Numerical verification of a new load alleviation technique for wind turbines in atmospheric turbulence
Final Report Abstract
In the present project, 2D and 3D CFD simulations of active and passive load alleviation systems for wind turbines were conducted. Those systems are designed to reduce load fluctuations, which occur due to variations of the inflow conditions (e.g. tower blockage, yawed inflow…) The results of the simulations, which were performed with the block-structured flow solver FLOWer, were compared to experimental results and results obtained from the lifting line free vortex wake code QBlade. Concerning the active trailing edge flap, the deflection was determined by QBlade simulations and prescribed in the CFD simulations. The passive system consists of a mechanically coupled leading and trailing edge flap (adaptive camber airfoil, ACA). Higher aerodynamic forces lead to a decambering of the airfoil, lower forces increase the camber of the airfoil. Consequently, the flap deflection counteracts the variation of the aerodynamic forces. bIn the CFD simulations, the deflection of the passive system was determined through fluid-structure interaction by coupling the flow solver to a multi body simulation (MBS) tool. To validate the setup, comparisons to experiments were conducted. Thereby, the complexity of the configuration and of the inflow conditions was increased step by step. First, a steady polar was simulated for the rigid airfoil and the airfoil equipped with the ACA, and the deflection angles showed a good accordance between simulation and experiment. A steady load reduction of ≈ 36 % - 37 % could be achieved in the considered (linear) part of the polar in the FLOWer simulation. Afterwards, the system was investigated under sinusoidal variations of the inflow angle. Although the measured and simulated flow fields around the airfoil were in good accordance, the forces showed deviations to the experiments. After comprehensive investigations, it was assumed that the wind tunnel model of the ACA might not be stiff enough, leading to impairment on the force measurement. Nevertheless, a load reduction of 15% could be achieved in the FSI simulation after including a damper to reproduce the friction of the real system. Due to unsteady effects, the steady load reduction could not be achieved. The spring system in the measurement was not optimized for the present case, leading to the assumption that the effectiveness of the system can be further improved. In the next step, the complexity was increased by investigating a 3D setup. In order to validate the fluidstructure coupling of the flaps, the Berlin Research Turbine (BeRT) was simulated. As the experimental setup is unusual (high blockage ratio, relative big tower, nozzle downstream of the turbine), the influence of wind tunnel walls, tower and nozzle on the aerodynamic loads was first investigated first. Afterwards, flow fields, on-blade velocity and AoA distributions as well as bending moments were compared between experiment, QBlade and FLOWer for the setup with rigid blades. The accordance for the on-blade velocity and AoA distributions was good, and the differences between the flow fields and bending moments could be explained. Afterwards, the active load alleviation system was investigated under yaw misalignment and a reduction of the global amplitude of the flapwise bending moment of 36% could be achieved in the FLOWer simulation under -30° yaw misalignment. The passive system was investigated under yaw misalignment, too, as well as under the influence of a local gust. In both cases, load alleviation could be attained (e.g. global load reduction of the flapwise bending moment of 17% under -15° yaw misalignment). The complexity was increased once again by simulating a full size turbine with ACA and flexible blades under extreme wind shear conditions. The turbine under investigation was the DTU 10 MW Reference Wind Turbine (RWT). With its large rotor diameter and slender blades, a simulation with stiff blades would have been unrealistic. Therefore, the blade deformation as well as the flap deflection were calculated through fluidstructure interaction. A structural model with a combination of flexible blades and coupled leading and trailing edge flaps was generated. A load reduction could be achieved (reduction of the global amplitude of the flapwise blade root bending moment of 11%). It has, however, to be kept in mind that the full potential of the system could not be realized as the ACA parameters were not specifically optimized for this particular turbine. To sum up, load reduction could be achieved for both the passive and active load alleviation systems. The CFD solver was successfully coupled to a multi body simulation tool. The time-accurate fluid-structure coupling could not only be used to determine the flaps deflections of the passive load alleviation system, but also to combine it with flexible blades. The airfoil study has shown a strong load reduction with appropriate ACA parameters, while the study of a multi-MW wind turbine equipped with ACA presented a smaller load reduction. It shows that, in order to achieve the full potential of the passive system, an optimization of the ACA parameters for each turbine and inflow conditions is necessary. This system should be further investigated for wind turbines in complex terrain, where strong load fluctuations occur due to the high turbulence intensity and the local flow inclination, leading to unequally balanced loads over the rotor area.
Publications
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"Numerical and experimental investigation of an airfoil with load control in the wake of an active grid." Journal of Physics: Conference Series. Vol. 753. No. 2. IOP Publishing, 2016
Fischer, A., Lutz T., Krämer E., Cordes, U., Hufnagel K., Tropea C., Kampers G., Hölling, M., Peinke, J.
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"Simulations of unsteady aerodynamic effects on innovative wind turbine concepts" in High Performance Computing in Science and Engineering´ 16, pages 529–543. Springer, 2016
Fischer, A., Klein, L., Lutz, T., and Krämer, E.
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"About the Influence of Wind Tunnel Walls, Tower and Nozzle on the Performance of a Model Wind Turbine" High Performance Computing in Science and Engineering’17, pp. 339–353, Springer, 2018
Klein, A., Zabel, S., Lutz, T., and Krämer, E.
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"About the suitability of different numerical methods to reproduce model wind turbine measurements in a wind tunnel with high blockage ratio." Wind Energ. Sci., 3, 439-460
Klein, A. C., Bartholomay, S., Martin, D., Lutz, T., Pechlivanoglou, G., Nayeri, C. N., Paschereit, C. O., Krämer, E.
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"Numerical and Experimental Investigation of Trailing Edge Flap Performance on a Model Wind Turbine." AIAA 2018-1246 Wind Energy Symposium. 2018
Marten, D., Bartholomay, S., Pechlivanoglou, G., Nayeri, C., Paschereit, C. O., Fischer, A., Lutz, T.
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Numerical Investigation of a Model Wind Turbine" New Results in Numerical and Experimental Fluid Mechanics XI, pp. 717–727, 2018
Fischer, A., Flamm, A., Jost, E., Lutz, T., and Krämer, E.
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“Cross-Talk Compensation for Blade Root Flapand Edgewise Moments on an Experimental Research Wind Turbine and Comparison to Numerical Results” ASME. Turbo Expo: Power for Land, Sea and Air, Volume 9: Oil and Gas Applications; Supercritical CO2 Power Cycles; Wind Energy ():V009T48A016
Batholomay, S., Marten, D., Sánchez Martínez, M., Alber, J., Pechlivanoglou, G., Nayeri, C. N., Paschereit, C. O., Klein, A., C., Lutz, T., Krämer, E.
