Mode coalescence in a two-fluid boundary-layer stability problem

The triple-deck analysis of a two-fluid boundary layer stability at high Re ynolds numbers in Timoshin [J. Fluid Mech. 353, 163 (1997)] is extended to include broader parameter variations, most notably variations in the scaled film thickness. The two underlying instabilities, the Tollmien-Schlichting and interfacial waves, are shown to have points of mode coalescence locate d in the stable or unstable part of the spectrum. An interpretation of the instability mechanisms operational in two-fluid triple-deck flows is propos ed in terms of the disturbance vorticity equation. (C) 2000 American Instit ute of Physics. [S1070-6631(00)01008-4].