Some Features of the Unsteady Pressure Field in Transonic Airfoil Buffeting

Comparisons are made between the unsteady transonic flowfields of two airfoils: a Whitcomb supercritical airfoil, and a conventional NACA 0012 section. Wind tunnel experiments on these airfoils included penetration into buffeting as a result of high cl and/or high M^. Fluctuating surface pressure, lift, and shock location were measured on both airfoils. Two-point pressure cross-correlations were used to determine coherence and propagation direction of pressure fluctuation patterns on the upper surface of each airfoil. Between the uppersurface shock and the trailing edge (a region with intense pressure fluctuations), pressure disturbances propagated upstream in attached flow, but traveled downstream when extensive separation existed. In the latter case, convection velocities were found to be frequency dependent. Another cross-correlation, relating surfacepressure fluctuations to unsteady lift, was employed to establish which regions of the pressure fields were of primary importance in producing buffeting forces. While many of the principal features of the pressure/lift cross-correlations were common to both airfoils, some specific differences were found. For example, the supercritical airfoil exhibited less periodicity in its cross-correlation. This result was attributed to the flattopped, aft-cambered shape of the supercritical airfoil section, which reduced the coupling between shock oscillations and lift fluctuations.