Hydrokinetic turbine blades experience a complete cycle of reversed stress during each revolution in the course of deployment and operation. Blades are usually designed as thin as possible to optimize the turbine power performance and minimize the cost of manufacturing. However, significant deflections (deformations) on the blade during its operation can cause oscillatory motions, such as fluttering, which could adversely affect its structural integrity and power performance. The presented study examines two different approaches to investigate the influence of blade deflection on the performance of a tidal turbine: (1) uncoupled CFD and (2) two-way coupled fluid-structure interaction (FSI) simulations.
Tidal current turbine design studies are often conducted by uncoupled Computational Fluid Dynamics (CFD) with a simple rigid blade assumption or Finite Element Analysis (FEA) with simplified hydrodynamic loads calculated from low-fidelity methods. This simplification can cause errors in predictions of the device reliability and the Levelized Cost of Energy (LCOE). To overcome this shortcoming, coupled FSI analysis is proposed. Coupled FSI analysis solves the fluid and structural equations separately, and boundary conditions are exchanged between the two solvers at the fluid-solid interface. The two-way coupled method has feedback between both the CFD and FEA. Within a time step, a displacement of a structure estimated from the FEA is interpolated to the CFD, and a CFD mesh, therefore, deforms as a response of the structure to the external loads.
A 0.5 m diameter model scale of a generic and open-source turbine design, the Department of Energy (DOE) Reference Model 1 (RM1) turbine is used for testing the CFD and two-way coupled FSI approaches. A CFD solver, ANSYS Fluent, is coupled with an FEA solver, ANSYS Mechanical, through System Coupling for the FSI analysis. Turbine blades are allowed to move and the surrounding mesh to deform by using an overset mesh scheme and a deforming zone option in the Fluent solver. The analysis is initiated by the uncoupled CFD and one-way coupled FSI simulations to assess the convergence of the grid and time step size for each solver. Uncoupled CFD simulation is validated with experimental data for the coefficient of performance at various Tip Speed Ratios (TSR). To impose a fully developed turbulence flow at the inlet of the CFD domain, velocity profiles are extracted from the CFD simulation on a 40 m long water tunnel. Finally, hydrodynamic parameters such as coefficients of performance, torque, and thrust obtained from each method are compared, and the influence of the blade deformation on the wake behind the turbine and performance is discussed.
Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.