Visualization Study of Flow Near the Trailing Edge of an Oscillating Airfoil

For the calculation of unsteady flow around an airfoil, the following assumptions are usually made: 1) Wake is straight, 2) Shed vorticities move downstream with uniform velocity, 3) Kutta-Joukowski's condition holds at the trailing edge. To estimate the validity of these assumptions, wake pattern of a translatorily oscillating airfoil in a uniform flow was visualized by Schlieren method. Instantaneous wake patterns were photographed at reduced frequency between 1 and 19. Observed wake patterns were classified into three types. The range of reduced frequency and oscillation amplitude for each type was given. Kutta-Joukowski's condition was found to be valid at any test condition in the sense that a dividing stream line detached the airfoil at the trailing edge.