Section: New Results

Validation of NaSCar at higher Reynolds numbers and Aeroelastic coupling

A beam finite element model has been implemented in order to study the dynamic behavior of the wind turbine blade. The structural model is linear and can describe bending, torsion and axial deformation. There is the possibility to take into account some coupling effects between bending-torsion and torsion-axial deformation. The implementation of the structural model has been validated by means of different static and dynamic tests. In 15 the Fast Fourier Transform of the tip deflection history is reported: the frequency of the predicted peaks is in good agreement with the theoretical values.

The structural model has been coupled with two different computational fluid dynamics codes: a cartesian code (NASCAR3d) and an octree code (developed by Claire Taymans during her PhD). The coupling requires to compute the loads for the structural model by performing an integral of the fluid forces on a surface mesh. The surface mesh is updated at each time step according to the displacement of the structure and this allows to update the level set which is used to impose the effects of the body on the fluid.

In order to focus the attention on a single blade of the rotor, the inertial terms (centrifugal and Coriolis forces) have been added in both the fluid solver and in the structural model. This makes it possible to perform a preliminary study of the behavior of a single elastic blade by neglecting the interactions between the different blades and the wind turbine's tower (see 16).

The turbulent flow around the blade is studied by means of the Vreman Large Eddy Simulation model (A.W. Vreman, "`An eddy-viscosity subgrid-scale model for turbulent shear flow: Algebraic theory and applications,"' Phys. Fluids 16,10 (2004)) which has been tested on the flow around a cylinder at Re=3900 and Re=140000. The validation of the model for high reynolds flows required the use of a very fine mesh in order to appropriately simulate turbulent disspation and accurately predict the mean flow field, the results obtained are in good agreement with the experimental data of Cantwell et al (B. Cantwell, D. Coles, "`An experimental study of entrainment and transport in the turbulent near wake of a circular cylinder,"' J. Fluid Mech. (1983), vol. 136, pp. 321-374), as reported in 17.

In order to extend the capability of the code to high Reynolds number a wall function approach has been implemented following the guidelines of De Tullio (De Tullio, M.D. (2006) Development of an Immersed Boundary method for the solution of the preconditioned Navier-Stokes equations (PhD thesis)). The main idea of this approach is to impose the value of the velocity in the first fluid cells close to the wall by performing a non-linear interpolation based on wall function which represents the velocity distribution in the turbulent boundary layer (see 18).