Flow topology and enstrophy production in chemically reacting compressible isotropic turbulence

Flow topology and enstrophy production in chemically reacting compressible isotropic turbulence are studied by using numerical simulations with solenoidal forcing at initial turbulent Mach numbers ranging from 0.2 to 0.6 and at initial Taylor Reynolds numbers ranging from 32 to 160. A detailed chemical kinetic mechanism including nine species and 19 elementary reactions is employed to represent the H-2/O-2 reaction in turbulence. It is found that heat release leads to an increase of internal energy and turbulent length scales. After strong heat release during the chemical reaction process, the instantly increased temperature results in the increase of viscosity and pressure work, leading to the decrease of turbulent Mach number and Taylor Reynolds number, as well as the decrease of intrinsic flow compressibility and density gradient magnitude. Various statistical properties of eight flow topologies based on the three invariants of velocity gradient tensor are investigated with a specific focus on the effect of reaction heat release and compressibility. The topologies unstable focus/compressing (UFC), unstable node/saddle/saddle (UN/S/S), and stable focus/stretching (SFS) are predominant flow patterns at three turbulent Mach numbers. The topologies UN/S/S and SFS have major contributions to the overall enstrophy production in expansion regions, while the topology UFC leads to evident destruction of enstrophy in compression regions. The strong compression motions cause the destruction of enstrophy by the interaction between the vorticity and strain rate tensor, while strong expansion motions significantly enhance the generation of enstrophy. The most probable eigenvalue ratios for the strain rate tensor at three turbulent Mach numbers are found to be -4:1:3 in the overall flow field. Overall, the heat release by chemical reaction significantly reduces the compressibility effect on the local flow topologies and enstrophy production in chemically reacting compressible turbulence.