JEC COMPOSITES MAGAZINE - Issue #70 - January/February 2012 - 43

Fig. 6: Coupled aerodynamic-structural iterative analysis for adaptive blade design

response of a wind turbine subjected to aerodynamic loads. The angle of attack changes as a result of the induced twist, which in return affects the aerodynamic load. A coupled aerodynamic-structural iterative analysis will better predict the aerodynamic loading and structural response. Figure 6 illustrates a coupled aerodynamic-structural iterative analysis approach to design large-scale adaptive wind turbine blades. External loads Wind turbine blades are designed to meet ultimate and fatigue strength requirements. Ultimate loads arise from extreme wind speeds, extreme wind

direction changes or specific events like start-ups, emergencies, shutdowns, etc. Fatigue load represents normal fluctuating full-load operation, i.e. aerodynamic, gravity, centrifugal and gyroscopic loads over the entire life of the blade. Industrial standards such as IEC 61400-1 provide the requirements of design load cases. For large-scale blades, the effect of gravitational load, in terms of edgewise moment, becomes more dominant due to increased weight. Most importantly, the aerodynamic and self-weight loads cause internal flapwise and edgewise bending moments and the twisting moment, which cause in-plane

normal and shear loads in the blade cross section, as shown in Figure 7. While the blade is rotating, the self-weight load alternatively changes between the axial and rotational directions, dominantly causing the edgewise moment. Its sign changes as shown in Figure 7. The mass centre and shear centre play important roles in the generation of the twisting moment due to the aerodynamic loads and self-weight. In fact, the twisting moment is caused as a result of the said aerodynamic lift and drag forces multiplied by the distance from the shear centre. The force table in Figure 8 shows the continuation of each external force to shear and normal laminate loads. Eventually, the ratio of the in-plane shear to normal loads affects the optimal fibre angle of the triaxial NCF. Failure-moment envelope During a typical blade design process, the failure indices or material resistance of each laminate are calculated under six resultant sectional loads along the blade. The major forces are the flapwise and edgewise moments. The buckling analysis should be performed to meet all the design load cases. A blade comprises varying cross sections with different geometrical properties and composite lay-up sequences, and each cross section needs to be analyzed separately. This is a long-term process for designers. A failure-moment envelope has been proposed to help designers grasp how each design variable affects the failure envelope and easily select the materials and lay-up-sequence with visual aid. The envelope represents a relationship of maximum flapwise and edgewise failure moments: the moment load cases inside the envelope will be safer while the moments beyond the envelope would cause material failure. Therefore, the moment loads, which are calculated from design loads provided, e.g., by IEC standards, should be enclosed by the envelope. The moment loads are not affected by the material properties and lay-up sequences. The moment envelopes depend on the selected materials and lay-up sequences, which designers should determine to

Feature Wind Energy

Fig. 7: Relationship between the in-plane shear and normal loads and the external aerodynamic loads, self-weight and centrifugal forces

No70 January - February 2012 /

jec composites magazine 43

JEC COMPOSITES MAGAZINE - Issue #70 - January/February 2012

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #70 - January/February 2012