Frequency analysis of the power output for a vertical axis marine turbine operating in the wake

Flow condition in rivers is highly turbulent and unsteady due to upstream structures or geometry of the river. In a hydrokinetic farm downstream turbines are subjected to the wake of upstream turbines. Operating in the wake not only affects the average power output of the turbine but also influences the quality of the power by imposing fluctuations to the power. The current study presents the water tunnel testing results of a vertical axis turbine operating in the vortex shedding behind a cylinder with various sizes located at different distances upstream the turbine. The vortex shedding from a circular cylinder has similar patterns with vortex shedding from blades of a vertical turbine. Results show that for certain sizes of upstream cylinder the maximum negative effect occurs when the upstream cylinder is 1.5 turbine diameters away. At closer or farther distances the negative effect is limited. The frequency analysis of the power data shows that when the turbine operates in the wake, the frequency strength at the rotational frequency of the turbine and its multiples decreases. At the same time, new frequency peaks appear in the power spectrum as the result of the vortex shedding behind the cylinder.