High Frequency Sloshing - Energy Dissipation and Viscous Damping through CFD

Liquid sloshing is an important issue in ground transportation, aerospace and automotive applications. Effects of sloshing in a moving liquid container can cause various issues related to vehicle stability, safety, component fatigue, audible noise and, liquid level measurement. The sloshing phenomenon is a highly nonlinear oscillatory movement of the free-surface of liquid inside a container under the effect of continuous or momentarily excitation forces. These excitation forces can result from sudden acceleration, braking, sharp turning or pitching motions. The sloshing waves generated by the excitation forces can impact on the tank surface and cause additional vibrations. For the loads with the frequencies between 2 to 200 Hz, the structural fatigue failure is a major concern for automotive applications. Also, for the dimensions associated with automotive, the fundamental frequency of the sloshing waves is usually a fraction of a Hertz, so frequencies above 2 Hz constitutes “high frequencies” for sloshing. Sloshing within liquid tanks causes rapid energy dissipation at the fluid resonant modes. Due to viscous effects (friction) the amplitude of the waves decreases over time when external excitation is stopped (liquid damping). The present work evaluates the liquid viscous damping through Computational Fluid Dynamics (CFD) at “high frequency” excitation conditions in automotive fuel tanks. These damping coefficients are important parameters for the accurate evaluation of the structural durability of fuel tank and its components. In this study, different liquid levels and liquid types were evaluated at numerous excitation frequencies in the range of 2-20 Hz. It was found that the excitation frequency of 10 Hz matches with the natural frequency of the systems with similar liquids (gasoline and water) in the particular container under study at 25% fill level. This can be observed by analyzing the Kinetic Energy (KE) after stopping the excitation. The dimensional damping constant tends to be proportional to the dynamic viscosity of the liquid.