Lagrangian Flow Separation in External Aerodynamics

Kinematic aspects of flow separation in external aerodynamics are investigated by revealing the initial motion of upwelling fluid material from the wall and its relation to the long-term attracting manifolds in the flow field. With direct numerical simulations of a circular cylinder and a cambered NACA 65(1)-412 airfoil, the location of initial fluid upwelling, the so-called spiking point, is determined from the curvature of advected material lines and from high-order numerical derivatives of the wall-normal velocity. While the short-time kinematics are governed by the formation of a material spike upstream of the zero-skin-friction point, over longer times the trajectories of the fluid tracers are guided by attracting ridges in the finite-time Lyapunov exponents once they leave the vicinity of the wall. The combination of initial fluid upwelling, asymptotic separation line, and attracting Lagrangian Coherent Structures draws a comprehensive picture of the mechanics of flow separation in external aerodynamics.