Full Aeroelastic Model Research Articles

Modal effects of span-layout types on coupled flutter of long-span suspension bridges

This study provides a theoretical explanation for a perplexing experimental phenomenon observed frequently in suspension bridge flutter testing, where the flutter critical wind speed (Ucr) predicted by two-dimensional (2D) section model testing is higher than that predicted by three-dimensional (3D) full aeroelastic model testing. Six long-span suspension bridges with either a single simply supported span or multiple continuously supported spans were taken as research examples, then the comparative studies on the flutter critical wind speeds, flutter frequencies, modal energy ratios and aerodynamic damping corresponding to 2D and 3D coupled flutter states were carried out to illustrate the flutter modal effects. The research findings indicate that the Ucr of 2D testing being higher than that of 3D testing is an inherent characteristic for single-span simply-supported suspension bridges, which can be attributed to the prominent aerodynamic negative damping associated with 2nd-order vertical bending mode. However, due to the aerodynamic positive damping associated with 3rd-order vertical bending mode, the Ucr of 2D testing is consistently lower than that of 3D testing in the cases of multi-spans continuously-supported suspension bridges. Additionally, to eliminate unsafe 2D evaluations, a reasonable modal matching principle is recommended for section model testing of single-span simply-supported suspension bridges.

Full-scale monitored wind and response characteristics of a suspension bridge compared with wind tunnel investigations at the design stage

Structural health monitoring of long-span bridges makes it possible to assess the assumptions and considerations in the design stage of the structures through measurements of their structural response and environmental conditions. In wind-resistant design of long-span bridges, the wind-induced dynamic buffeting response is difficult to assess as the uncertainty in the loading and the structural and aerodynamic characteristics is high. In that case, one of the best sources of information is wind tunnel tests, in addition to field measurements and analytical considerations. However, it is unclear if such methods can provide reliable information for the designer since, typically, many uncertainties and assumptions are involved in such a process. Here, we evaluate the scaled full aeroelastic and terrain model tests carried out during the design stage of the Gjemnessund Bridge in Norway through comparisons with full-scale monitoring data spanning five years. The monitoring system and the data are presented along with the wind tunnel study. The results from both campaigns are analyzed and critically discussed, emphasizing implications for the wind-resistant design of long-span bridges.

Aerodynamics and aeroelastic performance of a rigid-frame bridge with a bluff body girder subjected to short-rise-time gusts

Short-rise-time gusts in mountainous areas are extreme winds with the characteristics of rapidly varying wind speeds and directions, strong suddenness, and large angles of attack. Currently, studies on long-span bridges under short-rise-time gusts in mountainous areas, especially experimental studies, are limited. In this study, a typical long-span rigid-frame bridge with a bluff body girder located in a mountainous area suffering from frequent gusts was selected. Corresponding sectional and full aeroelastic model tests in a multiple-fan active control wind tunnel were performed to investigate the aerodynamic forces and wind-induced responses under strong gusts. Two aerodynamic measures (i.e., flange slab and stabilizing plate) were applied to mitigate the impact of gusts on the studied bridge. Moreover, numerical simulation analysis was also carried out to investigate the effects of gusts on the wind-induced stresses. The experimental results demonstrated that the gust parameters significantly affect the aerodynamic forces and wind-induced responses of the long-span rigid-frame bridge, especially under construction, although this type of bridge usually shows less wind-related sensitivity under normal monsoon climates. Furthermore, the proposed aerodynamic measures effectively mitigated the displacement responses of the studied bridge subjected to gusts in mountainous areas. This investigation can provide a more comprehensive understanding of the aerodynamics and aeroelastic performance of similar bridges under gusty conditions.

