Mixing enhancement characteristics and mechanism of supersonic Coandă jet

Abstract

The supersonic Coandă jet is a pivotal active flow control technique whose performance is closely tied to the evolution of shear layer mixing. Spanwise-distributed protrusions on the nozzle lip have been proven to significantly modulate mixing behavior, yet the influence mechanism of protrusion parameters (i.e., spacing and geometry) remains poorly understood. To address this research gap, the present study numerically investigates the effects of protrusion spacing (quantified as forcing wavelength λ) and geometry on the jet mixing characteristics, adopting the delayed detached-eddy simulation based on the two-equation shear stress transport k–ω turbulence model. The results reveal that protrusions induce initial streamwise vortices immediately downstream of the nozzle exit, which entrain ambient fluid and radially transport high-momentum core fluid, thus dominating the entire shear layer mixing process. The efficacy of this vortex-induced mixing enhancement depends strongly on vortex scale: larger vortices have longer persistence and a wider influence range, thus enabling more effective mixing. The vortex scale is directly determined by the imposed forcing wavelength. The effective wavelength initially coincides with the forcing wavelength, but evolves into three distinct development modes downstream: the self-sustaining mode occurs at longer forcing wavelengths (λ = 2h and 4h), while splitting or merging modes are observed at shorter forcing wavelengths (λ = 1h and 0.5h). Although vortex splitting and merging modify the effective wavelength, the eventual wavelength remains smaller than that of the unforced baseline case. Meanwhile, an excessively large forcing wavelength (λ = 4h) can induce unintended main flow deviation from the wall, which degrades overall mixing performance. Consequently, an optimal forcing wavelength exists (i.e., λ = 2h in the present study) that yields the maximum mixing efficiency. Furthermore, despite differences in vortex evolution at short forcing wavelengths, mixing characteristics are nearly identical for different protrusion geometries when the effective wavelength is the same. The only exception is triangular protrusions, which deliver inferior mixing performance due to the weaker initial flow perturbations they induce.