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The continuous spectrum of the Orr-Sommerfeld equation. Part 2. Eigenfunction expansions

Published online by Cambridge University Press:  20 April 2006

Harold Salwen  and
Chester E. Grosch
Harold Salwen
Affiliation:
Department of Physics and Engineering Physics, Stevens Institute of Technology, Hoboken, N. J. 07030
Chester E. Grosch
Affiliation:
Department of Oceanography and Department of Mathematics, Old Dominion University, Norfolk, Va. 23508
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Abstract

The expansion of an arbitrary two-dimensional solution of the linearized stream-function equation in terms of the discrete and continuum eigenfunctions of the Orr-Sommerfeld equation is discussed for flows in the half-space, y ε [0, ∞). A recent result of Salwen is used to derive a biorthogonality relation between the solution of the linearized equation for the stream function and the solutions of the adjoint problem.

For the case of temporal stability, the orthogonality relation obtained is equivalent to that of Schensted for bounded flows. This relationship is used to carry out the formal solution of the initial-value problem for temporal stability. It is found that the vorticity of the disturbance at t = 0 is the proper initial condition for the temporal stability problem. Finally, it is shown that the set consisting of the discrete eigen-modes and continuum eigenfunctions is complete.

For the spatial stability problem, it is shown that the continuous spectrum of the Orr-Sommerfeld equation contains four branches. The biorthogonality relation is used to derive the formal solution to the boundary-value problem of spatial stability. It is shown that the boundary-value problem of spatial stability requires the stream function and its first three partial derivatives with respect to x to be specified at x = 0 for all t. To be applicable to practical problems, this solution will require modification to eliminate disturbances originating at x = ∞ and travelling upstream to x = 0.

For the special case of a constant base flow, the method is used to calculate the evolution in time of a particular initial disturbance.

Type
Research Article
Information
Journal of Fluid Mechanics , Volume 104 , March 1981 , pp. 445 - 465
Copyright
© 1981 Cambridge University Press

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