In shear flows turbulence first occurs in the form of localized structures(puffs/spots) surrounded by laminar fluid. We here investigate such spatiallyintermittent flows in a pipe experiment showing that turbulent puffs have awell defined interaction distance, which sets the minimum spacing of puffs aswell as the maximum observable turbulent fraction. Two methodologies areemployed here. Starting from a laminar flow puffs can be created by locallyinjecting a jet of fluid through the pipe wall. When the perturbation isapplied periodically at low frequencies, as expected, a regular sequence ofpuffs is observed where the puff spacing is given by the ratio of the mean flowspeed to the perturbation frequency. On the other hand, at large frequenciespuffs are found to interact and annihilate each other. Varying the perturbationfrequency an interaction distance can be determined. In the second set ofexperiments, the Reynolds number is reduced suddenly from fully developedturbulence to the intermittent regime.The resulting flow reorganizes itself toa sequence of constant size puffs which, unlike in Couette and Taylor Couetteflow are randomly spaced. The minimum distance between the turbulent patches isidentical to the puff interaction length. The puff interaction length is foundto be in excellent agreement with the wavelength of regular stripe and spiralpatterns in plane Couette and Taylor-Couette flow. We propose that the sameinteraction mechanism is present in these flows.
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