histories is proposed to determine the optimal duration, i.e., low-frequency limit, for statistical parameters other than the peak. An analytical expression is proposed for the LES mesh size estimation from the target maximum frequency limit and corresponding spectral energies. The proposed validation metric is demonstrated with LES models of a low-rise building targeting respective BLWT tests in three wind directions and in three exposures. Despite some limitations, the peak Cp error was within ±10% of the BLWT value range. The optimal normalized durations from LES were one order of magnitude shorter than those from the BLWT cases. The peak values did not show a significant difference in trend from BLWT; hence the duration dependence of peak uncertainties remains relevant for LES. The BLWT data supports the previously proposed high-frequency limit of two. LES models can produce reasonably acceptable wind loads with proper attention to the existing and new requirements, as shown in the present study.

Use of Artificial Neural Networks to Predict Wind-Induced External Pressure Coefficients on a Low-Rise Building: A Comparative Study

Wind flow on a bluff body is a complex and nonlinear phenomenon that has been mainly studied experimentally or analytically. Several mathematical methods have been developed to predict the wind-induced pressure distribution on bluff bodies; however, most of them result unpractical due to the mathematical complexity required. Long-short term memory artificial neural networks with deep learning have proven to be efficient tools in the solution of nonlinear phenomena, although the choice of a more efficient network model remains a topic of open discussion for researchers. The main objective of this study is to develop long-short term memory artificial neural network models to predict the external pressure distribution of a low-rise building. For the development of the artificial neural network models, the multilayer perceptron and the recurrent neural network were also employed for comparison purposes. To train the artificial neural networks, a database with the external pressure coefficients from boundary layer wind tunnel tests of a low-rise building is employed. The analysis results indicate that the long-short term memory artificial neural network model and the multilayer perceptron neural network outperform the recurrent neural network.

Wind loading and its effects on photovoltaic modules: An experimental–Computational study to assess the stress on structures

Photovoltaic modules are one of the intensively used technologies that provide a renewable energy alternative to electricity generation. Consequently, these devices have been studied using different approaches in order to determine their aerodynamic characteristics. Nevertheless, there is still a lack of guidelines from the codes of practice that could enable users and manufacturers to chose/recommend different types of photovoltaic modules according to siting and operation conditions.In order to provide more data about the influence of the photovoltaic module aerodynamics on its constitutive structural elements, an interdisciplinary approach is advisable. In this study the subject is addressed through experimental measurements and numerical assessment of a standard photovoltaic module under different conditions. Boundary layer wind tunnel tests were performed to determine wind loads over ground mounted photovoltaic modules, considering two situations: stand-alone and forming an array of panels. Several wind directions and inclinations of the photovoltaic modules were taken into account in order to detect possible wind load combinations that may lead to a condition not prescribed by the codes of practice. All tests were carried out using rigid models of the photovoltaic modules, that is, the experimental analysis is limited to static wind tunnel testing. A detailed numerical evaluation is performed using the finite element method (FEM) to identify critical structural sections. For the stand-alone case, the most influential wind flow directions correspond to oblique directions for local pressures and along wind direction for overall forces. For the case of the photovoltaic module array, it is observed that the wind loading over the leading panels is decisive for the design. According to the numerical results, the central support device is the most critical structural component.

Thunderstorm wind load evaluation on storm shelters using wind tunnel testing

In Canada, there is a growing number of thunderstorms capable of generating intensive gust fronts and tornadoes, which drastically affect people living in rural areas. Currently, the influence of windstorms is not sufficiently accounted for in building codes such as the National Building Code of Canada (NBCC 2015). Moreover, people in rural areas are more prone to those weather-related fatalities due to the lack of suitable storm shelters. The current study aims at studying the aerodynamic behavior of storm shelters subjected to non-synoptic wind (i.e., downbursts). Wind tunnel tests were conducted at Ryerson University for the three commonly used storm shelter shapes in Canada for different angles of attack. Mean force and moment coefficients were found and benchmarked against similar data for the same models obtained from a turbulent boundary layer wind tunnel test. Afterwards, the maximum absolute design force and moment coefficients resulting from all angles of attack were identified, and a design example for storm shelters in Canada was given to compare between downburst and boundary layer design loads.

