Abstract
The simulation of very small vertical axis wind turbines is often a complex task due to the very low Reynolds number effects and the strong unsteadiness related to the rotor operation. Moreover, the high boundary layer instabilities, which affect these turbines, strongly limits their efficiency compared to micro horizontal axis wind turbines. However, as the scientific interest toward micro wind turbine power generation is growing for powering small stand-alone devices, Vertical Axis Wind Turbines (VAWTs)might be very suitable for this kind of application as well. Furthermore, micro wind turbines are widely used for wind tunnel testing, as the wind tunnel dimensions are usually quite limited. In order to obtain a better comprehension of the fluid dynamics of such micro rotors, in the present paper the authors demonstrate how to develop an accurate CFD 2D model of a micro H-Darrieus wind turbine, inherently characterized by highly unstable operating conditions. The rotor was tested in the subsonic wind tunnel, owned by the University of Catania, in order to obtain the experimental validation of the numerical model. The modeling methodology was developed by means of an accurate grid and time step sensitivity study and by comparing different approaches for the turbulence closure. The hybrid LES/RANS Delayed Detached Eddy Simulation, coupled to a transition model, demonstrated superior accuracy compared to the most advanced unsteady RANS models. Therefore, the CFD 2D model developed in this work allowed for a thorough insight into the unstable fluid dynamic operating conditions of micro VAWTs, leading the way for the performance improvement of such rotors.
Highlights
The interest for micro wind power generation is growing, since micro wind turbines appear to be very suitable for powering low-power devices such as wireless sensors, actuators, controllers, and small lightning systems
As the scientific interest toward micro wind turbine power generation is growing for powering small stand-alone devices, Vertical Axis Wind Turbines (VAWTs)might be very suitable for this kind of application as well
In general, the simulation of the unstable conditions of VAWTs, which arise at low tip speed ratio, is still a very complex problem
Summary
The interest for micro wind power generation is growing, since micro wind turbines appear to be very suitable for powering low-power devices such as wireless sensors, actuators, controllers, and small lightning systems. Bastankhah et al [10] designed and analyzed a miniature wind turbine with a rotor diameter of 15 cm They demonstrated that an accurate fluid dynamic design for specific low Reynolds number was of utmost importance for reaching high efficiency. The studies presented above demonstrate the scientific interest toward micro wind rotors and highlight the fact that small rotors need a very accurate fluid dynamic design in order to obtain high efficiency This is mainly due to two factors. This micro rotor operated at very low tip speed ratios and very low Re, which caused strong and sudden boundary layer instability (separation and unsteady vortex shedding) leading to early dynamic stall development and large lift losses on the blade. At these very low Re, the boundary layer is mostly laminar and, as is widely known, a laminar boundary layer is more sensitive to adverse pressure gradients that trigger laminar bubbles, separation, and sudden transition
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