VIV (vortex-induced vibration) suppression using aerodynamic countermeasures are critical for design of long-span bridges. Strong vertical VIVs were observed in a streamlined closed-box bridge girder ...subject to wind at an initial attack angle of +3°. CFD simulation, and pressures and displacements measured in wind tunnel tests on a large-scale model (approximately 1:20) were analyzed to reveal the VIV-triggering mechanism for a bridge girder, as well as the VIV-suppression mechanism for bridge girders modified with typical small-scale additional components such as spoilers on crash barriers or guide vanes near maintenance traces. Spoilers can almost eliminate VIVs, while guide vanes can moderately mitigate them. Large-scale vortexes generated from flow separation at leading barriers and leading maintenance trails, referred to as separated vortexes and secondary vortexes, respectively, and called "double vortex mode" together, are responsible for VIVs of an unmodified girder. When spoilers were set on the barriers, the separated vortexes, and then the "double vortex mode" observed on the unmodified bridge girder, could be broken. This greatly reduced the correlation between local aerodynamic forces and general vortex-excited forces (VEFs), and thus the contribution of local aerodynamic forces to general VEFs on the model surface, which caused the VIVs to disappear. As for the unmodified girder, the "double vortex mode" is also responsible for vertical VIVs of a girder modified with guide vanes. This is due to the strong correlation between local aerodynamic forces and general VEFs. However, the phase lags between them have an overall offset of approximately 90° from the approximate synchronous action. Furthermore, a significant decrease in RMS values of pressure coefficients on the model surface dramatically reduce the direct contribution of local aerodynamic forces to general VEFs, especially in the downstream region of the upper surface, thus clearly mitigating VIVs.
•CFD simulation, and pressures and displacements measured in wind tunnel tests on a large-scale reduced model reveal the VIV-triggering mechanism for the bridge girder.•The "double vortex mode" are proved to be responsible for VIVs in an original girder.•Further discussion of the mechanism involving phase lags and contribution about distributed pressure taps and flow visualization are introduced.
Based on 14 sets of records observed by 4 meteorological stations during 10 typhoons, a comprehensive investigation of gust characteristics, which are usually quantitatively expressed as the ...turbulence intensity and gust factor, for the near-ground typhoon winds was conducted. The direction- and mean-wind-speed-dependent features of these two gust parameters are analyzed at different heights. It is found that, in general, mean wind speed has little effect on gust parameters for off-sea winds coupled with a small number of over-land winds over 15 km of upstream fetch. Statistical characteristics, including the first four-order moments together with the probability distribution, are examined and turbulence intensity and gust factor profiles are compared with those given in various codes and standards for open-sea and flat-area conditions. The estimation of peak factor based on observations and a Gaussian distribution-based model is also presented, and they show generally good accordance for the level of mean values. The relationship between turbulence intensity and gust factor is also studied and compared with several pioneer results. Two typical upstream conditions, off-sea and over-land winds are separately analyzed, and they show an opposite tendency, i.e., the off-sea gust factor at 10 m is 8% higher than the over-land value while other heights show little difference. We attempt a rapid estimation of gust factor using the model coupled with the MODIS land-use roughness length data and compare the results with observations. Finally, some uncertainties that would induce scatter of gust characteristics and imperfections of the theoretical model are identified and discussed.
•Gust characteristics at different heights of 10 typhoons in coastal region are analyzed and discussed.•The direction- and mean-wind-speed-dependent features of turbulence intensity and gust factor are examined.•Statistical characteristics of turbulence intensity and gust factor are investigated.•A rapid estimation method of gust factor using the MODIS land-use roughness length data is presented.
