Abstract
Cross-flow turbines comprise an important sector of marine energy systems. Distinct from the more common axial flow turbines, they extract energy from moving water with the axis of rotation perpendicular to the freestream velocity. Previous research has investigated cross-flow turbine dynamics, both experimentally and through computational fluid dynamics (CFD) models, but most of this work utilizes a steady uniform freestream velocity to understand performance. In reality, many turbines are installed in locations where surface effects, surface waves, or other unsteady flow mechanisms may have significant impact. We investigate cross-flow turbines installed in relatively shallow waterways where waves may affect power generation by imposing oscillatory flow over the turbine blades.
Some research has started to explore the possibility that adjusting the rotation rate of cross-flow turbine blades based on the phase of surface waves can increase power generation (Zhang et al, 2014; Lust et al, 2021). To more fully understand these effects, we develop a CFD model combining steady uniform freestream flow and surface wave conditions that can accommodate a wider variation of parameters. The methodology solves the two-dimensional incompressible Navier-Stokes equations using the opensource OpenFOAM libraries. The waves are superimposed at the zeroth timestep and thereafter propagate from a boundary condition at the inlet. Wave velocities near the water surface are validated against linear wave theory approximations. The model simulates the complete dynamics of the air-water interface.
A two bladed turbine, placed horizontally in a channel, is simulated with and without the presence of waves under various operating conditions and wave parameters. We choose operating and wave parameters based on cases that have been previously studied experimentally (Lust et al, 2020), and we run simulations to compare performance of the computational model with these experimental results. We also compare these to baseline turbine simulations without waves that have previously been developed and used for comparison (Dave et al, 2021). We report on the results that include turbine performance and unsteady forces on the blades. After using the experimental data to confirm the CFD methodology, we plan to generate numerical data for multiple turbine and surface wave configurations using the model. This research will then be able to explore the effects of deep- vs shallow-water waves, turbine distance from the free surface, and other setup parameters on power generation. The results will help inform cross-flow turbine installation to maximize power generation in channels with surface waves.