Turbulence statistics were applied to the downwind flow of a Darrieus-type vertical axis wind turbine (VAWT) to provide insights about VAWT micro-flow characteristics. Micro-turbulence measurements of the flow in front of and behind a VAWT were made using a sonic anemometer. Turbulence data for downwind distances of 0.5 m and 2.0 m from the wind source were collected with and without the VAWT, respectively, using a blower (Type 1 flow) and industrial fans (Type 2 flow) as the wind source. The voltage V, generated by the VAWT was recorded every 15 min. Only the center line of the flow was considered. The Type 2 flow induced a constant spectral power at a high dimensionless frequency range of 0.007 < f < 0.04, perhaps due to the fan configuration, which had a larger wind-swept area. The results show an increase in lateral turbulence, σ v , downwind of the VAWT compared to its longitudinal, σ u , and vertical, σ w , counterparts. Using spectral analysis, the VAWT was found to reduce the horizontal (u and v) turbulent component energies in the dimensionless frequency range of 0.003-1. Interesting dip and peak features were observed for Type 1 flow, but only dip features were observed for Type 2 flow in the resulting spectra. Higher wind speeds increased the voltage generated at a rate of 0.5.
Turbulence intensity measurements of vertical axis wind turbine (VAWT) are essential to detemineitsefficiency and performance. In this study, the eddy covariance (EC) method was used to characterise the flow and directly measure turbulence of a VAWT in a controlled indoor laboratory using an ultrasonic anemometer (at 10 Hz) at specific grid positions of upwind and downwind (in the wake) of the VAWT. In constrast to numerical simulations, this method has the potential to accurately quantify turbulence of wind turbines and thus able to describe the flow patterns around a VAWTexperimentally. Results show that at the upwind position, some of the swept area of the VAWT obstructed the flow due to the counter current flow generated by the VAWT rotors' rotation while causing some flows to be diverted and concentrated to the concurrent flow area of the VAWT. In the wake of the VAWT, flow velocities and turbulence decreased and dispersed while at the centre line, vertical turbulence decreased towards the bottom (ground) swept area.This downward transfer of turbulence caused generally negative vertical momentum transfer. However, a slight increase of turbulence was observed at the ground surface due to flow deflection.
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