Commonwealth Advisory Aeronautical Research Council Research Articles

CFD wind tunnel investigation for wind loads of steel television tower with grid structure

Television(TV) towers with grid structures serve as broadcasting frequency modulation or television transmission signals and provide sightseeing opportunities. Accurate wind load information is critical to the design of these towers to ensure their resistance to wind. Due to the scaling ratio of the test model, wind tunnel test can only determine the overall wind load, resulting in the loss of wind load information on specific rods. However, there is currently a lack of research on the wind loads of the specific rods of a core tube with a grid structure. This article focuses on this special topic. Computational fluid dynamics (CFD) technology, which was validated based on the Commonwealth Advisory Aeronautical Research Council (CAARC) model, was employed in simulation for a towering television tower with a grid structure. Wind load distributions of the grid structure and core tube were studied, including flow streamlines, velocity profiles and wind pressure distributions. The results indicate that the wind pressure distribution on the outer surface of the cross rods is similar at different heights. Net wind pressures of the rods in the flow separation zone are close to 0, which significantly reduces the across-wind load. Different wind direction angles significantly influence the wind load of the grid structure but slightly influence the core tube. The simulation results can provide a reference for wind load design of similar grid structures.

An improved inflow turbulence generator for large eddy simulation evaluation of wind effects on tall buildings

Accurate inflow turbulence wind profile is a key premise of large eddy simulation (LES) methodology for super tall buildings. This paper presents an improved method for narrow band synthesis random flow generation to produce a more accurate inflow turbulence of LES based on superposition of harmonics. The proposed method considers time correlation by adopting time parameters, and an accurate expression of wind speed spectrum energy by a modified spectrum integral equation is used. These improvements could generate a more accurate turbulence intensity for the atmospheric boundary layer. A simple numerical example is applied to verify the spectrum characteristic, time correlation and spatial correlation of atmospheric numerical boundary layer. Meanwhile, the effects of the introduced parameters are investigated and the method is applied to the LES of the Commonwealth Advisory Aeronautical Research Council building (CAARC). The improved method could increase the accuracy of simulated turbulence intensity. Compared with the results of CAARC model obtained from the wind tunnel test, the improved method in this study decreases the errors of root-mean-square (rms) of the based bending moment from 5.3% to 4.1% in the along-wind direction and from 5.6% to 1.3% in the cross-wind direction.

Calibrated consistent flow generator for tall building aerodynamics using large eddy simulation

Ensuring proper matching of the target wind characteristics of the inflow is vital for the accuracy of both experimental and numerical wind studies. In the current study, a calibrated turbulent inflow generation technique was developed to accurately simulate the inflow boundary condition for large eddy simulation (LES) of the lower region of the atmospheric boundary layer (ABL) based on a newly developed technique, namely consistent discrete random flow generator (CDRFG). The proposed technique, namely Calibrated-CDRFG (C-CDRFG), targets specific incoming turbulence at distances downstream to account for dissipation of the turbulent kinetic energy. The proposed technique generates accurate turbulent velocities with proper turbulence intensity, spectra, coherency, and correlation at the target distance downstream for a range of terrain conditions. The technique was validated by evaluating the wind-induced loads and responses for the Commonwealth Advisory Aeronautical Research Council (CAARC) and comparing that to the existing wind tunnel (WT) wind-induced response for the angle of attack (AOA) ranging between 0° and 90°. It was found that the resulting wind-induced response from the C-CDRFG technique resulted in an average matching of 91% of the wind tunnel results. The proposed technique is believed to be computationally efficient, flexible, and easy to be sustainable for parallel computing environments and thus provides significant value for Large Eddy Simulation (LES) wind engineering applications.

