Compressibility-induced transition in momentum-based active flow control of a co-flow jet airfoil

Active flow control through momentum injection is an effective technique for suppressing boundary layer separation and enhancing aerodynamic performance; however, the influence of compressibility on its aerodynamic performance remains insufficiently understood. This study investigates the compressibility-induced transition in momentum-based active flow control using a co-flow jet (CFJ) airfoil over a Mach number range of 0.1–0.65, spanning incompressible, compressible subsonic, and near-transonic flow regimes. Reynolds-averaged Navier–Stokes (RANS)-based computational fluid dynamics simulations are performed by solving the RANS equations with the Spalart–Allmaras turbulence model using a validated structured O-grid mesh. The analysis examines how increasing freestream Mach number modifies jet boundary-layer interaction, flow separation, lift generation, and stall characteristics under different blowing ratios. The results show that CFJ provides substantial lift enhancement and stall delay in the incompressible and low-subsonic regimes, with the greatest improvement occurring near Mach 0.5, where stall delay reaches 31.3% and lift increases by 62.6%. Beyond this regime, increasing compressibility progressively weakens the effectiveness of momentum injection, reducing the ability of the injected jet to energize the boundary layer and suppress flow separation. These findings identify the physical transition governing the performance of momentum-based active flow control across compressibility regimes and provide new insight into the interaction between jet momentum, compressibility, and boundary layer dynamics, with aircraft operating conditions serving only as representative flow environments.