Investigation of vortex-induced vibration of a cable-stayed bridge without backstays based on wind tunnel tests

Cable-stayed bridges without backstays located in the abrupt valley are sensitive to wind-induced structural vibrations. The understanding of vortex-induced vibration for specific aerodynamic configuration with glass deck is of great importance for structure design. This paper thus aims to investigate the vortex-induced vibration of a cable-stayed bridges without backstays through the segmental model test and scaled full-aeroelastic model wind tunnel test. The results indicate that the bridge presents the vertical vortex-induced vibrations at the low wind speed whereas the maximum wind velocity causing the torsional vortex-induced vibrations increases to 41 m/s, which is much larger than that of the traditional bridge. The vertical and torsional vortex-induced vibrations in the smooth flow are observed at the positive wind angle of attack, owing to the responses for the bridge deck present a sharp single-frequency amplitude when the frequency of vortex-shedding is close to the flexural vibration frequency. The turbulence can effectively suppress the vortex-induced vibrations and only some small peaks are visible in the turbulent flow. The vortex-induced response of the bridge deck is significantly alleviated with increasing of damping ratio. Finally, the vortex-induced vibrations in the uniform flow disappear and the wind-induced responses are impaired with the ventilated railing in place of the airtight railing because the flow can go through the railing. It can provide a designing basis for engineering reference of cable-stayed bridges without backstays.

HFBB model test for tall buildings: A comparative benchmark with a full-aeroelastic model

A novel approach to full-aeroelastic model design is presented, considering continuous structures and coupled modes. Its validity is proven by building a full-aeroelastic CAARC standard tall building model featuring 12-degrees-of-freedom at 1:360 scale. A complete description of its design, manufacturing, tuning, and dynamic identification is reported. The model is tested in a novel wind-tunnel experimental campaign performed at the CRIACIV facility (Prato, Italy). A comparative study of aerodynamic and full-aeroelastic response in terms of base moment and accelerations shows that the aerodynamic model tends to overpredict the response along the broader side of the model. The contribution of second modes is found to be relevant for induced acceleration at the rooftop.

Case study of three-dimensional aeroelastic effect on critical flutter wind speed of long-span bridges

Three-dimensional (3-D) aeroelastic effect is known as the main factor that leads to the discrepancy between the critical flutter states predicted by two-dimensional (2-D) deck section and 3-D full structure. The 3-D aeroelastic effect on flutter performance of long-span bridges was investigated through historical experimental data statistics, numerical flutter analysis and wind tunnel tests. The critical flutter wind speeds of 16 long-span bridges were statistically analyzed. Subsequently, numerical flutter analysis were conducted towards a 300m-span quasi-simply supported beam bridge, and the 3-D structural effect and the effect of additional static wind angle of attack were analyzed. Corresponding section and full aeroelastic model tests were both carried out for further investigation of 3-D aeroelastic effect. Real-bridge experimental results show that the 3-D aeroelastic effect may lead to a maximum relative deviation in critical flutter wind speed of more than ±30%. Numerical-analysis-based 3-D aeroelastic effect with the contribution of both the 3-D structural effect and the effect of additional static wind angle of attack presents positive but weak influence, while the test results present more significant 3-D aeroelastic effect. Some potential factors, such as the spatial correlation and amplitude-dependency of aerodynamic forces, may also play an important role in 3-D aeroelastic effect.

Wind tunnel testing and frequency domain buffeting analysis of a 5000 m suspension bridge

Buffeting’s importance is highlighted in the case of super-long bridges. It remains unclear how buffeting will perform and whether the traditional methods still apply when bridge’s span is approaching its upper limitation. To figure out this, a full aeroelastic model testing has been carried out with the span arrangement 2000+5000+2000 m. A comparison with the existing bridges reveals the similarities and differences. It shows vertical and torsional buffetings are important for a 5000 m bridge while lateral response is almost static. Fundamental modes contribute most kinetic energy. With the increasing of wind speed, the aerodynamic coupling effect becomes prominent while the structural coupling effect keeps limited. Afterwards, four analytical methods, namely SRSS and CQC, with and without self-induced forces, are used for the buffeting displacements. The calculation indicates the multi-mode (CQC) analysis is still applicable on a 5000 m bridge but the mode-by-mode (SRSS) method is risky since the neglection of aerodynamic coupling effect. Parameter analysis shows aerodynamic admittance function and force coherence are decisive while the cross terms are of secondary importance. Since all data is experimentally measured, the reliability of analytical results can be ensured.

