Session Cc - General Instabilities.
ORAL session, Sunday, November 23
303, Moscone Center
The nonlinear interaction of instability waves may first occur within a thin critical layer where the basic mean velocity is equal to the phase speed of the instability wave. The resonant-triad interaction in an adverse-pressure-gradient boundary layer was considered. The resonant-triad is composed of a single two-dimensional mode and a pair of subharmonic oblique modes with equal and opposite spanwise wavenumbers. The frequency and streamwise wave number of the oblique modes are nearly equal to one halves of those of the two-dimensional mode. Our interest is in the case where the nonlinear interactions arise from the continued downstream growth of a triad of initially linear instability waves. The nonlinear effects in the critical layer produce a mean flow correction term which is periodic in the spanwise direction with the spanwise wavenumber equal to twice of that of the oblique modes. This mean flow distortion is very large in the sense that its magnitude is equal to that of the oblique waves.