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Asymptotic analysis of a fourth-order turning-point problem in hydrodynamic stability
( Department of Mathematical Sciences, Indiana University-Purdue University, Indianapolis, IN 46202-3216, USA )
An asymptotic analysis is made of the so-called Perkeris modes of the Orr-Sommerfeld problem for plane Poiseuille flow. These are damped modes of the 'centre' or 'fast' type for which cr
0 as
R
. The numerical results obtained by Orszag (J. Fluid Mech. 50 (1971) 689) for
= 1 and R = 10 000 showed that the eigenvalues for the even and odd modes of this type are very nearly equal, and one of the goals of this paper is to provide an analytical explanation for this rather striking result. Under certain simplifying assumptions we are led to a fourth-order equation which can be viewed as a generalization of Weber's equation for the parabolic cylinder functions. The eigenvalue problem is then posed on an infinite interval and we find that the eigenvalue relations for the even and odd modes are indeed the same even though the underlying analysis is significantly different in the two cases. Explicit results are also given for both the even and odd eigenfunctions. The even eigenfunctions are similar to the Whittaker functions Dn(x) but the odd eigenfunctions involve Dawson's integral and certain polynomials.