Simulation of Moving Trailing Edge Flaps on a Wind Turbine Blade using a Navier-Stokes based Immersed Boundary Method

Tim Behrens

Research output: Book/ReportPh.D. thesisResearch

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As the rotor diameter of wind turbines increases, turbine blades with distributed aerodynamic control surfaces promise significant load reductions. Therefore, they are coming into focus in relation to research in academia and industry. Trailing edge flaps are of particular interest in terms of control surfaces. The unsteady flow around such flaps is usually investigated by applying linearized unsteady aerodynamic models or by solving the two dimensional unsteady Reynolds averaged Navier-Stokes equations. The latter method is usually applied in combination with moving or interpolated body conforming meshes. A more flexible method would open up an opportunity to investigate the flow features of complex moving flap geometries in great detail. The immersed boundary method offers this flexibility, as the geometry is represented through the introduction of additional forcing terms in the governing equations. This approach allows for simulation of arbitrary geometries in fixed meshes that do not need to conform to the body geometry. The flow solver EllipSys has previously been extended with a base implementation of an immersed boundary method. The present work developed the necessary tools to handle trailing edge flap geometries in two and three dimensions. Validation cases were presented for the circular cylinder in a Cartesian mesh topology as well as in a topology similar to a standard body fitted mesh. To simulate trailing edge flaps, a hybrid approach was developed that modeled only the moving flap as an immersed boundary, while the rest of the airfoil was represented by a conventional body-fitted mesh. The results from the hybrid approach were validated against published wind tunnel measurements and improvement over a thin-airfoil based flow model was proven. A load alleviation control in a changing inflow was presented for a divided flap action, i.e. a segmented flap with independent actuation rates. It has been demonstrated that the total flap deflection can be divided into two separate deflections without deteriorating control authority. The results suggested that the combined use of two independent flap actuators was beneficial when dealing with complex inflows. Full scale turbine measurements were presented and indicated that the flap hinge moment provided suitable input for load control. A novel way of using the hinge moment of a moving flap for load alleviation control was presented. Simulations demonstrated the feasibility and robustness of the approach. The hybrid immersed boundary approach proved to be able to handle 3D airfoil sections with span-wise flap gaps. The flow around and in the wake of a deflected flap at a Reynolds number of 1.63mio was investigated for steady inflow conditions. A control for two span-wise independent flaps was implemented and first load reductions could be achieved. The hybrid method has demonstrated to be a versatile tool in the research of moving trailing edge flaps. The results shall serve as the basis for future investigations of the unsteady flow field around trailing edge flaps.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherDTU Mechanical Engineering
Number of pages131
Publication statusPublished - Nov 2011

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