The interfacial swirling motion of two-layer liquids in a tank under orbital excitations

The swirling motion of the interface of two-layer liquids in a tank with a square base is investigated experimentally in this study. The tank was fixed on a platform and the horizontal circular-orbital excitations were applied. Both resonant and off-resonant responses were studied and the profiles of the interface between two liquids along two adjacent vertical walls of the tank were recorded and processed by using the image processing method. When the frequency of the excitation was equal to the lowest natural frequency of the interface between two liquids, the resonant response of the interface was triggered as swirling waves with strong nonlinearity. Instead of being a circular shape, the parametric curve was more of a triangular shape. The modal analysis revealed that this is caused by the additional contributions from the nonlinear modes of sloshing waves with multiple times of the natural frequency. For off-resonant cases, the nonlinear contribution is weaker and the parametric curve is, thus, close to a circular shape.