Experimental study of libration-driven zonal flows in non-axisymmetric containers

Orbital dynamics that lead to longitudinal libration of celestial bodies also result in an elliptically deformed equatorial core–mantle boundary. The non-axisymmetry of the boundary leads to a topographic coupling between the assumed rigid mantle and the underlying low viscosity fluid. The present experimental study investigates the effect of non axisymmetric boundaries on the zonal flow driven by longitudinal libration. For large enough equatorial ellipticity, we report intermittent space-filling turbulence in particular bands of resonant frequency correlated with larger amplitude zonal flow. The mechanism underlying the intermittent turbulence has yet to be unambiguously determined. Nevertheless, recent numerical simulations in triaxial and biaxial ellipsoids suggest that it may be associated with the growth and collapse of an elliptical instability (Cébron et al., 2012). Outside of the band of resonance, we find that the background flow is laminar and the zonal flow becomes independent of the geometry at first order, in agreement with a non linear mechanism in the Ekman boundary layer (e.g., Calkins et al., 2010 and Sauret and Le Dizès, submitted for publication).

► We study experimentally the mean flow driven by longitudinal libration in axisymmetric and non-axisymmetric containers.
► We corroborate the analytical prediction of mean zonal flow driven by non-linear interaction in the Ekman boundary layer.
► We report intermittent instability in the bulk of the fluid associated with enhanced zonal flow.
► We suggest an elliptical instability mechanism may be the source of the intermittent turbulence.
► Theoretical growth rate of the elliptical instability are in good agreement with our measurements.