High-frequency Force Balance Research Articles

Wind-resistant structural optimization of supertall buildings based on high-frequency force balance wind tunnel experiment

The high-frequency force balance (HFFB) wind tunnel test is one of the most widely used techniques to estimate wind effects on supertall buildings. This paper introduces a wind-resistant optimal design method of supertall buildings based on HFFB wind tunnel experiments. Based on the HFFB wind tunnel test and wind-induced vibration analysis, it provides a reliable and efficient method for structural optimization of supertall buildings with different types of structural members and multiple constraints. Taking advantage of the classic linear mode shape assumption of the HFFB technique, this paper also presents an alternative routine that transforms all different types of constraints into a uniform frequency constraint, which significantly reduces the computational intensity while still offers acceptable accuracy of optimization results. Moreover, the sensitivity of the optimization results to the adopted basic wind pressure value is discussed. The total material cost of the optimized structure increases exponentially with the increase of the basic wind pressure, indicating that the structural costs of supertall buildings are quite sensitive to the adopted basic wind pressure for structural design and its value should be precisely determined by the wind climate analysis of the construction site.

Investigation of the spatial coherence function of wind loads on lattice frame structures

This study investigates the spatial coherence function of wind loads on the lattice frame structure through a simultaneous double high frequency force balance (HFFB) wind tunnel test. The wind forces on two segment models of the lattice frame structure are simultaneously measured by the wind tunnel test for several configurations of the models with different lateral and vertical distances. Coherence functions from the test are then calculated and compared with the existing coherence functions for tall buildings or tapered stacks. Results show that none of the existing coherence functions can provide an accurate enough estimate of the coherence function of wind loads on the lattice frame structure. Additionally, the coherence function of wind loads is affected by the frequency of wind loads, force direction, lateral and vertical distances. An expression of the coherence function specific for the lattice frame structure is then developed based on the wind tunnel test results. An example is presented to show the application and significance of the developed expression. The findings of this research can provide a more accurate estimate of the coherence function of wind loads on similar lattice frame structures, and hence the resultant generalized wind forces and wind performance of similar structures.

Thrust force and base bending moment acting on a horizontal axis wind turbine with a high tip speed ratio at high yaw angles

Onshore wind turbines may experience substantially different wind loads depending on their working conditions, i.e., rotation velocity of rotor blades, incoming freestream wind velocity, pitch angle of rotor blades, and yaw angle of the wind-turbine tower. In the present study, aerodynamic loads acting on a horizontal axis wind turbine were accordingly quantified for the high tip speed ratio (TSR) at high yaw angles because these conditions have previously not been adequately addressed. This was analyzed experimentally on a small-scale wind-turbine model in a boundary layer wind tunnel. The wind-tunnel simulation of the neutrally stratified atmospheric boundary layer (ABL) developing above a flat terrain was generated using the Counihan approach. The ABL was simulated to achieve the conditions of a wind-turbine model operating in similar inflow conditions to those of a prototype wind turbine situated in the lower atmosphere, which is another important aspect of the present work. The ABL and wind-turbine simulation length scale factors were the same (S=300) in order to satisfy the Jensen similarity criterion. Aerodynamic loads experienced by the wind-turbine model subjected to the ABL simulation were studied based on the high frequency force balance (HFFB) measurements. Emphasis was put on the thrust force and the bending moment because these two load components have previously proven to be dominant compared to other load components. The results indicate several important findings. The loads were substantially higher for TSR=10 compared to TSR=5.6. In these conditions, a considerable load reduction was achieved by pitching the rotor blades. For the blade pitch angle at 90°, the loads were ten times lower than the loads of the rotating wind-turbine model. For the blade pitch angle at 12°, the loads were at 50% of the rotating wind-turbine model. The loads were reduced by up to 40% through the yawing of the wind-turbine model, which was observed both for the rotating and the parked wind-turbine model.

