Two-dimensional transonic buffet in a supercritical wing section

Transonic buffet is a pre-stall aerodynamic instability caused by shock induced flow separation that results in self-sustained motion of the shock over the surface of the wing section. A numerical shock buffet investigation on the NASA SC(2)-0414 supercritical airfoil is presented. This airfoil forms the wing section of the NASA Benchmark Supercritical Wing (BSCW) for which wind-tunnel data exists at transonic Mach numbers. The flow is compressible and fully turbulent. The Reynolds number is 4.013x10^6. Two values of Mach number were simulated: M=0.8 for which the BSCW was observed to undergo shock buffet, and M=0.75 for which shock buffet oscillations were observed in the 2D simulations. At M=0.8, 2D RANS simulation using the Spalart-Allmaras (SA) turbulence model failed to not only capture shock-buffet but also stall at high angles of incidence. SA turbulence model with compressible mixing layer and Edwards-Chandra correction (SA-E-COMP) predicts static stall. 2D transonic shock buffet occurs at a lower Mach number of 0.75. Unsteady RANS (URANS) simulations at M=0.75 using SA-E-COMP were performed at different angles of attack to predict buffet onset, fully developed shock buffet, and buffet offset. Buffet onset occurs between 3-3.5deg. Fully developed shock buffet between 3.5−6deg. Buffet offset occurs between 6-7deg. At shock buffet onset, with an increase in angle of attack, shock moves upstream and is accompanied by an increase in shock strength. The shock buffet amplitude increases beyond buffet onset and then decreases closer to buffet offset. At buffet offset, flow downstream of the shock is fully separated. The shock buffet Strouhal number varies between 0.054-0.068 in the 3.5-6deg angle of attack range. This is within the 2D shock buffet Strouhal number limits observed experimentally. The coefficient of lift variation during shock buffet is dominated by a single frequency. Shockoscillations occur only on the suction surface of the airfoil and is present throughout the buffet cycle but varies in strength—Type IIA shock oscillation in Tijdeman [1] classification.