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Consistent section-averaged equations of quasi-one-dimensional laminar flow

Published online by Cambridge University Press:  01 July 2010

Dipartimento di Ingegneria Meccanica, Università di Salerno, 84084 Fisciano (SA), Italy
Institut de Mécanique des Fluides de Toulouse, CNRS – Université de Toulouse, 31400 Toulouse, France
Email address for correspondence:


Section-averaged equations of motion, widely adopted for slowly varying flows in pipes, channels and thin films, are usually derived from the momentum integral on a heuristic basis, although this formulation is affected by known inconsistencies. We show that starting from the energy rather than the momentum equation makes it become consistent to first order in the slowness parameter, giving the same results that have been provided until today only by a much more laborious two-dimensional solution. The kinetic-energy equation correctly provides the pressure gradient because with a suitable normalization the first-order correction to the dissipation function is identically zero. The momentum equation then correctly provides the wall shear stress. As an example, the classical stability result for a free falling liquid film is recovered straightforwardly.

Copyright © Cambridge University Press 2010

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