A design method for multi-degree-of-freedom aeroelastic model of super tall buildings

Wind tunnel test models for super tall buildings mainly include synchronized pressure models, high-frequency force balance models, forced vibration models and aeroelastic models. Aeroelastic models, especially MDOF aeroelastic models, are relatively accurate, and designing MDOF model is an important step in aero-model wind tunnel tests. In this paper, the authors propose a simple and accurate design method for MDOF model. The purpose of this paper is to make it easier to design MDOF models without unnecessary experimentation, which is of great significance for the use of the aero-model for tall buildings.

Applicability of random decrement technique in extracting aerodynamic damping of crosswind-excited tall buildings

At the vicinity of vortex lock-in wind speed, the nonlinear aerodynamic damping effect is significant. It can greatly promote the surge of crosswind response, even threaten structural safety and serviceability of tall buildings. Techniques dependent on experimental measurement to identify aerodynamic damping have been developed for estimating crosswind response from wind loading spectrum. Random decrement technique (RDT) as one frequently mentioned technique of system identification has been used in literature to extract aerodynamic damping from stochastic crosswind response, although it was originally suitable for linear single degree of freedom system with a constant damping ratio. This study investigates the applicability of RDT in extracting aerodynamic damping of tall buildings. The models of linear and nonlinear aerodynamic damping are applied to simulate crosswind response of a square section tall building. Accordingly, the wind loading spectra based on high-frequency-force-balance (HFFB) testing and the model recommended by AIJ are utilized respectively to consider the effect of random wind load on the characteristics of crosswind response. To examine the accuracy and efficiency of RDT, aerodynamic damping is extracted through two approaches and compared with target value. Moreover, the influence of varying wind loading spectra on amplitude-dependent aerodynamic damping is demonstrated.

Assessment of wind-induced responses for offshore jacket platforms based on high frequency force balance tests

This paper focuses on the wind sensitivity of large offshore jacket platforms subjected to hurricanes, and a 1:100 scaled model is tested in a boundary wind tunnel under 0~360° wind directions using the high frequency force balance (HFFB) technique. Based on the experimental results, a corrected frequency-domain calculation method for evaluating wind-induced responses is derived, which can eliminate the resonance effects of the test model. The results show that: the first seven or higher-order mode combinations of platforms should be adopted to evaluate the dynamic wind-induced responses; the different mode coupling effects show a complicated pattern, especially in the across wind direction, and the complete quadratic combination method is suggested; the root mean square (RMS) displacement and acceleration of the derrick present approximate O shapes at different heights, and the maximum values reach 9.35 cm and 49.98 cm/s2. Therefore, the wind-induced vibration of large offshore jacket platforms should be under stronger control to ensure their safety. [Received: April 15, 2018; Accepted: December 13, 2019]

Assessment of wind-induced responses for offshore jacket platforms based on high frequency force balance tests

Assessment of wind-induced responses for offshore jacket platforms based on high frequency force balance tests

Experimental error examination and its effects on the aerodynamic properties of wind turbine blades

Uncertainties associated with aerodynamic properties of wind turbine blades, such as the lift coefficient and its first derivative with respect to the angle of attack, measured in the wind tunnel, are not negligible. Uncertainties are due to random experimental laboratory errors. Uncertainties are examined by high frequency force balance tests in Northeastern University's wind tunnel. Experiments are conducted on various scale models of airfoil sections, derived from a benchmark wind turbine blade. Effects of angle of attack, Reynolds number and incoming turbulent flow conditions are considered. First, basic statistical descriptors and statistical inference are utilized to characterize experimental errors in lift coefficients and their first derivative. Second, two methods, based on polynomial representation and linear interpolation of empirical histograms, are investigated to synthetically generate random samples of the lift coefficient derivative. Suitability is verified by hypothesis testing. Finally, samples are employed for Monte-Carlo-based stochastic flutter analysis of a full-scale blade.

