Dear Colleagues,

Offshore winds and marine currents are deemed two of the most promising renewable energies, since the former have high intensity and low turbulence and the latter are highly predictable, being governed by tidal cycles. Horizontal axis or vertical axis turbines can be adopted, although unconventional devices based on flapping hydrofoils have also been tried for tidal currents. To cut the energy production costs, multidevice arrangements of efficient large size turbines must be adopted. Moreover, to reduce the occupied sea area, the turbines should be placed as close as possible, however avoiding harmful fluid dynamic interferences between rotors.

This Special Issue aims to collect experimental studies and CFD simulations giving a deep insight into the fluid dynamic mechanisms that govern the behavior of wind and hydrokinetic turbines, with the purpose of finding out practical solutions that improve the performance of the single turbine or reduce the sea area required for a certain overall power output. Just to name a few topics concerning the increase of the turbine efficiency, apart from the study of turbine or blade new concepts, there are devices for the flow local control to increase lift and to reduce tip losses and diffusers and devices for flow augmentation. With the goal to direct developers toward the most efficient turbine arrangement layouts, the following topics are also welcomed: wake structure and its recovery characteristics; fluid dynamic interactions between closely spaced rotors; and blockage effects due to the channel cross section dimension or to the presence of other rotors.

Moreover, papers contributing to understanding turbine performance under realistic operating conditions and environments will be appreciated. Offshore environments are characterized by complex physics: ice formation on the blades, turbulence, gusty winds and storms above the sea surface; bio-fouling, turbulence, and cavitation under the sea surface; and sea surface deformation and waves having consequences on both tidal and wind turbines. In fact, on one hand, waves significantly affect the fluid dynamic loads on the blades of tidal and floating wind turbines, increasing torque fluctuation and fatigue, while on the other hand, they can even support wake recovery mechanisms of tidal turbines.

Authors are also encouraged to submit papers on the prediction of aero/hydrodynamic forces and turbulent wakes by means of new CFD tools allowing a good balance between accuracy and computational expense in multiscale complex problems, for instance, tools based on blade element momentum approaches instead of geometry-resolved methods.

Dr. Stefania Zanforlin
Guest Editor

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• Offshore wind turbines
• Tidal turbines
• Fluid dynamic mechanisms
• Flow control
• Tip losses
• Ducted turbines
• Wake structure
• Clusters and arrays
• Interferences between rotors
• Blockage
• Effects of waves