In most of free vortex wake models (FVWMs), the induced velocity is computed by Biot-Savart law. But the details of velocity calculation are still incomplete in their self-integrated loss of adjacent segment's influence. Curved filament correction has already been studied to recover the FVWM in helicopter problems. In this work, an extended FVWM with the correction is developed intended to improve aerodynamic predictions of wind turbines. Numerical simulations are performed on ring vortices and practical modeling of flow state of both fixed and floating wind turbines. It has been shown that the newly-designed technique may generate higher fidelity.
Among multiple modeling methods in aerodynamics of wind turbines, vortex lattice method (VLM) with straight line segmentation have been commonly used. The trailing filaments generated by the blades are assumed to convect freely with material lines of concentrated vorticity in potential flow. Such force free motion is governed by the vortex transportation equation. The governing equation is a partial differential equation which can be solved by various numerical approximation with high-order accuracy in both time and space domain.
It has been studied that for the straight-line segmentation, the approximation of induced velocity is relatively accurate with respect to corresponding theoretical result with the exclusion of self-induced velocity. It means that the collocation points lie in nowhere in vicinity to the discrete vortex segments (Gupta and Leishman, 2005). When it comes to the case that collocation points are extremely close to the discrete segments, the self-induced velocities tend to be infinite. The solutions for this singularity can be eliminated by “cutoff’ process (Bhagwat and Leishman, 2001) and vortex core models (Leishman,2006). These solutions are initially introduced by core regularization to eliminate singularity of the collocation points or simply fulfill the physical mechanism. However, techniques with these processes are incomplete because they fail to add up the total induced velocity.
The aerodynamic and hydrodynamic performance of floating offshore wind turbines interact more than that of traditional fixed ones under the influence of structure oscillation and unsteady environments. The details of the aerodynamic performance still remain to be discussed and are challenging to predict accurately. Here included in this paper, the aerodynamic performances of floating offshore wind turbines affected by the hydrodynamic terms in simulation are studied and discussed for more accurate simulation and prediction results. The quasi-steady BEM theory is chosen as the key theory to discuss the effect on the aerodynamic performance of floating offshore wind turbines by their hydrodynamic terms. A series of different formulas for characterizing the calculation of local velocity in the axial and tangential direction are tested to summarize the inherent law of quasi-steady BEM theory. The formulas derived from low-frequency motion seem more reasonable than the classic ones in the simulation based on the quasi-steady assumption.
Wind energy has been occupying an important position of renewable energy around the world. Meanwhile, offshore wind energy was experiencing sustainable growth with an average annual increasing rate of 30% during the last five years. Now, most offshore wind turbines have been installed in shallow waters (less than 50 m). Compared with shallow waters, deep waters may supply much more sites for wind turbines and greater resource with stronger and more consistent wind, also with less turbulence intensity (Jonkman, 2007). However, fixed offshore wind turbines applied in shallow waters may not be suitable for deep waters out of economic consideration. Inspired by the development progression from fixed substructures to floating ones in offshore petroleum industry, floating offshore wind turbines were developed and expected as the future of offshore wind energy.
Although floating offshore wind turbines are evolved from fixed offshore wind turbines, their dynamic performance is quite different from that of fixed offshore wind turbines. In addition to the structure oscillation of tower and blades, also found in fixed offshore wind turbines, floating offshore wind turbines always run with irregular motion of floating substructures attributing to wind, waves and currents. As a result, the aerodynamic and hydrodynamic performance of floating offshore wind turbines interact more than that of fixed offshore wind turbines. On one hand, the motion of a floating substructure induced by waves and currents will change the relative wind velocity on the rotor which affects its aerodynamic performance. On the other hand, the thrust and torque from a rotor induced by wind will influence the motion of the floating substructure which affects its hydrodynamic performance. The interaction brings challenges to the study of floating offshore wind turbines, especially in simulation and prediction of their aerodynamic performance.
In this paper, a new controller is designed for OFWTs on the basis of the LPV control with the grid technique. It may cover both Region 2 and Region 3 instead of switching between the generator-torque controller and the blade-pitch controller. In addition, it may ensure the performance and stability at any point in the control region. Also, the additional motions induced by wind and waves are considered. Finally, the new controller is tested by a simulator in MATLAB/Simulink, and the model combining "NREL offshore 5-MW baseline wind turbine" with "OC3-Hywind platform" is chosen as an example for simulation.
In the paper, a coupled aero-hydrodynamic simulation tool in MATLAB/Simulink is developed for simulating the response and performance of offshore floating wind turbines (OFWTs) under wind and waves in the time domain. For aerodynamics, an unsteady blade element momentum (BEM) model and the free vortex wake method (FVM) are chosen to calculate the loads of the wind turbine. For hydrodynamics, the code uses the linearized classic marine hydrodynamic model with Morison’s equation to compute the loads of the platform. In addition, a variable speed controller is designed on the basis of the linear parameter-varying (LPV) technique for the wind between the cut-in and rated velocities. Finally, the simulation tool with the controller is tested by cases with different wind and waves.