Wind load evaluation on storm shelters using wind tunnel testing and North American design codes

In Canada, the number of extreme wind events is increasing due to climate change. Accordingly, storm shelters are becoming more popular, as it provides a life-saving solution during wind events. Currently, the National Building code of Canada (NBCC 2015) and the American Society of Civil Engineers (ASCE 7-16) codes consider storm shelters as low-rise buildings with high importance factor with no specific guidelines for their smaller scale. The current study aims at comparing North American codes for designing wind shelters to a detailed experimental wind simulation using boundary layer wind tunnel testing under the synoptic wind. Wind tunnel tests were conducted at Ryerson University for three commonly used shapes of storm shelters. The governing wind loads from the wind tunnel results were compared to the design codes for both structural forces and cladding pressures. The study also illustrates and quantifies critical zones that may exceed the codes provided values due to being flow separation regions.

Boundary layer wind tunnel tests of outdoor airflow field around urban buildings: A review of methods and status

Outdoor airflow fields have received increasing attention in the building aerodynamics community due to that the airflow distributions around outdoor buildings are closely related to issues such as thermal comfort, building ventilation, and pollutant dispersion. The focus of this paper is on the airflow distributions around buildings obtained through wind tunnel tests, and such studies are mostly conducted in boundary layer wind tunnel with long test section. This paper reviews current techniques for boundary layer wind tunnel tests of airflow distributions in urban outdoor environments. Then, the characteristics of airflow distributions around buildings in three typical configurations from previous studies (i.e. isolated building, street canyon, and building complexes) are reviewed. This review highlights that the proposed building models should be carefully assessed in combination with wind tunnel tests at the design stage. In addition, it is important to obtain wind tunnel test data for buildings with thermal effects, and the importance of arranging the underlying surfaces during the test is also emphasized.

Selection and Impact of an Aerofoil Leading Edge on Boundary Layer Transition

The choice of leading-edge aspect ratio (AR) plays a crucial role when planning boundary layer wind tunnel tests on a flat plate. Poor selection of the leading-edge profile hampers effectiveness of the experiment and increases testing costs associated with interchanging of leading edges to attain accurate results. Thus, the appropriate selection of the leading edge is a very crucial part of the wind tunnel experiment process. It is argued that the curvature of the leading edge and thus the AR is of paramount importance to achieve accurate results from the wind tunnel testing. In this project, seven different elliptical leading edges were tested, and their performance was compared with an ideal leading edge with zero thickness. Experiments and computation have been done for leading edges ranging from AR6 to AR20. Results were evaluated for boundary layer transition onset location, and it was found that AR20 has the least influence on the flow structure when compared to the ideal leading edge. A study of the flow structure at the stagnation point indicates an increase in adverse pressure gradient with an increase in the AR but also shows a decrease in the size of the stagnation region. The presence of a higher AR leading edge reduces the turbulent spot production rate, which is one of the primary causes of boundary layer transition. This paper presents a correlation that enables aerodynamicists to quantify the impact of the leading-edge AR on transition. A typical case is also presented to compare the relative performance of a wedge and the higher AR leading edge, which provides a choice between an elliptical or a wedge-shaped leading edge.

