Super-lift coefficient of active flow control airfoil: What is the limit?

This paper studies the enhancement of maximum lift coefficient and cruise efficiency using Co-Flow Jet (CFJ) active flow control airfoils. For potential flows, the maximum lift coefficient limit is derived as CLmax = 2π(1+t/c) for any airfoil with thickness of t/c. The present study indicates that the CFJ active flow control airfoil is able to achieve the maximum lift coefficient that far exceeds the theoretical limit. It is named super-lift coefficient. The research is based on validated CFD simulation, which employs 2D RANS solver with Spalart-Allmaras(S-A) turbulence model, 5th order WENO scheme for the inviscid fluxes, and 4th order central differencing for the viscous terms. The momentum coefficient Cμ studied is from 0.02 to 0.60 and the angle of attack (AoA) is from 0◦ to 74◦. Two CFJ airfoil configurations are created from the baseline NACA 6421 airfoil by translating the suction surface downward and adjusting the injection and suction slot sizes. One CFJ airfoil with smaller injection size is to achieve high CLmax for takeoff and landing. The other CFJ airfoil with larger injection size is to achieve high cruise efficiency. The maximum lift coefficient of 12.6 is achieved at AoA=70◦, M=0.063 and Cμ = 0.60. It is 66% higher than the theoretical limit of 7.6 for a 21% thickness airfoil with attached flow. The circulation achieved around the CFJ airfoil is so large that the stagnation point is detached from the airfoil solid body and the Kutta condition does not apply anymore. The CLmax appears to have no limit. It depends on how much energy can be added to the flow, which varies with the active flow control method. This study indicates that the CLmax increase is very sensitive to energy addition when the CLmax is at low level. There is almost a linear relationship between the CLmax increase and the CFJ power consumed at low CLmax level. The CLmax eventually becomes plateaued even with continuously increased consumption of CFJ power. The CLmax correlates very well with the CFJ power coefficient. For the super-lift condition at AoA of 70◦, the vortex structures in the CFJ injection region appear to include 4 vortex layers one next to each other from the airfoil wall surface to the far field freestream : 1) clockwise boundary layer vortex sheet on the airfoil suction surface; 2) counter clockwise CFJ vortex layer due to the high momentum jet and the shear layer shed from the upstream leading edge boundary layer; 3) clockwise induced vortex layer induced by the high momentum co-flow jet via the mixing shear layer; and 4) the last vortex layer is a counter clockwise vortex layer, through which the secondary induced jet transits to the slower freestream velocity. A new parameter named productivity efficiency defined as C L/CD is introduced to measure the cruise transportation capability of aircraft to carry a gross weight for maximum distance. For the second CFJ airfoil designed for cruise conditions with an assumed CFJ pumping efficiency of 80%, the peak aerodynamic efficiency (L/D)c that includes the CFJ power consumption is about 53% higher than that of the baseline airfoil. The productivity efficiency C L/CD of the CFJ airfoil is 109% higher. The CFJ airfoil is demonstrated to be able to achieve super-lift coefficient for takeoff/landing at very high angle of attack and ultra-high efficiency for cruise at low angel of attack. ∗ Ph.D. Candidate, AIAA student member † Professor, ASME Fellow, AIAA associate Fellow Approved for public release; distribution is unlimited. 1 D ow nl oa de d by G ec he ng Z ha o n M ar ch 3 , 2 01 7 | h ttp :// ar c. ai aa . o rg | D O I: 1 0. 25 14 /6 . 2 01 716 93 55th AIAA Aerospace Sciences Meeting 9 13 January 2017, Grapevine, Texas AIAA 2017-1693 Copyright © 2017 by all the authors of this paper. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. AIAA SciTech Forum