2011 年 6 巻 4 号 p. 477-486
We analytically examine nonlinear instabilities and breakup phenomena of a viscous compound liquid jet which consists of a core and a surrounding annular phase. Applying the long wave approximation to both phases, a set of reduced nonlinear equations is derived for large deformations of the jet. Breakup phenomena are numerically examined when sinusoidal disturbances are fed at the end of the semi-infinite jet. Typical breakup profiles are shown for surface tension ratios and Reynolds numbers. In particular, for sufficiently small Reynolds numbers, the core phase is found to be choked at a bottle neck when the jet is pinching, which is followed by the ballooning of the annular phase in the upstream. It is shown that there exit the most unstable frequencies of input disturbances for each parameters of the surface tension, viscosity and amplitudes of disturbances. From variations of the breakup time and distance for such frequencies, it is expected that there exist critical Weber numbers, above which the jet becomes convectively unstable and below which absolutely unstable.