“…Such neutral modes are readily distinguishable, because of the oo..illatory nature of the eigensolutions in the free sLream, while subsonic neutral modes exhibit exponential decay of their eigensolutions in the free stream. Mack (1963Mack ( , 1965aMack ( ,b, 1969Mack ( , 1984Mack ( , 1987Mack ( , 1990 has shown how such modes are extremely important in compressible boundary-layer flows, particularly in the case of boundary layers on cooled walls, where these modes are often associated with the largest amplification rates. Such modes have also been shown to exist and be important in other classes of flow by Tam and Hu (1989), Greenough (1989), Zhuang, Kubota, and Dimotakis (1990) (confined twodimensional supersonic mixing layers), Macaraeg (1990), and by Jackson and Grosch (1989) (compressible mixing layers).…”