To analyze the complex and unsteady aerodynamic flow associated with wind turbine functioning, computational fluid dynamics (CFD) is an attractive and powerful method. In this work, the influence of different numerical aspects on the accuracy of simulating a rotating wind turbine is studied. In particular, the effects of mesh size and structure, time step and rotational velocity have been taken into account for simulation of different wind turbine geometries. The applicative goal of this study is the comparison of the performance between a straight blade vertical axis wind turbine and a helical blade one. Analyses are carried out through the use of computational fluid dynamic ANSYS® Fluent® software, solving the Reynolds averaged Navier–Stokes (RANS) equations. At first, two-dimensional simulations are used in a preliminary setup of the numerical procedure and to compute approximated performance parameters, namely the torque, power, lift and drag coefficients. Then, three-dimensional simulations are carried out with the aim of an accurate determination of the differences in the complex aerodynamic flow associated with the straight and the helical blade turbines. Static and dynamic results are then reported for different values of rotational speed.

A numerical model, obtained by implementing a cyclic damage model in the framework of an interface element, is here proposed to reproduce the crack growth in laminated composites subjected to cyclically repeated loads. This model, which differs from the few studies in the literature on the topic, applies not only to single fracture modes but also to mixed modes, and to constant or variable crack growth rates. The applied load (in terms of force or displacement) is assumed to oscillate between a minimum and a maximum constant value. The Paris curve can be reproduced with accuracy once some parameters in the numerical model are tuned with experiments. These parameters are preliminarily found by simulating fatigue delamination in mode I (DCB test), in mode II (ELS or ENF test) and with a fixed mode mixture φ = GII/G (MMB test). A non-monotonic curve is then used to interpolate these preliminary results. Furthermore, tests where the mode mixture changes with crack length could also be reproduced. With this model it is possible to predict the crack growth rate with cycle of a generic structure without knowing a priori how the mode mixture φ changes during the crack propagation. This novel procedure gives a new opportunity in the design of composite structures subjected to repeatedly applied loads.