In the near future, the hydrokinetic energy of ocean currents, rivers and tides is expected to provide a growing contribution to our effort of decreasing the world dependence on fossil fuels, replacing them with cleaner, renewable and more sustainable energy resources. However, the research in this field is at its early stages and the relevant technology still has a strong potential for improvement.
Although hydrokinetic turbines share similarities with wind turbines, their working conditions are characterized by significant differences, affecting their performance, wake and mutual interaction between them. In particular, they often work in shallow waters, making the free-surface effects on both performance and wake development substantial.
Hydrokinetic turbines are usually required to operate in array configurations of multiple devices. Therefore, their wake properties are critical in defining the working conditions of downstream rows of turbines. The large tip and hub vortices shed by upstream turbines produce fatigue stresses on the downstream ones.
In addition, the literature on axial-flow rotors demonstrated extensively that the dynamics and eventual instability of the tip vortices have a major role in the process of wake recovery, which in turn affects turbulence levels, the momentum available to downstream devices and their performance. Free-surface effects are expected to have a significant impact on the dynamics of the tip vortices and, as a result, on the process of wake recovery.
In the present study, the project will evaluate this influence by means of high-fidelity, state-of-the-art, fluid dynamic computations, based on Detached Eddy Simulation, a dynamic overlapping grids technique and a single-phase level-set methodology. Geometry-resolving simulations will be conducted, dealing with an axial-flow hydrokinetic turbine, including its tower and nacelle, across changing depths of rotor immersion.
Their results will allow the project to assess the influence of the free-surface on the wake dynamics and recovery and to provide a useful reference to engineers and scientists adopting lower-fidelity methodologies for the design and the analysis of this class of turbines, when working in shallow waters. This study will also pave the way to our future research on the influence of waves on both performance and flow physics.