Numerical investigation of damage progressive in composite tidal turbine for renewable marine energies M.Nachtane1.2, M.Tarfaoui1, D.Saifaoui2, A. El Moumen1 1 2
ENSTA Bretagne, FRE CNRS 3744, IRDL, F-29200 Brest, France
Laboratory for Renewable Energy and Dynamic Systems, FSAC - UH2C (Morocco) Phone number: +33(0) 298348705, e-mail:
[email protected] Phone number: +33(0)751207871, e-mail:
[email protected]
Abstract -Marine energy is one of the most exciting emerging forms of renewable energy. Tidal turbines are used to extract this energy and installed on the seabed at locations with catastrophic loading. This investigation focuses on the numerical simulation of the damage progressive in tidal turbine used for marine energy application under impact loadings. The optimal design and the dynamic behavior of the turbine are studied. Finally, the hydrodynamic and hydrostatic pressures over the loading and the distribution of the stress, the deformation and damaged zone are presented.
obtained by using XFOIL software. By extension function of ABAQUS, the 3D structure of marine turbine is obtained as presented on figure 2. This structure is then introduced in the finite element code in order to control the evolution of damage and the design.
Keywords- Renewable marine energy, Marine turbine, Composite materials, Progressive damage, Impact loading.
I.
Introduction
The kinetic energy available within tidal currents is an important source of the renewable marine energy. If an effective technique of capturing this renewable energy can be developed, tidal currents can be considered to generate electrical power and then satisfy the world’s growing energy needs [1]. Depending on their constituents, various types of tidal turbine were considered. An experience of composites in marine structures especially for offshore application was tested in [23]. Because of their mechanical properties, composite materials offer new prospects for the renewable marine energy. However, the variability of their behavior, especially under catastrophic environmental loading plays a major obstacle to further development. This investigation describes the structural composite design by determination of extreme loads. The damage induces a degradation of properties [4]. For this purpose, it is essential to have the best possible knowledge of these turbines in terms of microstructure and mechanical properties.
II.
Structure, materials and properties 1. Geometry of structures
Figure 1 shows the profile of the turbine that is used to generate the 3D structure of the tidal turbine. Profile is
Fig 1: Hydrodynamic profile
(a)
(b)
Fig 2: Marine turbine: (a) real turbine and (b) simulated case. In order to simulate the dynamic behavior, the composite marine turbine structure was subjected to different impact loading. Three forms of impactor structures were considered (figure 3), with the dimension and the different properties are listed in table 1 and table 2.
Yc(MPa) St(MPa)= Sc(MPa)
III. (a)
(b)
(c)
Fig 3: Impactor forms: (a) Conical. (b) Flat (c) Hemispherical. Tab 1: Properties of conical and hemispherical Impactor
Radius (mm) 0.8 0.8 0.8
Mass (kg) 20 20 20
Velocity (m/s) 20 25 30
Energy (kJ) 4 6.25 9
Tab 2: Properties of flat impactor
Width & Length (mm) 5 5 5 2.
Mass (kg)
Velocity (m/s)
Energy (kJ)
20 20 20
20 25 30
4 6.25 9
171,8 35,3
Finites elements analysis
Numerical simulation of damage in turbine structures can be studied by means of finite element methods [7]. It should be noted that the damage is controlled by Hashin’s criteria [8] to estimate the fiber and matrix damage initiation. In our numerical model, the tidal turbine is modeled as a deformable structure with shell element (S4R) and the impactor as the rigid body [9]. In order to reduce computing times, by the reduction in the element number of models without harming the quality of simulation results, one strategy consists in optimizing the mesh model. Therefore, the mesh density is studied.
1. Mesh density The mesh density is defined as the minimum number of elements for which the convergence of properties was started [10-11]. Figure 4 illustrates different resolutions of model and figure 5 presents the variation of the maximum stress vs the number of elements. Convergence is obtained for a 0.1mm mesh size (129490 FE). The retained final mesh is given in figure 6.
