Table 2: Aeroelastic Simulations – Effect of Pre-sweeping on Power Production at 8 m/s Mean Wind Speed as Percentage of the Power Production of the Unswept Blade
a = 1 BEM Free-wake -1.85 BEM = blade element momentum.
induction factors must be corrected depending on the rotor loading (Glauert correction)12 blade.13
and the position from the tip and root of the
Finally, the geometry of the blade is described by only the chord and twist distributions while the form of the aerofoil shape itself is accounted for through the 2D aerodynamic data.
Next in the list of approximate flow solvers are the lifting line, the lifting surface and the full potential models.14,15
They all share
similar modelling of the wake which is based on vortex theory. The wake is considered as a freely moving and therefore deformable vortex sheet. The intensity of the vorticity released in the wake as well as the flow perturbation caused by the presence of moving blades in the flow field are determined by the blades. To this end, the no-penetration boundary condition on the blade surface and the Kutta condition along the trailing edge are used. Vortex-type models overcome most of the drawbacks BEM models have. The flow simulation includes the interaction between the blade sections as well as the blade–blade and blade–wake interactions; the flow in the wake is considered in detail; and the details of the blade geometry can be accounted for. However, the cost of vortex flow is significantly higher than that of BEM models which explains why they remain, to a large extent, research tools. User time can be drastically reduced if the currently available multiprocessor computer platforms are used and the software is appropriately programmed.
Once the overall flow information is obtained, the next step is to determine the aerodynamic loads. Due to blade rotation, the sectional aerodynamic properties deviate from the pure 2D situation. Scaling analysis and CFD have confirmed that close to the root there is lift augmentation close to and beyond the 2D stall angle. This augmentation can be approximated by means of the chord:radial position ratio (c/r) and expressions for this correction can be found in Chaviaropoulos et al.16
Finally, the effect of dynamic inflow is added.
To this end, several semi-empirical models have been formulated.17,18 The principle of these models is to define dynamic equations for the lift drag and moment coefficients that can simulate the hysteresis due to the time variation of the relative inflow. The coefficients of these equations are calibrated using wind tunnel testing but also contain the motion characteristics of the section.
Results and Discussion
In the above sections, the multi-component approach is first described as the general context in which the complete wind turbine dynamic behaviour can be formulated. The various existing structural and aerodynamic models are then briefly outlined. Among them, the model that combines first order beam theory and BEM aerodynamic modelling is considered as the baseline model. By comparing results from the baseline model against more detailed models, their differences can be assessed.
MODERN ENERGY REVIEW – VOLUME 3 ISSUE 2
-0.47 -3.05
a = 3
-1.60 -2.76 -4.90 -7.27 -11.60 -17.61
a = 4
b = 2 b = 4 b = 2 b = 4 b = 2 b = 4 -0.28
-8.17 -25.77
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