Wall-Resolved FSI Simulation of Modern Turbine Blades: Assessment of fidelity on the aerodynamic forces and deformation of the blade
Modern wind turbine blades feature long blades to increase the harvesting capacity of the turbine. However, with increasing the size of the blades, comes the flexibility which introduces new design aspects for modern wind turbines and requires aeroelastic and vortex-induced vibration (VIV) simulations to be considered in the design process. High-fidelity fluid-structure interaction (FSI) simulation of such blades is computationally challenging due to the combined effect of large deformations on the blade and the presence of locally or globally separated flows at high Reynolds numbers. Hence, distinct levels of fidelity are required when targeting various aspects of design such as the effect of flexibility in power generation or VIV of blades in parked conditions.
This work presents an assessment of the effect of fidelity on the captured flow features and predicted deformation of a highly deformable slender structure representing the typical turbine blades. Comparisons are made between the results of two FSI solvers, Nektar++/SHARPy FSI framework  and OpenFoam/Calculix FSI solver.
Nektar++/SHARPy FSI solver is an open-source code developed in Nektar++  framework where a quasi-3D sectional approach, thick strip method [1,3] is used for representing the full 3D domain with a series of smaller domains distributed over the structural span. Each of these strips has a finite thickness in spanwise direction to enable capturing the local 3D turbulent wake near the structure while reducing the computational cost. Incompressible Navier-Stokes equations are discretized and solved using high-order spectral/hp element method with the Fourier expansion in the spanwise direction in each strip. Navier-Stokes equations are solved in a non-inertial body-fitted frame to avoid dynamic remeshing due to high deformation of the structure. Large-Eddy simulation is used to resolve turbulent structures in fluid flow. Large-deformation dynamics of the structure is modelled using a geometrically-exact nonlinear composite beam finite-element model .
The OpenFoam/CalculiX FSI solver is developed in OpenFoam 2006 library  where the OpenFoam native flow solver is used for solving flow equations and structural dynamics are dealt with using CalculiX FEM package . The two solvers are coupled using the PreCiCE coupling package . Unsteady Reynolds-Averaged Navier-Stokes equations (URANS) are used for the full 3D simulation of turbulent flow, where a second-order spatial and temporal discretization is adopted with transient PISO method for pressure-velocity coupling. Second-order solid elements are used in CalculiX and proper material values are calculated to model the simulated object structural characteristics.
Both solvers will be first validated against the UVLM-based simulations at low angle of attack and simulation results of the two FSI solvers at several angles of attack are compared for a highly deformable slender structure. Effect of simulation fidelity, i.e. large-eddy simulation and URANS, are investigated on the turbulent flow structure near the blade and final deformation of the blade. Moreover, the effect of quasi-3D approach versus full 3D simulation is discussed and recommendations on the range of applicability of each FSI approach are made.
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