The lift of an aircraft can be effectively enhanced by circulation control (CC) technology at subsonic speeds, but the efficiency at transonic speeds is greatly decreased. The underlying mechanism of this phenomenon is not fully understood. In this study, Reynolds averaged Navier—Stokes simulation with k−ω shear stress transport model was utilized to investigate the mechanism of lift enhancement by CC in transonic flow. For validation, the numerical CC results were compared with the NASA experimental data obtained for transonic CC airfoil. Thereafter, the RAE2822 airfoil was modified with a Coanda surface. The lift enhancement effects of CC via steady blowing with different momentum coefficients were tested at Ma=0.3 and 0.8 at α=3∘, and various fluid mechanics phenomena were investigated. The results indicate that the flow structure of the CC jet is insensitive to the incoming flow conditions because of the similarity to the local static pressure field around the trailing edge of the airfoil. Owing to the appearance of shockwaves on the airfoil surface in the transonic regime, the performance of the CC jet is restricted to the trailing edge of the airfoil. Transonic CC achieved a slight improvement in aerodynamic performance owing to a favorable shift in the shockwave pattern and accelerated flow in the separation region on the airfoil surfaces. Revealing the mechanism of lift enhancement of CC in the transonic regime can facilitate the rational design of new fluidic actuators with high activity and expand the potential applications of CC technology.

The performance of a single expansion ramp nozzle (SERN) drastically declines on over-expanded conditions. A numerical code can accurately predict nozzle performance in the over-expanded state, which is crucial for the SERN configuration design. A Reynolds-averaged Navier–Stokes (RANS) simulation of the SERN jet in an over-expanded state was performed to verify the numerical performance of the well-established commercial CFD solver (ANSYS FluentTM v202) and rhoCentralFoam solver in OpenFOAM. The wall pressure distributions and flow field characteristics including the shock structures and the width of the jet were studied in detail with an inlet nozzle pressure ratio (NPR) of 1.5, 3, 4, and 8. The SERN aerodynamic performance with an inlet NPR ranging from 1.5 to 9 was then calculated. The results showed that the Fluent 3D simulation could qualitatively predict the characteristics of the internal and external flow of the nozzle, because it overestimated the wall pressure and shock wave position. Two-dimensional (2D) simulations made it difficult to capture the external flow structure due to the 3D effects. The simulation results of rhoCentralFoam for over-expanded SERN flow were not ideal. The Fluent can produce physical solutions, and it achieved limited success. The existing errors were mainly caused by the inlet boundary setting.