A parametric study of vertical axis turbines of the H-Darrieus type is conducted using state-of-the-art Computational Fluid Dynamics (CFD) and the k-ω Shear Stress Transport RANS model in its unsteady form. Although most parameters have previously been investigated individually, the effect of solidity, number of blades, tip speed ratio, Reynolds number, fixed blade pitch angle, and blade thickness on the aerodynamic efficiency of the turbine is evaluated using the same performance evaluation set-up in order to determine what would be the best aerodynamic configuration and operation parameter in a given application. The quantitative impact of 3D effects associated with the blade aspect ratio and the use of end-plates is also investigated. For high-Reynolds applications, optimal radius-based solidity is found to be around σ=0.2, while higher solidities show a lower maximum efficiency than what was previously published using simpler streamtube based methods. In 3D, a small blade aspect ratio (AR=7) leads to a relative efficiency drop of nearly 60% compared to the 2D prediction. Longer blades improve the 3D efficiency greatly. End-plates are found to have a positive effect on power extraction performances, as long as their size and thus their drag are limited.

No abstract

No abstract

We have developed a simple, two-dimensional piezoelectric tube device which uses the inertial slider method to achieve remote micropositioning in the vertical and horizontal directions. The motion of the slider car occurs with respect to a quartz rod which is attached to, and accelerated by the piezo tube. The vertical motion, against or with gravity, is achieved by activating the longitudinal mode of the piezo tube. The horizontal motion is a rotation about the quartz rod, achieved by activating the bending mode of the piezo tube. The device is very compact, works at cryogenic temperatures and is ultrahigh vacuum compatible. Step sizes from 10 to 3000 nm and speeds up to 0.2 mm/s are possible. A computer simulation model of the inertial slider process has been developed also, to investigate the effectiveness of various activating waveforms. It confirms the advantage of a cycloidal-like waveform recently reported in the literature. Also, the model predictions are consistent with the results obtained in the laboratory. A diagram of the analog circuit developed to produce a cycloidal-like waveform to drive the piezoelectric is included. We briefly describe an alternative design where the activating piezoelectric is attached to the slider car which moves vertically on a fixed quartz rod.

No abstract