Session Cc - General Instabilities.
ORAL session, Sunday, November 23
303, Moscone Center
The primary instability of a liquid-gas mixing layer is investigated. The frequency, wavelength and convection velocity of the primary unstable two-dimensional waves are measured on a plane mixing layer set-up for broad ranges of liquid and gas velocities. Particular attention is paid to the liquid physical properties (e.g. surface tension, viscosity) and to the gas stream initial velocity profile so that the characteristic parameters of the instability are precisely identified. The convection velocity, U_c, is found to be in good agreement with a relation used for gas shear layers with density differences, U_c=\sqrt\rho_1\,U_1+\sqrt\rho_2\,U_2øver\sqrt\rho_1+\sqrt\rho_ 2 where the subscripts 1 and 2 correspond to the liquid and to the gas respectively. Whatever the gas exit conditions may be, i.e. laminar or turbulent, the instability wavelength is found to be proportional to the vorticity thickness of the initial gas stream velocity profile, \delta_ømega, and the instability frequency, f, measured in a fixed frame, is given by f\proptoU_cøver\delta_ømega. Liquid viscosity and surface tension play no role. A linear stability analysis is performed for velocity and density profiles similar to the experimental ones, and the computed eigenvalues are found to be in good agreement with the observed ones within numerical accuracy: f\delta_ømegaøverU_c\simeq 8\times10^-3. In particular, the limiting trends for f are, for laminar and turbulent exit conditions, f\sim U_2^3/2 and f\sim U_2^7/4, respectively.