Effects of inertia-induced surface deformation on the transient lift force of oscillating airfoils*

The effect of surface deformation on the lift force production of a sinusoidally heaving and pitching airfoil is experimentally investigated for reduced frequencies of k = fc/U = 0.10 – 0.18, pitching amplitude of θ = 70° and heaving amplitude of h₀/c = 0.6. The finite-domain impulse theory is used to estimate the transient lift force from the velocity fields measured using particle image velocimetry. To achieve surface deformation, the leading or the trailing one-third of the airfoil was attached to the main body using a hinging mechanism based on a torsion rod. The results show that airfoil deformation at the trailing edge increases the lift force production relative to the rigid airfoil, by enhancing the strength and advection velocity of the leading edge vortex. In addition, it is shown that at low reduced frequencies, the deforming trailing edge can suppress the formation of the trailing edge vortex, thereby decreasing its lift-diminishing effects. Furthermore, surface deformation at the leading edge is shown to negatively influence the lift force when compared to the rigid airfoil, by causing a premature leading edge vortex detachment from the airfoil surface.