Further observations on transition in a pipe.

Fully developed Poiseuille flow in a pipe was artificially disturbed at x/D 400 and 1700 Re 4000. Puffs and slugs generated by the disturbance were identical to the structures observed when the flow in the inlet region underwent transition (Wygnanski and Champagne 1973). It was established that a slug which has all the attributes of a fully developed turbulent pipe flow is generated by the coalescence of puffs. The puff, which seems to contain a small number of toroidal eddies appears to be a fundamental coherent structure in a fully developed turbulent pipe flow. Artificially generated successions of puffs, which were allowed to interact, closely resembled a slug. The evolution of a slug from puffs was thus established.