Momentum and heat flux events in compressible turbulent channel flows

In the present study, the momentum and heat fluxes (MF and HF) in subsonic or supersonic channel flows are studied and compared via resorting to the established analysis tools developed for incompressible flows, such as the conditional sampling and the spectral linear stochastic estimation, by leveraging the newly built database. Particular attention is paid to clarifying the effects of the inner-outer scale interactions on the statistical characteristics of MF and HF in the near-wall and logarithmic regions. To this end, by employing the spectral linear stochastic estimation, the near-wall fluxes are decomposed into large- and small-scale components, and the logarithmic-region fluxes are decomposed into active and inactive parts, respectively. For the near-wall region, the large-scale component is found to be the footprints of large-scale eddies and rather uniform in physical space, whereas the remaining small-scale component is uneven in space and includes the strong transports of momentum and heat in the near-wall region. For the logarithmic region, both the inactive and active components of MF and HF are found to contribute to their mean flux. In the outer region, the ejections of HF are remarkably stronger than those of the MF, and the former is more sparse in the physical space. Reynolds number is shown to have a minor effect on the statistical characteristics of the two fluxes, and the enlargement of the Mach number only appears to lessen the linkages between the inner and outer region fluxes, adjust the proportions of the inactive and active components in the logarithmic region, and rarely alter the overall properties of them. The findings of the present study may contribute to the development of the modeling approach in compressible wall turbulence.