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On the Lagrangian description of unsteady boundary-layer separation. Part 1. General theory

Published online by Cambridge University Press:  26 April 2006

Leon L. Van Dommelen  and
Stephen J. Cowley
Leon L. Van Dommelen
Affiliation:
Department of Mechanical Engineering, FAMU/FSU College of Engineering, PO Box 2175, Tallahassee, FL 32316-2175, USA
Stephen J. Cowley
Affiliation:
Department of Mathematics, Imperial College of Science, Technology and Medicine, Huxley Building, 180 Queen's Gate, London SW7 2BZ, UK
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Abstract

Although 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 three-dimensional separating flows follow and depend on the symmetry properties of the flow (e.g. line symmetry, axial symmetry). In the absence of any symmetry, the singularity structure just prior to separation is found to be quasi two-dimensional with a displacement thickness in the form of a crescent-shaped ridge. Physically the singularities can be understood in terms of the behaviour 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.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 210 , January 1990 , pp. 593 - 626
Copyright
© 1990 Cambridge University Press

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