ANALYSES ON GENERATION MECHANISM OF SKIN FRICTION IN HIGH ENTHALPY TURBULENT BOUNDARY LAYER

When flying in low or medium attitude at very high Mach number, the surface of new hypersonic vehicles will encounter the interaction between turbulence and chemical non-equilibrium, which makes the flying environment more complicated. Generation mechanism of skin friction in such high enthalpy turbulent boundary layer is the fundamental scientific problem. The clarification of this mechanism can serve guidance for the drag reduction design, which has a significant engineering practical value. This work chose the flow condition after the leading shock of a cone in hypersonic flight, and performed direct numerical simulation (DNS) of turbulent boundary including chemical non-equilibrium effect. The low enthalpy case under the same boundary condition was set as a comparison. The RD (Renard & Deck) decomposition was utilized to analyse the dominant generation process of skin friction. The profiles of the integrand functions of main contributors were compared in detail. The influence of chemical non-equilibrium on the generation mechanism of skin friction was investigated. Furtherly, quadrant analysis technique was utilized to analyse the dominant flow events of turbulence kinetic energy production term in RD decomposition. The results show that the steaks scales of skin friction fluctuation are reduced both in streamwise and spanwise directions due to the chemical non-equilibrium effect. The molecular viscous dissipation term and the turbulence kinetic energy production term are the two main contributors to the generation of skin friction. The former mainly works in the near wall region, and the influence of high enthalpy is applied through its average portion. The profile of the integrand function of the molecular viscous dissipation term is different between high- and low enthalpy cases. The results of quadrant analysis show that the ejection and sweep events are the dominant processes for the latter term.