Dense gas effect on small-scale structures of compressible isotropic turbulence

The small-scale statistics and local flow topology of compressible homogeneous isotropic turbulence of dense gas are numerically investigated with the turbulent Mach number and Taylor Reynolds number, respectively, nearly equaling 1.0 and 153.0. The initial state of the flow field is in the inversion zone, where the fundamental derivative of gas dynamics is negative. After reaching the stationary state, the flow field includes three different gas regions: a Bethe-Zel'dovich-Thompson (BZT) region, a classical dense gas (CDG) region, and a usual gas region. In the present study, the effects of different gas regions on the statistical properties of the enstrophy production term are investigated. Based on Helmholtz decomposition, it is found that the enstrophy production mainly comes from its solenoidal component. The dense gas effect reduces the production of enstrophy in the compression region and weakens the loss of enstrophy in the expansion region. Furthermore, the properties of flow topology based on the three invariants of the velocity gradient tensor are studied. The expansion region is mainly occupied by the BZT and CDG regions. In the expansion region, the dense gas effect significantly reduces the expansive vortex structure and weakens the contribution of this structure to the enstrophy loss.