To deal with the structural complexity, the wind turbine blades are modeled using finite elements. Vibrations at natural frequencies is an important part of blade designing. Therefore, for the validation of the structural design and the dynamic or modal response of the blades, finite elemental analysis technique was used. The analysis was done with varying degrees of complexity. Hence, the approach used by Professor Mostapha Tarfaoui and Dr. Owaisur Rahman Shah from ENSTA Bretagne & Institut de Recherche Dupuy de Lôme (France) in their studies, was to model the blade and to define the material. Furthermore, to validate the proposed design they applied a static bending test on the full scale blade. The results from these static tests, using strain gauges, were then used to validate the sub-modeling regime in terms of strains measured at the blade surface.
The researchers considered the sub-models as sections or portions of the larger model. The larger complete model; the parent (as it can be a sub-model of a higher and larger, more complex model) and the child as sub-model. They compared the global model with the experimental results to validate the approach of sub-modeling and then later all the sub-models were validated successively.
Sub-modeling technique reduces the domain size of a finite element model to a more manageable size. From the different methods of sub dividing the problem domain into simpler smaller domains, they used the method of transfer of nodal displacement from one parent model to its child model. The method of nodal displacement was used to predict failure in large structures and to identify the sensitive zones.
Only ‘delamination’ was studied as the criteria for the failure instead of different criteria of failure of plies. They used the criteria of combination of failure as it would be necessary to simulate accurately and appropriately the failure of these structures.
In their experiments, the wind turbine blade was subjected to buckling at its compression side and that too, at its critically loaded section at 7.55m from the root which has produced waviness in the blade’s structure due to which the plies have inter-ply transverse tensile stresses in the stacking direction. So this caused the delamination type failure.
Sub-modeling results corresponded well from the parent level to its child level or sub-level. Therefore, the researchers concluded that the nodal displacement based sub-modeling can be used to locate weak points in a structure under extreme loading conditions. They further validated the global model in strain calculations when compared to full scale physical tests.
Due to the consistent results of sub-models, Shah and Tarfaoui concluded that the finest sub-models are an accurate representation of the failure that would take place eventually.
Owaisur Rahman Shah, Mostapha Tarfaoui. The identification of structurally sensitive zones subject to failure in a wind turbine blade using nodal displacement based finite element sub modeling. Renewable Energy, 2016, Volume87, pp 168-181.
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