Aerodynamic loads on a wind turbine rotor in axial motion
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Original versionAerodynamic loads on a wind turbine rotor in axial motion by Lene Eliassen, Stavanger : University of Stavanger, 2015 (PhD thesis UiS, no. 245)
This study investigates the unsteady aerodynamics of attached flow on a two-dimensional airfoil. The unsteady aerodynamics introduces aerodynamic damping of the offshore wind turbine structure and is thus important for the turbine structural integrity. This includes an impact on the fatigue damage of the structure and, consequently, an effect on the total cost of energy. Unsteady aerodynamics can be studied using a variety of methods. In this thesis, a panel vortex method was developed to estimate the aerodynamic forces. This method is based on potential theory, which can’t account for the viscosity in the fluid. Consequently, dynamic stall, which is an important unsteady aerodynamic effect, can not be modeled, and we are limited to attached flow conditions. Despite this limitation, the vortex method is in some situation the preferred option when investigating unsteady aerodynamics. The vortex method has the advantage of considering the wake history in the estimation of the aerodynamic forces. Using the panel vortex method developed in this study, one is not dependent on look-up tables since the aerodynamic loads are calculated by direct modelling of flow conditions on an airfoil of a given geometry. However, the computational time of the vortex method is long and is therefore often not used. There is a possibility to reduce the computational time of the vortex method. By using a graphic processing unit, it is demonstrated how the computational time can be reduced for a two-dimensional panel vortex code. A significant reduction in computational time can be achieved for the simulation, depending on the number of vortex elements in the analysis. For a low amount of vortex elements, the computation is faster on a central processing unit, CPU. The panel vortex method is used to investigate the motion induced aerodynamic loads on an offshore wind turbine. Studying the flow conditions on an airfoil oscillating in plunge motion at frequencies similar to the eigenfrequencies for a floating spar type wind turbine, the aerodynamic damping for eigenmodes represented is estimated. Including the neighbouring airfoils and their wakes in the analysis has a relatively large effect on the estimated aerodynamic damping. The aerodynamic damping is reduced when the period of the oscillating airfoil is equal to the time it takes for one airfoil to travel from its original position to the neighbouring airfoil’s original position. One example where this can occur is if the eigenfrequency of the tower is equal to the blade passing frequency. This effect has previously been studied by other researchers, but mostly for helicopter rotors. The change in the wind-structure interaction effects is studied with regards to the fatigue damage of the tower using a single degree of freedom model. Comparing the fatigue damage results using different computational methods to estimate the aerodynamic forces can be useful when evaluating the effect of the aerodynamic model chosen on the cost. This study only focuses on one unsteady plunging motion, and is therefore limited. It is found that the unsteady aerodynamic models that are most commonly used may overestimate the damping, and thus estimate a too low fatigue damage. This will have a negative impact on the cost if the wind turbine fails.
PhD thesis in Offshore technology