11. Characterization of Fluid-Filled Tank and Mode Shape Identification: Approach via Cryogenic Fluid Substitution by Granular Meta-material

Considering the extending use of hydrogen as a propellant in terrestrial and aerospace applications with subsequent growing needs for storage, as well as containers for transport, reserve, and distribution, cryogenic tank scrutiny holds an upmost significance. This work thus focuses on characterizing the dynamic behavior of a modeled but representative cryogenic tank to help the certification process of actual containers used in aerospace applications. The core objective of the work is the implementation of an experimental setup and ensuing structural modal analysis of a suspended vibrated tank. However, due to hazardous handling of liquid hydrogen, actual up-to-scale experimental testing is often impossible or prohibited to perform in regular conditions. To counter this hindrance, the innovative approach and main postulate of this work is to consider granular materials as substitutes to cryogenic fluids.

The aim is to obtain a modal behavior similar in terms of mode shapes and natural frequencies to the behavior of a tank filled with liquid hydrogen. Most of the studies regarding this issue are using water as a surrogate material for gas in tank testing. However, since using water as a substitute could not respect isomass and isovolume of liquefied hydrogen simultaneously, an innovative method was attempted via substitution of the gas by a granular meta-material. Apart from the initiated work on this topic, no other study was found to display this approach. The current goal is therefore to explore further possibilities in terms of material substitution as well as filling rate influence, system fixation impact, and change of excitation modes. For that purpose, investigations are carried out on an empty, fully, and partially filled tank subjected to vibration. The natural frequency reflecting the dynamic behavior of the tank for each vibration mode is measured for the different configurations. Notable frequencies of modal deformed shapes are occurring at 555, 1036, 1333, 1500, and 1600 Hz on an empty suspended aluminum tank, with associated shapes of flexion, flexion-torsion, ovalization, trefoil, quadrifoil, lobe modes, and then combined modes. Pre-trials on filled tank display the same deformed shape occurrence but at lower frequencies: 260, 539, 701, 726, and 1020 Hz. First results also show that low-density materials help reach the flexural modes, pointing the surrogate material density as important. Further results will help validate the new methodology.