The present investigation used numerical simulations to study the vortex induced vibrations (VIVs) of a 96 m long wind turbine blade. The results of this baseline shape were compared with four additional geometry variants featuring different tip extensions. The geometry of the tip extensions was generated through the variation of two design parameters: the dihedral angle bending the blade out of the rotor plane and the sweep angle bending the blade in the rotor plane. The applied numerical methods relied on a fluid structure interaction (FSI) approach, coupling a computational fluid dynamics solver with a multi-body structural solver. The methodology followed for locating VIV regions was based on the variation of the inclination angle. This variable was defined as the angle between the freestream velocity and the blade axis, being 0 ○ when these vectors were normal and positive when a velocity component from tip to root was introduced. For the baseline geometry, the FSI simulations predicted significant blade vibrations for inclination angles between 47.5 ○ and 60 ○ with a maximum peak-to-peak amplitude of 2.3 m. The installation of the different tip extensions on the blade geometry was found to significantly modify the inclination angles where VIV was observed. In particular, the simulations of three of the tip designs showed a shifting of several degrees for the point where the maximum vibrations were recorded. For the specific tip geometry where only the sweep angle was taken into account, a total mitigation of the VIV was observed.

Floating wind turbines have the potential to enable global exploitation of offshore wind energy, but there is a need to further understand the complex aerodynamic phenomena they can encounter due to floater induced rotor motion. Aerodynamic models traditionally used in the wind energy sector, like the Blade Element Momentum (BEM) theory, may not be capable of capturing the dynamic phenomena that occur when the rotor moves in and out of its own wake. In the present paper, we therefore compare an industry standard BEM‐based code to a state of the art vortex solver, to investigate the phenomena in detail and further clarify the capabilities and limitations of both methods. An initial benchmark of the two codes using the IEA Wind 15 MW RWT mounted on the WindCrete spar‐buoy floater is carried out. Three different scenarios are taken into account: a bottom‐fixed, a floating case, and a floating case subject to regular waves. Growing discrepancies between the codes have been observed with the increasing complexity of the simulations. Moreover, large differences between the wake generated by a bottom fixed and a floating turbine have been observed, with the latter one experiencing a faster recovery. To further explore the floating turbines behaviors that can affect the rotor performance and wake, a systematic investigation of the mean tilt angle influence in the wake development has been carried out. Further, to account for the oscillatory motion of a floating turbine, a parametric study where the floater motion is prescribed in both pitch and surge degrees of freedom (DoF) is designed. The study covers a large variety of scenarios; a wide range of relevant frequencies and amplitudes are taken into account in under and above‐rated wind conditions. A total of more than 28 unique cases have been defined and simulated with both fidelity models. The results include the downstream evolution of the wake recovery and intensity of the turbulent disturbances induced by the rotor. The generic nature of the study allows to characterize the flow and performance effects and enables subsequent generalization to floater designs of given natural frequency and motion amplitude. It has been found that the BEM and LL predictions of the maximum loading in the blade root and tower bottom compare quite well, except for the case of large oscillation frequency in above rated conditions, where the BEM method under‐predicts the loads. Moreover, the use of a vortex solver makes it possible to look in depth into the wake characteristics, in which large differences are observed between the bottom‐fixed and the floating case in non‐turbulent inflow conditions. It has been found that the frequency and amplitude of the turbine oscillations can have a strong impact on the recovery of the wake. Moreover, we have found a link between short time intervals of large FA blade tip displacements and a faster break down of the wake. Finally it has been shown that a floating wind turbine with large and fast oscillations can transition between the differ...

Abstract. In the present work, a computationally efficient engineering model for the aerodynamic load calculation of non-planar wind turbine rotors is proposed. The method is based on the vortex cylinder model, and can be used in two ways: either as a correction to the currently widely used blade element momentum (BEM) method, or used as the main model, replacing the BEM method in the engineering modelling complex. The proposed method needs the same order of computational effort as the ordinary BEM method, which makes it ideal for time-domain aero-servo-elastic simulations. The results from the proposed method are compared with results from two higher-fidelity aerodynamic models: a lifting-line method and a Navier-Stokes solver. For planar rotors, the aerodynamic loads are identical to the current BEM model when the drag force is excluded during the calculation of the induced velocities. For non-planar rotors, the influence of the blade out-of-plane shape, measured by the difference of the load between the non-planar rotor and the planar rotor, is in very good agreement with higher-fidelity models. Meanwhile, the existing BEM methods, even with a correction of radial induction included, show relatively large deviations from the higher-fidelity method results.

The manuscript presents a novel aero‐hydro‐servo‐elastic coupling framework, MIRAS‐HAWC2. In this coupling, the wind turbine blades and rotor‐wake aerodynamics are modeled using a modified lifting‐line theory which accounts for blade curvature, combined with a hybrid vortex method. The wind turbine structure and foundation are modeled using a finite‐element and multi‐body system approach. Last, hydrodynamics are modeled using Airy wave theory together with Morison's equation. An initial assessment of the performance of the aeroelastic coupling framework has been performed for steady rotor‐only cases, assuming laminar inflow without shear. This included a comparison against fully resolved computational fluid dynamics, for both stiff and flexible blades showing an excellent agreement. In a second stage, the aero‐hydro‐servo‐elastic coupling is used, comparing MIRAS‐HAWC2 as well as blade‐element momentum‐based simulations with selected results from the Offshore Code Comparison Collaboration projects (OC3 and OC4), which study the NREL 5 MW turbine mounted on different offshore support structures. A good agreement has been obtained for the simulations of a monopile with rigid foundation, a tripod, and jacket support structures.

Abstract. Advanced aeroelastically optimized tip extensions are among rotor innovation concepts which could contribute to the higher performance and lower cost of wind turbines. A novel design optimization framework for wind turbine blade tip extensions based on surrogate aeroelastic modeling is presented. An academic wind turbine is modeled in an aeroelastic code equipped with a near-wake aerodynamic module, and tip extensions with complex shapes are parametrized using 11 design variables. The design space is explored via full aeroelastic simulations in extreme turbulence, and a surrogate model is fitted to the data. Direct optimization is performed based on the surrogate model seeking to maximize the power of the retrofitted turbine within the ultimate load constraints. The presented optimized design achieves a load-neutral gain of up to 6 % in annual energy production. Its performance is further evaluated in detail by means of the near-wake model used for the generation of the surrogate model and compared with a higher-fidelity aerodynamic module comprising a hybrid filament-particle-mesh vortex method with a lifting-line implementation. A good agreement between the solvers is obtained at low turbulence levels, while differences in predicted power and flapwise blade root bending moment grow with increasing turbulence intensity.