  Scaling and mechanism of the propagation speed of turbulent fronts in pipe
flow with the Reynolds number has been a long-standing problem in the past
decades. Here, we derive an explicit scaling law of the upstream front speed,
which approaches to a power-law scaling at high Reynolds numbers and explain
the underlying mechanism. Our data show that the average wall distance of
low-speed streaks at the tip of the upstream front, where transition occurs,
appears to be constant in local wall units in the wide bulk-Reynolds-number
range investigated, between 5000 and 60000. By further assuming that the axial
propagation of velocity fluctuations at the front tip, resulting from streak
instabilities, is dominated by the advection of the local mean flow, the front
speed can be derived as an explicit function of the Reynolds number. The
derived formula agrees well with the measured speed by front tracking. Our
finding reveals the relationship between the structure and speed of a front,
which enables to obtain a close approximation of the front speed based on a
single velocity field without having to track the front over time.