Characterizing wind pressure on CAARC standard tall building with various façade appurtenances: An experimental study

Tall buildings are usually considered as wind-sensitive structures. While extensive studies have been conducted concerning the wind load characteristics on tall buildings, the effect of façade appurtenances has not been fully understood. In this study, a 1:300 scaled CAARC (Commonwealth Advisory Aeronautical Research Council) standard tall building was tested in wind tunnel to diagnose the effect of various façade appurtenances on wind pressure characteristics. It is shown that, the change of façade appurtenances can significantly influence the fluctuating wind pressure on the windward and leeward surfaces, with the maximum reduction reaching 21%. By contrast, the effect of façade appurtenance on mean wind pressure is somewhat less pronounced. The results also indicated that adding vertical strips is likely to reduce the horizontal coherence of surface wind pressure, with the maximum reduction of horizontal coherence coefficient reaching 38%. Comparatively, the effect of façade appurtenance on vertical coherence is less significant.

Machine learning based algorithms for wind pressure prediction of high-rise buildings

In recent years, machine learning (ML) techniques have been used in various fields of engineering practice. In order to evaluate the feasibility of machine learning algorithms for prediction of wind-induced effects on high-rise buildings, four ML algorithms including ridge regression, decision tree, random forest and gradient boosting regression tree are adopted in this study to predict wind pressures on Commonwealth Advisory Aeronautical Research Council standard tall building. The gradient boosting regression tree model is proved to be well performed in predicting both mean wind pressures and fluctuating wind pressures. Compared to expensive wind tunnel tests and time-consuming computational fluid dynamic simulations, it is expected that the gradient boosting regression tree model is an efficient and economical alternative for predicting wind pressures on high-rise buildings.

Probability Characteristics, Area Reduction, and Wind Directionality Effects of Extreme Pressure Coefficients of High-Rise Buildings

Wind tunnel tests are carried out for the Commonwealth Advisory Aeronautical Research Council (CAARC) high-rise building with a scale of 1:400 in exposure categories D. The distribution law of extreme pressure coefficients under different conditions is studied. Probability distribution fitting is performed on the measured area-averaged extreme pressure coefficients. The general extreme value (GEV) distribution is preferred for probability distribution fitting of extreme pressure coefficients. From the comparison between the area-averaged coefficients and the value from GB50009-2012, it is indicated that the wind load coefficients from GB50009-2012 may be non-conservative for the CAARC building. The area reduction effect on the extreme wind pressure is smaller than that on the mean wind pressure from the code. The recommended formula of the area reduction factor for the extreme pressure coefficient is proposed in this study. It is found that the mean and the coefficient of variation (COV) for the directionality factors are 0.85 and 0.04, respectively, when the orientation of the building is given. If the uniform distribution is given for the building’s orientation, the mean value of the directionality factors is 0.88, which is close to the directionality factor of 0.90 given in the Chinese specifications.

Refined Mathematical Models for Across-Wind Loads of Rectangular Tall Buildings with Aerodynamic Modifications

This paper aims to systematically study the across-wind loads of rectangular-shaped tall buildings with aerodynamic modifications and propose refined mathematic models accordingly. This study takes the CAARC (Commonwealth Advisory Aeronautical Research Council) standard tall building as a benchmark model and conducts a series of pressure measurements on the benchmark model and four CAARC models with different round corner rates (5%, 10%, 15% and 20%) in a boundary layer wind tunnel to investigate the across-wind dynamic loads of the typical tall building with different corner modifications. Based on the experimental results of the five models, base moment coefficients, power spectral densities and vertical correlation coefficients of the across-wind loads are compared and discussed. The analyzed results shown that the across-wind aerodynamic performance of the tall buildings can be effectively improved as the rounded corner rate increases. Taking the corner round rate and terrain category as two basic variables, empirical formulas for estimating the across-wind dynamic loads of CAARC standard tall buildings with various rounded corners are proposed on the basis of the wind tunnel testing results. The accuracy and applicability of the proposed formulas are verified by comparisons between the empirical formulas and the experimental results.

