The Effect of the Nutation Angle on the Flow in a Precessing Cylinder: Experimental Results

Precession-driven flows are discussed as possible sources for dynamo action in the Earth [1], the ancient moon, and in some asteroids. A precession-driven dynamo experiment is currently under construction at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) as part of the DRESDYN project. It consists of a liquid sodium filled cylinder with a radius of 1 m and a height of 2 m. The cylinder rotates at a frequency of up to 10 Hz and precesses at a frequency of up to 1 Hz around the second axis [2].
A downscaled 1:6 water mockup with the same aspect ratio and rotation and precession frequencies was developed to better understand the hydrodynamics in a precessing cylinder. The typical non-axisymmetric Kelvin mode, which initially increases as the precession ratio increases, is alone not suitable for dynamo action in the experiment. However, a secondary axisymmetric mode that appears in a narrow region of the precession ratio was demonstrated to be particularly promising for dynamo action in the sodium experiment [3].
To be able to anticipate dynamo behaviour for various precession ratios and precession angles, a complete understanding of the flow structure in the precessing cylindrical vessel is required. For that purpose, we conducted a series of precession measurements using Ultrasonic Doppler velocimetry (UDV) on the downscaled water experiment at various precession angles of 60o, 75o, and 90o. We present the effect of precession angle and rotation direction (i.e. prograde or retrograde) on the dominant flow modes, and quantify this behaviour in dependence on the rotation rate, which is parameterized by the Reynolds number Re = ΩcR2/ν, and the precession ratio Po = Ωp/Ωc, where ν is the viscosity and Ωp = 2πfp is the angular frequency of the precession. The experimental results are compared with numerical simulations [4].