  We investigated the propagation of turbulent fronts in pipe flow at high
Reynolds numbers by direct numerical simulation. We used a technique combining
a moving frame of reference and an artificial damping to isolate the fronts in
short periodic pipes, which enables us to explore the bulk Reynolds number up
to Re = $10^5$ with affordable computation power. We measured the propagation
speed of the downstream front and observed that a fit of
$1.971-(Re/1925)^{-0.825}$ (in unit of bulk speed) well captures this speed
above $Re\simeq 5000$. The speed increases monotonically as Re increases, in
stark contrast to the decreasing trend above $Re\simeq 10000$ reported by
Wygnanski & Champagne (1973). The speed of the upstream front overall agrees
with the former studies and $0.024 + (Re/1936)^{-0.528}$ well fits our data and
those from the literature. Based on our analysis of the front dynamics, we
proposed that both front speeds would keep their respective monotonic trends as
the Reynolds number increases further. We show that, at high Reynolds numbers,
the local transition at the upstream front tip is via high-azimuthal-wavenumber
structures in the high-shear region near the pipe wall, whereas at the
downstream front tip is via low-azimuthal-wavenumber structures in the
low-shear region near the pipe center. This difference is possibly responsible
for the asymmetric speed scalings between the upstream and downstream fronts.