Numerical investigation of interference effects on the critical wind velocity of tall buildings

The main aim of this study is to evaluate the upstream and downstream interference effect of two CAARC (Commonwealth Advisory Aeronautical Research Council) standard tall buildings using Computational Fluid Dynamics (CFD). To that end, the Large Eddy Simulation (LES) turbulence model is employed for the three-dimensional simulations of turbulent wind flow with Reynolds numbers of 1.4×104<Re<8×104. The aerodynamic response of the principal building in different interference cases is investigated using mean and fluctuating coefficients of lift and drag forces, pressure distribution and Strouhal number. To better understand the involved physics, streamlines and vorticity contours are presented and their relationship with the aerodynamic results is interpreted. In addition, an isolated CAARC standard tall building is considered for comparison with interference cases and to validate our results compared to previous experimental and numerical studies. In contrast with the isolated case, in most of the interference cases, the shielding effect of the interfering building results in a lower mean drag coefficient of the principal building. Also, compared to the isolated building and, depending on the location of the interfering building, the fluctuating lift coefficient either increases or decreases. While the mean pressure coefficient at the windward surface is not significantly sensitive to the interference states, it is strongly influenced by the different states of the interference at the lateral and leeward surfaces. Also, the principal building’s Strouhal numbers for different interference states and different Reynolds numbers are calculated using spectral analysis of lift signals. It is then shown that Strouhal numbers are highly dependent on the location of the interfering building and are inversely related to the width of the wake region. Our results, furthermore, demonstrate that the aerodynamic parameters have little sensitivity to Reynolds number variations in the considered range. Finally, the critical wind velocity corresponding to the aeroelastic instabilities is estimated based on Strouhal numbers and it is shown that upstream interference cases, will increase the critical velocity of the wind, thereby reducing the likelihood of occurrence of aeroelastic instabilities.

The effect of soil–structure interaction on the along-wind response of high-rise buildings

Rapid population growth tends to lead to the construction of tall buildings. It is thus necessary to study the influential parameters on the behaviour of tall structures due to wind action. Soil deformability is one parameter that can affect the aerodynamic behaviour of tall structures. In this work, the impact of wind on the standard tall building of the Commonwealth Advisory Aeronautical Research Council, placed on a raft footing, was numerically modelled using Abaqus software. The wind was defined by a power function in the boundary layer of the atmosphere. Stream turbulence was modelled using the implicit large eddy simulation technique and a co-simulation was used to exchange non-uniform loads from the fluid domain to the structural domain. The mechanical reaction of the footing–soil framework was modelled using a direct strategy. Infinite boundary conditions were added to the numerical model for simulation of free boundaries and appropriate contact elements for sliding and separation between subsurface components were considered. The fluid and structural solutions were compared with the mean and root mean square values of experimental estimations for an extensive range of reduced speeds. Numerical results for the framework with soil–structure interaction (SSI) were compared with those without SSI. The dynamic properties and reactions of the building subjected to wind loading were affected by the SSI and therefore engineers should consider these parameters to guarantee practical designs.

Effects of internal angle between limbs of “Y” plan shaped tall building under wind load

Y plan shaped building is usually a triaxially symmetrical building where three separate limbs are bridged in a central core portion. Due to some engineering or architectural requirements, the symmetricity among the wings is not maintained properly, and the limb angle modification is unavoidable. This paper demonstrates a comprehensive study on mean pressure, force and moment coefficients of different types of “Y” plan shaped tall buildings by varying the internal angles between limbs by 30° for various Wind incidence angles (WIAs) from 0° to 180° at an interval of 30°. To maintain the same plan area, the limb sizes are slightly changed accordingly. Numerical analysis has been carried out to generate a similar type of flow condition as per terrain category II of IS:875 (Part 3) −2015. ANSYS CFX is used for the simulation. For validation of the present computational setup, a graphical comparison is made on the Commonwealth Advisory Aeronautical Research Council (CAARC) building model. Some previous wind tunnel results on “Y” shaped building are also compared with our numerical data. The distribution of pressure over the surfaces, mean pressure coefficients and force coefficients are evaluated for each “Y” type building model, and the results are represented graphically to understand the extent of nonconformities due to such angular modifications in the plan. Finally, Fourier expressions of WIA are proposed for obtaining force and moment coefficients for different building models. The accuracies of the fitted models are measured by R2 value.

