Trailing Edge Conditions for Unsteady Flows at High Reduced Frequency

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In the prediction of time-variant flows past isolated airfoils and airfoil cascades, the steady-state KuttaJoukowsky condition is generally assumed to be valid. Recent experimental investigations on isolated airfoils however, have indicated that this assumption is only appropriate for flows characterized by low reduced frequency values. For the case of airfoil cascades, only low reduced frequency data exist, but these limited data indicate the same trend, i.e., the Kutta-Joukowsky condition is appropriate only for low reduced frequency values. Hence, the objective of the experimental investigation described herein was to determine the applicability of the Kutta-Joukowsky condition for isolated airfoils and airfoil cascades at high reduced frequency values over a range of incidence angles. This was accomplished by measuring the time-variant surface pressure distribution in the trailing edge region of a classical isolated flat plate, a classical flat plate cascade, and a cambered airfoil cascade, and correlating these data with an appropriate state-of-the-art zero incidence flat plate theoretical model that applies the Kutta-Joukowsky condition. The results obtained indicate that at these high reduced frequency values, the Kutta-Joukowsky condition is appropriate for the classical isolated flat plate and flat plate cascade over a wide range of incidence angles. However, for the cambered airfoil cascade, this condition does not appear to be satisfied for any value of incidence angle.