Asymptotic solution of the Orr-Sommerfeld equation in the outer region of the boundary layer with allowance for nonparallelism

There have been many publications devoted to the investigation of the hydrodynamic stability of nonparallel flows on the basis of the modified Orr-Sommerfeld equation 1–4]. Taking into account the additional terms associated with the presence in the flow of a transverse component of velocity and acceleration can lead not only to a significant quantitative discrepancy as compared with calculations based on the usual Orr-Sommer-feld equation but also to qualitatively new results (nonclosure of the neutral curves for flow on a permeable surface in the presence of strong injection 4]). In this paper an asymptotic solution of the Orr-Sommer-feld equation, valid in the outer region of boundary layer flow, is constructed for self-similar gradient flow over a surface (Falkner-Skan flow). The continuity of the eigenvalue spectrum for an unbounded increase in the perturbation propagation velocity is demonstrated on the basis of the solution obtained. For the ordinary Orr-Sommerfeld equation a continuous transition of the spectrum through the value of the perturbation propagation velocity C_r=1 (which coincides with the velocity of the external flow) is impossible 5].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 1, pp. 171–173, January–February, 1987.