Investigation on passive simulation method and factors influencing the type-C-terrain wind profile of a structural wind-resistant moving-vehicle tester

Wind-profile simulation is fundamental to structural wind engineering. The transiting test method is used to test the wind-resistance performance of structures via vehicle-driving wind. Herein, we establish a novel method of simulating the wind profile of Commonwealth Advisory Aeronautical Research Council (CAARC) high-rise building standard model type-C-terrain by using flat plates through a transiting test method. Effects of several factors (i.e., the distance between flat plates and Pitot tubes, the length of flat plates along the wind direction, the distance between adjacent flat plates, the number of flat plates, and the vehicle speed) on the simulation results of type-C-terrain wind profiles were investigated. The results confirm the feasibility of applying a transiting test method to simulate wind profiles. The vehicle speed is 72 km/h (20 m/s). When the distance between the flat plates and the Pitot tubes is properly adjusted from 300 mm to 340 mm, the length of the flat plates along the wind direction is 200 or 300 mm. The results well agree with the wind-profile curves corresponding to the type-C-terrain in Chinese Load Code GB 50009-2012. Overall, this study provides a novel method of simulating type-C-terrain wind profiles.

An uncoupled fluid structure interaction method in the assessment of structural responses of tall buildings

This paper presents an innovative and efficient numerical approach for predicting wind induced dynamic responses of a tall building. The method uses an uncoupled fluid structure interaction (FSI) approach, where wind flow data obtained from a validated Computational Fluid Dynamics (CFD) analysis was used to obtain structural responses of the 184 m tall CAARC (Commonwealth Advisory Aeronautical Research Council) building with a typical façade. An innovative method of converting recorded pressures on the building’s surface, to time history nodal loads, was presented. This new method of pressure to load conversion enabled the use of an implicit modal time history analysis where the structure’s dynamic behaviour was predicted. This new method was critically evaluated with a full transient structural analysis. It was shown that the proposed mapping and implicit analysis technique took 45 s to perform and was able to provide similar numerical accuracy to a full transient analysis which took 2 clock hours. Also, the proposed method was capable of predicting building responses accurately in terms of mean, maximum and standard deviation quantities of displacement where comparison were made to experimental measurements. Other important characteristics such as the impact of using higher number of modes for the proposed modal superposition method is presented and discussed.

Performance-Based Wind-Resistant Optimization Design for Tall Building Structures

This paper presents a novel framework for the performance-based wind-resistant design of tall buildings accounting for the joint probability distribution of wind speed and direction obtained from long-term wind measurements of local meteorological stations. The joint probability distribution of the wind speed and direction, as well as the uncertainties and probability distributions of natural frequencies and damping ratios, are integrated to investigate the realistic structural performance of optimized target buildings under the wind action. A procedure on the performance-based wind-resistant optimization design of tall buildings is then proposed by adopting the optimality criteria algorithm. The proposed framework is applied to the Commonwealth Advisory Aeronautical Research Council building; the total weight and sizes of structural members are selected as the objective function and design variables in the optimization process, respectively. Compared with the deterministic constraints adopted in the conventional optimization procedure, the optimization constraint conditions in this paper are expressed by the failure probabilities in terms of the maximum displacement, interstory drift, and wind-induced acceleration responses at different design levels, which are associated with different return periods required by the strength and serviceability criteria in the performance-based wind engineering design. The mapping transformation method and checking point algorithm in structural reliability theory are adopted to conduct reliability analysis on the probability constraints in the optimization procedure. The optimized results are analyzed and discussed in the numerical example to verify the effectiveness of the proposed performance-based wind-resistant optimization design of tall buildings.

Sensitivity analysis of wind pressure coefficients on CAARC standard tall buildings in CFD simulations