The clarification of the nonlinear and unsteady aerodynamic characteristics of a quasi-flat plate is of great importance to predict its wind-induced post-flutter behavior. Forced motion wind tunnel ...tests on a quasi-flat plate sectional model with a width-over-thickness ratio of 62.5:1 are conducted using the synchronous measurement of the dynamic forces and torsional displacements at various initial angles of attack (AoAs), vibration amplitudes and incoming wind speeds. The flutter derivatives related to torsional degree of freedom (DOF) are identified and compared. The nonlinear and unsteady characteristics of self-excited forces (SEFs) in terms of the frequency components and phase lag between the force and motion are quantitatively examined. The wind-induced energy maps of the quasi-flat plate at various initial AoAs are analyzed by introducing the aerodynamic work done by the self-excited lifting moment. A polynomial model is proposed to describe the nonlinear characteristics of the SEFs. It is validated that the flutter derivatives related to the torsional DOF are almost a single-valued function with respect to the reduced frequency, even at large vibration amplitudes. At large initial AoAs and oscillation amplitudes, the high-order frequency components of SEFs are significant, and the nonlinearity of the self-excited lifting moment is stronger than that of the self-excited lift force for torsional motion. This provides a reasonable explanation for the significant amplitude dependence of the flutter derivatives related to the lifting moment. The proposed polynomial nonlinear SEF model can achieve satisfactory accuracy in reproducing the SEFs in both time and frequency domains.
Structural robustness is defined by the international engineering community as the capabilities of structural systems that enable them to survive unforeseen or unusual circumstances. In order to ...highlight the unforeseen and unusual characteristics of wind hazards, this study introduces the concept of structural robustness into the wind-resistant design and evaluation of bridges and proposes the robustness evaluation of aerodynamic flutter and aerostatic torsional stability of long-span bridges. Furthermore, the return period of the design wind speed that a bridge can resist is used to represent wind-resistant robustness. Aiming at the problem of aerodynamic and aerostatic stability, the analysis methods of aerodynamic flutter stability robustness and aerostatic torsional stability robustness of long-span suspension bridges are respectively established. Based on the established methods of aerodynamic flutter stability and aerostatic torsional stability robustness evaluation, robustness analysis is carried out on eight completed long-span suspension bridges and two long-span suspension bridges to be built. The evaluation method proposed in this study makes it possible to measure the ability of bridge structures to resist multiple disasters using the same index.
During the erection stage of the suspension bridge, its critical flutter wind speed is continuously varying due to changing structural dynamic properties including mass and stiffness. On the other ...hand, the wind climate is also varying greatly seasonally or even monthly during the bridge erection procedure. This work proposes an analytical framework to optimize the deck erection timeline under complex wind climate attacks. Xihoumen Bridge built at Chinese southeastern coast is taken as a calculation example. Full-bridge aeroelastic model wind tunnel test was conducted to examine the critical flutter speed at different erection stages. Extreme wind speed for tropical cyclones and synoptic wind are analyzed by tropical cyclone simulation and meteorological records respectively. Results show that the optimal timeline for Xihoumen Bridge is starting in August if a one-month duration is required for each stage. However, if the worst timeline is selected, the flutter risk will be increased 40 times larger. If the construction timeline is flexible for the construction schedule, this research proves that timeline optimization is a more economical and safe approach than structural approaches like storm ropes.
•An framework to optimize the optimal deck erection timeline under complex wind climate attack.•Deck erection timeline plays a key role for flutter risk. 40 times difference on flutter risk exists between the best and worst timeline.•Construction timeline optimization is a more economical and safe approach to reduce flutter risk than structural approaches like storm ropes.
The nonlinear vortex-induced vibration of a rectangular section with an aspect ratio
B
/
D
= 6 is investigated in this study, aiming at explanation of the phenomenon of double lock-in ranges. First, ...numerical results by two-dimensional CFD simulation are compared with experimental results; then flow field characteristics, aerodynamic loadings and structural motion properties are presented and discussed; finally, an energy-trapping-based model for motion stability is brought forward, based on which the observed aeroelastic phenomena are discussed. The present study shows the motion-induced lock-in range is able to be explained qualitatively with the proposed principle describing the free-oscillation stability. Within the motion-induced lock-in range, the 1-DOF system can experience two motion components. The dominance of the free vibration fueled by feedback lift can dwarf or even eliminate the vortex-shedding-induced motion. Further, it is demonstrated that the phase angle between the lift and motion velocity, instead of the load amplitude, dominates the motion stability.
