Aerodynamic characterization of a Griffith-type transonic, laminar-flow airfoil

An experimental investigation was conducted at the University of Illinois’ Transonic Wind Tunnel Facility on a Griffith-type transonic airfoil to evaluate the effectiveness of its laminar flow qualities and boundary-layer flow-control characteristics in the transonic regime. Airfoil surface and wake pressure data were acquired to characterize the aerodynamic performance across a range of α = -2˚ to 2˚ and M = 0.3 to 0.7. In addition, surface-oil flow visualization, PIV, and Schlieren imaging were performed to identify the suction influence on boundary-layer transition, momemtum deficit alleviation in the wake, and stability of transonic shocks. It was observed from the pressure distributions that the flow control application had a beneficial influence, allowing for a more aggressive pressure recovery downstream of the suction slot resulting in higher recovery pressure values at the trailing edge. At the design conditions of M = 0.7 and α = 0˚, a net profile drag reduction of 10.70% and an increase in the lift-to-drag ratio of 14.68% were observed when compared to no-suction conditions. Velocity flow field contours obtained from the PIV data showed an increase in the wake velocity magnitude for all Mach numbers at α = 0˚, displaying excellent agreement for the drag data at these conditions. Surface-oil flow visualization experiments revealed that the airfoil experienced extensive laminar flow runs regardless of suction application due to the low-Reynolds number test condition. The laminar flow was found to be shock limited followed by a laminar separation bubble in most cases. From the Schlieren experiments, a characteristic frequency of 22.38 Hz for the shock oscillatory process was identified at the design conditions which also was observed to stabilize under the influence of suction. In general, the improvements in aerodynamic efficiency and stability of the shock resulting from suction were observed to be greatest at higher angles of attack where the boundary layer was subjected to stronger-unforced pressure gradients