Constructing tall buildings has been a common trend for many cities due to rapid increase of population, therefore, it is essential to analyze wind pressure around and over these tall buildings. CFD simulation is an effective approach to realize this, in which CAARC (Commonwealth Advisory Aeronautical Research Council) standard tall building has been a well-acknowledged model to validate and calibrate tall building models. Nevertheless, less work has been conducted to study sensitivity of wind pressures over CAARC standard tall building to geometric and computational parameters in CFD simulation. This paper is therefore designed to fill this gap by analysing impacts of various parameters like turbulence model, approaching-flow speed and grid type on wind pressure coefficients over CAARC buildings. In this paper, wind pressure coefficients over CAARC models subjected to four wind directions were numerically analyzed for validating the correctness and effectiveness of numerical model. Sensitivity of wind pressures to geometric and computational parameters was analyzed through comparisons of deviation and absolute deviation. Results indicated that various phenomena such as fluid separation, vortex, wake effect and reattachment were sensitive to wind direction, resulting in great variations in wind pressure. Maximum positive wind pressure coefficients emerged around 0.8–0.85H of windward surface, while maximum negative one occurred at foreside of top surface. Furthermore, turbulence model had significant influences on the accuracy of numerical results, where Realizable k-ε and SST were the most accurate turbulence models, and standard k-ε and RNG k-ε models were the second accurate ones, but BSL was not recommended for wind resistance design. Approaching-flow speed and grid type exerted insignificant influences on pressure coefficient distribution, while grid resolution had significant effects on negative wind pressure distribution. In general, conclusions obtained in this study can assist structural engineers to select practical geometric and computational parameters in predicting wind pressure distribution against tall buildings.

Design optimization on high-rise buildings considering occupant comfort reliability and joint distribution of wind speed and direction

A new procedure for the wind resistant optimization of high-rise buildings considering the uncertainties of the wind speed, structural natural frequency and damping ratio, and the joint distribution of the wind speed and direction is proposed. The novelty of the procedure includes three aspects. Firstly, the modal acceleration responses of buildings are employed to evaluate the occupant comfort in the building instead of the commonly used total acceleration responses. This enhances the applicability of the proposed method and reduces the computation error by including the effect of the reoccurrence period of wind and avoiding calculation of the peak factor of response. Secondly, because the current method such as the modified Hasofer-Lind-Rackwitz-Fiessler (HLRF) algorithm is not suitable to the optimization design considering multiple wind directions, to overcome this problem, the design point method in generalized random space (DPG method) is used to establish equations of reliability for converting modal acceleration constraints to natural frequency constraints, in which an approach of mapping transformation is used to treat the non-normal variables. Thirdly, by improving the method of determining the limit of the wind speed based on the joint distribution of the wind speed and direction, the failure probabilities of modal accelerations under all wind directions are combined based on the joint probability distribution of the wind speed and direction to obtain the limit of natural frequency constraints. The parameters of the joint probability distribution are determined from the meteorological observation data of the wind speed. In association with the Optimality Criterion (OC) algorithm, the proposed method is applied in the wind resistant optimization of a 60-stories standard model of Commonwealth Advisory Aeronautical Research Council (CAARC). The investigations show that the proposed method can effectively decrease the structural total weight subject to reliability frequency constraints, displacement constraints and inter-story drift constrains. Considering the randomness of parameters and joint distribution of the wind speed and direction in the comfort constraint could enhance the design space for the structural total weight.

Steady suction for controlling across-wind loading of high-rise buildings

Summary To reduce across-wind effects on high-rise buildings, this paper introduces a new active aerodynamic control named steady suction. To test its effect, the control mechanism of steady suction is discussed first, and then, a synchronization pressure test was conducted in a wind tunnel to measure the across-wind loading on a high-rise model (Commonwealth Advisory Aeronautical Research Council standard high-rise building model). A series of analytical methods were used to compare the different effects on across-wind aerodynamic forces caused by different parameters. The results show that when the wind blows straight on the wide side of the model, steady suction arranged on the narrow side close to the leading edge can effectively reduce the fluctuating base moment. When the wind blows straight on the narrow side, steady suction arranged on the middle of the wide side effectively reduces the fluctuating base moment. Copyright © 2016 John Wiley & Sons, Ltd.

Suction-based active aerodynamic control of wind loads on super high-rise buildings

This article introduces a new method, active flow field control through steady suction, to reduce wind loads on buildings. To test its effect, suction equipment was developed and installed on a Commonwealth Advisory Aeronautical Research Council super high-rise model. A series of experiments were then conducted in a wind tunnel. The effectiveness of this method for reducing wind loads is discussed through comparison of the suction bottom moment ratio under different wind incidence angles and different positions of steady suction. The results of the experiment demonstrate that steady suction reduces wind loads effectively.

