Wall heat flux analysis in a hypersonic turbulent boundary layer over a compression ramp

The wall heat flux (WHF) in a hypersonic turbulent boundary layer over a 24° compression ramp at Mach 6.0 is studied based on direct numerical simulation with the grid point number up to 3.5 billion and the Reynolds number based on momentum thickness of 8200. An apparent spanwise non-homogeneity of the mean WHF is observed on the compression ramp, caused by large-scale Görtler-type streamwise vortices. The pre-multiplied spanwise energy spectra of the fluctuating WHF reveal two energetic spanwise length scales in the interaction region, which are associated with the elongated streaky structures and Görtler-type vortices, respectively. The probability density functions of the fluctuating WHF in the interaction region are much more negatively skewed than that in the upstream flat-plat boundary layer, and the probability that the extremely high WHF occurs is significantly increased, both of which are found to be caused by the spanwise large-scale vortical structures via applying the low (high)-wavenumber-pass filter technique. The mean WHF in the interaction region is significantly increased, and two local maximum peaks are obtained. The upstream one lies just downstream of the reattachment position, arising from the significantly increased temperature peak value nearby and the strong downward motion that transports the relatively high-temperature fluid toward the wall. The other one is associated with the combined influence of the strong mean convection and turbulent mixing as well as the high viscous dissipation. The Reynolds analogy factor reveals an obvious variation in the interaction region, and its streamwise distribution downstream of the reattachment position is similar to the ratio of the total viscous dissipation and its component related to the wall-normal gradient of the streamwise velocity.