A Characterization of Vertical Axis Wind Turbines
Vertical axis wind turbines (VAWT) have a number of advantages over the horizontal axis wind turbines (HAWT), however the performance is a little smaller, so that researches for their development should be continued for the implementation of these turbines in urban environment for electrical energy generation. This paper provides an overview of characterization of the VAWT, through a literature review, making comparisons with the HAWT and highlighting the advantages and disadvantages of this type of turbines. The paper also defines and compares the main exist types of VAWT for application in urban environment, so that aims to be a first step towards development of a VAWT research. Finally, this paper features how the aerodynamic optimization of a VAWT can be obtained, defining the experimental and the computational numerical analysis, by the relationship between them.
CFD Investigation on the Aerodynamic Interferences between Medium-solidity Darrieus Vertical Axis Wind Turbines
Abstract The present study contributes to understand physical mechanisms involved in an achievable power enhancement by setting vertical axis wind turbines in close proximity. The turbines are straight-bladed Darrieus micro-turbines characterized by medium-high solidity and therefore low tip-speed ratio. Preliminary CFD simulations of the isolated turbine explain the reasons why it has a low power output, namely which are laminar flow and laminar separation bubbles on the blades. This fact is expected also considering the low Reynolds number. Subsequently a campaign of CFD simulations has been performed to analyse the aerodynamic interferences in two-rotor configurations. The behaviour of counter-rotating and co-rotating arrangements is analysed at different distances between rotor axes. The simulations show an increasing of power production of about 10% compared to results for the isolated turbine, independently of the sense of rotation. In order to verify wheter vortex shedding suppression might be the cause of the enhanced performance interactions has been simulated between two closely spaced Magnus spinning cylinders with the same tip-speed ratio of the turbines. These last results don’t show reasonable analogies with VAWT wake structures and interactions. Our main conclusion is that accelerated free-stream flow between the turbines is the principle cause of the power extraction enhancement by means of contraction and re-energisation of the turbine wakes. CFD predictions of a four-rotor configuration confirm our hypothesis, nevertheless the wind direction strongly affects the overall efficacy.
A new method of near field analysis
The problem of the accurate determination of the total electromagnetic field produced by a current distribution on a circular disc type aperture is examined. For comparison of the near and Fresnel region solutions, an exact Maxwellian integral is used as the starting point. Aperture and space coordinates are separated by the use of Bessel function addition theorems, allowing the field to be expressed as a sum of terms containing radial and angular integrals. The radial integrals involve only the disc (aperture) distribution and coordinates while the angular integrals involve only the elevation angle. The distance from origin appears implicitly in the series coefficients. These integrals are suitable for high speed computations and have been evaluated for a number of cases. The results are compared with the small angle and general Fresnel formula values for various observation angles, distances from the aperture, and aperture distributions.