REFINE

The flow of air over a cylinder at sufficiently high Reynolds number ( Re ) leads to the formation of vortices. The vortices formed undergoes the process of shedding, exerting lift and drag forces onto the cylinder causing the cylinder to oscillate. These oscillations are termed as Vortex Induced Vibrations (VIV). VIV’s can induce very high fatigue loads onto the structure, leading to its failure. Hence, the suppression/mitigation of these VIV’s is of utmost importance in practical situations. Vortex suppression techniques such as active, passive or compound techniques can be used for the same. This paper focuses on providing a brief overview on the available passive techniques with particular interest in the flow modification, flow separation control devices and their implementation on a wind turbine tower. From this study, it was observed that most of the experimental research related to the testing of the flow modification devices were carried out in a flow channel with water as the flow medium. Therefore, their effectiveness on a wind turbine tower is questionable. However, flow separation control devices were tested in air but these devices have been tested at different Re and therefore their effectiveness cannot be comprehensively compared. The paper concludes by giving a brief overview of the work that will be carried out in the future to tackle this particular problem.

The importance of wind energy in Germany can hardly be overstated. According to the current German energy policy, it is the most important green energy source. Enormous financial investments are needed if the agreed upon reductions of 55 % of CO2 emission in comparison with 1990's emissions are to be met. Hence, the German government currently plans to install additional 4,500 MW of wind power capacity in 2021 alone and furthermore plans on adding 2,900 MW each year from 2022 onwards. With modern wind turbines, size increases an thus efficiency as well. This in turn results in reduced levelized cost of electricity, engineering challenges, however, arise. Many innovations in the last years have been achieved in blade design, power train design and wind farm management. Of great significance for the success of modern wind turbines is the design of the towers as they can make up 28 % of overall turbine cost. Towers are subject of a certain dichotomy: on one hand, they are produced in series. On the other hand, tower designs often need to be modified in order to account for site-and turbine specific requirements, hence driving up complexity and cost. A crucial factor for a tower's design are the vibrations a tower is subjected to over the span of its life cycle. During installation of wind turbines, for instance, oscillations induce large tower top motions, inhibiting-if not even preventing-the installation of blades under certain environmental conditions. During regular operations, the number of vibration cycles as well as the vibration magnitude determine the cumulative fatigue damage and thus the life span of a turbine. During maintenance operations, vibration magnitude needs to be bearable by maintenance personnel. The joint research project REFINE follows a three tier architecture to better understand wind turbine vibrations: 1. Based upon innovative IoT measurement technology, developed at University of Bremen, a large fleet of onshore wind turbines shall be monitored over an extensive amount of time. This measurement campaign will improve the understanding of the types of vibrations occurring throughout a turbines' live cycle. Rare events, such as vortex-induced vibrations, may be better captured, if a large fleet of turbines is monitored. 2. Utilizing the recorded conditions at the sensorized wind farm, high-fidelity computational fluid dynamics simulations coupled with structural models of the wind turbines, will allow detailed analysis of the how vibrations come to be. Simulations will be validated against the measured structural response of the turbines. Additionally, the deployment of aerodynamic devices to reduce aerodynamic loading of turbines shall be investigated. 3. Holistic economic simulations will complement the project with detailed estimates of possible cost reductions due to e.g. the deployment of aerodynamic devices. The consortium consists of Nordex, a leading OEM of onshore wind turbines and the Institute for integrated product development at University of Bremen.