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A strongly coupled Eulerian Lagrangian method applied in unsteady 3D external flows around Wind Turbine rotors

This paper presents a hybrid CFD solver that couples a standard Eulerian approach that solves the compressible Navier-Stokes equations under a cell-centered finite volume discretization, with a Lagrangian one, based on particles representation which carry mass, pressure, dilatation and vorticity. The velocity field is calculated using the Helmholtz’s decomposition theorem. Computational performance is enhanced by employing the Particle Mesh (PM) technique in order to solve the Poisson equations for the scalar potential ϕ and the stream function ψ. Even though validated for 2D flows over airfoils, this specific solver is used for the first time in order reproduce the flow around a wind turbine rotor. The validation simulations concern axial flow over the wind turbine model rotor used in the New MEXICO experimental campaign. Results of the hybrid solver, presented as blade pressure distribution and axial flow velocities are compared against the ones produced by its pure Eulerian counterpart and experimental measurements. PM grids of up to 5 points per chord of the blade section at 75% radius have been used. Comparison with the standard Eulerian solver suggest that the produced blade loads are over-predicted by approximately 7% near the tip and 14% near the root. However, the calculated velocity field is much closer to the experimental measurements as compared to the one produced by the Eulerian approach, which is attributed to the reduced numerical diffusion of the Lagrangian-vorticity formulation.