Skin-friction and heat-transfer decompositions in hypersonic transitional and turbulent boundary layers

The decompositions of the skin-friction and heat-transfer coefficients based on the twofold repeated integration in hypersonic transitional and turbulent boundary layers are analysed to give some major reasons of the overshoot phenomena of the wall skin friction and heat transfer. It is shown that the overshoot of the skin-friction coefficient is mainly caused by the drastic change of the mean velocity profiles, especially the strong negative streamwise gradient of the mean streamwise velocity far from the wall; and the overshoot of the heat-transfer coefficient is primarily due to the viscous dissipation, especially the strong positive vertical gradient of the mean streamwise velocity near the wall. These observations are different from the previous observations that the Reynolds shear stress and Reynolds heat flux are the reasons, respectively. Further investigations show that the above observations are independent of the set-up of the wall blowing and suction parameters, which indicates the universality of the major reasons of the overshoot phenomena in our numerical simulations. In the hypersonic turbulent boundary layers, it is observed that the strongly cooled wall temperature and the high Mach number can slightly enhance the contribution of the Reynolds shear stress, and weaken the contribution of the mean convection, mainly due to the strong compressibility effect. Moreover, the magnitudes of the relative contributions of the mean convection, pressure dilatation, viscous dissipation and the Reynolds heat flux increase as the wall temperature increases.

Automated summary

The analysis of the decompositions of the skin-friction and heat-transfer coefficients in hypersonic transitional and turbulent boundary layers reveals that the overshoot phenomena in wall skin friction and heat transfer are mainly caused by the change in mean velocity profiles and the viscous dissipation, respectively. These observations differ from previous findings that attribute the overshoot to the Reynolds shear stress and Reynolds heat flux. Additionally, it is observed that the strongly cooled wall temperature and high Mach number can slightly affect the contribution of the Reynolds shear stress and weaken the contribution of mean convection due to compressibility effects. Moreover, the relative contributions of mean convection, pressure dilatation, viscous dissipation, and the Reynolds heat flux increase with wall temperature.