Several models have been proposed to predict the shaft output characteristics of the vertical axis wind turbine from the aerodynamic characteristics of the blade. However, it has not predicted the output characteristics of the small wind turbine, turbine diameter less than 1 m, even the multiple stream tube model included effect of the momentum reduction. A solidity of the small wind turbine become higher than large, i.e. blades interval of small wind turbine becomes short. Therefore, it expects that the wake of preceding blade influence strongly aerodynamic character of following blade. Thus, we have investigated the relation between wake of rotating turbine and its shaft output characteristics. From result obtain, we had cleared the shaft output character agree with variation of momentum loss of the wake of turbine. Moreover, the relevance between the shaft output characteristics and velocity fluctuation in the wake had been found. In the low tip speed ratio region increasing the shaft output, the periodical component synchronized with motion of the blade remarkably appear in velocity fluctuation, and it disappear near the peak of shaft output. Meanwhile, in the high tip speed ratio region decreasing the shaft output, the turbulent component is dominant in velocity fluctuation. It indicates that the blade-wake interference was affected on shaft output of the small wind turbine. Therefore, the purpose of this study was to find out a similarity parameter of the shaft output characteristics based on blade-wake interference. The larger the scale of blade wake or slower the advection velocity of it, the higher the possibility that blade wake generated from previous blade interfere to following blade become. For blade-wake interaction, these two parameters which the scale and the advection velocity of blade wake, are important. Thus, I estimated these parameter in this study using the wind turbine model with single blade. From measurement of the blade wake, I found that the scale of the blade wake depends on the ratio of the rotational speed of the turbine to the free stream velocity, and the advection velocity of blade wake depends on the free stream velocity. From these characteristics of blade wake, I suggest a new parameter based on the blade-wake interaction using tip speed ratio, solidity, number of blade, and turbine diameter. The shaft output characteristics of the small wind turbine indicates good similarity for the new parameter.

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