This project used model-scale tank testing and fluid-structure-interaction (FSI) simulations to investigate the behavior of foils with large deflections and the effect of these deflections on crossflow turbine performance with the goal of determining the maximum allowable deflections consonant with efficiency and a robust, durable structure. A validated modeling and simulation approach was used in the design of an Ocean Renewable Power Company (ORPC) full-scale turbine. The methodology is applicable to the design of other marine hydrokinetic (MHK) devices, and to hydrokinetic turbines which experience significant deflections during operation.
Simulation and validation work focused on developing engineering tools for predicting the effect of high deflection on performance and structural longevity. The tank tests were modeled using computational fluid dynamics (CFD) and finite element analysis (FEA) simulations. Methodologies were investigated and refined to allow for load transfer between the CFD and FEA models. A fluid structure interaction (FSI) methodology was validated using the tank test data. Data and analysis show that:
• As the struts are moved, the strain range experienced by the foil changes significantly, with high strains experienced in cases with low level of structural support for the foil. Strain reflects patterns generally expected from distributed loads on beams. Strain is symmetric between inner and outer surfaces, indicating a pure bending loading.
• The hydrodynamic performance reduction from carbon to glass composite foils is very small and may well be within the error bounds of the test. This indicates that while stiffness is important from a structural viewpoint, its impact on hydrodynamic performance may be less than expected.
• The helical turbine is hydrodynamically very different from the straight foil turbines. The helical turbine tested is a highly twisted turbine and may as a result be introducing additional hydrodynamic effects when compared with the straight foil turbines.
• Testing of the titanium helical turbine indicates that surface roughness is a critical parameter for testing at the model scale.
Using the analytical methodologies developed in this work, a new high-deflection rotor for ORPC’s turbines was designed, making use of the lessons learned in model testing and analytical methods explored in the project. This improved rotor design implements improvements in foil geometry which lead to improved turbine performance and reduced structural loading, using alternative turbine strut placement and design. This updated rotor design provides a 24 percent increase in energy efficiency over the baseline efficiency. This represents a 24 percent increase in annual energy production (AEP) and a 19 percent reduction in levelized cost of energy (LCOE) for a representative ORPC Power System.