Backward guided modes with double zero-group-velocity points in liquid-filled pipes

For an elastic circular hollow-pipe, a backward wave possesses only one zero-group-velocity (ZGV) point at which the group velocity vanishes at a non-zero wavenumber. Based on the ZGV resonance, a non-contact laser technique has been developed to measure material properties and the wall-thickness of hollow pipes. In this paper, dispersion, excitation properties, and mode structures of backward waves and associated ZGV modes in liquid-filled pipes are investigated, in order to explore whether this ZGV-resonance technique could be applied for the inspection of pipelines that are transmitting oils. Moreover, the generation of backward waves with double ZGV points has been studied.