Keywords: Maglev, Hyperloop, vehicle-guideway interaction, wave-induced instability, parametric resonance, coupled resonance.

The high target speeds of Maglev and Hyperloop pose unique challenges for the vibrations of the vehicle and guideway. This paper discusses the nature and importance of two instability phenomena and one instability mechanism relevant to vehicle-guideway interaction. Regarding the wave-induced instability mechanism, we show using simplified models that it is mostly activated at high speeds. It typically gives rise to general oscillatory instability, and it is relevant for both active and passive levitation systems. To suppress the mechanism through guideway and levitation system design, dedicated research is needed, including experimental validation. This is especially important in view of the high target speeds and of the development of slender guideways. Speed and slenderness also necessitate thorough understanding and analysis of the parametric resonance and coupled resonance phenomena. Parametric resonance is an instability type that can occur when the guideway is periodic in space, irrespective of the levitation system type. The size of the instability zones depends on the vehicle speed and on magnitude of the periodic variation: the smaller the guideway’s bending stiffness, the larger the support spacing and/or the support stiffness, the larger the zones. To date, the severity of the parametric resonances for realistic systems is far from clear. Regarding the coupled resonance of vehicles and guideways, it is known to occur typically for small vehicle speeds and at standstill, for active levitation systems only. The phenomenon seems to be (a self-excited vibration resulting in) a limit-cycle oscillation, implying that it is not an instability type, it rather occurs when vibrations are already unstable. Detailed investigations should clarify, the outcome may even require the name “coupled resonance” to be reconsidered. We emphasize that, as it critically depends on the guideway stiffness that typically reduces significantly with increasing speed, the phenomenon may occur at high speeds even when the system is tuned to suppress it at standstill.

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