Comparative Numerical and Experimental Modal Analysis of Aluminum and Composite Plates

The paper presents the comparative analysis of the dynamic behavior of two rectangular plates of different material, aluminum and composite. While their global geometric dimensions (length, width and thickness) are similar, their inner structures are quite different. Whereas aluminum plate can be considered isotropic, the composite plate is a unidirectional carbon-epoxy laminate. Modal characteristics of the two plates were determined both numerically and experimentally and a comparative analysis of the obtained results was performed. Responses of the plates were documented by an optical, contactless 3D digital image correlation (DIC) system that contains a set of high-speed cameras capable of recording the movement of the white-and-black stochastic pattern applied to the upper surfaces of the plates. Numerical simulations were performed by the finite element method (FEM) in the commercial software package ANSYS. The plates were excited by a modal hammer and allowed to freely oscillate. In order to determine the natural frequencies of the plates the recorded time-domain responses were post-processed, i.e. converted to the frequency domain by fast Fourier transform (FFT). The first three natural modes were successfully experimentally established and compared to the corresponding numerical values. Since the differences between the two sets of results are less than 5%, the applied experimental technique can be considered valid and suitable for a wide range of engineering problems involving vibrations

Computing aerodynamic performances of small-scale vertical-axis wind turbines: Possibilities and challenges

The past few decades have been marked by an immense interest of the scientific community in making better use of renewable energy resources, particularly wind energy. One of the suggestions is to increase the number of small-scale vertical-axis wind turbines (VAWTs) in densely populated areas. Given that VAWTs primarily operate in adverse conditions (irregular wind speeds, Earth’s boundary layer, vortex trail of surrounding objects), it is necessary to pay special attention to the numerical and experimental estimation of their aerodynamic performances. The conceptual design of small-scale wind turbines usually begins with detailed simulations of the encompassing flow field. There are three categories of most often employed computational methods for Darrieus-type VAWTs: i) quasi 1D momentum models frequently upgraded by blade element theory, ii) vortex models and iii) computational fluid dynamics (CFD) approach that enables modeling the complete flow field. Each of these methods is founded on a particular set of assumptions, has its advantages and disadvantages and can provide different numerical results, although engineers are usually mostly interested in accurately estimating power coefficient curves. This paper describes and references some of the realized and tested small-scale VAWTs. It also accentuates modeling possibilities as well as several dilemmas, challenges and errors that may appear while performing complex, unsteady aerodynamic analyses of fluid flows encompassing VAWTs. The main goal of the paper is to provide useful guidelines for successful design of small-scale VAWTs for urban environments