Published online by Cambridge University Press: 05 August 2009
The instability of a fluid inside a precessing cylinder is studiedtheoretically and experimentally. This study is motivated byaeronautics and geophysics applications. Precessional motion forceshydrodynamics waves called Kelvin modes whose structure andamplitude are predicted by a linear inviscid theory. When a forcedKelvin mode is resonant, a viscous and weakly nonlinear theory hasbeen developed to predict its saturated amplitude. We show that thisamplitude scales as Re1/2 for low Reynolds numbers and asθ1/3 (where θ is the precessing angle) for highReynolds numbers. These scalings are confirmed by PIV measurements.For Reynolds numbers sufficiently large, this forced flow becomesunstable. A linear stability analysis based on a triadic resonancebetween a forced Kelvin mode and two free modes has been carriedout. The precessing angle for which the flow becomes unstable ispredicted and compared successfully to experimental measurements. Aweakly nonlinear theory was developed and allowed to show that thebifurcation of the instability of precession is subcritical. It alsoshowed that, depending on the Reynolds number, the unstable flow canbe steady or intermittent. Finally, this weakly nonlinear theoryallowed to predict, with a good agreement with experiments, the meanflow in the cylinder; even if it is turbulent.