Geometry and dynamics of passive scalar structures in compressible turbulent mixing

We present a structure-based numerical analysis of passive scalar mixing in decaying homogeneous isotropic turbulence (DHIT) and shock-turbulence interaction canonical configurations. The analysis focuses on the temporal evolution of ensembles of passive scalar structures, initialized as spheres of different sizes relative to the Taylor microscale. An algorithm is introduced to track the evolution of each individual structure and the interactions with other structures in the ensemble, relating changes in the surface geometry and the underlying physical processes (turbulent transport, scalar dissipation, and shock compression). The tracking algorithm is applied to datasets from shock-capturing direct numerical simulations of DHIT, with Taylor microscale Reynolds number Reλ=40 and turbulence Mach number Mt=0.2, and STI cases in which the turbulence is processed by a shock wave at Mach numbers M = 1.5 and 3.0. Temporal surface convolution increases for initially larger structures, resulting in a higher probability of locally hyperbolic geometries where breakup into smaller structures occurs. Shock-induced deformation of the structures amplifies breakup processes, enhancing mixing, particularly for larger structures. Mixing enhancement by the shock is manifested as an amplification of the surface-averaged scalar gradient, which increases for initially larger structures. The alignment between the scalar gradient and the most extensional strain-rate eigendirection on the scalar isosurfaces also increases across the shock. Larger magnitudes of the scalar gradient and its alignment with the most compressive strain-rate eigendirection correlate with flatter surface regions. Shock-induced structure compression increases the area coverage of flat regions, where the amplification of scalar gradient is localized. © 2021 Author(s).     «

We present a structure-based numerical analysis of passive scalar mixing in decaying homogeneous isotropic turbulence (DHIT) and shock-turbulence interaction canonical configurations. The analysis focuses on the temporal evolution of ensembles of passive scalar structures, initialized as spheres of different sizes relative to the Taylor microscale. An algorithm is introduced to track the evolution of each individual structure and the interactions with other structures in the ensemble, relating c...     »

Funding text Financial support was provided by the Army Research Office (Grant No. W911NF2010096, Program Manager M. Munson) and the 2018 Summer Program at the Center for Turbulence Research (Stanford University). Computation for the work described in this paper was supported by the University of Southern California’s Center for High-Performance Computing and an award of computer time provided by the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. This research used resources of the Argonne Leadership Computing Facility, which is a DOE Office of Science User Facility supported under Contract No. DE-AC02-06CH11357. The authors are grateful to Professor Johan Larsson for providing the Hybrid numerical flow solver upon which extensions for passive scalar transport were implemented. Fruitful discussions with Profs. Sanjiva Lele and Lin Fu during the 2018 Summer Program at the Center for Turbulence Research, Stanford University are gratefully acknowledged.     «

Funding text Financial support was provided by the Army Research Office (Grant No. W911NF2010096, Program Manager M. Munson) and the 2018 Summer Program at the Center for Turbulence Research (Stanford University). Computation for the work described in this paper was supported by the University of Southern California’s Center for High-Performance Computing and an award of computer time provided by the Innovative and Novel Computational Impact on Theory and Experiment (INCITE) program. This resea...     »