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On the Lagrangian description of unsteady boundary-layer separation. I - General theoryAlthough unsteady, high-Reynolds number, laminar boundary layers have conventionally been studied in terms of Eulerian coordinates, a Lagrangian approach may have significant analytical and computational advantages. In Lagrangian coordinates the classical boundary layer equations decouple into a momentum equation for the motion parallel to the boundary, and a hyperbolic continuity equation (essentially a conserved Jacobian) for the motion normal to the boundary. The momentum equations, plus the energy equation if the flow is compressible, can be solved independently of the continuity equation. Unsteady separation occurs when the continuity equation becomes singular as a result of touching characteristics, the condition for which can be expressed in terms of the solution of the momentum equations. The solutions to the momentum and energy equations remain regular. Asymptotic structures for a number of unsteady 3-D separating flows follow and depend on the symmetry properties of the flow. In the absence of any symmetry, the singularity structure just prior to separation is found to be quasi 2-D with a displacement thickness in the form of a crescent shaped ridge. Physically the singularities can be understood in terms of the behavior of a fluid element inside the boundary layer which contracts in a direction parallel to the boundary and expands normal to it, thus forcing the fluid above it to be ejected from the boundary layer.
Document ID
Document Type
Reprint (Version printed in journal)
Van Dommelen, Leon L.
(Florida State University Tallahassee, United States)
Cowley, Stephen J.
(Imperial College of Science, Technology, and Medicine London, United Kingdom)
Date Acquired
August 14, 2013
Publication Date
January 1, 1990
Publication Information
Publication: Journal of Fluid Mechanics
Volume: 210
ISSN: 0022-1120
Subject Category
Fluid Mechanics And Heat Transfer
Accession Number
Distribution Limits

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