Assessing Free Surface Effects inTidal Turbine Simulations
Pál Schmitt, Milo Feinberg, Christian Windt, Josh Davidson
For tidal turbines operating in shallow water, interactions with the free surface can have a strong effect on performance and structural loading. Accounting for blockage and scaling effects alone is insufficient. Design tools that do not consider free surface effects will reliably fail to produce accurate results, and in many cases show large variation from collected data. Especially cross-flow turbines, like the RivGen and TidGen devices developed by ORPC, operatec lose to the surface and any design tool which does not consider free surface effects, and thepossibly dramatic effect on power output and loading, will yield useless results. By far the most common way to take free surface effects into account, for example for wave propagation problems, are volume of fluid (VoF) methods. The OpenFOAM C++ framework for computational fluid dynamics offers two useful VoF implementations, inter Foam and inter IsoFoam. However, the additional computational burden when compared to a single phase fluid solver like pimple Foam is signficant, since additional variable fields, equations for species transport, and high mesh resolution in the vicinity of the free surface are required. Potential Free Surface FOAM is essentially an extension of the classic incompressible, single phase flow solver pimple Foam. A special boundary condition relates pressure p to surface elevation zetausing the hydrostatic condition. The variable zeta is only evaluated on the patch describing the free surface, allowing free surface effects to be accounted for with virtually no extra cost. Comparison with field data from ORPC trials indicates that changes in power output, thrust and surface elevation, including the development of a bore above the turbine, are captured correctly. Detailed validation against experimental tests are still required as suitable data is not yet available. The origin of potential Free Surface FOAM solver is unclear and the authors are not aware of any treatment in academic literature, but we suggest it as a useful and efficient tool for many marine and offshore engineering problems.