Kinematic and mixing characteristics of vortex interaction induced by a vortex generator model: a numerical study

The underlying effect of vortex interaction characterized by the merging and non-merging on mixing enhancement is of fundamental significance to understand the flow dynamics of strut injectors in scramjets. Starting from a simplified configuration of a vortex generator, this study focuses on the influence of geometric parameters on vortex structures and fluid mixing through compressible Navier-Stokes (NS) simulations. By adjusting the induction of outer vortices, the inner co-rotating vortex pair exhibits two modes of interaction (merging/separation regime) reflected by closer/farther vortex centers. Defined by the zero variation rate of the inner vortex spacing, the critical state of equilibrium is determined. The critical condition is well predicted by a theoretical model based on the Biot-Savart law. Through the introduction of mixedness and mixing time, the intrinsic impact of interaction modes on fluid mixing is revealed. Compared with the vortex dynamics in the merging regime, the one in the separation regime is more effective for passive scalar mixing augmentation. With efficient material stretching characterized by the higher interface stretching factor and averaging finite-time Lyapunov exponent (FTLE), the mixing time is shortened by as much as 2.5 times in the separation regime. The implication of the present two interaction regimes in mixing enhancement physically reflected by the averaging FTLE has the potential to improve the combustion performance and shorten the combustor chamber.