Computational investigation of wind tunnel wall effects on buffeting flow and lock-in for an airfoil at high angle of attack

Wind tunnel walls may significantly affect the aerodynamic characteristics of an airfoil if the distance between walls is not large enough and the experimental data may be influenced by the confinement effect. The purpose of this paper is to investigate the wall effects on a NACA 0012 airfoil at high angle of attack (the flow over the airfoil is assumed to be fully separated). The effects on both a static airfoil and also an oscillating airfoil (pitching motion) are studied. Reynolds number varies from 10⁵ to 3×10⁵. For a static airfoil, there is a dominant frequency of the flow oscillations called the Strouhal or buffet frequency. Strouhal number of an airfoil in confined flow is affected by blockage ratio and angle of attack. A new wall correction formula for the Strouhal number has been determined. For an oscillating airfoil, lock-in is determined by a combination of excitation frequency and amplitude of the oscillating airfoil. The reason of difference between computational and experimental lock-in regions are explained. Lock-in region decreases with increase of blockage ratio. Reynolds number has slight impact on lock-in region, both in unconfined flow and confined flow.