Helicity transfer in compressible turbulent flows

The dual-channel characteristics of large-scale helicity transfer in compressible turbulent flows, including subgrid-scale (SGS) and viscosity terms, are investigated. After selecting a suitable definition for large-scale helicity, we confirm the existence of the dual channel of SGS and viscosity terms of large-scale helicity governing equations and theoretically prove that no dual pressure term channel exists. The second channel of the SGS and viscosity terms also consists of two terms, which originate from the rotation of the SGS stress and the baroclinic of the velocity and density gradients, respectively. The identical relationship of the ensemble averages of the dual channel of SGS and viscosity terms can be theoretically and numerically confirmed, whereas their second channel which is associated with shocklets is more intermittent. For the SGS term, the compression regions are dominant in contrast to the expansion regions, and the strain regions are dominant in contrast to the rotation regions in the inertial scale range. The viscous dissipation mechanism of large-scale helicity differs from that of large-scale kinetic energy. It is dominated by the first channel on the inside of the vortex structure and by the second channel on the outside. The further decompositions of the second channel of the SGS and viscosity terms provide a possible mechanism for the inverse helicity transfer. This means that expansion motions promote inverse helicity transfer through the second terms of their second channels.