Materials and properties
The considered structure is made of composite materials based on Polyester matrix reinforced with glass fibers. The orthotropic behavior of the composite can be defined as a 3D stiffness matrix consisting of nine independent constants. We recall that, all of the properties of composites, used for simulation, are listed in table 3 and 4. Tab3: Properties of the composite glass-polyester fiber [5]
Properties ρ E1(MPa) E2(MPa)= E3(MPa) Nu12 Nu13= Nu23 G12(MPa)= G13(MPa) G23(MPa)
Fig 4: Maximum stress according to the mesh size
Values 1960 * 10-9 48160 11210 0,270 0,096 4420 9000
Tab4: Ultimate Stress of glass-polyester composite [6]
Properties Xt(MPa) Xc(MPa) Yt(MPa)
Values 1021,3 978 29,5
Fig6: Final mesh for numerical simulation
2. Simulation of damage in marine turbine Mechanical properties of turbine structures and damage initiation properties of specimens were listed in table 1. The considered model is presented in figure 7. This model shows the turbine part and the impactor region. Many situations of accidental impact were treated as: Effect of impactor geometry: (a) hemispherical (b) flat (c) conical.
Effect of impactor velocity: 20 m/s (a) ; 25 m/s (b) and 30m/s (c)
Fig 7 : Visualization of the overall system
Figure 8 illustrates the variation of energies system as a function of the time. This figure shows that, during impact test, ETOTAL is equal to the summation of the ALLKE and ALLIE energies. This figure also shows that the ALLSE energy simultaneously varies as ALLIE until 1 ms. When the ALLDMD appears, after 1 ms, a small difference is observed between two energies because the structure damage is started. Finally, the hypotheses of impact energy were satisfied. Therefore the created numerical model can estimate the progressive damage and can be used to control the damage of marine turbine. For models that treat the dynamic aspects, we always make sure that the Hourgalss phenomena are low (ALLAE less than 10% of ALLIE).
Fig 8: Variation of energies system versus time
4. Damaged area 3. Results and discussion In this part, we have interested to analysis the impact energy and the damaged area after loading.
3-1 Energy conservation In order to validate the finite element approach, the conservation of energy [12] was controlled during the impact test. Three types of energies were measured and compared. The total energy (ETOTAL) equivalents to the summation of the kinetic energy (ALLKE) and the internal energy (ALLIE). Several energy types were distinguished as for example: Total energy (ETOTAL) Kinetic energy (ALLKE) Strain energy (ALLSE) Internal energy (ALLIE) Damaging energy (ALLDMD) Artificial energy (ALLAE)
In this part, we focus on the damage appeared during the impact. Hashin’s criteria [13], for fiber and matrix damages, were introduced. Figure 9 shows the force-time curves for different impactors and different velocities. The effects of impact geometry and velocities impact are noticeable. The maximum force is obtained at higher velocity and decreases with decreasing the velocity. The maximum force corresponding to each impactor type is: 550 kN for conical, 450 kN for Hemispheric and 350 kN for Flat impactor. In the case of an elastic collision, the force-time history has a parabolic curve, takes a symmetrical form, with a maximum pick. Consequently, the loading and unloading parts are identical. In our case, no symmetry between loading and unloading phases is observed, because the apparition of damages which are marked by the sharp decline of the force. At this moment the failure of composites is started.
(a) Conical impactor (a) Flat impactor
(b) Flat impactor Fig 10: Damage of the nozzle under impact
(b) Hemispheric impactor
IV.
Conclusion
We could define this paper modeling (3D Shell) simple and robust implemented in Abaqus for composite permits rapid global approach and not time-consuming for calculating the impact tests and have a size and shape damaged area quickly and accurately. This approach based on the stacking sequence and Hashin type of damage criteria allows the rapid preliminary design of the state of the structure.
(c) Conical impactor
The study presented briefly here has yielded several conclusions:
The Performance of composites impacted in seawater depends heavily on the type of fiber, and especially of the matrix formulation.
The Establishment of numerical models is a first step in integrating the response of the material in the design of marine energy recovery structures.
Information lacking to date is a good knowledge of offshore loading conditions. The prototype study in ponds provides some of this information but field trials are essential.
Fig 9: Force-time curves Figure 10 shows a snapshot of the damaged turbine. It appears that the damage initiation depends on the impactor geometry. The maximum damaged zone is obtained by conical geometry because of their tip. A small damaged zone is observed in the case of flat impactor.
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