Dynamics and Energy Extraction of a Surging and Plunging Airfoil at Low Reynolds Number

We investigate the unsteady aerodynamic forces and energy transfer associated with harmonic surging (streamwise) and plunging (transverse) motion of a thin airfoil at low Reynolds number. Two-dimensional unsteady flows are simulated over a large range of amplitude and reduced-frequency of the oscillatory motion using the immersed boundary projection method, and the computational results are compared to inviscid flow models and experiments. At low angle of attack there is reasonable agreement with inviscid theory for the amplitude and phase of lift fluctuations, despite the low Reynolds number. At high angle of attack, the separated flow leads to larger lift and drag fluctuations not captured by inviscid models. At frequencies below the vortex shedding frequency, lift fluctuations are first enhanced and then attenuated depending on the phase between the freestream velocity and the forming leading-edge vortex. Resonance with the vortex shedding frequency also occurs. The time-averaged forces and power supplied by the oscillating airfoil are also evaluated to find frequency ranges that are favorable for the airfoil.