  Pipe flow often traverses a regime where laminar and turbulent flow co-exist.
Prandtl and Tietjens explained this intermittency as a feedback between the
fluctuations of the internal flow resistance and the constant pressure drop
driving the flow. However, because the focus has moved towards studying
intermittency without flow fluctuations near the universal critical Reynolds
number, their explanation has largely disappeared. Here we refine the
mechanism, which has never been put to a quantitative test, to develop a model
that agrees with experiments at higher Reynolds numbers, enabling us to
demonstrate that Prandtl and Tietjens' mechanism is, in fact, intrinsic to
flows where both the pressure gradient and perturbation are constant.
