  In this work, the stability and transition to turbulence over an experimental
blunt flat plate with different leading-edge radii are investigated. The
freestream Mach number is 5, the unit Reynolds number is $6\times10^7$
m$^{-1}$, and the maximum nose-tip radius 3 mm exceeds the experimental
reversal value. High-resolution numerical simulation and stability analysis are
performed. Three-dimensional broadband perturbation is added on the farfield
boundary to initiate the transition. The highlight of this work is that the
complete physical process is considered, including the three-dimensional
receptivity, linear and nonlinear instabilities, and transition. The
experimental reversal phenomenon is favourably reproduced in the numerical
simulation for the first time. Linear stability analysis shows that unstable
first and second modes are absent in the blunt-plate flows owing to the
presence of the entropy layer, although these modes are evident in the
sharp-leading-edge case. Therefore, the transition on the blunt plate is due to
nonmodal instabilities. Numerical results for all the blunt-plate cases reveal
the formation of streamwise streaky structures downstream of the nose (stage I)
and then the presence of intermittent turbulent spots in the transitional
region (stage II). In stage I, a preferential spanwise wavelength of around 0.9
mm is selected for all the nose-tip radii, and low-frequency components are
dominant. In stage II, high-frequency secondary instabilities appear to grow,
which participate in the eventual breakdown. By contrast, leading-edge streaks
are not remarkable in the sharp-leading-edge case, where transition is induced
by oblique first and Mack second modes. The transition reversal beyond the
critical nose-tip radius arises from an increasing magnitude of the streaky
response in the early stage, while the transition mechanism keeps similar
qualitatively.
