Fluid-structure interaction of flexible lifting bodies with multi-body dynamics of order-reduced models and the actuator-line method

The development of complex energy converters generates currently unresolved demands on simulation tools. In this thesis, a model-based method to represent fluid-structure interaction is developed using the example of vertical-axis turbines. The kinematics of a turbine are simulated with a multi-body approach supporting flexible bodies. The dynamics of the flexible bodies are based on order-reduced models.

The fluid domain is represented by Reynolds-averaged Navier-Stokes equations combined with an actuator-line method, a reduced model of the blade approximated by external forces in the momentum equations. Coupling of mechanical and fluid solver is achieved by a two-way iterative approach. The convergence rate is increased by usage of a step prediction based on quasi-Newton methods. An acceleration using a proper orthogonal decomposition to decompose the system dynamics achieves the fastest convergence rates. As an alternative to the partitioned approach a monolithic approach is outlined and its restrictions by using the selected solvers are identified.

The simulation method is validated with experimental data and applied to vertical-axis turbines, both in air and water domains. Extensive validation of uncoupled flow problems and coupled dynamic response of turbines is accomplished. The results are in good agreement with experimental data and analytic solutions. Comparison to potential flow and streamtube methods shows the enhanced accuracy of the developed tool chain. Compared to available, fully meshed solutions, the computational costs are significantly lower.

Conclusively, a vertical-axis tidal turbine with flexible blades is simulated and deflections are compared to a linear solution. The local blade deflections are examined and linked to wake-body interaction.