Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

1 Publication
Instability of precession driven Kelvin modes: Evidence of a detuning effect
Herault, J.; Giesecke, A.; Gundrum, T.; Stefani, F.;
We report an experimental study of the instability of a nearly resonant Kelvin mode forced by precession in a cylindrical vessel. The instability is detected above a critical precession ratio via the appearance of peaks in the temporal power spectrum of pressure fluctuations measured at the end walls of the cylinder. The corresponding frequencies can be grouped into frequency sets satisfying resonance conditions with the forced Kelvin mode. We show that one set forms a triad that is associated with a parametric resonance of Kelvin modes. We observe a significant frequency variation of the unstable modes with the precession ratio, which can be explained by a detuning mechanism due to the slowdown of the background flow. By introducing a semianalytical model, we show that the departure of the flow from the solid body rotation leads to a modification of the dispersion relation of Kelvin modes and to a detuning of the resonance condition. The second frequency set includes a very low frequency and does not exhibit the properties of a parametric resonance between Kelvin modes. Interestingly, this frequency set always emerges before the occurrence of the triadic resonances, i.e., at a lower precession ratio, which implies that it may correspond to a different type of instability. We discuss the relevance of an instability of a geostrophic mode described by Kerswell [Kerswell, J. Fluid Mech. 382, 283 (1999)], although other mechanisms cannot be completely ruled out.
Keywords: Precession, Dynamo, DRESDYN


Publ.-Id: 28944 - Permalink