Unsteady Aerodynamic Response of a Two-Dimensional Airfoil at High Reduced Frequency

An experimental technique is described which corroborates the predictions of several new analyses of the unsteady response of an airfoil to high frequency flow fluctuations. The periodically fluctuating flowfield was produced by the natural shedding of vortices from a transverse cylinder to yield a reduced frequency of 3.9 based on airfoil semichord. Unsteady pressure measurements were made on an instrumented airfoil mounted downstream and above the turbulent wake of the cylinder. These unsteady pressures were found to be in good agreement with current compressible theories and show a chordwise variation of pressure phase angle which is not predicted by the incompressible analysis of Sears. Large reductions of the unsteady lift and phase angle were also observed for large airfoil incidence angles.