Numerical Study of Reverse Thrust Generation Using Co-Flow Jet

This paper studies a new reversed thrust generation method for airfoils by use of active flow control altered from the regular Co-Flow Jet (CFJ) airfoil. This reverse thrust is generated by a CFJ injection slot duct that is redirected to flow against the freestream flow direction, rather than injecting flow tangentially to the main flow behind the leading edge. Such an injection jet can disrupt the flow with considerable flow separation, thereby decreasing the lift coefficient (C₁) and increasing the drag coefficient (C_d). Three configurations of the reverse thrust CFJ concept are derived from a CFJ-NACA 6421 CFJ airfoil to compare the performance characteristics and determine the most effective approach to achieving maximal C_dand minimal C₁ with low energy expenditure. The three configurations studied include an injection slot directed upward, an injection slot directed directly outward against the freestream, and an injection slot directed downward. The regular CFJ airfoil and the baseline airfoil with no flow control are also simulated for comparison. All three configurations are simulated, along with a regular CFJ airfoil, at a jet momentum coefficient (C_µ) of 0.03, 0.1, 0.2, 0.3, and 0.4, at angles of attack between 0° and 70°, and at a Mach number M = 0.15. The results for the CFJ configurations are compared with the baseline NACA 6421 airfoil. It reveals the forward-facing injection to be the most effective configuration in general, particularly at high AoA, between 15° and 60°, and C_µ = 0.2~0.3, with inverse aerodynamic efficiencies C_d ⁄C_l increased by up to 993% compared with the baseline NACA 6421 airfoil. The forward injection also requires substantially lower CFJ power than the downward and upward injection. At low angles of attack, AoA = 0°~10°, downward injection appears to be more effective, using C_µ = 0.2~0.3, with C_d ⁄C_l increased by 330% to 480% compared with the baseline airfoil C_d ⁄C_l .