Abstract
The open centre turbine can be easily deployed with a kite-like mooring system, which is promising to harvest renewable marine resources due to its higher energy conversion efficiency, and lower cost, as well as the minimum impacts on the submarine environment. However, the multi-turbines deployment is still a great challenge due to the interaction between the wake flow generated by each device. Understanding the wake development is critical to implementing a strategy for multi-turbine deployment and minimizing the impact of the array on the submarine environment, the shore bed and the coast.
This work aims to define the wake morphology and the multi-device configuration for the open centre turbines by applying the computational fluid dynamic (CFD) analysis, and the Jensen model, validated by scaled experiments for traditional turbines.
The first step of the research deals with a stand-alone fully resolved turbine geometry. The annular rotor works like a Venturi channel: the flow passing through the central hole rises its velocity and reduces the pressure behind the rotor plane. The induced suction effect reduces the tangential flow velocity components, containing either the wake radial expansion to 1.6R (turbine radius) and axial extension to 6D (turbine diameter). The wake takes a cylindrical shape and the flow field outside this cylinder can be assumed as undisturbed.
The second step of research deals with the study of an optimal multi-device layout. The parameters to be found are the distance between rotors’ rows and turbines’ wheelbase, under the condition that the power of each turbine is almost equal. For a 2 staggered turbines layout, a wheelbase of 3D and a distance between rows of 5D allow for keeping the devices’ performances constant, being, in both cases, the array Cp = 0.414. For 3 turbines in two staggered rows, the optimal configuration is characterized by a wheelbase of 1.5D and a distance between rows of 3D, with an array Cp = 0.413.
The key to this performance is the cylindrical wake generated by the open centre rotor geometry: in the multi-device configuration any turbine is decoupled, so there is no mutual disturbance even at reduced inter-device clearances.