FORCING LEVEL EFFECTS OF INTERNAL ACOUSTIC EXCITATION ON THE IMPROVEMENT OF AIRFOIL PERFORMANCE

The effects of internal acoustic excitation on the leading-edge, separated boundary layers and the aerodynamic performance of NACA 63(3)-018 cross section airfoil are examined as a function of forcing level and forcing frequency of the introduced acoustics. Tests are separately conducted in two suction, open-typed wind tunnels at the Reynolds number of 3.0 x 10(5) for the measurements and 1.0 x 10(4) for the visualization. Results indicate that the flow separation is delayed at the angles of attack higher than the stalled angle of small level excitation with the forcing frequency f(e) near the shear layer instability frequency f(t). As the forcing level is increased to some extent, the velocity fluctuations around the slot exit are demonstrated to be the primary governing parameter for modifying the separated boundary layers. Data also show that the effective forcing frequency (and the Strouhal number, St) extends over wider range as compared to the lower level excitation. Meanwhile, the pressure distributions on the airfoil surface exhibit recovery behaviors with different forcing frequencies. The corresponding boundary layers are visualized to be reattached to the surface to form a recirculation region when the airfoil is around at the stalled angles.