We numerically study precession driven flows in a cylindrical container whose nutation angle varies between 60 and 90 degrees for prograde and retrograde precession. For prograde precession we observe sharp transitions between a laminar and a turbulent flow state with low and high geostrophic axisymmetric flow components related with a centrifugal instability, while for retrograde precession a rather smooth transition between a low state and a high state occurs. At the same time prograde and perpendicular precession shows an abrupt breakdown of the flow directly excited by the forcing mechanism, which is not the case for retrograde motion. We characterize the corresponding flow states in terms of the directly driven, non-axisymmetric Kelvin mode, the axisymmetric geostrophic mode, and an axisymmetric poloidal flow which is promising for precession-driven dynamo action. The latter issue is discussed with particular view on an optimal parameter choice for the DRESDYN dynamo project.

The fluid flow in a precessing cylinder is investigated numerically with focus on the Ekman boundary layers in the strongly forced regime. Not surprisingly, in that regime, we find deviations from the linear theory due to significant modifications of the base flow in terms of an axisymmetric geostrophic mode whose rotation is opposite to that of the container. The transition of the bulk flow from a three-dimensional non-axisymmetric base flow to a geostrophic axisymmetric pattern is reflected in the scaling of both the sidewall boundary layers and the Ekman boundary layers on top and bottom of the cylinder. In our simulations, the Ekman layers surpass the threshold of the first instability (class A) and show an increase in the thickness together with a marked vertical flow advection inside the boundary layer in a limited range of the forcing magnitude. However, due to numerical restrictions in our simulations, which limit the range of achievable Ekman numbers, no developed boundary layer turbulence is found. An estimation by extrapolation shows that, for this purpose, Ekman numbers smaller by a factor of two have to be achieved.

The effect of the nutation angle on the flow inside a precessing cylinder is experimentally explored and compared with numerical simulations. The focus is laid on the typical breakdown of the directly forced m =  1 Kelvin mode for increasing precession ratio (Poincaré number) and the accompanying transition between laminar and turbulent flows. Compared to the reference case with a 90° nutation angle, prograde rotation leads to an earlier breakdown, while in the retrograde case, the forced mode continues to exist also for higher Poincaré numbers. Depending largely on the occurrence and intensity of an axisymmetric double-roll mode, a kinematic dynamo study reveals a sensitive dependence of the self-excitation condition on the nutation angle and the Poincaré number. Optimal dynamo conditions are found for 90° angle which, however, might shift to slightly retrograde precession for higher Reynolds numbers.

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