Lagrangian visualization of mixing enhancement induced by finite-time stretching in compressible vortex interaction

The investigation of the coupling relationship between mixing enhancement and convective stretching bears practical and scientific significance. In this paper, Lagrangian perspective is adopted for compressible vortex interaction visualization to emphasize the effect of inner flow dynamics other than the vorticity structure on mixing performance. Transport characteristics of the flow are studied through a blob of passive particles. The results demonstrate that Lagrangian coherent structures are faithfully tracked throughout the moving of particles, and the blob initially arranged upon the ridge of Lagrangian coherent structures exhibits efficient stretching. In this sense, the physical meaning of Lagrangian coherent structure as a material barrier and maximum stretching direction is highlighted. On this basis, mixing characteristics are investigated through the introduction of helium concentration blob considering viscous diffusion. Characterized by high growth rate of mixing and low mixing time, mixing enhancement is achieved with the aid of significant stretching directly reflected by high average finite-time Lyapunov exponent in the region of blob deposition, offering an optimal strategy for fluid mixing.