MS24.6 - Wind Induced Vibrations of Slender Structures

Wind energy plays an increasingly important role in promoting renewable energy development. Therefore, much focus has been naturally given to wind turbines under working conditions. Meanwhile, a standstill condition is another critical state of a wind turbine. When the turbine is at standstill, the blade is often pitched by a large angle to cut out the wind. In this situation, vortices could form along the blade, creating strong vibrations considering the increasing size of wind turbines. This vibration is called vortex-induced vibration (VIV). VIV has a high potential to increase the fatigue load of the wind turbine blade. Meanwhile, the high wind sometimes discourages the installation of wind turbine blades, and workers have to wait until wind speed drops to a certain limit to guarantee a safe rotor installation. Therefore, more labor cost, time cost, rental cost, and in all, Levelized Energy Cost (LEC) increase. Concerning the load and cost problem induced by VIV in the industry, it is essential to understand the unsteady aerodynamics of a wind turbine blade at different inclination angles, especially under large angles of attack (AOA). This research aims to experimentally study the unsteady aerodynamic characteristics of vortex shedding of flat plates. The campaign was carried out in the W Tunnel at TU delft. Three flat plate models were built with the same length of 50 cm and the same thickness of 3mm. They have different chord lengths of 3 cm, 5 cm, and 10 cm, which brings three different aspect ratios of 16.7, 10, and 5, respectively. The ones with aspect ratios of 5 and 16.7 were made of aluminum and the one with the aspect ratio of 10 was made of steel for higher stiffness. In the first step, a force balance was mounted under the models to measure the streamwise and crossflow load at a wind speed of 6.4 m/s. Seven AOAs (0°, 15°, 30°, 45°, 60°, 75° and, 90°) under five inclination angles (30°, 60°, 90°, 120° and 150°) were tested. In the second step, stereoscope PIV was set up to measure the three component velocity field in different spanwise positions of the steel flat plate with the aspect ratio of 10. The main outcomes of this research will be: 1) The influence of inclination angle on the forces and velocity field. 2) The influence of aspect ratio on the forces and velocity field. 3) The difference between tip vortex and leading/trailing edge vortex in terms of structure, influence on the force, etc. 4) The 3D tip effect at different inclination angles and AOAs. These outcomes will provide a fundamental understanding of the unsteady aerodynamics of the flat wing. In addition, the understanding of unsteady vortex shedding paves the way for further research on vortex-induced vibrations on wind turbine blades.

A wind engineering database specifically for bridges has been built by using Access Database software and Java programming language based on the previous wind tunnel test results of long span bridges. The connection between the underlying database and the foreground application is driven by a local protocol to achieve platform independence and high execution efficiency. All the data can be summarized into three modules: basic information, dynamic characteristics, and aerodynamic parameters. The machine learning models for identifying flutter derivatives of closed box girders are trained and developed via gradient boosting decision tree based on this database. 20 sets of wind tunnel test data from this database are used for the machine learning modeling, but it is difficult to obtain a good training effect for flutter derivatives under 20 sets of data because the potential distribution characteristics of flutter derivatives are not very clear and the wind tunnel test data usually fluctuates greatly. Therefore, another data pattern was used: the flutter derivatives of these 20 sets of cross sections were re calculated by CFD numerical simulation, combined with the additional 35 sets of numerical simulation data as mixed datasets to jointly drive the training process of machine learning. The machine learning models can explore the underlying distribution of dataset. The trained models have good fitting and generalization ability under the current data conditions. In this way, the present research work can make the identification of flutter derivatives separated from tedious wind tunnel tests and complex numerical simulations to some extent. It can also provide a convenient and feasible option for expanding data sets of aerodynamic parameters. In addition, it 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 the aerodynamic shape optimization by modifying local geometric features of the cross section to evaluate the influence of the aerodynamic shape on flutter performance. Before that, it was analyzed which flutter derivatives have the major effect on the flutter critical wind velocity so that the flutter performance analysis process can be simplified. Then the relationship between the shape of cross section and the flutter critical wind velocity can be analyzed. For closed box girder, there are not many factors affecting the aerodynamic shape without considering the influence of the auxiliary facilities on the flutter critical wind velocity in the construction stage. It is time consuming and may not lead to better calculation results if every detail of closed box girder is taken into account. Therefore, this study only discusses three important parameters: width to height ratio, wind fairing angle and inclined web slope. The calculation results show that flutter critical wind velocity decreases with the increase of width to height ratio. It first increases and then decreases with the change of wind fairing angle, and it decreases with the increase of inclined web slope, which is almost linear.

Load-bearing cables in bridges frequently have problems with excessive vortex-induced vibrations, requiring intervention by additional damping devices to reduce vibration amplitudes. This contribution presents results from monitoring of acceleration responses in hangers on the Hålogaland suspension bridge, a long-span bridge in northern Norway. Since its construction, this bridge has had problems with hanger vibrations believed to be problematic for long-term service life. The hangers have later been equipped with Stockbridge dampers, consisting of two rigid masses suspended from two steel-coil wires. Surprisingly, several of the dampers have been damaged during their first year for reasons largely unknown. This further strengthens the need to verify the combined dynamic behavior of the hangers and the dampers. This contribution represents a first characterization of the monitoring results. The hangers considered are between 120 and 100 m long. Data from two configurations are presented: hangers with and without dampers. Thus, the effectiveness of vibration mitigation is quantified. It is shown that the dampers reduce the vibration amplitudes by a factor of 2 to 10 – depending on the wind conditions and the dominant frequency range of vibration. Secondly, an analysis using measured wind velocities is carried out. By using spectrograms of the accelerations of hangers without dampers, it is clearly shown that the time-varying vibration frequencies strongly correlate with the mean wind speed. This is further evidence that the vibrations can be characterized by a shedding frequency (fs) commonly given by the relation fs=St*V/D, where St=0.18 is the Strouhal number, D is the diameter, and V is the wind speed. It is also clear that the vibrations distinctly shift from one cable vibration mode to the next concurrently with variations in the mean wind speed. Still, there are also cases where additional higher-order modes are excited, occurring at twice the shedding frequency. Lastly, it is presented some future work further characterizes the in-situ behavior of the damper: accelerometers have been attached on the Stockbridge damper masses. To the best of the authors' knowledge, none or few existing studies of dampers have combined data from wind, cable vibrations, and damper mass vibrations. The work is expected to reach recommendations for the effective and durable use of Stockbridge dampers on cables.

Aerodynamic derivatives are widely used to model the self-excited forces on bridges and for estimating the corresponding flutter stability. It is currently unclear to which degree the aerodynamic derivatives of a cross section depend on the presence of turbulence in a flow, however it is believed to depend on the cross section. Wind tunnel experiments were conducted with forced vibration testing to investigate whether aerodynamic derivatives for a twin deck cross section changed with increasing turbulence and whether longer measurement periods would yield less uncertainty in the data. Numerical simulations were run to investigate the uncertainty of aerodynamic derivatives related to turbulence and length of measurement period. It was shown the uncertainty of the aerodynamic derivatives decrease for longer measurement periods and less turbulence.