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Further Observations on Transition in a Pipe.

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Interim rept. Mar 78-Mar 79,

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Fully developed Poiseuille flow in a pipe was artificially disturbed at xD 400 and 17RE4000. Puffs and slugs generated by the disturbance were identical to the structures observed when the flow in the inlet region had undergone transition Wygnanski and Champagne 1973. Since the disturbance was sufficiently strong to cause transition even at low Reynolds numbers the appearance of either puffs or slugs depended on the Reynolds number only. Velocity measurements in the pipe were taken with rakes of hot wires using digital acquisition methods and in this way each realization could be observed in its entirety. The coherence of the large structures was studied in radial and azimuthal directions. Puffs and slugs generated by the disturbance were mapped and found to be identical to the structures observed at the inlet region of the pipe. It was established that a slug which has all the attibutes of a fully developed turbulent pipe flow is generated by a 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. Previous observations, which were based on a single-point measurement and ensemble-averaged data did not reveal the full structure of the puff in the same detail as the present techniques. Single realizations were analysed showing instantaneous velocity profiles, vorticity perturbation contours, as well as streamlines moving with the structure. Artificially generated successions of puffs which were allowed to interact, closely resembled a slug. The evolution of a slug from puffs was thus established. Author

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  • Fluid Mechanics

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