Aerodynamic Behavior of the X-56A Airfoil During Oscillatory Plunging Motion

The effects of structural motion on the aerodynamics of the X-56A airfoil are investigated experimentally. An oscillatory plunging mechanism provides a sinusoidal plunging motion. The static aerodynamic characteristics of the X-56A airfoil model are verified against computational results from XFLR5 and thin airfoil theory. The plunging experiments are carried out at a Reynolds number of 200,000 to be comparable to CFD simulations and future 1/2 scale flight test experiments. Two nominal angles of attack were used in the plunging experiments: 10 degrees and 12 degrees. At both angles, the oscillation parameters used (k = 0.61 and 0.70, h = 0.030 - 0.048) provide effective angles of attack that extend up to and past the region associated with static stall. For the case of a nominal angle of 10 degrees, the phase-averaged lift cycles are seen to oscillate symmetrically about the static CL value and increasing the oscillation amplitude increases the magnitude of the CL variation. A comparison with CFD and Theodorsen's theory shows fair agreement, but the comparison to theory worsens as the oscillation amplitude increases. This is due to limitations in Theodorsen's theory and uncertainty in the experimental CL. An FFT of the lift shows the primary frequency to be the same as the plunging frequency for all cases. For the case of a nominal angle of 12 degrees, the phase-averaged lift cycle is no longer symmetric about the static lift value. A comparison of CL between the experimental data and CFD simulations shows reasonable agreement. Observing the pressure distribution around the airfoil at this higher angle reveals a growth-reduction cycle experienced by the laminar separation bubble which is not seen to occur in the 10 degree case. An FFT of the lift at this angle again shows the primary frequency to be equivalent to the plunging frequency. Additionally, the first harmonic is more prominent at this angle. Hot wire measurements behind the airfoil show an oscillation about the free stream value for the 10 degree cases and a drastic periodic drop in velocity for the 12 degree cases. This drop in velocity is thought to be associated with the passage of a vortex which can also be seen in CFD visualizations