  In shear flows turbulence first occurs in the form of localized structures
(puffs/spots) surrounded by laminar fluid. We here investigate such spatially
intermittent flows in a pipe experiment showing that turbulent puffs have a
well defined interaction distance, which sets the minimum spacing of puffs as
well as the maximum observable turbulent fraction. Two methodologies are
employed here. Starting from a laminar flow puffs can be created by locally
injecting a jet of fluid through the pipe wall. When the perturbation is
applied periodically at low frequencies, as expected, a regular sequence of
puffs is observed where the puff spacing is given by the ratio of the mean flow
speed to the perturbation frequency. On the other hand, at large frequencies
puffs are found to interact and annihilate each other. Varying the perturbation
frequency an interaction distance can be determined. In the second set of
experiments, the Reynolds number is reduced suddenly from fully developed
turbulence to the intermittent regime.The resulting flow reorganizes itself to
a sequence of constant size puffs which, unlike in Couette and Taylor Couette
flow are randomly spaced. The minimum distance between the turbulent patches is
identical to the puff interaction length. The puff interaction length is found
to be in excellent agreement with the wavelength of regular stripe and spiral
patterns in plane Couette and Taylor-Couette flow. We propose that the same
interaction mechanism is present in these flows.