The Impact of Interphase Spacers on Galloping Control of Three-Phase Iced Eight-Bundled Transmission Lines: An Experimental Study

This study aims to conduct galloping experiments using a full aeroelastic model to investigate the galloping controlling effects of interphase spacers with different kinds of arrangements on three-phase iced eight-bundled transmission lines. A low-speed wind tunnel was designed and built to assess the compact transmission lines on a single span. The similarity conditions between the aeroelastic model and the prototype were determined by the similarity theory and the proposed modification method. Four common arrangements of interphase spacers were considered. Compared with the situation without interphase spacers, the composite amplitude reduction rate of the first two modes and amplitude reduction rate varying with wind speed were evaluated. Then, the anti-galloping effects of interphase spacers were investigated and the controlling effects of interphase spacers on the interphase distance were analyzed. Besides, the corresponding arrangements under different conditions were determined. In addition, the galloping phenomena varying with wind speed were explored. The results suggested that their galloping phenomena were changed because of the effects of interphase spacers, and coupling effects of interphase spacers with different arrangements were different. This research could be used to estimate the positions of interphase spacers and offered a theoretical foundation for the anti-galloping design of transmission lines.

Systems Engineering for Lidar-Assisted Control: A Sequential Approach

This work presents a sequential approach to explore and optimize the benefits of lidar-assisted control for wind turbines. The optimization is divided in three steps: lidar hardware, lidar data processing, and feedback controller optimization. Appropriate optimization criteria and computational efficient models are used for the intermediate steps and energy production is optimized in the last step with a full aero-elastic model to provide an estimation of lidar-assisted control without the need of a detailed cost model. The case study shows that lidar-assisted control together with an adjustment of the power level are promising to extend the life-time of wind turbines and finally increase the energy capture.

Wind tunnel test of the influence of an interphase spacer on the galloping control of iced eight-bundled conductors

To study the anti-galloping effect of interphase spacers with different kinds of arrangements on an iced transmission line, wind tunnel experiments using a full aeroelastic model are carried out in this work. Using similarity theory, the similarity criteria between the aeroelastic model and the prototype have been deduced. A triangular arrangement of three phases is considered, and the arrangements of iced eight-bundled conductors with and without the spacers are divided into six cases. In the experiments, laser sensors at different positions along the span of each phase are used to measure the variation of dynamic displacements with wind speed. Furthermore, galloping characteristics, such as the responses, frequencies, vibration modes, amplitudes and distance between two phases, are obtained for different wind speeds and arrangements. The obtained results show that interphase spacers have a certain inhibitory effect on the first symmetric mode and the first anti-symmetric mode galloping, but not all of the arrangements have a positive effect. The regularity of the variation with wind speed of the anti-galloping effect with interphase spacers is summarized. The galloping amplitude curves indicate that the amplitude is likely to be larger when a modal transformation occurs, in contrast to the results obtained without interphase spacers. The motion of the phase C located close to the diffusion section is different from those of the other two phases. Moreover, due to interphase spacers, the distance between the two phases is increased. The obtained conclusions can be used to predict the positions of interphase spacers and provide a fundamental tool for anti-galloping of eight-bundled conductors.

Full Aeroelastic Model Testing for Examining Wind-Induced Vibration of a 5,000 m Spanned Suspension Bridge

This paper presents an experimental investigation on wind-induced vibrations of a suspension bridge with ultimate main span length, including flutter and buffeting. Since the upper limit of a suspension bridge’s main span can reach more than 5,000m based on strength and weight of steel main cables, a feasible prototype with a span arrangement of 2,000m+5,000m +2,000m has been proposed. In order to improve flutter stability, a twin box girder has been designed with a 40m slot in center and two 20m wide decks suspended by four main cables. Its full aeroelastic model was designed and manufactured with the geometrical scale of 1:620, and the wind tunnel testing was carried out under smooth flows and turbulent flows for different angles of attack. A big difference in critical flutter speed was found between the proposed slotted girder and corresponding slot-sealed girder, which verifies the necessity of a widely-slotted twin box girder for a super-long suspension bridge. Among three angles of attack, the critical flutter speed under smooth flow has the maximum value of 80.9 m/s at -3° and the minimum value of 51.4m/s at 3°. On-coming turbulence will not only cause buffeting responses but also influence the critical flutter speed in an unfavorable way.