Experimental investigation on non-stationary wind loading effects generated with a multi-blade flow device

Overcoming the spatial constraints of the small-scale wind tunnel at Northeastern University, a multi-blade flow device (MBFD) has been installed within the facility’s test chamber to generate ramps in measured wind velocities by redirecting horizontally driven flows. As a continuation feasibility study, this paper analyzes the aerodynamic loads imparted on a building model from these simulated non-stationary outflows. Base forces are recorded with a high-frequency force balance sensor (HFFB) and compared to digital simulations using a modified quasi-steady aerodynamic load approach. Under certain conditions, the forces obtained from both physical and numerical procedures coincide well with each other, although differences arise due to assumptions and deficiencies in the modeling. Nonetheless, results indicate that altering horizontal flows with this device is suitable for replicating non-stationary aerodynamic loads, within the confines of a small-scale wind, “straight” tunnel.

Design and Verification of Tuned Liquid Column Dampers for High-rise Buildings Using CFD

BMT Fluid Mechanics conducted a comprehensive flow study on the effectiveness of a pair of Tuned Liquid Column Dampers (TLCDs) aimed at mitigating wind-induced accelerations in a high-rise building. The study successfully combined Computational Fluid Dynamics (CFD) with physical model tests performed at the 6 degree-of-freedom shake table facility located at the Department of Civil Engineering of the University of Bristol. The original TLCD concept discussed in this work was proposed at the early design stages of a slender 42-storey high-end residential building located in the Middle East. On-site full-scale measurements and High Frequency Force Balance (HFFB) wind tunnel tests performed by BMT Fluid Mechanics showed that the highest occupied floors could experience excessive wind-induced motion. Depending on the inherent damping of the finished structure, this motion had the potential to exceed standard occupant comfort criteria. A new CFD methodology was developed to assess the performance of the TLCD design using the multi-phase Volume-of-Fluid (VOF) solver and dynamic mesh motion libraries available in the CFD software tool HELYX®. The damping effects introduced by the array of internal porous baffles in the TLCD were approximated using a Darcy-Forchheimer porosity model. Comparisons of free-decay damping performance between CFD results and shake table experiments for a 1:20 scale model of a 3 baffles’ TLCD configuration were found to be satisfactory for design purposes. The results for the amplitude of the net forces acting on the TLCD, as well as the frequency response measured using the free-decay logarithmic decrement approach, confirmed that the proposed CFD methodology provides an accurate representation of real operating conditions. The same CFD approach was successfully applied to model the full-scale TLCD device. With the introduction of a pair of identical purposely designed TLCDs, a 30% increase performance of the structural response of the 42-storey building to wind loading excitation was achieved.

Method for Decoupling and Correction of Dynamical Signals in High-Frequency Force Balance Tests

AbstractHigh-frequency force balance (HFFB) technology is a popular experimental method for investigating dynamic wind loading and wind-induced response of high-rise buildings. A decoupling matrix ...

Experimental investigation of aerodynamic forces and flow structures of bionic cylinders based on harbor seal vibrissa

In the present study, we investigated a new type of non-uniform surface cylinder based on a harbor seal vibrissa to reduce aerodynamic forces and suppress vortex shedding. Three vibrissa-based bionic cylinder models (i.e., models #1, #2, and #3) with different undulation wavelengths were manufactured and tested in a wind tunnel. The wind tunnel experiments were conducted at a Reynolds number of Re ≈ 50,000 based on the hydrodynamic diameter (Dh) and incoming airflow speed. The control effects on the aerodynamic forces and flow structures around the bionic cylinders were studied and compared with those of a baseline circular cylinder. In addition to measuring the surface pressure distributions using an array of digital pressure transducers, a high frequency force balance was used to determine the aerodynamic forces acting on the test models. Moreover, a digital particle image velocimetry (PIV) system was utilized to quantify the stream-wise flow structure and span-wise flow field to assess the effectiveness of the bionic cylinder models. The results revealed that a bionic non-uniform surface distribution could reduce the mean drag and suppress the fluctuating lift to a certain extent. Bionic cylinder model #2, which had a wavelength of two minor axes, worked best among the three models, and it was found to achieve a drag reduction of 15% and a maximum fluctuating lift suppression of 58% when the angle of the incoming airflow was 0°. The PIV results indicated that the non-uniform surface structure would disrupt the consistency of span-wise flow and decrease the span-wise correlation in the near wake of the test models, which could decrease the turbulent kinetic energy, mean drag, and fluctuating lift of the test models.

