Aerodynamic-Structural Wind Turbine Blade Optimization
5.10 Discussion of the Optimization Results
The main geometric features and aerodynamic loads of the original blade and of the (second) optimized one are presented in Tables 5.9 and 5.10.
rrr[m][m][m] ccc[m][m][m] θθθ[[[◦◦◦]]] ααα [[[◦◦◦]]] F [N]FF [N][N]
0.279 0.4570 8.10◦ - 0 0.775 0.5303 8.10 39.60 19 0.889 0.5462 7.60 36.90 11 1.092 0.5745 6.72 33.86 43 1.702 0.6568 6.24 26.10 38 2.311 0.7173 5.94 19.66 135 3.124 0.7400 5.53 13.39 236 3.937 0.6920 4.86 9.39 301 4.750 0.6258 4.10 8.69 376 5.563 0.5572 3.28 7.55 435 6.603 0.4780 2.37 6.67 366 7.059 0.4410 1.95 5.56 224 7.490 0.4060 1.54 - 109
Table 5.9: Geometric features and load conditions of the original AOC 15/50 blade; no angle of attack is computed for the root section (being characterized by a junction between an airfoil and an oval section) and for the tip one (due to Prandtl's tip loss factor, see [41])
Figure 5.15 shows the evolution of the chord distribution for both the original model and the (second) optimized one: chord values are increased along the whole blade span, up to the maximum value allowed from the constraint imposed on the blade surface. As a consequence, the blade results to be more loaded in almost every section (see Tables 5.9 and 5.10 and Figure 5.18): the global amount of aerodynamic forces is 2293 N in the original blade and increases to 2418 N in the (second) optimized one. Moreover, as shown in Figure 5.16, the generalized increment in the twist angle θ determines a corresponding decrease in the angle of attack α for all the considered sections (the trend is shown in Figure 5.17). In the second part of the optimized blade, the values of α result to be closer to the angle that determines the highest aerodynamic e ciency of the adopted airfoils (6◦ for the S812 and
5.10 Discussion of the Optimization Results
Table 5.10: Geometric features and load conditions of the (second) optimized AOC 15/50 blade;
no angle of attack is computed for the root section (being characterized by a junction between an airfoil and an oval section) and for the tip one (due to Prandtl's tip loss factor, see [41])
5◦ for the S813). The twist distribution of AOC 15/50 reveals to be not optimized for the nominal wind velocity. The blade root has small in uence on aerodynamic performance as shown in Figure 5.18, however the non-optimized twist of the baseline AOC 15/50 probably causes detachments of the boundary layer and a diuse stall in the sections near the root. Adopting a higher twist in the root zone helps to reduce these issues, however it does not greatly aect the overall performances, due to the small in uence the root zone on the power production.
Figure 5.15: Comparison between the chord distributions for both the original AOC 15/50 blade and the (second) optimized one
The composite skin layout of the (second) optimized blade is changed by the genetic algorithm.
The +45◦lamina (gene A), that covered the entire blade span in the baseline con guration, is removed and replaced by the duplication of the more resistant 0◦ laminas (Genes F, G, I and L). A great
Figure 5.16: Comparison between the twist distributions for both the original AOC 15/50 blade and the (second) optimized one
Figure 5.17: Comparison between the angle of attack distributions for both the original AOC 15/50 blade and the (second) optimized one
Figure 5.18: Comparison between the aerodynamic force distributions evaluated in 12 blade sections for both the original AOC 15/50 blade and the (second) optimized one
improvement in the structural characteristics is obtained, passing from a 219.46 mm deformation of the original blade to the 148.90 mm of the (second) optimized one (-32.15%).
In order to better understand the in uence of blade geometry on its structural behaviour, a further investigation is hereby proposed: the (second) optimized blade is analysed using the original Sandia
5.10 Discussion of the Optimization Results
layout. A total deformation of 187.99 mm is registered (-14.34%), con rming that a structural im-provement can also be achieved by means of a proper twist distribution. It is just the case of reminding that the solution of extending the twisted portion of the blade up to its root can be nd in all com-mercial Enercon models [78]: as is clearly proved in this work, such architecture, besides increasing the aerodynamic e ciency of the blade portion close to the nacelle, presents also a not negligible structural bene t.
The present ndings prove that the registered enhancement in the (second) optimized blade are to be ascribed to two contributions:
a blade stiening due to the higher values of both chord and twist angles along the blade span, responsible for a 14.34% reduction in the blade deformation;
a more e cient layer distribution, responsible for a 17.81% reduction in the blade deformation.
Figure 5.19 shows the evolution of the exural rigidity EI along the blade span, computed as the ratio between the moment M acting on a given cross section and the rate of rotation dΘ/dz of the section itself (see [54] [66]) for the original AOC 15/50 blade, the (second) optimized one and the (second) optimized geometry coupled with the original Sandia layout. A marked improvement can be observed over the whole blade span, particularly in the central blade portion.
Figure 5.19: Comparison of the exural rigidity distribution for the original AOC 15/50 blade, the (second) optimized one and the (second) optimized geometry coupled with the original Sandia layout
The surface density distribution along the blade is also presented in Figure 5.20. The overall mass of the blade has been increased, particularly in the root zone. The two optimized solutions present a similar trend of the surface density. The results show improvements in the power generation reducing the blade deformation, however the mass of the blade is increased: a further deeper analysis should take into account also the Cost Of Energy, not considered in the present optimization.
Figure 5.20: Comparison between the surface density distribution for the original AOC 15/50 blade and the optimized solutions