In 2011, John Dabiri proposed the use of counter-rotating vertical-axis wind turbines to achieve enhanced power output per unit area of a wind farm. Since then, various studies in the wind energy and marine hydrokinetic (MHK) literature have been dedicated to pairs of vertical axis turbines in both co-rotating and counter-rotating configurations, in terms of their power production, wake characterization, and optimal array design. Previous experimental works suggest an enhancement of up to 27.9% in the system power coefficient of pair configurations compared to a single turbine. Additionally, previous numerical studies have indicated that the increased power output is correlated with higher torque on the turbine blades which correspondingly produces a stronger leading edge vortex. This paper presents an extended investigation into a pair of laboratory scaled cross-flow hydrokinetic turbines in counter-rotating configurations. Experiments were conducted to observe, compare, and discuss the leading edge vortex shedding from the turbine blades during their positive torque phase. The turbines operated in a small water flume at the diameter-based Reynolds number of 22,000 with a 0.316 m/s freestream velocity and 4% turbulent intensity. Using a monoscopic particle image velocimetry setup, multiple realizations of the water flow around each blade at their positive torque phase were recorded and phase-averaged. Results show consistent leading vortex shedding at these turbine angles while a correlation between the turbine power performance and the vortex size and strength was observed.

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