Axisymmetric and lateral free surface oscillations of ferrofluids in microgravity

The term sloshing refers to the forced movement of liquids in partially filled containers. In a low-gravity environment, the liquid mixes with pressurizing gas bubbles and adopts a random position inside the vessel, resulting in unwanted perturbations and complicated tank design. Liquid sloshing has consequently represented a major concern for space engineers since the beginning of the space era. The sloshing of magnetic liquids has distinctive characteristics that suggest an alternative approach to this problem. Magnetic fields can be used to control the position, shift the natural frequencies, and increase the damping ratios of a susceptible oscillating fluid. This concept may be implemented in addition to or in substitution of classical propellant management devices, reducing the inert mass of the spacecraft, enhancing the stability properties, and minimizing attitude control disturbances. The future implementation of magnetic sloshing technologies for spacecraft control passes through the accurate understanding of basic physics and modeling capability of magnetic liquid dynamics. In this context, a coupled quasi-analytical magnetic free surface oscillation model has been recently introduced by the authors [1]. Measurements collected at ZARM's drop tower with ferrofluids during the ESA Drop Your Thesis! 2017 campaign suggest that this theoretical framework overestimates the axisymmetric natural frequencies of the system [2]. This behavior is attributed to the linearization of the theoretical solution, highlighting the need for advanced models and additional experiments. In this paper, results from the UNOOSA DropTES 2019 campaign, that studied the axisymmetric and lateral sloshing of a ferrofluid solution at ZARM's drop tower, are presented and compared with theoretical predictions. Although a qualitatively similar deviation as in the ESA Drop Your Thesis! 2017 experiment is observed for the axisymmetric sloshing frequencies, simultaneous lateral frequency measurements are in excellent agreement with theoretical predictions. This result validates the quasi-analytical model for the lateral sloshing case and motivates the development of axisymmetric Computational Fluid Dynamic simulation frameworks accounting for the complex magnetohydrodynamic coupling of the problem. Further phenomena of scientific interest, such as the magnetically-induced decrease in the hysteresis parameter at the liquid-wall contact line, are also addressed. [1] Á. Romero-Calvo, G. Cano-Gómez, E. Castro-Hernández, and F.Maggi. Free and Forced Oscillations of Magnetic Liquids Under Low-Gravity Conditions. Journal of Applied Mechanics 87.2(2020). pp. 021010 [2] Á. Romero-Calvo, M. Herrada, T. H. Hermans, L. P. Benítez, G. Cano-Gómez, and E. Castro-Hernández. Axisymmetric ferrofluid oscillations in a cylindrical tank in microgravity. Microgravity Science and Technology, submitted to the journal (2020)