Vortex-shedding characteristics in the wake of an oscillating airfoil at low Reynolds number

An experimental investigation was carried out to study the unsteady characteristics of vortex shedding in the near wake of an oscillating airfoil. The airfoil was given a harmonic pitching motion about the quarter-chord axis at four reduced frequencies: 0.1, 0.2, 0.3, and 0.4. The mean incidence and the oscillating amplitude were 0° and 3°, respectively. The velocity in the wake was measured by hot-wire anemometry. Smoke-wire visualization was also carried out to look into the link between the boundary layer flow and the shedding frequency variation. To analyze the frequency characteristics of the vortex shedding, we composed a short-time spectrum by the autoregressive (AR) method and a moving window. It was found that the shedding frequency in the wake of oscillating airfoil was much different from that for the case of steady airfoil at a given angle of attack, implying that the state of the boundary layer dictated the vortex-shedding characteristics. The vortex-shedding frequency was found to vary with the phase angle of oscillation, not with the angle of attack, for a given reduced frequency. The range of variation of the shedding frequency was diminished as the reduced frequency of oscillation increased.