Studies of combustion effects on near-wall turbulence in supersonic flows by large eddy simulation

In scramjet combustors, combustion occurring inside supersonic boundary layers can affect the near-wall turbulence, which can further reduce wall skin friction substantially. To reveal the mechanisms of the skin-friction reduction by boundary-layer combustion, high-accuracy large eddy simulations (LES) of the Burrows-Kurkov supersonic reacting wall-jet experiment are conducted in the present research. The LES results are then employed to investigate combustion effects on characteristics of near-wall turbulence, including flow structures, intensity of turbulent fluctuations, wall skin friction and heat flux, et al. The studies show that ignition generates turbulent kinetic energy and Reynolds shear stress dramatically but they will be depressed quickly by heat release at the downstream stable combustion region. The combustion chemical reactions affect the characteristics of the near-wall turbulence to further influence wall skin friction and heat flux. Due to volumetric expansion and turbulence-generation effects of ignition, distributions of wall skin friction at the ignition region exhibit the feature of a sudden decrease following by a drastic increase. Furthermore, the changes of wall skin friction and heat flux at the ignition region and downstream stable combustion region are mainly due to the changes of wall normal gradients of mean velocity and temperature. It is also found that ignition is a dominant factor for combustion effects on near-wall turbulence, which was generally ignored in the previous studies. Therefore, ignition reaction should be taken into consideration seriously in future application of the skin-friction reduction techniques by boundary-layer combustion.