Cyber-physical aerodynamic shape optimization of a tall building in a wind tunnel using an active fin system

This study explores the use of a cyber-physical systems (CPS) framework for the design and optimization of the aerodynamics of a tall building through aeroelastic boundary layer wind tunnel (BLWT) testing. The framework is a fully-automated combination of traditional experimental wind tunnel testing with numerical optimization strategies to evaluate a wide range of candidate designs both accurately and quickly. In this study, candidate designs are achieved through the physical adjustment of the aerodynamic shape of a multi-degree-of-freedom (MDOF) aeroelastic tall building model. BLWT testing is carried out at the University of Florida Natural Hazard Engineering Research Infrastructure (NHERI) Experimental Facility (EF). The specimen is equipped with an actuation system consisting of a series of individually-controllable slotted fins comprising an active fin system (AFS), which enable precise modifications of the aerodynamic shape of the envelope. The wind-induced building response of the specimen is directly captured using a series of accelerometers and laser displacement sensors. A stochastic optimization algorithm is then employed in the aerodynamic CPS framework to evaluate the fitness of each candidate design based on specified performance criterion related to either occupant comfort or building drift. Optimization results indicate that the CPS framework can reliably achieve the optimal solution which minimizes the response (i.e., horizontal accelerations and lateral displacements) of the aeroelastic specimen.

Cyber-physical design and optimization of tall building dynamics using aeroelastic wind tunnel modeling

This study explores the use of an automated cyber-physical system (CPS) framework for the design and optimization of tall building dynamics through aeroelastic boundary layer wind tunnel (BLWT) testing. The framework is fully automated, integrating experimental BLWT modeling with numerical optimization algorithms to rapidly evaluate a wide range of candidate designs. In this study, candidate tall building designs are generated by physically adjusting the dynamic properties of a multi-degree-of-freedom (MDOF) aeroelastic model. The specimen is equipped with an actuation system consisting of a series of variable stiffness devices (VSD), which enable precise control of the lower natural frequencies. The specimen is also instrumented with accelerometers and displacement sensors to capture wind-induced building response. A stochastic optimization algorithm evaluates the fitness of each candidate design based on code-based and/or user-specified serviceability limit states related to occupant comfort and building drift (e.g., sway). Wind tunnel experiments were conducted at the University of Florida Natural Hazard Engineering Research Infrastructure (NHERI) Experimental Facility. Results show that the CPS framework can reliably drive the aeroelastic specimen to the optimal solution which minimizes stiffness (i.e., natural frequency) while satisfying multiple acceleration and drift constraints.

Numerical investigation of scale resolved turbulence models (LES, ELES and DDES) in the assessment of wind effects on supertall structures

Scale resolved turbulence models, such as large eddy simulation (LES), have been increasingly utilised in applications of computational wind engineering (CWE) for tall buildings, due to their proven numerical accuracy. The demand for computational time and resources in LES is still a concern in practical applications of CWE, where it is necessary to explore alternative modelling techniques. The main objective of this study is to critically investigate the possibility of using a computationally efficient hybrid RANS/LES approach, specifically embedded-LES (ELES), to predict wind effects on a supertall 406 m slender tower of non-standard geometry. Numerical predictions of LES and ELES are compared and critically analysed with experimental results obtained by performing boundary layer wind tunnel (BLWT) tests. It was shown that the choice of sub-grid scale (SGS) modelling in LES has an impact on the wind effects predicted on the structure. Out of the five SGS models tested, the wall adapting local eddy viscosity (WALE) model was the most suitable due to its treatment of the near wall region. An optimum domain configuration based on a parametric study was proposed for the application of ELES. The importance of overall mesh quality and its impact on fluctuating pressures and dynamic properties of wind is presented. Wind flow behaviour on the structure, such as Von-Karman vortex shedding and its dynamic implications, is also discussed. Finally, using the proposed modelling techniques, ELES can generate similar numerical accuracy to that of an LES model at a lower computational cost.

