Motion of unstable two interfaces in a three-layer fluid with a non-zero uniform current

The nonlinear motion of two interfaces in a three-layer fluid with density stratification is investigated theoretically and numerically. We consider the situation such that a uniform current is present in one of the three layers. The linear dispersion relation is calculated by the Newton's method, from which the initial conditions for numerical computations are determined. When the uniform current is present in the upper (lower) layer, strong vorticity is induced on the upper (lower) interface, and it rolls up involving the other interface at the late stage of computations. When the current is present in the middle layer, a varicose wave appears at the initial stage, and it evolves into an asymmetric heart-shaped vortex sheet at the last computed stage. These phenomena are presented using the vortex sheet model with and without regularizations. (paper)