Modeling vortex-shedding effects for the stochastic response of tall buildings in non-synoptic winds

This study derives a model for the vortex-induced vibration and the stochastic response of a tall building in strong non-synoptic wind regimes. The vortex-induced stochastic dynamics is obtained by combining turbulent-induced buffeting force, aeroelastic force and vortex-induced force. The governing equations of motion in non-synoptic winds account for the coupled motion with nonlinear aerodynamic damping and non-stationary wind loading. An engineering model, replicating the features of thunderstorm downbursts, is employed to simulate strong non-synoptic winds and non-stationary wind loading. This study also aims to examine the effectiveness of the wavelet-Galerkin (WG) approximation method to numerically solve the vortex-induced stochastic dynamics of a tall building with complex wind loading and coupled equations of motions. In the WG approximation method, the compactly supported Daubechies wavelets are used as orthonormal basis functions for the Galerkin projection, which transforms the time-dependent coupled, nonlinear, non-stationary stochastic dynamic equations into random algebraic equations in the wavelet space. An equivalent single-degree-of-freedom building model and a multi-degree-of-freedom model of the benchmark Commonwealth Advisory Aeronautical Research Council (CAARC) tall building are employed for the formulation and numerical analyses. Preliminary parametric investigations on the vortex-shedding effects and the stochastic dynamics of the two building models in non-synoptic downburst winds are discussed. The proposed WG approximation method proves to be very powerful and promising to approximately solve various cases of stochastic dynamics and the associated equations of motion accounting for vortex shedding effects, complex wind loads, coupling, nonlinearity and non-stationarity.

Statistical extremes and peak factors in wind-induced vibration of tall buildings

In the structural design of tall buildings, peak factors have been widely used to predict mean extreme responses of tall buildings under wind excitations. Vanmarcke’s peak factor is directly related to an explicit measure of structural reliability against a Gaussian response process. We review the use of this factor for time-variant reliability design by comparing it to the conventional Davenport’s peak factor. Based on the asymptotic theory of statistical extremes, a new closed-form peak factor, the so-called Gamma peak factor, can be obtained for a non-Gaussian resultant response characterized by a Rayleigh distribution process. Using the Gamma peak factor, a combined peak factor method was developed for predicting the expected maximum resultant responses of a building undergoing lateral-torsional vibration. The effects of the standard deviation ratio of two sway components and the inter-component correlation on the evaluation of peak resultant response were also investigated. Utilizing wind tunnel data derived from synchronous multi-pressure measurements, we carried out a wind-induced time history response analysis of the Commonwealth Advisory Aeronautical Research Council (CAARC) standard tall building to validate the applicability of the Gamma peak factor to the prediction of the peak resultant acceleration. Results from the building example indicated that the use of the Gamma peak factor enables accurate predictions to be made of the mean extreme resultant acceleration responses for dynamic serviceability performance design of modern tall buildings.

Integrated aerodynamic load determination and stiffness design optimization of tall buildings

AbstractModern tall steel buildings are wind sensitive and are prone to dynamic serviceability problems. Although wind tunnel techniques have emerged as valuable tools in providing reliable prediction of the wind‐induced loads and effects on tall buildings, current design practice normally considers the wind tunnel‐derived loads as constant static design loads. Such practice does not take into account the change in wind‐induced structural loads while the dynamic properties of a building are modified during the design synthesis process. This paper presents a computer‐based technique that couples together an aerodynamic wind tunnel load analysis routine and an element stiffness optimization method to minimize the cost of tall steel buildings subject to the lateral drift design criteria, while allowing for instantaneous prediction and updating of wind loads during the design synthesis process. Results of a full‐scale steel building framework with the same geometric shape of the Commonwealth Advisory Aeronautical Research Council (CAARC) standard building indicate that not only is the proposed technique able to produce the cost‐effective element stiffness distribution of the structure satisfying the serviceability wind drift design criteria, but a potential benefit of reducing the design wind loads can also be achieved by the stiffness optimization method. Copyright © 2007 John Wiley &amp; Sons, Ltd.