A representation and reconstruction model for the time-variant flow features of non-stationary flow system is proposed. Different from snapshot-based methods, flow-time-history (FTH) data are used to ...extract and learn the hidden non-stationary features via self-supervised deep learning strategy. First, the high-dimensional non-stationary flow system is reduced to a low-dimensional latent representation using the flow-time-history autoencoder (FTH-AE). Second, a mapping from flow physical space to the latent code space is established using multi-layer perceptron neural network. Finally, FTH data for positions that have not been measured are generated using the FTH generator model. The proposed method is validated using data from non-stationary flow around a cylinder at
Re
= 200. High-resolution results of the flow development process are simulated using sparse available FTH data. The results demonstrate that the proposed model is a new low-dimensional representation method for non-stationary flow systems. It is a self-supervised model that does not require paired or labeled datasets and is suitable for the abundant FTH data from simulations and experiments.
Flutter derivatives (FDs) of the bridge deck are basic aerodynamic parameters by which flutter analysis determines critical flutter velocity (CFV), and they are traditionally identified by sectional ...model wind tunnel tests or computational fluid dynamics (CFD) numerical simulation. Based on some wind tunnel testing results and numerical simulation data, the machine learning models for identifying FDs of closed-box girders are trained and developed via a gradient boosting decision tree in this study. The models can explore the underlying input–output transfer relationship of datasets and realize rapid intelligent identification of FDs without wind tunnel tests or numerical simulation. This method also provides a convenient and feasible option for expanding datasets of FDs, and the distribution of FDs can be analyzed through the post-interpretation of trained models. Combined with FD sensitivity analysis, the models can be verified by the calculation error of CFV. In addition, the proposed method can help determine the appropriate shape of the box girder cross-section in the preliminary design stage of long-span bridges and provide the necessary reference for aerodynamic shape optimization by modifying the local geometric features of the cross-section.
WRF simulation of surface wind in high latitudes Dong, Haotian; Cao, Shuyang; Takemi, Tetsuya ...
Journal of wind engineering and industrial aerodynamics,
August 2018, 2018-08-00, Letnik:
179
Journal Article
Recenzirano
Odprti dostop
High-resolution Weather Research and Forecasting (WRF) simulations of surface wind, pressure, temperature, humidity and snow depth in Arctic coastal regions are performed in this study. We compare ...simulation results for different elevation resolutions, surface roughness lengths, PBL schemes, initialization data and snow cover conditions with meteorological observations and validate WRF's usage in high latitudes. WRF performs well in simulations of high-latitude strong wind if the roughness length is carefully modified considering site's real vegetation type, though WRF produces insufficient wind fluctuations. Noah LSM is effective in updating surface roughness length according to snow depth. Higher elevation resolution increases surface pressure accuracy. MYNN3/YSU PBL scheme is recommended for wind speed/temperature simulation. In addition, the vertical structure of the Arctic atmospheric boundary layer is discussed, and a statically stable and turbulent surface layer is the Arctic winter is reported.
•WRF simulation is carried out to study the surface wind in high latitudes.•Roughness length modification according to snow cover is essential in WRF study of Arctic wind.•Combination of ASTER elevation, MYNN3/YSU PBL and modified roughness leads to best simulation results.
Shape optimization of single box girder allows the bridge to achieve the most favourable aerodynamic performance. Currently, aerodynamic optimization of bridge deck is usually achieved by traversing ...all potential schemes using wind tunnel tests or computational fluid dynamics (CFD), which are time-consuming, laborious, and restricted to few numbers of geometric parameter. This study proposed an aerodynamic optimization strategy using CFD technique, uniform design, Kriging surrogate model and hybrid infill sampling criteria to achieve the most favourable girder shape with the largest flutter wind speed. The effect of the variation of section shape on the dynamic characteristics of a real suspension bridge is considered. The Kriging surrogate model is then applied to approximately describe the aerodynamic parameters including static force coefficients and flutter derivatives as well as dynamic characteristics of the bridge. The cross-validation and samples infill are conducted to validate and improve the accuracy of the model. The optimal ranges, influence degree of each factor and their interactions on flutter performance are examined by statistical analysis. The contributions of aerodynamic parameters and dynamic characteristics to flutter performance are compared and discussed. The girder shape with the best flutter performance is obtained in updated surrogate model.
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