Unsteady Evolution of a Laminar Separation Bubble Subjected to Wing Structural Motion

Wind tunnel experiments and direct numerical simulations are employed to investigate the laminar separation bubble that forms on the suction side of a static and plunging wing section for a modified NACA64(3)−618 airfoil at a chord Reynolds number of Re=200k. A plunging motion with an amplitude of ℎ=6% chord and a reduced frequency of k=0.67 is imposed on the wing at zero degrees angle of attack. Surface pressure, 2D Particle Image Velocimetry and Infrared Thermography measurements are used to track the unsteady evolution of the bubble along the plunging cycle and its effect on the wing loading. Modal analysis of the flow, using Proper Orthogonal Decomposition, shows that the transition process in the separated shear layer is mainly due to the amplification of Kelvin-Helmholtz instabilities, followed by shedding of spanwise coherent structures that lead to turbulent reattachment. A hysteretic behaviour is observed for the bubble size and location during the cycle. No bubble bursting is observed at these conditions, thus having a small impact on the global lift and pitching moment coefficients. Active flow control using dielectric barrier discharge plasma actuators is explored for reducing separation and extending the laminar flow region.