Review: The Structure of Design: An Engineer's Extraordinary Life in Architecture, by Leslie Earl Robertson

Leslie Earl Robertson The Structure of Design: An Engineer's Extraordinary Life in Architecture New York: Monacelli Press, 2017, 336 pp., 313 color and 23 b/w illus. $60, ISBN 9781580934299 The Structure of Design is a compelling and unique contribution to the discussion of an underrepresented topic: the relationship between architecture and structural engineering. It presents the truly remarkable life and career of its author, Leslie Earl Robertson, a dynamic and pathbreaking structural engineer. The book is, as Robertson states, a product of himself—it is both by him and about him. This autobiographical effort not only documents Robertson's past projects and innovations, as well as the architectural relationships that defined his professional career, but it also allows him to recall remarkably personal experiences and directly share his views with the next generation of architects and engineers. His stories—organized both chronologically and by architectural partnership—oscillate between in-depth discussions of specific buildings and technical achievements and more casual anecdotes and recollections of personal relationships. Robertson provides vital insight into an important period of architectural and structural history but leaves a sense that there is still much more of the story to be told. Robertson's professional work supports the claim that he was among the most impactful structural engineers of the last half of the twentieth century. He was instrumental in introducing the digital production of construction documents (on punch cards) and played an important role in the beginnings of computer optimization in structural design. He also pioneered the use of steel-and-concrete composite columns, the use of boundary-layer wind tunnel testing and engineering, and a variety of other specific technological leaps in structural engineering. These technologies emerged not through isolated invention, however, but through individual projects that Robertson worked on in collaboration with architects and firms such …

Towards LES as a design tool: Wind loads assessment on a high-rise building

The accurate evaluation of wind loads on high-rise buildings represents a key point in their design process. Traditionally, atmospheric boundary layer wind tunnel tests, conceived in order to be representative of the wind conditions expected on site, are used for this purpose. Recently, owing to the increase in computational power, Computational Fluid Dynamics (CFD) techniques have gained interest as a complementary tool to wind tunnel testing. Unfortunately, wind flow around bluff bodies is often very complex and substantial research efforts are still needed in order to assess the accuracy and reliability of CFD results. In this paper, Large Eddy Simulations (LES) are performed aimed at evaluating the wind loads on an isolated high-rise building. In order to assess the capabilities of LES for adoption as a design tool, the results are analysed in terms of both pressure distributions and internal forces on the structural elements. It is found that the accuracy of LES in reproducing the fluctuating pressure field is not necessarily maintained when internal forces are taken into account. Nevertheless, the design values predicted by LES can be still considered satisfactory, in particular when maximum and minimum values over different angles of attack are considered.

In situ measurement of wind pressure loadings on pedestal style rooftop photovoltaic panels

The installation of rooftop photovoltaic (PV) arrays is increasing throughout the US. Until recently, pedestal type PV framing systems for rooftops were basically designed using procedures from the ASCE7-10 Components and Cladding Standard for rooftop equipment. The 2011 Japanese Standard Load design guide on structures for photovoltaic arrays was useful in characterizing the pressure coefficients on rooftops, but the Standard employs different wind speed and importance factors, making its use in the US quite limited. Even the updated 2017 version is written for a different audience. Because rooftop pressure loadings are high due to flow separation, SEAOC and other organizations contracted boundary layer wind tunnel tests of panels attached to rooftops to ascertain if the ASCE7-10 equipment loadings were appropriate. The investigations resulted in new standards for pedestal-style arrays that appear in Chapter 29 of ASCE7-16. However, the new standards are limited to simple geometries and orientations, and the dynamics of the simply-supported thin PV plates do not appear to be considered. Questions regarding the ability of the boundary tunnels to simulate accurately the turbulence at the scale required for the attached panels have been raised. In response, very limited full-scale investigations in large-scale tunnels and in situ have been undertaken to calibrate the tunnel results. The results of this paper represent one of these calibration investigations. Specifically, in situ full-scale net wind pressure loadings on a rooftop PV array in a pedestal-style framing system located on the three story Hogue Technology Building of Central Washington University (CWU) in Ellensburg, Washington were measured. The CWU campus has a rural setting in a region with steady winds: Ellensburg is located in the Kittitas Breezeway portion of the Northwest wind power region. Indeed, the Wild Horse Wind and Solar Farm is located on the outskirts of town. The data described here were collected from April through August 2014. The measured net pressure coefficient time series were similar to those for rooftop pressure loadings for low-rise buildings described in the literature such as the Wind Engineering Research Field Laboratory at Texas Tech University (Ham and Bienkiewicz, 1998 [1]; Levitan and Mehta, 1992 [2]). The analysis of the net pressure time series data included an examination of the minimum, maximum, mean, and RMS values. Preliminary results suggest that the range of the values is larger than assumed in the ASCE7 Standard, and that the magnitude of the loadings vary considerably spatially over the multiple panel array. The pressure loading measurements are ongoing.