Model-Selection Uncertainty Quantifications in HFFB Dynamic Analyses of a Complex Tall Building

AbstractThe high-frequency force balance (HFFB) technique is widely adopted for the assessment of wind loads acting on a tall building because of its simplicity and versatility. Although a number o...

Use of modal interference approach (MIA) to reduce wind-induced building motions

A new method of structural optimization to reduce across-wind responses of tall buildings, named MIA (modal interference approach), is proposed in this paper. This method is based on an observation that if there are multiple modes participating in vortex-induced oscillations, these modes tend to interrupt each other, resulting in considerably smaller amplitude of vibration in comparison with a single mode response. A theoretical analysis was performed to describe this phenomenon, based on which MIA was developed. The new method is to stimulate multi-mode responses to vortex excitations by rotating the modal directions away from the wind-susceptible building directions and separating the frequencies between the participated modes. Wind tunnel studies of a high-frequency force balance model and a stick aeroelastic model verified the effectiveness of the proposed method.

Reduction of Mean Wind Forces on Photovoltaic System using Aerodynamic Appendages

This paper describes the results of scale-model wind-tunnel testing of a unique ground-mounted solar photovoltaic (PV) conducted in smooth flow for a single wind direction. The mean wind forces and moments were measured using a six-component High Frequency Force Balance (HFFB). The measured wind loads on the PV system were comparable to those in the Japanese standards. The mitigation effects of aerodynamic appendages such as hanging walls and eaves are reported. The maximum reduction of mean-wind-force and moment coefficients occurred with an exterior hanging wall plate with a 70% solidity ratio.

An Aerodynamic Load Correction Method for HFFB Technique Based on Signal Decoupling and an Intelligent Optimization Algorithm

In high‐frequency force balance (HFFB) wind tunnel tests, the aerodynamic wind loads at the base of the building model are usually amplified by the model‐balance system. This paper proposes a new method for eliminating such an amplification effect. Firstly, the measured base bending moment signals are decoupled into independent components. Then, an optimization model is established to represent the problem of identifying the natural frequencies and damping ratios for the different modes of the model‐balance system. Finally, the genetic algorithm (GA) is employed to seek the solution to the optimization problem, and the base bending moment is corrected through the identified dynamic parameters of the model‐balance system. Compared to the conventionally used knocking method, the proposed method requires no extra knocking tests and can take the aerodynamic damping of the model‐balance system into account. An engineering case, the Guangzhou East Tower (GZET), is taken as an example to show the effectiveness of the method.

Wind forces on circular steel tubular lattice structures with inclined leg members

The wind forces on the steel lattice structures have drawn increasing concerns in the structural design of high-rise and long-span buildings and structures due to their low damping, light self-weight, and high flexibility. The aim of this paper is to assess the aerodynamic forces on circular steel tubular lattice structures with vertical and inclined leg members using the wind tunnel tests. In order to elucidate the underlying mechanism of wind loads contributed by the individual members of lattice structures, a new type of rigid experimental model for High-frequency force balance (HFFB) wind tunnel tests is proposed in the study. And the individual member of lattice structures can be sequentially removed until remaining vertical or inclined leg members, which is referred to as “member subtraction”. The experimental results reveal that with the same solidity ratio, the drag coefficient of lattice structure with inclined leg members is approximately 9% smaller than that with vertical ones. This indicates that the configuration of steel lattice structures, such as inclined leg members, might exert indispensable effects on the drag forces, which has yet been considered in the current wind codes and standards. In addtion, in order to clarify the interference mechanism between the leg and diagonal members, the wind tunnel tests of a three-cylinder model are performed and the wind tunnel results indicate that the presence of diagonal members in lattice structures would increase the overall drag coefficients in the scenario of small separation distances. Finally, the advantages and disadvantages of structural configuration (with and without horizontal or diagonal members) and their implications on structural design are discussed.

