Mechanism Analysis of the Effect of Medium Molecular Weight on Monatomic Jet-Induced Transition Promotion on Compression Surfaces

Abstract To enhance the resistance of the compression surface boundary layer to adverse pressure gradients and mitigate the risk of non-start in air-breathing engines, promoting an early transition to turbulence is crucial. This study investigates the use of wall-injected jets to trigger boundary layer transition on inlet compression surfaces, proposing a novel analytical framework that incorporates jet shear layer development analysis. The methodology involves establishing grid independence using the γθ–Reθ transition model and validating numerical simulations against wind tunnel data. Key flow parameters, including the streamwise velocity and the density, were extracted from the near-wall region downstream of micro-jets injecting monatomic gases of varying molecular weights. Analysis of the evolving jet shear layers revealed how the molecular weight of the jet injectant influences the transition process and the wall heat flux. By examining the underlying mechanisms through the shear layer flow characteristics and vorticity theory, it was determined that higher molecular weights intensify the development of the jet shear layer and associated flow perturbations. These enhanced disturbances accelerate the boundary layer transition, leading to increased skin friction and wall heat flux. Compared to helium (the lightest gas), krypton (the heaviest) shifted the transition onset and completion locations upstream by up to 32.8 and 55.7%, respectively. Despite the transition promotion, the jets reduced wall skin friction relative to the no-jet case by diminishing the Reynolds stress and the wall-normal gradient of streamwise velocity, thereby partially counteracting the associated increase in heat flux. These findings provide valuable theoretical support for advancing active flow control strategies based on jet-induced transition.