Contribution of flow topology to the kinetic energy flux in hypersonic turbulent boundary layer

The contribution of various flow topologies to the subgrid-scale (SGS) flux of kinetic energy in hypersonic turbulent boundary layer for different Mach numbers and wall temperature ratios is investigated by direct numerical simulation. In the far-wall region (approximately y(+) = y/delta(nu) > 50, where y is the wall-normal location and delta(nu) is the viscous length scale), the volume fractions of flow topologies unstable focus/compressing (UFC) and stable focus/stretching (SFS) increase with the increase in filter width, resulting in the dominance of UFC and SFS in the inertial range; while in the near-wall region, the volume fractions of flow topologies unstable/saddle/saddle (UN/S/S), stable node/ saddle/saddle (SN/S/S), stable focus/compressing (SFC), and unstable focus/stretching (UFS) increase with the increase in filter width, leading to the majority of UN/S/S and SN/S/S in the inertial range. In the inertial range, the SGS flux of kinetic energy is mainly contributed by UFC and SFS far from the wall (approximately y(+) > 50) and is primarily contributed by UN/S/S and SN/S/S near the wall. The wall temperature has a significant effect on the contributions of various flow topologies in the near-wall region. As the wall temperature decreases, the contributions by SN/S/S and SFC to the SGS kinetic energy flux increase in the compression region, and those by UN/S/S and UFS increase in the expansion region. Moreover, the direct transfer of fluctuating kinetic energy from large scales to small scales is mainly characterized by UN/S/S, SFS, and SFC in the compression region, while the reverse transfer of fluctuating kinetic energy is primarily characterized by UFC, SN/S/S, and UFS in the expansion region.

Automated summary

This study investigates the contributions of various flow topologies to the subgrid-scale flux of kinetic energy in hypersonic turbulent boundary layer. The results show that the dominance of different flow topologies varies with Mach numbers, wall temperature ratios and filter widths. Wall temperature has a significant effect on the contributions of different flow topologies in the near-wall region.