Super-Lift and Thrusting Airfoil of Coflow Jet Actuated by Micro-Compressors

Abstract

This paper presents the wind tunnel experimental study of coflow jet (CFJ) active flow control airfoils actuated by micro-compressors embedded inside the airfoils. This is the first time that a CFJ airfoil is successfully controlled by the self-contained zero-net mass-flux (ZNMF) system. It is a crucial step to bringing the CFJ airfoil to practical aerospace applications. Furthermore, this study proves for the first time in experiment that a CFJ airfoil can achieve a Super-Lift Coefficient (SLC), which exceeds the theoretical limit of potential flow theory defined by CLmax = 2π(1 + t/c). The CFJ airfoils studied in this research were modified from the NACA 6421 airfoil geometry with a size of 0.72 m × 2.1 m (chord × span). Two airfoils were tested, one with larger injection slot size for high cruise efficiency and low CFJ power consumption, the other with smaller injection size to achieve high CLmax for takeoff/landing. The freestream velocity varies from about 4.8m/s to 16.2m/s while the Reynolds number varies from 208,000 to 691,000. The CLmax of 8.6 is achieved by the high lift takeoff/landing configuration at the low freestream speed of 4.8m/s. The CFJ airfoil also generates very high thrust with the thrust coefficient up to about 1.0. The thrust is maintained up to the airfoil stall at 40◦ AoA with a drag of CD = −0.5. Since the micro-compressors and the CFJ airfoil were designed separately, they do not work optimally together in the experiment. The micro-compressor operating line is substantially lower the the designed operating line with a severe penalty to the compressor efficiency. Future micro-compressor design needs to be tightly incorporated with the CFJ airfoil operating conditions to make use of the high compressor efficiency.