NUMERICAL STUDY OF SURFACE SUCTION EFFECTS ON THE AERODYNAMIC PERFORMANCE OF A NACA 4412 AIRFOIL AT LOW REYNOLDS NUMBERS

Abstract

Early separation of the boundary layer and the appearance of a Laminar Separation Bubble (LSB) can result in reduced lift and increased drag at low Reynolds numbers. The aerodynamic performance of a National Advisory Committee for Aeronautics (NACA) 4412 airfoil is numerically studied in the present work, under the influence of suction at its surface, with two-dimensional Computational Fluid Dynamics (CFD) simulations in ANSYS Fluent. The influence of suction on lift coefficient, drag coefficient, pressure coefficient distribution and LSB characteristics was assessed by simulations performed at Reynolds numbers of 25,000, 35,000, 45,000 and 50,000 for various angles of attack. The Transition SST turbulence model was used to properly simulate the transition behaviour and separation phenomena. Suction was applied through a 0.002 w/c slot located on the upper surface over 0.20 < x/c < 0.40, with suction coefficients (C_q) of 0.002, 0.003 and 0.005. The results show that the aerodynamics of the airfoil can be significantly enhanced by the use of surface suction, which can energise the boundary layer and inhibit flow separation. At 25,000 Re and an angle of attack of 10°, the lift coefficient increased from 0.833 to 1.020 (22.5%), while the drag coefficient decreased from 0.200 to 0.170 (15.0%). Using pressure coefficient analysis, at 50,000 Re and angle of attack of 8^o, it was found that the suction peak at the leading edge was increased by 9–15% and the pressure plateau at the LSB was completely removed. At a suction coefficient (C_q) of 0.002, the length of the LSB was decreased to 0.10c (66.7%), but the suction coefficients of 0.003 and 0.005 fully eliminated the LSB. Overall, surface suction progressively suppressed the laminar separation bubble, reducing its length by 66.7% at 0.002 C_q and completely eliminating it at 0.003 C_q and 0.005 C_q, which corresponded to improved aerodynamic performance through increased lift and reduced drag.