Investigation of trailing edge stall on 2D airfoils

Abstract

A combined approach of time resolved particle image velocimetry and a proper orthogonal decomposition (POD) method is adopted to investigate the complex topological structure of the flow separating from an airfoil at large angle of attack. The application of global time resolved imaging of the trailing edge separation region yields significant insight into the formation and evolution of large-scale vortical structures. The topological structure of the separated flow over an airfoil depends on the type of stall. Trailing edge stall in a supercritical flow regime is associated with the formation of a laminar separation bubble, i.e. a localized region of laminar separation, transition and reattachment, followed by separation of the turbulent boundary layer at the trailing edge. The wake region is characterized by alternately shedding of counter rotating vortices as a result of the interaction between the shear layer due to the separating boundary layer and the shear layer instability that develops at the trailing edge. Measurements have been carried out in the wake of a NACA 4415 airfoil section at high angle of attack. The ensemble of PIV snapshots has been analysed using a proper orthogonal decomposition method in order to identify and characterize coherent structures.