The performance and flow around an oscillating foil device for current energy extraction (a wingmill) was studied through numerical simulations. OpenFOAM was used in order to study the two-dimensional (2D) flow around a wingmill. A closed loop control law was coded in order to follow a reference angle of attack. The objective of this control law is to modify the angle of attack in order to enhance the lift force (and increase power extraction). Dimensional analysis suggests a compromise between the generator (or damper) stiffness and actuator/control gains, so a parametric study was carried out while using a new dimensionless number, called B, which represents this compromise. It was found that there is a maximum on the efficiency curve in terms of the aforementioned dimensionless parameter. The lessons that are learned from this fluid-structure and feedback coupling are discussed; this interaction, combined with the feedback dynamics, may trigger dynamic stall, thus decreasing the performance. Moreover, if the control strategy is not carefully selected, then the energy spent on the actuator may affect efficiency considerably. This type of simulation could allow for the system identification, control synthesis, and optimization of energy harvesting devices in future studies.
The dynamics of a closed loop self controlled underwater oscillating foil device for energy extraction (a wingmill) is studied through numerical simulations. The viscous two and three dimensional flow around the foil was computed using OpenFOAM and a Lattice-Boltzmann Equation model, respectively. Heaving is driven by the computed hydrodynamic lift and a damper, that extracts energy, while pitching is driven by the hydrodynamic torque and a feedback control torque that leads the foil to a given angle of attack. Unlike most of the related work found in the literature, the heaving and pitching motion of the foil is not prescribed. Dimensional analysis suggests a compromise between the generator and control gains, so a parametric study was carried out. The effect of a three dimensional finite wingspan on the performance of the device, and the flow is compared with the two dimensional case. This fully coupled fluid-solid-body interaction configuration will allow for the system identification, control and optimization of energy harvesting devices in future studies.
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