Characterization of a micro-hydrokinetic turbine in close proximity to the free surface

• Hydrokinetic turbine operation with free surface interaction.
• Volume of Fluid (VOF) multiphase model used in capturing transient effects.
• 32.2% reduction in power at supercritical flows.
• Transition from subcritical to supercritical flow is predicted as Fr exceeds unity.
• Traditional and propeller-based hydrokinetic turbine designs were compared.

Predicting hydrokinetic turbine power generation is difficult due to complex geometry, highly turbulent conditions, and difficulty capturing the transient interface existing between air and water. A three-dimensional finite volume solver was used to capture the effects resulting from free surface interaction with the aid of a Volume of Fluid (VOF) multiphase solver. Depths from free surface level to blade tip with corresponding Froude numbers of 0.71, 0.92, 1.04, and 1.31 were modeled specifically to capture the transition from subcritical to supercritical flow conditions. A sharp decrease in performance was observed at the critical Froude number (Fr=1.0). Results at subcritical conditions showed acceptable agreement with previously published single phase results where the turbine is assumed to be operating in an infinite medium. At subcritical conditions, the propeller-based turbine studied was compared to numerical and experimental results obtained for a traditional marine current turbine (MCT). As the flow became critical, a 32.2% decrease in the power coefficient was predicted and significant wake-free surface interaction was observed.