Evaluation of a griffith-type transonic laminar flow airfoil

An experimental test campaign was conducted on a Griffith-type laminar flow airfoil with a suction slot located between 82.5%-87.5% of the chord. This airfoil was designed to operate at a freestream M = 0.7, and experiments were performed in order to evaluate the effectiveness of its laminar flow qualities and boundary-layer flow control characteristics in the transonic regime. Wind tunnel experiments were performed in the transonic wind tunnel facility at the University of Illinois at Urbana-Champaign across an angle-of-attack range of-2° to 2° and freestream Mach numbers of 0.3-0.7. From the pressure distributions it was observed that the flow control application had a beneficial influence on the airfoil performance, as it permitted a more aggressive pressure recovery aft of the suction slot and also allowed the flow to recover to an overall higher pressure at the trailing edge. When compared to the no-suction cases, the application of moderate suction levels were observed to produce lower C_pvalues upstream of the suction slot, up to approximately x/c = 0.55 at the design Mach number. At the condition of M = 0.7 and α = 0°, a net profile drag reduction of 19.9% and an increase in the L/D ratio of 27.9% was observed. PIV experiments were performed in order to quantify the variation in wake characteristics produced with and without suction for which good agreement with the drag performance was found across every Mach number condition. Surface-oil flow visualization experiments were also performed at the design Mach number of 0.7 to find that for both suction and no-suction cases the airfoil experienced extensive laminar flow runs, which were followed by low-shear regions and in some cases a small separation region due to compressibility effects. The suction, however, delayed and decreased the extent of the low-shear region for each angle of attack while also alleviating weak shocks present at 1° and 2° angles of attack.