Numerical simulation of the critical reduced velocity (U*) in a 2-D pitching airfoil (NACA 0012) using fluid-structure interaction.

One of the most important assemblies in a wind turbine is the pitch control system that is responsible of feathering the wind turbine blades in strong winds. In this study we present the numerical analysis of the interaction between a laminar flow and an oscillating symmetric airfoil. This oscillations are generated using an undamped linear axial spring. The shear stresses and the pressure distribution of the flow on the surface of the airfoil are responsible for the spring behavior. This causes a deformation in the spring that modifies the airfoil motion hence the fluid flow is disturbed. This interaction is a function of different parameters as the angular position θ, the spring stiffness k, the reduced velocity U^*=U_∞/f_sC where U_∞ is the flow velocity, f_s is the natural frequency of the spring, and C is the chord of the airfoil, as well as the Reynolds number. The equations are solved in OpenFOAM using PIMPLE and sixDoFRigidBodyMotion. The first case is analyzed without a flow field, where a damping in the amplitude of the oscillation as a function of time is observed. As the Re increases, the damping also increases. As the Re number increases further, at the beginning the flow prevents the airfoil motion however as time passes the oscillation suddenly begins to increase.