Experimental study of wind effects on two neighboring super-tall buildings

With high-frequency force balance technique, wind tunnel tests have been carried out to study the wind effects on two iconic super-tall buildings located in South China, the Guangzhou East Tower and the Guangzhou West Tower. It was found that the normally used method, which calculates the background response and the resonant response separately and then combines them to get the total response, could give overestimated or underestimated response results. Neglecting the correlation between the background response and the resonant response is the very reason for the inaccuracy. Afterward, the wind-induced responses of Guangzhou East Tower and Guangzhou West Tower are computed by considering four different reference wind speeds obtained from different wind climate models, including the GB, GMB, ZL, and ARA models. It was found that different reference wind speeds from different wind climate models created significantly different structural response results. But, the computational maximum acceleration response by adopting any one of the four wind reference wind speeds is much greater than the field measured maximum instantaneous acceleration at the top of Guangzhou West Tower in the past 7 years, which would suggest that all of the four reference wind speeds from different wind climate models are conservative. If the wind-induced structural response is computed by adopting the field measured wind speed, the computational peak acceleration response is close to the field measured result, indicating the wind tunnel test itself is reliable.

Dynamic Wind Load Combination for a Tall Building Based on Copula Functions

Dynamic wind loads on tall buildings can be decomposed into three components, i.e. two translational components and one torsional component. When one component reaches its maximum, the other two components have low probability to take their maximum values. It is common to use combination coefficients for estimating the mean extremes of linearly combined wind loads. The traditional design practice for determining wind load combinations relies partly on some approximate combination rules and lacks a systematic and reliable method. Based on the high frequency force balance (HFFB) testing results, wind loads can be acquired in terms of time history data, which provides necessary information for the more rigorous determination of combination coefficients by probabilistic methods. In this paper, a 3D copula-based approach is proposed for determining the combination coefficients for three stochastic wind loads associated with a specific exceedance probability and a set of 3D realizable equivalent static wind loads (ESWLs) on tall buildings. Using the measured base moment and torque data by the HFFB wind tunnel test, a case study is presented to illustrate the effectiveness of the proposed framework to determine the dynamic wind load combinations and associated 3D realizable ESWLs on a full-scale 60-story building.

Examination of experimental variability in HFFB testing of a tall building under multi-directional winds

Examination of wind loading uncertainty is necessary for structural fragility analysis of tall buildings in the context of performance-based wind engineering. One of the sources of uncertainly is represented by random experimental error, which may occur during a standard test in wind tunnel, such as the high frequency force balance (HFFB) test. Few examples of systematic experimental error analysis are available. This study examines a series of experiments on a model of a standard benchmark building under simulated turbulent flow. HFFB experiments were conducted in the Northeastern University's small-scale wind tunnel. Three model equations are proposed to represent the power spectral density (PSD) functions of the above three forces in the frequency domain.Furthermore, effects of experimental errors are incorporated by allowing the parameters in each function to vary randomly. Variability is evaluated by considering the second-order statistical moments of the parameters (standard deviations and correlations). At last, this paper introduces a data-driven Monte-Carlo approach to reconstruct the generalized PSD of the wind loading through model equations. Experimental variability is accounted for by exploiting information on the probability distributions of the model parameters. Verification of the reconstructed PSD functions against experiments suggests that the Monte-Carlo approach can effectively generate synthetic PSD curves.