Investigations of adverse wind loads on a large cooling tower for the six-tower combination

Wind-induced pressures are measured on a group of six cooling towers during a series of boundary layer wind tunnel tests, which varied among two tower arrangements, three tower spacings and sixteen wind directions. Based on the statistical results of wind pressures on the throat level, twenty-seven types of pressure distribution curves including nine symmetrical distributions and eighteen asymmetrical distributions are proposed and fitted by the fifteen-term trigonometric function with seven sine terms, seven cosine terms and one constant term. The local buckling factor and construction cost are adopted to compare wind effects induced by the twenty-seven types of wind loads. The results show that there are three symmetrically distributed and three asymmetrically distributed wind loads, which can lead to more severe wind effects. The most adverse wind load pattern of all is the symmetrical distribution with the largest pressures around the stagnation point area, largest suctions in both sideward regions and lowest suctions in the leeward region.

LES of ABL flow in the built-environment using roughness modeled by fractal surfaces

The available methods to model ground roughness are not suitable for practical unsteady flow simulations in built-environment, or are limited to slightly rough terrains, or do not provide enough details on the flow structure close to the ground. In the current study, a robust model that simulates rough terrains and provides reasonable transient flow details close to the ground is proposed. The model can handle wide variety of rough exposure encountered in the built-environment. It is based on coupling two existing models in the literature; (i) surface gradient drag-based and (ii) canopy models. In this study the terrains are represented by equivalent fractal surfaces generated from random Fourier modes (RFM). Further, in a boundary layer wind tunnel testing, it is a common practice to use code-based prescribed aerodynamic roughness, z0, to define the terrain exposure type and to select the inflow mean velocity profiles and turbulent intensity levels. For a similar numerical application, there is no guidance on how to simulate turbulent flow corresponding to a specific prescribed z0. For such applications a new scaling technique is developed to scale arbitrary fractal surfaces in order to produce the prescribed z0. ABL flow, over three types of terrain exposures, is investigated using LES employing the new model. It is observed that the resulting ABL flow characteristics in terms of mean and fluctuating velocity profiles as well as velocity spectra match very well with the target engineering data, which validates the proposed method.

Investigation on wind tunnel tests of a full aeroelastic model of electrical transmission tower-line system

A full aeroelastic model with one tower two lines to simulate electrical transmission tower-line system was established. Displacements and accelerations of the aeroelastic model of the tower with and without lines under various wind speeds were investigated through boundary layer wind tunnel tests, and the maximum wind loads on, and wind-induced destroyed mode of, the tower-line model were also observed. Meanwhile, the static and dynamic loads on electrical transmission tower transferred from lines were acquired by measuring the strains of the insulators at the ends of lines. The effects of coupling between electrical transmission tower and line on wind-induced vibrations of electrical transmission towers were investigated, and the pattern of wind induced responses along with wind speeds, as well as the characteristics of spectra of wind induced dynamic responses, of the aeroelastic model of the tower with and without line were analyzed. The experimental results indicate that the effects of coupling between transmission tower and line on wind-induced responses of the tower, as well as the across-wind vibration of the tower, must take into consideration in the wind-resistant design of electrical transmission tower; and the reinforcement at the weak parts of electrical transmission tower could effectively increase the survival ability of the tower under strong wind.