Enstrophy transfer and local topology at the interfaces of large-scale structures in spatially developing compressible mixing layers
Phys. Rev. Fluids 10, 124603 – Published 4 December, 2025
DOI: https://doi.org/10.1103/txks-trgs
Abstract
Direct numerical simulations (DNSs) of spatially developing compressible mixing layers are conducted to investigate enstrophy transfer at the interfaces of high- and low-speed large-scale structures (LSSs) at convective Mach numbers () of 0.3 and 0.8. The isosurfaces of zero fluctuating streamwise velocity are detected within the turbulent region and defined as interfaces of LSSs. The spatial evolution of the interfaces of LSSs is investigated. Compared with the self-similar region, there are more intense turbulent fluctuations and vorticity in the transition region. Conditional averages at the interface coordinates exhibit a characteristic velocity jump and vorticity bump, which are larger at the leading edge than at the trailing edge. Enstrophy is mainly produced at the interfaces and transferred toward the surrounding areas by viscous diffusion. As the convective Mach number increases, the strength of the enstrophy transfer is suppressed. The two-dimensional patterns of the conditional averages of the enstrophy transfer terms near the interfaces reveal small-scale flow behaviors: the vortex is stretched at the leading edge and compressed at the trailing edge along the spanwise direction, which is related to the local topology at the interfaces of LSSs. The proportion of focal topology at the interfaces of LSSs is significantly larger than that in other turbulent regions. The proportion of unstable focus/compressing to stable focus/stretching increases at the trailing edge. Furthermore, the relative contribution of the topology regions to enstrophy transfer terms indicates that the production and diffusion terms have a preference in topology space, which is different between their positive and negative values, whereas the enstrophy dissipation shows no obvious preference. Compressibility stabilizes interfaces, reducing velocity jumps, vorticity magnitudes, and enstrophy transfer terms. Dilatation significantly modulates enstrophy transfer, with expansion enhancing and compression attenuating net enstrophy change near the interfaces of LSSs.