Effect of stabilizer on flutter stability of truss girder suspension bridges

An aerodynamic optimization measure of the flutter stability of long-span suspension bridges with truss girder is presented in this paper. At first, the improvement of several kinds of central stabilizers and horizontal stabilizers on flutter stability is examined through series of section model and full aeroelastic model wind tunnel tests. Subsequently, the flutter derivatives of the truss girder with and without stabilizer are identified based on two degrees of freedom coupling free vibration method. Furthermore, based on the identified flutter derivatives, the critical flutter velocities of the truss girder section with and without stabilizer are analyzed through two dimensional flutter analysis method and the critical flutter velocities of the full bridge with and without stabilizer are analyzed through three dimensional method. Afterwards, the influence of each flutter derivative on the flutter stability of the truss girder is investigated. The results indicate that central upper stabilizer can effectively increase the critical flutter velocity of the truss girder. In contrast, the central lower stabilizer and horizontal stabilizer have less influence. Setting up central upper stabilizer leads to an obvious decrease in the value of the flutter derivatives A2* and H2*, while the flutter derivatives H1*, H4*, A1* and A3* are little influenced. The two dimensional and three dimensional flutter analysis results agree well with the sectional model and full model wind tunnel test results respectively.

Aeroelastic stability of two long-span arch structures: A collaborative experience in two wind tunnel facilities

In this paper, a rare example of comparison between sectional and full-aeroelastic model tests is presented. Interestingly, the experiments were conducted in two very different wind tunnel facilities by different research teams. The study concerns two long-span steel arch structures recently built in Milan, Italy, for Expo 2015 World Fair. The structures have only aesthetic purposes and are therefore very flexible and light, which makes them sensitive to wind-induced excitation and prone to aeroelastic instabilities. In particular, in smooth flow an interesting phenomenon of interference between vortex-induced vibration and galloping was observed up to high values of the Scruton number. This aeroelastic instability is very dangerous as large-amplitude vibrations can occur in wind speed ranges where they are not expected, at least for what classical theories for vortex-induced vibration and quasi-steady galloping are concerned. Moreover, the provisions of Eurocode 1 resulted clearly unsuitable and non-conservative to address such a phenomenon. Despite the differences in the facilities and in the models, a good agreement was found between the results obtained in the two laboratories. The major discrepancies were observed in the transitional behavior for intermediate values of the Scruton number, the sectional model showing a more unstable behavior. The tests on the full-aeroelastic model also allowed considering the effect of the angle of wind exposure of the structures, both the in-plane and the out-of-plane vibrations of the arches and the dynamic response to turbulent wind. In particular, a set of tests in smooth flow was performed accounting for the presence of the other arch and of the surrounding buildings. A particular dynamic excitation of the in-plane flexural modes of the structures was observed in well defined ranges of flow speeds when one arch is in the wake of the other. Finally, both experimental campaigns highlighted the need for the installation of tuned mass dampers on the real structures to guarantee their safety. The effectiveness of these devices against the observed galloping-type instability was also verified through wind tunnel tests on the full-aeroelastic model.

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.

Turbulence Integral Scale Corrections to Experimental Results of Aeroelastic Models with Large Geometric Scales: Application to Gust Loading Factor of a Transmission Line Tower

Full aeroelastic model tests are often carried out to investigate the buffeting responses and the corresponding gust loading factor (GLF) of flexible structures. The geometric model scale for complicated structures may sometimes be excessively large in comparison with the ratio between turbulence integral scales measured in wind tunnel and in real atmospheric environments. This inconsistency in scale modeling leads to severe distortions of turbulence integral scale similarity whose influence manifests itself by modifications in resonant component of buffeting response and in resulting GLF as well, and numerical correction is needed when these testing results are converted to full-scale. This paper presents a correction procedure in terms of correction factors for experimental GLF obtained from full aeroelastic model tests to account for the inconsistency in scale modeling. The correction is simply made on analyzing the prototype in two air flows with different turbulence integral scales, namely the nominal value in the real atmosphere environment and the full-scale value correspondent to aeroelastic model test. The correction procedure is applied to the full aeroelastic model experiments of a transmission line tower with an unusually large geometric scale of 1/40 in order to examine the effect of turbulence integral scale on GLF. An appreciable effect is observed. It is found that the GLF obtained from wind tunnel experiments tends to be conservative for most structures and becomes unsafe for structures with extremely low modal frequencies when disregarding the correction. Correction factors are then derived to accommodate the inconsistency in modeling turbulence integral lengths. As a first generalization, a set of more general correction factors defined in terms of modal damping ratio and frequency ratio, the ratio between modal frequency and the dominant frequency in wind power spectra, are further developed through parametric analysis. The correction factors decrease with the augment of modal damping ratio and frequency ratio.

