Non-intrusive estimation of the buffet loads on a supercritical airfoil with SCBs

This study experimentally investigates the effect of three-dimensional shock control bumps (SCBs) on the aerodynamic loads of a supercritical airfoil under transonic buffet conditions. The experiments consisted in planar particle image velocimetry (PIV) measurements and have been carried out in the transonic-supersonic wind tunnel of TU Delft under fully developed buffet conditions (Ma=0.7 and alpha=3.5 degrees). The bumps are wedge-shaped and have been placed in the center of the shockwave oscillation range. Shock detection and phase-averaged velocity fields confirm that properly designed and spaced (Delta y(SCB)/c=25%) SCBs reduce the shockwave oscillation range (compared to the clean case). The velocity data have been further used to evaluate the pressure field around the entire airfoil, and afterward, lift and drag coefficients have been retrieved, respectively, from momentum contour and wake integral approaches. Results demonstrate that SCBs have a beneficial effect on the aerodynamic loads with an increase in lift and a decrease in drag under fully developed buffet conditions. More importantly, a strong reduction of the amplitude of oscillations of both lift and drag coefficient, within the different buffet phases, was noted. Tests at multiple spanwise locations revealed relevant differences, with lower drag and higher lift values being achieved in the symmetry plane of a SCB, while a worse performance (with values comparable to the clean case) was achieved in the symmetry plane in between two adjacent bumps.