Unsteady fluid physics and surrogate modeling of low Reynolds number, flapping airfoils

To improve our understanding of the fluid physics related to micro air vehicles (MAVs), the current work investigates the low chord Reynolds (Re) number, between 10² and 10³, fluid physics of a 2D flapping airfoil via direct numerical simulation and surrogate modeling. Addressed are the impacts of kinematic parameters and Re number under freestream/hovering conditions. The kinematic parameters include plunging amplitude, angular amplitude, and pitching/plunging phase angle. Composite surrogate models are constructed and global sensitivity evaluations of these variables are analyzed. Wake capture, delayed stall, and interaction with a jet-like flow feature all influence the performance of the airfoil. It is found that the plunging amplitude and reduced frequency play a surprisingly small role in determining the airfoil performance in the design space examined. Interestingly, in normal hovering studied here, the kinematic variables are largely uncoupled, and while the aerodynamics are complex, that the cumulative effect can be largely explained with a linear superposition of individual influences. Furthermore, delayed stall and the jet interaction exhibit major influence on the overall lift. As expected, the kinematics requiring the least amount of power occurred at high angular amplitudes, with minimum delayed stall and angle of attack at the maximum translational velocity (φ=90).