Flutter Mode Transition of a Double-Main-Span Suspension Bridge in Full Aeroelastic Model Testing

AbstractThe Maanshan Bridge over the Yangtze River in China is a new long-span suspension bridge with double main spans of 2×1,080 m and a closed streamlined cross section of single box deck. The flutter performance of the bridge was investigated via a wind tunnel test of a full bridge aeroelastic model at a geometric scale of 1:211. The test was conducted in both smooth and simulated boundary layer wind fields with various combinations of wind yaw and inclination angles. A unique and interesting flutter-mode transition behavior was observed in smooth flow. As wind speed approaches the flutter threshold, the model oscillation with the first antisymmetric torsional mode (A-T-1) transfers to alternate torsional oscillation between double spans, then transfers to oscillation with the first symmetric torsional mode (S-T-1), and then violent oscillation occurs. The mechanism of the phenomenon is unknown, and there may be no numerical method to simulate it to date. One possible reason for the flutter-mode trans...

Flatness-based Feedforward Control of Wind Turbines Using Lidar

Current lidar technology is offering a promising opportunity to take a fresh look at wind turbine control. This work evaluates a flatness-based feedforward approach, that allows to calculate the control action based on trajectories of the rotor speed and tower motion using wind measurements. The trajectories are planned online considering actuator constrains to regulate the rotor speed and minimize tower movements. The feedforward signals of the collective pitch and generator torque update can be combined with conventional feedback controllers. This facilitates the application on commercial wind turbines. Simulations using a realistic lidar simulator and a full aero-elastic model show considerable reduction of tower and shaft loads.

Investigation on vortex-induced vibration of a suspension bridge using section and full aeroelastic wind tunnel tests

Obvious vortex induced vibration (VIV) was observed during section model wind tunnel tests for a single main cable suspension bridge. An optimized section configuration was found for mitigating excessive amplitude of vibration which is much larger than the one prescribed by Chinese code. In order to verify the maximum amplitude of VIV for optimized girder, a full bridge aeroelastic model wind tunnel test was carried out. The differences between section and full aeroelastic model testing results were discussed. The maximum amplitude derived from section model tests was first interpreted into prototype with a linear VIV approach by considering partial or imperfect correlation of vortex-induced aerodynamic force along span based on Scanlan\'s semi-empirical linear model. A good consistency between section model and full bridge model was found only by considering the correlation of vortex-induced force along span.

Aerodynamic Stability of a Three-Tower Suspension Bridge during Erection via Aeroelastic Model Test

As a new long-span suspension bridge with double main spans and a typical closed streamline cross-section of single box deck, the flutter performance of the Maanshan Bridge during erection was investigated via a full bridge aeroelastic model test. Critical flutter wind speeds of 13 testing cases with different percentage of deck completion are much higher than the flutter checking wind speeds, and the bridge is hence proven to be stable enough during erection in aerodynamics. The case with the percentage of deck completion of 86.4% gets the lowest flutter critical wind speed, perhaps because frequency ratio gets the minimum value at this case.

Wind-induced response and loads for the Confederation Bridge -Part I: on-site monitoring data

This is the first of two companion papers that analyse ten years of on-site monitoring data for the Confederation Bridge to determine the validity of the original wind speeds and wind loads predicted in 1994 when the bridge was being designed. The check of the original design values is warranted because the design wind speed at the middle of Northumberland Strait was derived from data collected at shore-based weather stations, and the design wind loads were based on tests of section and full-aeroelastic models in the wind tunnel. This first paper uses wind, tilt, and acceleration monitoring data to determine the static and dynamic responses of the bridge, which are then used in the second paper to derive the static and dynamic wind loads. It is shown that the design ten-minute mean wind speed with a 100-year return period is 1.5% less than the 1994 design value, and that the bridge has been subjected to this design event once on November 7, 2001. The dynamic characteristics of the instrumented spans of the bridge including frequencies, mode shapes and damping are in good agreement with published values reported by others. The on-site monitoring data show bridge response to be that of turbulent buffeting which is consistent with the response predicted at the design stage.